draft-ietf-opsec-ip-security-02.txt   draft-ietf-opsec-ip-security-03.txt 
Operational Security Capabilities F. Gont Operational Security Capabilities F. Gont
for IP Network Infrastructure UK CPNI for IP Network Infrastructure UK CPNI
(opsec) February 20, 2010 (opsec) April 13, 2010
Internet-Draft Internet-Draft
Intended status: Informational Intended status: Informational
Expires: August 24, 2010 Expires: October 15, 2010
Security Assessment of the Internet Protocol version 4 Security Assessment of the Internet Protocol version 4
draft-ietf-opsec-ip-security-02.txt draft-ietf-opsec-ip-security-03.txt
Abstract Abstract
This document contains a security assessment of the IETF This document contains a security assessment of the IETF
specifications of the Internet Protocol version 4, and of a number of specifications of the Internet Protocol version 4, and of a number of
mechanisms and policies in use by popular IPv4 implementations. It mechanisms and policies in use by popular IPv4 implementations. It
is based on the results of a project carried out by the UK's Centre is based on the results of a project carried out by the UK's Centre
for the Protection of National Infrastructure (CPNI). for the Protection of National Infrastructure (CPNI).
Status of this Memo Status of this Memo
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Table of Contents Table of Contents
1. Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Scope of this document . . . . . . . . . . . . . . . . . . 7 1.2. Scope of this document . . . . . . . . . . . . . . . . . . 7
1.3. Organization of this document . . . . . . . . . . . . . . 7 1.3. Organization of this document . . . . . . . . . . . . . . 7
2. The Internet Protocol . . . . . . . . . . . . . . . . . . . . 7 2. The Internet Protocol . . . . . . . . . . . . . . . . . . . . 8
3. Internet Protocol header fields . . . . . . . . . . . . . . . 8 3. Internet Protocol Header Fields . . . . . . . . . . . . . . . 8
3.1. Version . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Version . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. IHL (Internet Header Length) . . . . . . . . . . . . . . . 9 3.2. IHL (Internet Header Length) . . . . . . . . . . . . . . . 10
3.3. Differentiated Services field . . . . . . . . . . . . . . 10 3.3. Type of Service . . . . . . . . . . . . . . . . . . . . . 11
3.4. Explicit Congestion Notification (ECN) . . . . . . . . . . 11 3.3.1. Original Interpretation . . . . . . . . . . . . . . . 11
3.5. Total Length . . . . . . . . . . . . . . . . . . . . . . . 12 3.3.2. Standard Interpretation . . . . . . . . . . . . . . . 12
3.6. Identification (ID) . . . . . . . . . . . . . . . . . . . 13 3.3.2.1. Differentiated Services field . . . . . . . . . . 12
3.6.1. Some workarounds implemented by the industry . . . . . 14 3.3.2.2. Explicit Congestion Notification (ECN) . . . . . 13
3.6.2. Possible security improvements . . . . . . . . . . . . 14 3.4. Total Length . . . . . . . . . . . . . . . . . . . . . . . 14
3.7. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.5. Identification (ID) . . . . . . . . . . . . . . . . . . . 15
3.8. Fragment Offset . . . . . . . . . . . . . . . . . . . . . 18 3.5.1. Some Workarounds Implemented by the Industry . . . . . 16
3.9. Time to Live (TTL) . . . . . . . . . . . . . . . . . . . . 19 3.5.2. Possible security improvements . . . . . . . . . . . . 17
3.9.1. Fingerprinting the operating system in use by the 3.5.2.1. Connection-Oriented Transport Protocols . . . . . 17
source host . . . . . . . . . . . . . . . . . . . . . 20 3.5.2.2. Connectionless Transport Protocols . . . . . . . 18
3.9.2. Fingerprinting the physical device from which the 3.6. Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 19
packets originate . . . . . . . . . . . . . . . . . . 20 3.7. Fragment Offset . . . . . . . . . . . . . . . . . . . . . 21
3.9.3. Locating the source host in the network topology . . . 20 3.8. Time to Live (TTL) . . . . . . . . . . . . . . . . . . . . 22
3.9.4. Evading Network Intrusion Detection Systems . . . . . 22 3.8.1. Fingerprinting the operating system in use by the
3.9.5. Improving the security of applications that make source host . . . . . . . . . . . . . . . . . . . . . 24
use of the Internet Protocol (IP) . . . . . . . . . . 22 3.8.2. Fingerprinting the physical device from which the
3.10. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 23 packets originate . . . . . . . . . . . . . . . . . . 24
3.11. Header Checksum . . . . . . . . . . . . . . . . . . . . . 24 3.8.3. Mapping the Network Topology . . . . . . . . . . . . . 24
3.12. Source Address . . . . . . . . . . . . . . . . . . . . . . 24 3.8.4. Locating the source host in the network topology . . . 25
3.13. Destination Address . . . . . . . . . . . . . . . . . . . 25 3.8.5. Evading Network Intrusion Detection Systems . . . . . 26
3.14. Options . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.8.6. Improving the security of applications that make
3.14.1. General issues with IP options . . . . . . . . . . . . 26 use of the Internet Protocol (IP) . . . . . . . . . . 27
3.14.1.1. Processing requirements . . . . . . . . . . . . . 26 3.8.7. Limiting spread . . . . . . . . . . . . . . . . . . . 28
3.14.1.2. Processing of the options by the upper layer 3.9. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 28
protocol . . . . . . . . . . . . . . . . . . . . 27 3.10. Header Checksum . . . . . . . . . . . . . . . . . . . . . 28
3.14.1.3. General sanity checks on IP options . . . . . . . 27 3.11. Source Address . . . . . . . . . . . . . . . . . . . . . . 28
3.14.2. Issues with specific options . . . . . . . . . . . . . 29 3.12. Destination Address . . . . . . . . . . . . . . . . . . . 29
3.14.2.1. End of Option List (Type = 0) . . . . . . . . . . 29 3.13. Options . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.14.2.2. No Operation (Type = 1) . . . . . . . . . . . . . 29 3.13.1. General issues with IP options . . . . . . . . . . . . 31
3.14.2.3. Loose Source Record Route (LSRR) (Type = 131) . . 29 3.13.1.1. Processing requirements . . . . . . . . . . . . . 31
3.14.2.4. Strict Source and Record Route (SSRR) (Type = 3.13.1.2. Processing of the options by the upper layer
137) . . . . . . . . . . . . . . . . . . . . . . 32 protocol . . . . . . . . . . . . . . . . . . . . 32
3.14.2.5. Record Route (Type = 7) . . . . . . . . . . . . . 34 3.13.1.3. General sanity checks on IP options . . . . . . . 32
3.14.2.6. Stream Identifier (Type = 136) . . . . . . . . . 36
3.14.2.7. Internet Timestamp (Type = 68) . . . . . . . . . 36 3.13.2. Issues with specific options . . . . . . . . . . . . . 33
3.14.2.8. Router Alert (Type = 148) . . . . . . . . . . . . 39 3.13.2.1. End of Option List (Type=0) . . . . . . . . . . . 34
3.14.2.9. Probe MTU (Type =11) . . . . . . . . . . . . . . 40 3.13.2.2. No Operation (Type=1) . . . . . . . . . . . . . . 34
3.14.2.10. Reply MTU (Type = 12) . . . . . . . . . . . . . . 40 3.13.2.3. Loose Source and Record Route (LSRR)
3.14.2.11. Traceroute (Type = 82) . . . . . . . . . . . . . 40 (Type=131) . . . . . . . . . . . . . . . . . . . 34
3.14.2.12. DoD Basic Security Option (Type = 130) . . . . . 41 3.13.2.4. Strict Source and Record Route (SSRR)
3.14.2.13. DoD Extended Security Option (Type = 133) . . . . 42 (Type=137) . . . . . . . . . . . . . . . . . . . 37
3.14.2.14. Commercial IP Security Option (CIPSO) (Type = 3.13.2.5. Record Route (Type=7) . . . . . . . . . . . . . . 39
134) . . . . . . . . . . . . . . . . . . . . . . 42 3.13.2.6. Stream Identifier (Type=136) . . . . . . . . . . 40
3.14.2.15. Sender Directed Multi-Destination Delivery 3.13.2.7. Internet Timestamp (Type=68) . . . . . . . . . . 40
(Type = 149) . . . . . . . . . . . . . . . . . . 43 3.13.2.8. Router Alert (Type=148) . . . . . . . . . . . . . 43
3.15. TOS . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.13.2.9. Probe MTU (Type=11) (obsolete) . . . . . . . . . 44
4. Internet Protocol Mechanisms . . . . . . . . . . . . . . . . . 45 3.13.2.10. Reply MTU (Type=12) (obsolete) . . . . . . . . . 44
4.1. Fragment reassembly . . . . . . . . . . . . . . . . . . . 45 3.13.2.11. Traceroute (Type=82) . . . . . . . . . . . . . . 44
4.1.1. Security Implications of Fragment Reassembly . . . . . 46 3.13.2.12. DoD Basic Security Option (Type=130) . . . . . . 44
4.1.1.1. Problems related with memory allocation . . . . . 46 3.13.2.13. DoD Extended Security Option (Type=133) . . . . . 46
3.13.2.14. Commercial IP Security Option (CIPSO)
(Type=134) . . . . . . . . . . . . . . . . . . . 46
3.13.2.15. Sender Directed Multi-Destination Delivery
(Type=149) . . . . . . . . . . . . . . . . . . . 47
4. Internet Protocol Mechanisms . . . . . . . . . . . . . . . . . 47
4.1. Fragment reassembly . . . . . . . . . . . . . . . . . . . 47
4.1.1. Security Implications of Fragment Reassembly . . . . . 48
4.1.1.1. Problems related with memory allocation . . . . . 48
4.1.1.2. Problems that arise from the length of the IP 4.1.1.2. Problems that arise from the length of the IP
Identification field . . . . . . . . . . . . . . 48 Identification field . . . . . . . . . . . . . . 50
4.1.1.3. Problems that arise from the complexity of 4.1.1.3. Problems that arise from the complexity of
the reassembly algorithm . . . . . . . . . . . . 48 the reassembly algorithm . . . . . . . . . . . . 51
4.1.1.4. Problems that arise from the ambiguity of the 4.1.1.4. Problems that arise from the ambiguity of the
reassembly process . . . . . . . . . . . . . . . 49 reassembly process . . . . . . . . . . . . . . . 51
4.1.1.5. Problems that arise from the size of the IP 4.1.1.5. Problems that arise from the size of the IP
fragments . . . . . . . . . . . . . . . . . . . . 50 fragments . . . . . . . . . . . . . . . . . . . . 53
4.1.2. Possible security improvements . . . . . . . . . . . . 50 4.1.2. Possible security improvements . . . . . . . . . . . . 53
4.1.2.1. Memory allocation for fragment reassembly . . . . 50 4.1.2.1. Memory allocation for fragment reassembly . . . . 53
4.1.2.2. Flushing the fragment buffer . . . . . . . . . . 51 4.1.2.2. Flushing the fragment buffer . . . . . . . . . . 54
4.1.2.3. A more selective fragment buffer flushing 4.1.2.3. A more selective fragment buffer flushing
strategy . . . . . . . . . . . . . . . . . . . . 52 strategy . . . . . . . . . . . . . . . . . . . . 55
4.1.2.4. Reducing the fragment timeout . . . . . . . . . . 54 4.1.2.4. Reducing the fragment timeout . . . . . . . . . . 57
4.1.2.5. Counter-measure for some IDS evasion 4.1.2.5. countermeasure for some IDS evasion techniques . 57
techniques . . . . . . . . . . . . . . . . . . . 55 4.1.2.6. countermeasure for firewall-rules bypassing . . . 57
4.1.2.6. Counter-measure for firewall-rules bypassing . . 55 4.2. Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 58
4.2. Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2.1. Precedence-ordered queue service . . . . . . . . . . . 58
4.2.1. Precedence-ordered queue service . . . . . . . . . . . 55 4.2.2. Weak Type of Service . . . . . . . . . . . . . . . . . 59
4.2.2. Weak Type of Service . . . . . . . . . . . . . . . . . 56 4.2.3. Impact of Address Resolution on Buffer Management . . 59
4.2.3. Address Resolution . . . . . . . . . . . . . . . . . . 57 4.2.4. Dropping packets . . . . . . . . . . . . . . . . . . . 60
4.2.4. Dropping packets . . . . . . . . . . . . . . . . . . . 58 4.3. Addressing . . . . . . . . . . . . . . . . . . . . . . . . 60
4.3. Addressing . . . . . . . . . . . . . . . . . . . . . . . . 58 4.3.1. Unreachable addresses . . . . . . . . . . . . . . . . 61
4.3.1. Unreachable addresses . . . . . . . . . . . . . . . . 58 4.3.2. Private address space . . . . . . . . . . . . . . . . 61
4.3.2. Private address space . . . . . . . . . . . . . . . . 58 4.3.3. Former Class D Addresses (224/4 Address Block) . . . . 61
4.3.3. Class D addresses (224/4 address block) . . . . . . . 59 4.3.4. Former Class E Addresses (240/4 Address Block) . . . . 62
4.3.4. Class E addresses (240/4 address block) . . . . . . . 59 4.3.5. Broadcast/Multicast addresses, and
4.3.5. Broadcast and multicast addresses, and Connection-Oriented Protocols . . . . . . . . . . . . 62
connection-oriented protocols . . . . . . . . . . . . 59 4.3.6. Broadcast and network addresses . . . . . . . . . . . 62
4.3.6. Broadcast and network addresses . . . . . . . . . . . 60 4.3.7. Special Internet addresses . . . . . . . . . . . . . . 62
4.3.7. Special Internet addresses . . . . . . . . . . . . . . 60 5. Security Considerations . . . . . . . . . . . . . . . . . . . 65
5. Security Considerations . . . . . . . . . . . . . . . . . . . 62 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 65
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 62 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.1. Normative References . . . . . . . . . . . . . . . . . . . 65
7.1. Normative References . . . . . . . . . . . . . . . . . . . 63 7.2. Informative References . . . . . . . . . . . . . . . . . . 67
7.2. Informative References . . . . . . . . . . . . . . . . . . 64 Appendix A. Advice and guidance to vendors . . . . . . . . . . . 76
Appendix A. Advice and guidance to vendors . . . . . . . . . . . 72 Appendix B. Changes from previous versions of the draft . . . . . 76
Appendix B. Changes from previous versions of the draft (to B.1. Changes from draft-ietf-opsec-ip-security-02 . . . . . . . 76
be removed by the RFC Editor before publishing B.2. Changes from draft-ietf-opsec-ip-security-01 . . . . . . . 76
this document as an RFC) . . . . . . . . . . . . . . 73 B.3. Changes from draft-ietf-opsec-ip-security-00 . . . . . . . 77
B.1. Changes from draft-ietf-opsec-ip-security-01 . . . . . . . 73 B.4. Changes from draft-gont-opsec-ip-security-01 . . . . . . . 77
B.2. Changes from draft-ietf-opsec-ip-security-00 . . . . . . . 73 B.5. Changes from draft-gont-opsec-ip-security-00 . . . . . . . 77
B.3. Changes from draft-gont-opsec-ip-security-01 . . . . . . . 73 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 77
B.4. Changes from draft-gont-opsec-ip-security-00 . . . . . . . 73
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 73
1. Preface 1. Preface
1.1. Introduction 1.1. Introduction
The TCP/IP protocols were conceived in an environment that was quite The TCP/IP protocols were conceived in an environment that was quite
different from the hostile environment they currently operate in. different from the hostile environment they currently operate in.
However, the effectiveness of the protocols led to their early However, the effectiveness of the protocols led to their early
adoption in production environments, to the point that, to some adoption in production environments, to the point that, to some
extent, the current world's economy depends on them. extent, the current world's economy depends on them.
While many textbooks and articles have created the myth that the While many textbooks and articles have created the myth that the
Internet protocols were designed for warfare environments, the top Internet protocols were designed for warfare environments, the top
level goal for the DARPA Internet Program was the sharing of large level goal for the DARPA Internet Program was the sharing of large
service machines on the ARPANET [Clark1988]. As a result, many service machines on the ARPANET [Clark1988]. As a result, many
protocol specifications focus only on the operational aspects of the protocol specifications focus only on the operational aspects of the
protocols they specify, and overlook their security implications. protocols they specify, and overlook their security implications.
While the Internet technology evolved since it inception, the While the Internet technology evolved since its inception, the
Internet's building blocks are basically the same core protocols Internet's building blocks are basically the same core protocols
adopted by the ARPANET more than two decades ago. During the last adopted by the ARPANET more than two decades ago. During the last
twenty years, many vulnerabilities have been identified in the TCP/IP twenty years, many vulnerabilities have been identified in the TCP/IP
stacks of a number of systems. Some of them were based in flaws in stacks of a number of systems. Some of them were based on flaws in
some protocol implementations, affecting only a reduced number of some protocol implementations, affecting only a reduced number of
systems, while others were based in flaws in the protocols systems, while others were based on flaws in the protocols
themselves, affecting virtually every existing implementation themselves, affecting virtually every existing implementation
[Bellovin1989]. Even in the last couple of years, researchers were [Bellovin1989]. Even in the last couple of years, researchers were
still working on security problems in the core protocols still working on security problems in the core protocols
[I-D.ietf-tcpm-icmp-attacks] [Watson2004] [NISCC2004] [NISCC2005]. [I-D.ietf-tcpm-icmp-attacks] [Watson2004] [NISCC2004] [NISCC2005].
The discovery of vulnerabilities in the TCP/IP protocols led to The discovery of vulnerabilities in the TCP/IP protocols led to
reports being published by a number of CSIRTs (Computer Security reports being published by a number of CSIRTs (Computer Security
Incident Response Teams) and vendors, which helped to raise awareness Incident Response Teams) and vendors, which helped to raise awareness
about the threats and the best mitigations known at the time the about the threats and the best mitigations known at the time the
reports were published. Unfortunately, this also led to the reports were published. Unfortunately, this also led to the
skipping to change at page 6, line 18 skipping to change at page 6, line 18
Producing a secure TCP/IP implementation nowadays is a very difficult Producing a secure TCP/IP implementation nowadays is a very difficult
task, in part because of the lack of a single document that serves as task, in part because of the lack of a single document that serves as
a security roadmap for the protocols. Implementers are faced with a security roadmap for the protocols. Implementers are faced with
the hard task of identifying relevant documentation and differentiate the hard task of identifying relevant documentation and differentiate
between that which provides correct advisory, and that which provides between that which provides correct advisory, and that which provides
misleading advisory based on inaccurate or wrong assumptions. misleading advisory based on inaccurate or wrong assumptions.
There is a clear need for a companion document to the IETF There is a clear need for a companion document to the IETF
specifications that discusses the security aspects and implications specifications that discusses the security aspects and implications
of the protocols, identifies the possible threats, discusses the of the protocols, identifies the possible threats, discusses the
possible counter-measures, and analyzes their respective possible countermeasures, and analyzes their respective
effectiveness. effectiveness.
This document is the result of an assessment the IETF specifications This document is the result of an assessment of the IETF
of the Internet Protocol (IP), from a security point of view. specifications of the Internet Protocol (IP), from a security point
Possible threats were identified and, where possible, counter- of view. Possible threats were identified and, where possible,
measures were proposed. Additionally, many implementation flaws that countermeasures were proposed. Additionally, many implementation
have led to security vulnerabilities have been referenced in the hope flaws that have led to security vulnerabilities have been referenced
that future implementations will not incur the same problems. in the hope that future implementations will not incur the same
Furthermore, this document does not limit itself to performing a problems. Furthermore, this document does not limit itself to
security assessment of the relevant IETF specifications, but also performing a security assessment of the relevant IETF specifications,
provides an assessment of common implementation strategies found in but also provides an assessment of common implementation strategies
the real world. found in the real world.
This document does not aim to be the final word on the security of This document does not aim to be the final word on the security of
the Internet Protocol (IP). On the contrary, it aims to raise the Internet Protocol (IP). On the contrary, it aims to raise
awareness about many security threats based on the IP protocol that awareness about many security threats based on the IP protocol that
have been faced in the past, those that we are currently facing, and have been faced in the past, those that we are currently facing, and
those we may still have to deal with in the future. It provides those we may still have to deal with in the future. It provides
advice for the secure implementation of the Internet Protocol (IP), advice for the secure implementation of the Internet Protocol (IP),
but also provides insights about the security aspects of the Internet but also provides insights about the security aspects of the Internet
Protocol that may be of help to the Internet operations community. Protocol that may be of help to the Internet operations community.
Feedback from the community is more than encouraged to help this Feedback from the community is more than encouraged to help this
document be as accurate as possible and to keep it updated as new document be as accurate as possible and to keep it updated as new
threats are discovered. threats are discovered.
This document is heavily based on the "Security Assessment of the This document is heavily based on the "Security Assessment of the
Internet Protocol" [CPNI2008] released by the UK Centre for the Internet Protocol" [CPNI2008] released by the UK Centre for the
Protection of National Infrastructure (CPNI), available at: Protection of National Infrastructure (CPNI), available at:
http://www.cpni.gov.uk/Products/technicalnotes/3677.aspx . http://www.cpni.gov.uk/Products/technicalnotes/3677.aspx .
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Scope of this document 1.2. Scope of this document
While there are a number of protocols that affect the way in which IP While there are a number of protocols that affect the way in which IP
systems operate, this document focuses only on the specifications of systems operate, this document focuses only on the specifications of
the Internet Protocol (IP). For example, routing and bootstrapping the Internet Protocol (IP). For example, routing and bootstrapping
protocols are considered out of the scope of this project. protocols are considered out of the scope of this project.
The following IETF RFCs were selected for assessment as part of this The following IETF RFCs were selected as the primary sources for the
work: assessment as part of this work:
o RFC 791, "Internet Protocol. DARPA Internet Program. Protocol o RFC 791, "Internet Protocol. DARPA Internet Program. Protocol
Specification" (51 pages). Specification" (51 pages).
o RFC 815, "IP datagram reassembly algorithms" (9 pages). o RFC 815, "IP datagram reassembly algorithms" (9 pages).
o RFC 919, "BROADCASTING INTERNET DATAGRAMS" (8 pages).
o RFC 950, "Internet Standard Subnetting Procedure" (18 pages)
o RFC 1112, "Host Extensions for IP Multicasting" (17 pages)
o RFC 1122, "Requirements for Internet Hosts -- Communication o RFC 1122, "Requirements for Internet Hosts -- Communication
Layers" (116 pages). Layers" (116 pages).
o RFC 1812, "Requirements for IP Version 4 Routers" (175 pages). o RFC 1812, "Requirements for IP Version 4 Routers" (175 pages).
o RFC 2474, "Definition of the Differentiated Services Field (DS o RFC 2474, "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers" (20 pages). Field) in the IPv4 and IPv6 Headers" (20 pages).
o RFC 2475, "An Architecture for Differentiated Services" (36 o RFC 2475, "An Architecture for Differentiated Services" (36
pages). pages).
o RFC 3168, "The Addition of Explicit Congestion Notification (ECN) o RFC 3168, "The Addition of Explicit Congestion Notification (ECN)
to IP" (63 pages). to IP" (63 pages).
o RFC 4632, "Classless Inter-domain Routing (CIDR): The Internet
Address Assignment and Aggregation Plan" (27 pages).
1.3. Organization of this document 1.3. Organization of this document
This document is basically organized in two parts: "Internet Protocol This document is basically organized in two parts: "Internet Protocol
header fields" and "Internet Protocol mechanisms". The former header fields" and "Internet Protocol mechanisms". The former
contains an analysis of each of the fields of the Internet Protocol contains an analysis of each of the fields of the Internet Protocol
header, identifies their security implications, and discusses the header, identifies their security implications, and discusses the
possible counter-measures. The latter contains an analysis of the possible countermeasures. The latter contains an analysis of the
security implications of the mechanisms implemented by the Internet security implications of the mechanisms implemented by the Internet
Protocol. Protocol.
2. The Internet Protocol 2. The Internet Protocol
The Internet Protocol (IP) provides a basic data transfer function, The Internet Protocol (IP) provides a basic data transfer function
in the form of data blocks called "datagrams", from a source host to for passing data blocks called "datagrams" from a source host to a
a destination host, across the possible intervening networks. destination host, across the possible intervening networks.
Additionally, it provides some functions that are useful for the Additionally, it provides some functions that are useful for the
interconnection of heterogeneous networks, such as fragmentation and interconnection of heterogeneous networks, such as fragmentation and
reassembly. reassembly.
The "datagram" has a number of characteristics that makes it The "datagram" has a number of characteristics that makes it
convenient for interconnecting systems [Clark1988]: convenient for interconnecting systems [Clark1988]:
o It eliminates the need of connection state within the network, o It eliminates the need of connection state within the network,
which improves the survivability characteristics of the network. which improves the survivability characteristics of the network.
o It provides a basic service of data transport that can be used as o It provides a basic service of data transport that can be used as
a building block for other transport services (reliable data a building block for other transport services (reliable data
transport services, etc.). transport services, etc.).
o It represents the minimum network service assumption, which o It represents the minimum network service assumption, which
enables IP to be run over virtually any network technology. enables IP to be run over virtually any network technology.
3. Internet Protocol header fields 3. Internet Protocol Header Fields
The IETF specifications of the Internet Protocol define the syntax of The IETF specifications of the Internet Protocol define the syntax of
the protocol header, along with the semantics of each of its fields. the protocol header, along with the semantics of each of its fields.
Figure 1 shows the format of an IP datagram. Figure 1 shows the format of an IP datagram.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length | |Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset | | Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum | | Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address | | Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address | | Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding | | [ Options ] | [ Padding ] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Internet Protocol header format Figure 1: Internet Protocol header format
Even when the minimum IP header size is 20 bytes, an IP module might Even though the minimum IP header size is 20 bytes, an IP module
be handed an (illegitimate) "datagram" of less than 20 bytes. might be handed an (illegitimate) "datagram" of less than 20 bytes.
Therefore, before doing any processing of the IP header fields, the Therefore, before doing any processing of the IP header fields, the
following check should be performed by the IP module on the packets following check should be performed by the IP module on the packets
handed by the link layer: handed by the link layer:
LinkLayer.PayloadSize >= 20 LinkLayer.PayloadSize >= 20
where LinkLayer.PayloadSize is the length (in octets) of the datagram
passed from the link layer to the IP layer.
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented this event should be logged (e.g., a counter could be incremented
reflecting the packet drop). reflecting the packet drop).
The following subsections contain further sanity checks that should The following subsections contain further sanity checks that should
be performed on IP packets. be performed on IP packets.
3.1. Version 3.1. Version
This is a 4-bit field that indicates the version of the Internet This is a 4-bit field that indicates the version of the Internet
skipping to change at page 9, line 29 skipping to change at page 10, line 9
module, it does so based on a "Protocol Type" field in the data-link module, it does so based on a "Protocol Type" field in the data-link
packet header. packet header.
In theory, different versions of IP could coexist on a network by In theory, different versions of IP could coexist on a network by
using the same "Protocol Type" at the Link-layer, but a different using the same "Protocol Type" at the Link-layer, but a different
value in the Version field of the IP header. Thus, a single IP value in the Version field of the IP header. Thus, a single IP
module could handle all versions of the Internet Protocol, module could handle all versions of the Internet Protocol,
differentiating them by means of this field. differentiating them by means of this field.
However, in practice different versions of IP are identified by a However, in practice different versions of IP are identified by a
different "Protocol Type" number in the link-layer protocol header. different "Protocol Type" (EtherType) number in the link-layer
For example, IPv4 datagrams are encapsulated in Ethernet frames using protocol header. For example, IPv4 datagrams are encapsulated in
a "Protocol Type" field of 0x0800, while IPv6 datagrams are Ethernet frames using an EtherType of 0x0800, while IPv6 datagrams
encapsulated in Ethernet frames using a "Protocol Type" field of are encapsulated in Ethernet frames using an EtherType of 0x86DD
0x86DD [IANA2006a]. [IANA2006a].
Therefore, if an IPv4 module receives a packet, the Version field Therefore, if an IPv4 module receives a packet, the Version field
must be checked to be 4. If this check fails, the packet should be must be checked to be 4. If this check fails, the packet should be
silently dropped, and this event should be logged (e.g., a counter silently dropped, and this event should be logged (e.g., a counter
could be incremented reflecting the packet drop). could be incremented reflecting the packet drop).
3.2. IHL (Internet Header Length) 3.2. IHL (Internet Header Length)
The IHL (Internet Header Length) indicates the length of the internet The IHL (Internet Header Length) indicates the length of the internet
header in 32-bit words (4 bytes). As the minimum datagram size is 20 header in 32-bit words (4 bytes). As the minimum datagram size is 20
skipping to change at page 10, line 11 skipping to change at page 10, line 39
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented this event should be logged (e.g., a counter could be incremented
reflecting the packet drop). reflecting the packet drop).
For obvious reasons, the Internet header cannot be larger than the For obvious reasons, the Internet header cannot be larger than the
whole Internet datagram it is part of. Therefore, the following whole Internet datagram it is part of. Therefore, the following
check should be enforced: check should be enforced:
IHL * 4 <= Total Length IHL * 4 <= Total Length
This needs to refer to the size of the datagram as specified by
the sender in the Total Length field, since link layers might have
added some padding (see Section 3.4).
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented this event should be logged (e.g., a counter could be incremented
reflecting the packet drop). reflecting the packet drop).
The above check allows for Internet datagrams with no data bytes in The above check allows for Internet datagrams with no data bytes in
the payload that, while nonsensical for virtually every protocol that the payload that, while nonsensical for virtually every protocol that
runs over IP, it is still legal. runs over IP, are is still legal.
3.3. Differentiated Services field 3.3. Type of Service
3.3.1. Original Interpretation
Figure 2 shows the original syntax of the Type of Service field, as
defined by RFC 791 [RFC0791], and updated by RFC 1349 [RFC1349].
This definition has been superseded long ago (see Section 3.3.2.1 and
Section 3.3.2.2), but it is still assumed by some deployed
implementations.
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| PRECEDENCE | D | T | R | C | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
Figure 2: Type of Service Field (Original Interpretation)
+----------+----------------------------------------------+
| Bits 0-2 | Precedence |
+----------+----------------------------------------------+
| Bit 3 | 0 = Normal Delay, 1 = Low Delay |
+----------+----------------------------------------------+
| Bit 4 | 0 = Normal Throughput, 1 = High Throughput |
+----------+----------------------------------------------+
| Bit 5 | 0 = Normal Reliability, 1 = High Reliability |
+----------+----------------------------------------------+
| Bit 6 | 0 = Normal Cost, 1 = Minimize Monetary Cost |
+----------+----------------------------------------------+
| Bits 7 | Reserved for Future Use (must be zero) |
+----------+----------------------------------------------+
Table 1: TOS-bits Semantics
+-----+-----------------+
| 111 | Network Control |
+-----+-----------------+
| 110 | Internetwork |
+-----+-----------------+
| 101 | CRITIC/ECP |
+-----+-----------------+
| 100 | Flash Override |
+-----+-----------------+
| 011 | Flash |
+-----+-----------------+
| 010 | Immediate |
+-----+-----------------+
| 001 | Priority |
| 000 | Routine |
+-----+-----------------+
Table 2: Precedence Field Values
The Type of Service field can be used to affect the way in which the
packet is treated by the systems of a network that process it.
Section 4.2.1 ("Precedence-ordered queue service") and Section 4.2.3
("Weak TOS") of this document describe the security implications of
the Type of Service field in the forwarding of packets.
3.3.2. Standard Interpretation
3.3.2.1. Differentiated Services field
The Differentiated Services Architecture is intended to enable The Differentiated Services Architecture is intended to enable
scalable service discrimination in the Internet without the need for scalable service discrimination in the Internet without the need for
per-flow state and signaling at every hop [RFC2475]. RFC 2474 per-flow state and signaling at every hop [RFC2475]. RFC 2474
[RFC2474] defines a Differentiated Services Field (DS Field), which [RFC2474] redefined the IP "Type of Service" octet, introducing a
is intended to supersede the original Type of Service field. Figure Differentiated Services Field (DS Field). Figure 3 shows the format
5 shows the format of the field. of the field.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| DSCP | CU | | DSCP | CU |
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
Figure 2: Structure of the DS Field Figure 3: Revised Structure of the Type of Service Field (RFC 2474)
The DSCP ("Differentiated Services CodePoint").is used to select the The DSCP ("Differentiated Services CodePoint") is used to select the
treatment the packet is to receive within the Differentiated Services treatment the packet is to receive within the Differentiated Services
Domain. The CU ("Currently Unused") field was, at the time the Domain. The CU ("Currently Unused") field was, at the time the
specification was issued, reserved for future use. The DSCP field is specification was issued, reserved for future use. The DSCP field is
used to select a PHB, by matching against the entire 6-bit field. used to select a PHB (Per-Hop Behavior), by matching against the
entire 6-bit field.
Considering that the DSCP field determines how a packet is treated Considering that the DSCP field determines how a packet is treated
within a DS domain, an attacker send packets with a forged DSCP field within a DS domain, an attacker could send packets with a forged DSCP
to perform a theft of service or even a Denial of Service attack. In field to perform a theft of service or even a Denial-of-Service
particular, an attacker could forge packets with a codepoint of the attack. In particular, an attacker could forge packets with a
type '11x000' which, according to Section 4.2.2.2 of RFC 2474 codepoint of the type '11x000' which, according to Section 4.2.2.2 of
[RFC2474], would give the packets preferential forwarding treatment RFC 2474 [RFC2474], would give the packets preferential forwarding
when compared with the PHB selected by the codepoint '000000'. If treatment when compared with the PHB selected by the codepoint
strict priority queuing were utilized, a continuous stream of such '000000'. If strict priority queuing were utilized, a continuous
pockets could perform a Denial of Service to other flows which have a stream of such packets could cause a Denial of Service to other flows
DSCP of lower relative order. which have a DSCP of lower relative order.
As the DS field is incompatible with the original Type of Service As the DS field is incompatible with the original Type of Service
field, both DS domains and networks using the original Type of field, both DS domains and networks using the original Type of
Service field should protect themselves by remarking the Service field should protect themselves by remarking the
corresponding field where appropriate, probably deploying remarking corresponding field where appropriate, probably deploying remarking
boundary nodes. Nevertheless, care must be taken so that packets boundary nodes. Nevertheless, care must be taken so that packets
received with an unrecognized DSCP do not cause the handling system received with an unrecognized DSCP do not cause the handling system
to malfunction. to malfunction.
3.4. Explicit Congestion Notification (ECN) 3.3.2.2. Explicit Congestion Notification (ECN)
RFC 3168 [RFC3168] specifies a mechanism for routers to signal RFC 3168 [RFC3168] specifies a mechanism for routers to signal
congestion to hosts sending IP packets, by marking the offending congestion to hosts exchanging IP packets, by marking the offending
packets, rather than discarding them. RFC 3168 defines the ECN packets, rather than discarding them. RFC 3168 defines the ECN
field, which utilizes the CU unused field of the DSCP field described field, which utilizes the CU field defined in RFC 2474 [RFC2474].
in Section 3.14 of this document. Figure 6 shows the syntax of the Figure 4 shows the current syntax of the IP Type of Service field,
ECN field, together with the DSCP field used for Differentiated with the DSCP field used for Differentiated Services and the ECN
Services. field.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
| DS FIELD, DSCP | ECN FIELD | | DS FIELD, DSCP | ECN FIELD |
+-----+-----+-----+-----+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+-----+-----+-----+
Figure 3: The Differentiated Services and ECN fields in IP Figure 4: The Differentiated Services and ECN fields in IP
As such, the ECN field defines four codepoints: As such, the ECN field defines four codepoints:
+-----------+-----------+ +-----------+-----------+
| ECN field | Codepoint | | ECN field | Codepoint |
+-----------+-----------+ +-----------+-----------+
| 00 | Not-ECT | | 00 | Not-ECT |
+-----------+-----------+ +-----------+-----------+
| 01 | ECT(1) | | 01 | ECT(1) |
+-----------+-----------+ +-----------+-----------+
| 10 | ECT(0) | | 10 | ECT(0) |
+-----------+-----------+ +-----------+-----------+
| 11 | CE | | 11 | CE |
+-----------+-----------+ +-----------+-----------+
Table 1: ECN codepoints Table 3: ECN codepoints
ECN is an end-to-end transport protocol mechanism based on
notifications by routers through which a packet flow passes. To
allow this interaction to happen on the fast path of routers, the ECN
field is located at a fixed location in the IP header. However, its
use must be negotiated at the transport layer, and the accumulated
congestion notifications must be communicated back to the sending
node using transport protocol means. Thus, ECN support must be
specified per transport protocol.
The security implications of ECN are discussed in detail in a number The security implications of ECN are discussed in detail in a number
of Sections of RFC 3168. Of the possible threats discussed in the of Sections of RFC 3168. Of the possible threats discussed in the
ECN specification, we believe that one that can be easily exploited ECN specification, we believe that one that can be easily exploited
is that of host falsely indicating ECN-Capability. is that of a host falsely indicating ECN-Capability.
An attacker could set the ECT codepoint in the packets it sends, to An attacker could set the ECT codepoint in the packets it sends, to
signal the network that the endpoints of the transport protocol are signal the network that the endpoints of the transport protocol are
ECN-capable. Consequently, when experiencing moderate congestion, ECN-capable. Consequently, when experiencing moderate congestion,
routers using active queue management based on RED would mark the routers using active queue management based on RED would mark the
packets (with the CE codepoint) rather than discard them. In the packets (with the CE codepoint) rather than discard them. In this
same scenario, packets of competing flows that do not have the ECT same scenario, packets of competing flows that do not have the ECT
codepoint set would be dropped. Therefore, an attacker would get codepoint set would be dropped. Therefore, an attacker would get
better network service than the competing flows. better network service than the competing flows.
However, if this moderate congestion turned into heavy congestion, However, if this moderate congestion turned into heavy congestion,
routers should switch to drop packets, regardless of whether the routers should switch to drop packets, regardless of whether the
packets have the ECT codepoint set or not. packets have the ECT codepoint set or not.
A number of other threats could arise if an attacker was a man in the A number of other threats could arise if an attacker was a man in the
middle (i.e., was in the middle of the path the packets travel to get middle (i.e., was in the middle of the path the packets travel to get
to the destination host). For a detailed discussion of those cases, to the destination host). For a detailed discussion of those cases,
we urge the reader to consult Section 16 of RFC 3168. we urge the reader to consult Section 16 of RFC 3168.
3.5. Total Length There also is ongoing work in the research community and the IETF to
define alternate semantics for the CU / ECN field of IP TOS octet
(see [RFC5559], [RFC5670], and [RFC5696]). The application of these
methods must be confined to tightly administered domains, and on exit
from such domains, all packets need to be (re-)marked with ECN
semantics.
3.4. Total Length
The Total Length field is the length of the datagram, measured in The Total Length field is the length of the datagram, measured in
bytes, including both the IP header and the IP payload. Being a 16- bytes, including both the IP header and the IP payload. Being a 16-
bit field, it allows for datagrams of up to 65535 bytes. RFC 791 bit field, it allows for datagrams of up to 65535 bytes. RFC 791
[RFC0791] states that all hosts should be prepared to receive [RFC0791] states that all hosts should be prepared to receive
datagrams of up to 576 bytes (whether they arrive as a whole, or in datagrams of up to 576 bytes (whether they arrive as a whole, or in
fragments). However, most modern implementations can reassemble fragments). However, most modern implementations can reassemble
datagrams of at least 9 Kbytes. datagrams of at least 9 Kbytes.
Usually, a host will not send to a remote peer an IP datagram larger Usually, a host will not send to a remote peer an IP datagram larger
than 576 bytes, unless it is explicitly signaled that the remote peer than 576 bytes, unless it is explicitly signaled that the remote peer
is able to receive such "large" datagrams (for example, by means of is able to receive such "large" datagrams (for example, by means of
TCP's MSS option). However, systems should assume that they may be TCP's MSS option). However, systems should assume that they may
sent datagrams larger than 576 bytes, regardless of whether they receive datagrams larger than 576 bytes, regardless of whether they
signal their remote peers to do so or not. In fact, it is common for signal their remote peers to do so or not. In fact, it is common for
NFS [RFC3530]implementations to send datagrams larger than 576 bytes, NFS [RFC3530] implementations to send datagrams larger than 576
even without explicit signaling that the destination system can bytes, even without explicit signaling that the destination system
receive such "large" datagram. can receive such "large" datagram.
o Additionally, see the discussion in Section 4.1 "Fragment Additionally, see the discussion in Section 4.1 ("Fragment
reassembly" regarding the possible packet sizes resulting from reassembly") regarding the possible packet sizes resulting from
fragment reassembly. fragment reassembly.
Implementations should be aware that the IP module could be handed a Implementations should be aware that the IP module could be handed a
packet larger than the value actually contained in the Total Length packet larger than the value actually contained in the Total Length
field. Such a difference usually has to do with legitimate padding field. Such a difference usually has to do with legitimate padding
bytes at the link-layer protocol, but it could also be the result of bytes at the link-layer protocol, but it could also be the result of
malicious activity by an attacker. Furthermore, even when the malicious activity by an attacker. Furthermore, even when the
maximum length of an IP datagram is 65535 bytes, if the link-layer maximum length of an IP datagram is 65535 bytes, if the link-layer
technology in use allows for payloads larger than 65535 bytes, an technology in use allows for payloads larger than 65535 bytes, an
attacker could forge such a large link-layer packet, meaning it for attacker could forge such a large link-layer packet, meaning it for
the IP module. If the IP module of the receiving system were not the IP module. If the IP module of the receiving system were not
prepared to handle such an oversized link-layer payload, an prepared to handle such an oversized link-layer payload, an
unexpected failure might occur. Therefore, the memory buffer used by unexpected failure might occur. Therefore, the memory buffer used by
the IP module to store the link-layer payload should be allocated the IP module to store the link-layer payload should be allocated
according to the payload size reported by the link-layer, rather than according to the payload size reported by the link-layer, rather than
according to the Total Length field of the IP packet it contains. according to the Total Length field of the IP packet it contains.
The IP module could also be handled a packet that is smaller than the The IP module could also be handed a packet that is smaller than the
actual IP packet size claimed by the Total Length field. This could actual IP packet size claimed by the Total Length field. This could
be used, for example, to produce an information leakage. Therefore, be used, for example, to produce an information leakage. Therefore,
the following check should be performed: the following check should be performed:
LinkLayer.PayloadSize >= Total Length LinkLayer.PayloadSize >= Total Length
If this check fails, the IP packet should be dropped, and this event If this check fails, the IP packet should be dropped, and this event
should be logged (e.g., a counter could be incremented reflecting the should be logged (e.g., a counter could be incremented reflecting the
packet drop). As the previous expression implies, the number of packet drop). As the previous expression implies, the number of
bytes passed by the link-layer to the IP module should contain at bytes passed by the link-layer to the IP module should contain at
least as many bytes as claimed by the Total Length field of the IP least as many bytes as claimed by the Total Length field of the IP
header. header.
o [US-CERT2002] is an example of the exploitation of a forged IP [US-CERT2002] is an example of the exploitation of a forged IP
Total Length field to produce an information leakage attack. Total Length field to produce an information leakage attack.
3.6. Identification (ID) 3.5. Identification (ID)
The Identification field is set by the sending host to aid in the The Identification field is set by the sending host to aid in the
reassembly of fragmented datagrams. At any time, it needs to be reassembly of fragmented datagrams. At any time, it needs to be
unique for each set of {Source Address, Destination Address, unique for each set of {Source Address, Destination Address,
Protocol}. Protocol}.
In many systems, the value used for this field is determined at the In many systems, the value used for this field is determined at the
IP layer, on a protocol-independent basis. Many of those systems IP layer, on a protocol-independent basis. Many of those systems
also simply increment the IP Identification field for each packet also simply increment the IP Identification field for each packet
they send. they send.
This implementation strategy is inappropriate for a number of This implementation strategy is inappropriate for a number of
reasons. First, if the Identification field is set on a protocol- reasons. Firstly, if the Identification field is set on a protocol-
independent basis, it will wrap more often than necessary, and thus independent basis, it will wrap more often than necessary, and thus
the implementation will be more prone to the problems discussed in the implementation will be more prone to the problems discussed in
[Kent1987] and [RFC4963]. [Kent1987] and [RFC4963]. Secondly, this implementation strategy
opens the door to an information leakage that can be exploited in a
number of ways.
Additionally, this implementation strategy opens the door to an [Sanfilippo1998a] examined to determine the packet rate at which a
information leakage that can be exploited to in a number of ways. given system is transmitting information. Later, [Sanfilippo1998b]
[Sanfilippo1998a] originally pointed out how this field could be described how a system with such an implementation can be used to
examined to determine the packet rate at which a given system is perform a stealth port scan to a third (victim) host.
transmitting information. Later, [Sanfilippo1998b] described how a [Sanfilippo1999] explained how to exploit this implementation
system with such an implementation can be used to perform a stealth strategy to uncover the rules of a number of firewalls.
port scan to a third (victim) host. [Sanfilippo1999] explained how [Bellovin2002] explains how the IP Identification field can be
to exploit this implementation strategy to uncover the rules of a exploited to count the number of systems behind a NAT. [Fyodor2004]
number of firewalls. [Bellovin2002] explains how the IP is an entire paper on most (if not all) the ways to exploit the
Identification field can be exploited to count the number of systems information provided by the Identification field of the IP header.
behind a NAT. [Fyodor2004] is an entire paper on most (if not all)
the ways to exploit the information provided by the Identification
field of the IP header.
3.6.1. Some workarounds implemented by the industry Section 4.1 contains a discussion of the security implications of
the IP fragment reassembly mechanism, which is the primary
"consumer" of this field.
3.5.1. Some Workarounds Implemented by the Industry
As the IP Identification field is only used for the reassembly of As the IP Identification field is only used for the reassembly of
datagrams, some operating systems (such as Linux) decided to set this datagrams, some operating systems (such as Linux) decided to set this
field to 0 in all packets that have the DF bit set. This would, in field to 0 in all packets that have the DF bit set. This would, in
principle, avoid any type of information leakage. However, it was principle, avoid any type of information leakage. However, it was
detected that some non-RFC-compliant middle-boxes fragmented packets detected that some non-RFC-compliant middle-boxes fragmented packets
even if they had the DF bit set. In such a scenario, all datagrams even if they had the DF bit set. In such a scenario, all datagrams
originally sent with the DF bit set would all result in fragments originally sent with the DF bit set would all result in fragments
that would have an Identification field of 0, which would lead to with an Identification field of 0, which would lead to problems
problems ("collision" of the Identification number) in the reassembly ("collision" of the Identification number) in the reassembly process.
process.
Linux (and Solaris) later set the IP Identification field on a per- Linux (and Solaris) later set the IP Identification field on a per-
IP-address basis. This avoids some of the security implications of IP-address basis. This avoids some of the security implications of
the IP Identification field, but not all. For example, systems the IP Identification field, but not all. For example, systems
behind a load balancer can still be counted. behind a load balancer can still be counted.
3.6.2. Possible security improvements 3.5.2. Possible security improvements
Contrary to common wisdom, the IP Identification field does not need Contrary to common wisdom, the IP Identification field does not need
to be system-wide unique for each packet, but has to be unique for to be system-wide unique for each packet, but has to be unique for
each {Source Address, Destination Address, Protocol} tuple. each {Source Address, Destination Address, Protocol} tuple.
o For instance, the TCP specification defines a generic send() For instance, the TCP specification defines a generic send()
function which takes the IP ID as one of its arguments. function which takes the IP ID as one of its arguments.
We provide an analysis of the possible security improvements that We provide an analysis of the possible security improvements that
could be implemented, based on whether the protocol using the could be implemented, based on whether the protocol using the
services of IP is connection-oriented or connection-less. services of IP is connection-oriented or connection-less.
Connection-oriented protocols 3.5.2.1. Connection-Oriented Transport Protocols
To avoid the security implications of the information leakage To avoid the security implications of the information leakage
described above, a pseudo-random number generator (PRNG) could be described above, a pseudo-random number generator (PRNG) could be
used to set the IP Identification field on a {Source Address, used to set the IP Identification field on a {Source Address,
Destination Address} basis (for each connection-oriented transport Destination Address} basis (for each connection-oriented transport
protocol). protocol).
o [Klein2007] is a security advisory that describes a weakness in [Klein2007] is a security advisory that describes a weakness in
the pseudo random number generator (PRNG) in use for the the pseudo random number generator (PRNG) in use for the
generation of the IP Identification by a number of operating generation of the IP Identification by a number of operating
systems. systems.
While in theory a pseudo-random number generator could lead to While in theory a pseudo-random number generator could lead to
scenarios in which a given Identification number is used more than scenarios in which a given Identification number is used more than
once in the same time-span for datagrams that end up getting once in the same time-span for datagrams that end up getting
fragmented (with the corresponding potential reassembly problems), in fragmented (with the corresponding potential reassembly problems), in
practice this is unlikely to cause trouble. practice this is unlikely to cause trouble.
By default, most implementations of connection-oriented protocols, By default, most implementations of connection-oriented protocols,
such as TCP, implement some mechanism for avoiding fragmentation such as TCP, implement some mechanism for avoiding fragmentation
(such as the Path-MTU Discovery mechanism described in [RFC1191]). (such as the Path-MTU Discovery mechanism described in [RFC1191]).
Thus, fragmentation will only take place sporadically, when a non- Thus, fragmentation will only take place sporadically, when a non-
RFC-compliant middle-box is placed somewhere along the path that the RFC-compliant middle-box is placed somewhere along the path that the
packets travel to get to the destination host. Once the sending packets travel to get to the destination host. Once the sending
system is signaled by the middle-box that it should reduce the size system is signaled by the middle-box that it should reduce the size
of the packets it sends, fragmentation would be avoided. Also, for of the packets it sends, fragmentation would be avoided. Also, for
reassembly problems to arise, the same Identification field should be reassembly problems to arise, the same Identification value would
reused very frequently, and either strong packet reordering or packet need to be reused very frequently, and either strong packet
loss should take place. reordering or packet loss would need to take place.
Nevertheless, regardless of what policy is used for selecting the Nevertheless, regardless of what policy is used for selecting the
Identification field, with the current link speeds fragmentation is Identification field, with the current link speeds fragmentation is
already bad enough to rely on it. A mechanism for avoiding already bad enough to rely on it. A mechanism for avoiding
fragmentation should be implemented, instead. fragmentation (such as [RFC1191] or [RFC4821] should be implemented,
instead.
Connectionless protocols 3.5.2.2. Connectionless Transport Protocols
Connectionless protocols usually have these characteristics: Connectionless transport protocols often have these characteristics:
o lack of flow-control mechanisms, o lack of flow-control mechanisms,
o lack of packet sequencing mechanisms, and, o lack of packet sequencing mechanisms, and/or,
o lack of reliability mechanisms (such as "timeout and retransmit"). o lack of reliability mechanisms (such as "timeout and retransmit").
This basically means that the scenarios and/or applications for which This basically means that the scenarios and/or applications for which
connection-less transport protocols are used assume that: connection-less transport protocols are used assume that:
o Applications will be used in environments in which packet o Applications will be used in environments in which packet
reordering is very unlikely (such as Local Area Networks), as the reordering is very unlikely (such as Local Area Networks), as the
transport protocol itself does not provide data sequencing. transport protocol itself does not provide data sequencing.
o The data transfer rates will be low enough that flow control will o The data transfer rates will be low enough that flow control will
be unnecessary. be unnecessary.
o Packet loss is not important and probably also unlikely. o Packet loss is can be tolerated and/or is unlikely.
With these assumptions in mind, the Identification field could still With these assumptions in mind, the Identification field could still
be set according to a pseudo-random number generator (PRNG). In the be set according to a pseudo-random number generator (PRNG). In the
event a given Identification number was reused while the first event a given Identification number was reused while the first
instance of the same number is still on the network, the first IP instance of the same number is still on the network, the first IP
datagram would be reassembled before the fragments of the second IP datagram would be reassembled before the fragments of the second IP
datagram get to their destination. datagram get to their destination.
In the event this was not the case, the reassembly of fragments would In the event this was not the case, the reassembly of fragments would
result in a corrupt datagram. While some existing work result in a corrupt datagram. While some existing work
[Silbersack2005] assumes that this error would be caught by some [Silbersack2005] assumes that this error would be caught by some
upper-layer error detection code, the error detection code in upper-layer error detection code, the error detection code in
question (such as UDP's checksum) might be intended to detect single question (such as UDP's checksum) might not be able to reliably
bit errors, rather than data corruption arising from the replacement detect data corruption arising from the replacement of a complete
of a complete data block (as is the case in corruption arising from data block (as is the case in corruption arising from collision of IP
collision of IP Identification numbers). Identification numbers).
o In the case of UDP, unfortunately some systems have been known to In the case of UDP, unfortunately some systems have been known to
not enable the UDP checksum by default. For most applications, not enable the UDP checksum by default. For most applications,
packets containing errors should be dropped. Probably the only packets containing errors should be dropped. Probably the only
application that may benefit from disabling the checksum is application that may benefit from disabling the checksum is
streaming media, to avoid dropping a complete sample for a single- streaming media, to avoid dropping a complete sample for a single-
bit error. bit error.
In general, if IP Identification number collisions become an issue In general, if IP Identification number collisions become an issue
for the application using the connection-less protocol, then use of a for the application using the connection-less protocol, then use of a
different transport protocol (which hopefully avoids fragmentation) different transport protocol (which hopefully avoids fragmentation)
should be considered. should be considered.
It must be noted that an attacker could intentionally exploit It must be noted that an attacker could intentionally exploit
collisions of IP Identification numbers to perform a Denial of collisions of IP Identification numbers to perform a Denial-of-
Service attack, by sending forged fragments that would cause the Service attack, by sending forged fragments that would cause the
reassembly process to result in a corrupt datagram that would either reassembly process to result in a corrupt datagram that would either
be dropped by the transport protocol, or would incorrectly be handed be dropped by the transport protocol, or would incorrectly be handed
to the corresponding application. This issue is discussed in detail to the corresponding application. This issue is discussed in detail
in section 4.1 ("Fragment Reassembly"). in section 4.1 ("Fragment Reassembly").
3.7. Flags 3.6. Flags
The IP header contains 3 control bits, two of which are currently The IP header contains 3 control bits, two of which are currently
used for the fragmentation and reassembly function. used for the fragmentation and reassembly function.
As described by RFC 791, their meaning is: As described by RFC 791, their meaning is:
Bit 0: reserved, must be zero o Bit 0: reserved, must be zero (i.e., reserved for future
standardization)
Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment o Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment
Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments
o Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments
The DF bit is usually set to implement the Path-MTU Discovery (PMTUD) The DF bit is usually set to implement the Path-MTU Discovery (PMTUD)
mechanism described in [RFC1191]. However, it can also be exploited mechanism described in [RFC1191]. However, it can also be exploited
by an attacker to evade Network Intrusion Detection Systems. An by an attacker to evade Network Intrusion Detection Systems. An
attacker could send a packet with the DF bit set to a system attacker could send a packet with the DF bit set to a system
monitored by a NIDS, and depending on the Path-MTU to the intended monitored by a NIDS, and depending on the Path-MTU to the intended
recipient, the packet might be dropped by some intervening router recipient, the packet might be dropped by some intervening router
(because of being too big to be forwarded without fragmentation), (because of being too big to be forwarded without fragmentation),
without the NIDS being aware of it. without the NIDS being aware of it.
(still to be added) +---+
(See Figure 3 in Page 13 of the CPNI document) | H |
+---+ Victim host
|
Router A | MTU=1500
|
+---+ +---+ +---+
| R |-----| R |---------| R |
+---+ +---+ +---+
| MTU=17914 Router B
+---+ |
| S |-----+
+---+ |
|
NIDS Sensor |
|
_ ___/---\______ Attacker
/ \_/ \_ +---+
/ Internet |---------| H |
\_ __/ +---+
\__ __ ___/ <------
\___/ \__/ 17914-byte packet
DF bit set
Figure 4: NIDS evasion by means of the Internet Protocol DF bit Figure 5: NIDS evasion by means of the Internet Protocol DF bit
In Figure 3, an attacker sends a 17914-byte datagram meant to the In Figure 3, an attacker sends a 17914-byte datagram meant to the
victim host in the same figure. The attacker's packet probably victim host in the same figure. The attacker's packet probably
contains an overlapping IP fragment or an overlapping TCP segment, contains an overlapping IP fragment or an overlapping TCP segment,
aiming at "confusing" the NIDS, as described in [Ptacek1998]. The aiming at "confusing" the NIDS, as described in [Ptacek1998]. The
packet is screened by the NIDS sensor at the network perimeter, which packet is screened by the NIDS sensor at the network perimeter, which
probably reassembles IP fragments and TCP segments for the purpose of probably reassembles IP fragments and TCP segments for the purpose of
assessing the data transferred to and from the monitored systems. assessing the data transferred to and from the monitored systems.
However, as the attacker's packet should transit a link with an MTU However, as the attacker's packet should transit a link with an MTU
smaller than 17914 bytes (1500 bytes in this example), the router smaller than 17914 bytes (1500 bytes in this example), the router
that encounters that this packet cannot be forwarded without that encounters that this packet cannot be forwarded without
fragmentation (Router B) discards the packet, and sends an ICMP fragmentation (Router B) discards the packet, and sends an ICMP
"fragmentation needed and DF bit set" error message to the source "fragmentation needed and DF bit set" error message to the source
host. In this scenario, the NIDS may remain unaware that the host. In this scenario, the NIDS may remain unaware that the
screened packet never reached the intended destination, and thus get screened packet never reached the intended destination, and thus get
an incorrect picture of the data being transferred to the monitored an incorrect picture of the data being transferred to the monitored
systems. systems.
o [Shankar2003] introduces a technique named "Active Mapping" that [Shankar2003] introduces a technique named "Active Mapping" that
prevents evasion of a NIDS by acquiring sufficient knowledge about prevents evasion of a NIDS by acquiring sufficient knowledge about
the network being monitored, to assess which packets will arrive the network being monitored, to assess which packets will arrive
at the intended recipient, and how they will be interpreted by it. at the intended recipient, and how they will be interpreted by it.
Some firewalls are known to drop packets that have both the MF (More Some firewalls are known to drop packets that have both the MF (More
Fragments) and the DF (Don't fragment) bits set. While in principle Fragments) and the DF (Don't fragment) bits set. While in principle
such a packet might seem nonsensical, there are a number of reasons such a packet might seem nonsensical, there are a number of reasons
for which non-malicious packets with these two bits set can be found for which non-malicious packets with these two bits set can be found
in a network. First, they may exist as the result of some middle-box in a network. First, they may exist as the result of some middle-box
processing a packet that was too large to be forwarded without processing a packet that was too large to be forwarded without
fragmentation. Instead of simply dropping the corresponding packet fragmentation. Instead of simply dropping the corresponding packet
and sending an ICMP error message to the source host, some middle- and sending an ICMP error message to the source host, some middle-
boxes fragment the packet (copying the DF bit to each fragment), and boxes fragment the packet (copying the DF bit to each fragment), and
also send an ICMP error message to the originating system. Second, also send an ICMP error message to the originating system. Second,
some systems (notably Linux) set both the MF and the DF bits to some systems (notably Linux) set both the MF and the DF bits to
implement Path-MTU Discovery (PMTUD) for UDP. These scenarios should implement Path-MTU Discovery (PMTUD) for UDP. These scenarios should
be taken into account when configuring firewalls and/or tuning be taken into account when configuring firewalls and/or tuning
Network Intrusion Detection Systems (NIDS). Network Intrusion Detection Systems (NIDS).
3.8. Fragment Offset Section 4.1 contains a discussion of the security implications of the
IP fragment reassembly mechanism.
3.7. Fragment Offset
The Fragment Offset is used for the fragmentation and reassembly of The Fragment Offset is used for the fragmentation and reassembly of
IP datagrams. It indicates where in the original datagram the IP datagrams. It indicates where in the original datagram payload
fragment belongs, and is measured in units of eight bytes. As a the payload of the fragment belongs, and is measured in units of
consequence, all fragments (except the last one), have to be aligned eight bytes. As a consequence, all fragments (except the last one),
on an 8-byte boundary. Therefore, if a packet has the MF flag set, have to be aligned on an 8-byte boundary. Therefore, if a packet has
the following check should be enforced: the MF flag set, the following check should be enforced:
(Total Length - IHL * 4) % 8 == 0 (Total Length - IHL * 4) % 8 == 0
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented this event should be logged (e.g., a counter could be incremented
reflecting the packet drop). reflecting the packet drop).
Given that Fragment Offset is a 13-bit field, it can hold a value of Given that Fragment Offset is a 13-bit field, it can hold a value of
up to 8191, which would correspond to an offset 65528 bytes within up to 8191, which would correspond to an offset 65528 bytes within
the original (non-fragmented) datagram. As such, it is possible for the original (non-fragmented) datagram. As such, it is possible for
skipping to change at page 18, line 40 skipping to change at page 22, line 5
when the fragmentation mechanism would seem to allow fragments that when the fragmentation mechanism would seem to allow fragments that
could reassemble into such large datagrams, the intent of the could reassemble into such large datagrams, the intent of the
specification is to allow for the transmission of datagrams of up to specification is to allow for the transmission of datagrams of up to
65535 bytes. Therefore, if a given fragment would reassemble into a 65535 bytes. Therefore, if a given fragment would reassemble into a
datagram of more than 65535 bytes, the resulting datagram should be datagram of more than 65535 bytes, the resulting datagram should be
dropped, and this event should be logged (e.g., a counter could be dropped, and this event should be logged (e.g., a counter could be
incremented reflecting the packet drop). To detect such a case, the incremented reflecting the packet drop). To detect such a case, the
following check should be enforced on all packets for which the following check should be enforced on all packets for which the
Fragment Offset contains a non-zero value: Fragment Offset contains a non-zero value:
Fragment Offset * 8 + (Total Length - IHL * 4) <= 65535 Fragment Offset * 8 + (Total Length - IHL * 4) + IHL_FF * 4 <= 65535
In the worst-case scenario, the reassembled datagram could have a where IHL_FF is the IHL field of the first fragment (the one with a
size of up to 131043 bytes. Fragment Offset of 0).
o Such a datagram would result when the first fragment has a If a fragment does not pass this check, it should be dropped.
If IHL_FF is not yet available because the first fragment has not yet
arrived, for a preliminary, less rigid test, IHL_FF == IHL should be
assumed, and the test is simplified to:
Fragment Offset * 8 + Total Length <= 65535
Once the first fragment is received, the full sanity check described
earlier should be applied, if that fragment contains "don't copy"
options.
In the worst-case scenario, an attacker could craft IP fragments such
that the reassembled datagram reassembled into a datagram of 131043
bytes.
Such a datagram would result when the first fragment has a
Fragment Offset of 0 and a Total Length of 65532, and the second Fragment Offset of 0 and a Total Length of 65532, and the second
(and last) fragment has a Fragment Offset of 8189 (65512 bytes), (and last) fragment has a Fragment Offset of 8189 (65512 bytes),
and a Total Length of 65535. Assuming an IHL of 5 (i.e., a header and a Total Length of 65535. Assuming an IHL of 5 (i.e., a header
length of 20 bytes), the reassembled datagram would be 65532 + length of 20 bytes), the reassembled datagram would be 65532 +
(65535 - 20) = 131047 bytes. (65535 - 20) = 131047 bytes.
Additionally, the IP module should implement all the necessary Additionally, the IP module should implement all the necessary
measures to be able to handle such illegitimate reassembled measures to be able to handle such illegitimate reassembled
datagrams, so as to avoid them from overflowing the buffer(s) used datagrams, so as to avoid them from overflowing the buffer(s) used
for the reassembly function. for the reassembly function.
o [CERT1996c] and [Kenney1996] describe the exploitation of this [CERT1996c] and [Kenney1996] describe the exploitation of this
issue to perform a Denial of Service (DoS) attack. issue to perform a Denial-of-Service (DoS) attack.
3.9. Time to Live (TTL) Section 4.1 contains a discussion of the security implications of the
IP fragment reassembly mechanism.
The Time to Live (TTL) field has two functions: to bind the lifetime 3.8. Time to Live (TTL)
The Time to Live (TTL) field has two functions: to bound the lifetime
of the upper-layer packets (e.g., TCP segments) and to prevent of the upper-layer packets (e.g., TCP segments) and to prevent
packets from looping indefinitely in the network. packets from looping indefinitely in the network.
Originally, this field was meant to indicate maximum time a datagram Originally, this field was meant to indicate the maximum time a
was allowed to remain in the internet system, in units of seconds. datagram was allowed to remain in the internet system, in units of
As every internet module that processes a datagram must decrement the seconds. As every internet module that processes a datagram must
TTL by at least one, the original definition of the TTL field became decrement the TTL by at least one, the original definition of the TTL
obsolete, and it must now be interpreted as a hop count. field became obsolete, and in practice it is interpreted as a hop
count (see Section 5.3.1 of [RFC1812]).
Most systems allow the administrator to configure the TTL to be used Most systems allow the administrator to configure the TTL to be used
for the packets sent, with the default value usually being a power of for the packets they originate, with the default value usually being
2. The recommended value for the TTL field, as specified by the IANA a power of 2, or 255 (see e.g. [Arkin2000]). The recommended value
is 64 [IANA2006b]. This value reflects the assumed "diameter" of the for the TTL field, as specified by the IANA is 64 [IANA2006b]. This
Internet, plus a margin to accommodate its growth. value reflects the assumed "diameter" of the Internet, plus a margin
to accommodate its growth.
The TTL field has a number of properties that are interesting from a The TTL field has a number of properties that are interesting from a
security point of view. Given that the default value used for the security point of view. Given that the default value used for the
TTL is usually a power of eight, chances are that, unless the TTL is usually a power of two or 255, chances are that, unless the
originating system has been explicitly tuned to use a non-default originating system has been explicitly tuned to use a non-default
value, if a packet arrives with a TTL of 60, the packet was value, if a packet arrives with a TTL of 60, the packet was
originally sent with a TTL of 64. In the same way, if a packet is originally sent with a TTL of 64. In the same way, if a packet is
received with a TTL of 120, chances are that the original packet had received with a TTL of 120, chances are that the original packet had
a TTL of 128. a TTL of 128.
o This discussion assumes there was no protocol scrubber, This discussion assumes there was no protocol scrubber,
transparent proxy, or some other middle-box that overwrites the transparent proxy, or some other middle-box that overwrites the
TTL field in a non-standard way, between the originating system TTL field in a non-standard way, between the originating system
and the point of the network in which the packet was received. and the point of the network in which the packet was received.
Asserting the TTL with which a packet was originally sent by the Asserting the TTL with which a packet was originally sent by the
source system can help to obtain valuable information. Among other source system can help to obtain valuable information. Among other
things, it may help in: things, it may help in:
o Fingerprinting the operating system being used by the source host. o Fingerprinting the originating operating system.
o Fingerprinting the physical device from which the packets o Fingerprinting the originating physical device.
originate.
o Locating the source host in the network topology. o Mapping the network topology.
Additionally, it can be used to perform functions such as: o Locating the source host in the network topology.
o Evading Network Intrusion Detection Systems. o Evading Network Intrusion Detection Systems.
However, it can also be used to perform important functions such as:
o Improving the security of applications that make use of the o Improving the security of applications that make use of the
Internet Protocol (IP). Internet Protocol (IP).
3.9.1. Fingerprinting the operating system in use by the source host o Limiting spread of packets.
3.8.1. Fingerprinting the operating system in use by the source host
Different operating systems use a different default TTL for the Different operating systems use a different default TTL for the
packets they send. Thus, asserting the TTL with which a packet was packets they send. Thus, asserting the TTL with which a packet was
originally sent will help to reduce the number of possible operating originally sent will help heuristics to reduce the number of possible
systems in use by the source host. operating systems in use by the source host.
3.9.2. Fingerprinting the physical device from which the packets However, these defaults may be configurable (system-wide or per
protocol) and managed systems may employ such opportunities for
operational purposes and to defeat the capability of fingerprinting
heuristics.
3.8.2. Fingerprinting the physical device from which the packets
originate originate
When several systems are behind a middle-box such as a NAT or a load When several systems are behind a middle-box such as a NAT or a load
balancer, the TTL may help to count the number of systems behind the balancer, the TTL may help to count the number of systems behind the
middle-box. If each of the systems behind the middle-box use a middle-box. If each of the systems behind the middle-box uses a
different default TTL for the packets they send, or they are located different default TTL value for the packets it sends, or each system
in a different place of the network topology, an attacker could is located at different distances in the network topology, an
stimulate responses from the devices being fingerprinted, and each attacker could stimulate responses from the devices being
response that arrives with a different TTL could be assumed to come fingerprinted, and responses that arrive with different TTL values
from a different device. could be assumed to come from a different devices.
o Of course, there are many other and much more precise techniques Of course, there are many other (and much more precise) techniques
to fingerprint physical devices. Among drawbacks of this method, to fingerprint physical devices. One weakness of this method is
while many systems differ in the default TTL they use for the that, while many systems differ in the default TTL value that they
packets they send, there are also many implementations which use use, there are also many implementations which use the same
the same default TTL. Additionally, packets sent by a given default TTL value. Additionally, packets sent by a given device
device may take different routes (e.g., due to load sharing or may take different routes (e.g., due to load sharing or route
route changes), and thus a given packet may incorrectly be changes), and thus a given packet may incorrectly be presumed to
presumed to come from a different device, when in fact it just come from a different device, when in fact it just traveled a
traveled a different route. different route.
3.9.3. Locating the source host in the network topology However, these defaults may be configurable (system-wide or per
protocol) and managed systems may employ such opportunities for
operational purposes and to defeat the capability of fingerprinting
heuristics.
3.8.3. Mapping the Network Topology
An originating host may set the TTL field of the packets it sends to
progressively increasing values in order to elicit an ICMP error
message from the routers that decrement the TTL of each packet to
zero, and thereby determine the IP addresses of the routers on the
path to the packet's destination. This procedure has been
traditionally employed by the traceroute tool.
3.8.4. Locating the source host in the network topology
The TTL field may also be used to locate the source system in the The TTL field may also be used to locate the source system in the
network topology [Northcutt2000]. network topology [Northcutt2000].
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| A |-----| R |------| R |----| R |-----| R | | A |-----| R |------| R |----| R |-----| R |
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
/ | / \ / | / \
/ | / \ / | / \
/ | / +---+ / | / +---+
skipping to change at page 21, line 27 skipping to change at page 25, line 32
+---+ +---+ +---+ \ +---| +---+ +---+ +---+ \ +---|
| R |----------| R |-- \ | R |----------| R |-- \
+---+ +---+ \ \ +---+ +---+ \ \
| \ / \ +---+| +---+ | \ / \ +---+| +---+
| \ / ----| R |------| R | | \ / ----| R |------| R |
| \ / +---+ +---+ | \ / +---+ +---+
+---+ \ +---+ +---+ +---+ \ +---+ +---+
| B | \| R |----| C | | B | \| R |----| C |
+---+ +---+ +---+ +---+ +---+ +---+
Figure 5: Tracking a host by means of the TTL field Figure 6: Tracking a host by means of the TTL field
Consider network topology of Figure 5. Assuming that an attacker Consider network topology of Figure 6. Assuming that an attacker
("F" in the figure) is performing some type of attack that requires ("F" in the figure) is performing some type of attack that requires
forging the Source Address (such as a TCP-based DoS reflection forging the Source Address (such as for a TCP-based DoS reflection
attack), and some of the involved hosts are willing to cooperate to attack), and some of the involved hosts are willing to cooperate to
locate the attacking system. locate the attacking system.
Assuming that: Assuming that:
o All the packets A gets have a TTL of 61. o All the packets A gets have a TTL of 61.
o All the packets B gets have a TTL of 61. o All the packets B gets have a TTL of 61.
o All the packets C gets have a TTL of 61. o All the packets C gets have a TTL of 61.
skipping to change at page 22, line 5 skipping to change at page 26, line 10
o All the packets D gets have a TTL of 62. o All the packets D gets have a TTL of 62.
Based on this information, and assuming that the system's default Based on this information, and assuming that the system's default
value was not overridden, it would be fair to assume that the value was not overridden, it would be fair to assume that the
original TTL of the packets was 64. With this information, the original TTL of the packets was 64. With this information, the
number of hops between the attacker and each of the aforementioned number of hops between the attacker and each of the aforementioned
hosts can be calculated. hosts can be calculated.
The attacker is: The attacker is:
o Three hops away from A. o Four hops away from A.
o Three hops away from B. o Four hops away from B.
o Three hops away from C. o Four hops away from C.
o Two hops away from D. o Four hops away from D.
In the network setup of Figure 3, the only system that satisfies all In the network setup of Figure 3, the only system that satisfies all
these conditions is the one marked as the "F". these conditions is the one marked as the "F".
The scenario described above is for illustration purposes only. In The scenario described above is for illustration purposes only. In
practice, there are a number of factors that may prevent this practice, there are a number of factors that may prevent this
technique from being successfully applied: technique from being successfully applied:
o Unless there is a "large" number of cooperating systems, and the o Unless there is a "large" number of cooperating systems, and the
attacker is assumed to be no more than a few hops away from these attacker is assumed to be no more than a few hops away from these
a systems, the number of "candidate" hosts will usually be too systems, the number of "candidate" hosts will usually be too large
large for the information to be useful. for the information to be useful.
o The attacker may be using a non-default TTL value, or, what is o The attacker may be using a non-default TTL value, or, what is
worse, using a pseudo-random value for the TTL of the packets it worse, using a pseudo-random value for the TTL of the packets it
sends. sends.
o The packets sent by the attacker may take different routes, as a o The packets sent by the attacker may take different routes, as a
result of a change in network topology, load sharing, etc., and result of a change in network topology, load sharing, etc., and
thus may lead to an incorrect analysis. thus may lead to an incorrect analysis.
3.9.4. Evading Network Intrusion Detection Systems 3.8.5. Evading Network Intrusion Detection Systems
The TTL field can be used to evade Network Intrusion Detection The TTL field can be used to evade Network Intrusion Detection
Systems. Depending on the position of a sensor relative to the Systems. Depending on the position of a sensor relative to the
destination host of the examined packet, the NIDS may get a different destination host of the examined packet, the NIDS may get a different
picture from that of the intended destination system. As an example, picture from that of the intended destination system. As an example,
a sensor may process a packet that will expire before getting to the a sensor may process a packet that will expire before getting to the
destination host. A general counter-measure for this type of attack destination host. A general countermeasure for this type of attack
is to normalize the traffic that gets to an organizational network. is to normalize the traffic that gets to an organizational network.
Examples of such traffic normalization can be found in [Paxson2001]. Examples of such traffic normalization can be found in [Paxson2001].
OpenBSD Packet Filter is an example of a packet filter that includes
TTL-normalization functionality [OpenBSD-PF]
3.9.5. Improving the security of applications that make use of the 3.8.6. Improving the security of applications that make use of the
Internet Protocol (IP) Internet Protocol (IP)
In some scenarios, the TTL field can be also used to improve the In some scenarios, the TTL field can be also used to improve the
security of an application, by restricting the hosts that can security of an application, by restricting the hosts that can
communicate with the given application . For example, there are communicate with the given application [RFC5082]. For example, there
applications for which the communicating systems are typically in the are applications for which the communicating systems are typically in
same network segment (i.e., there are no intervening routers). Such the same network segment (i.e., there are no intervening routers).
an application is the BGP (Border Gateway Protocol) between utilized Such an application is the BGP (Border Gateway Protocol) utilized by
by two peer routers. two peer routers (usually on a shared link medium).
If both systems use a TTL of 255 for all the packets they send to If both systems use a TTL of 255 for all the packets they send to
each other, then a check could be enforced to require all packets each other, then a check could be enforced to require all packets
meant for the application in question to have a TTL of 255. meant for the application in question to have a TTL of 255.
As all packets sent by systems that are not in the same network As all packets sent by systems that are not in the same network
segment will have a TTL smaller than 255, those packets will not pass segment will have a TTL smaller than 255, those packets will not pass
the check enforced by these two cooperating peers. This check the check enforced by these two cooperating peers. This check
reduces the set of systems that may perform attacks against the reduces the set of systems that may perform attacks against the
protected application (BGP in this case), thus mitigating the attack protected application (BGP in this case), thus mitigating the attack
vectors described in [NISCC2004] and [Watson2004]. vectors described in [NISCC2004] and [Watson2004].
o This same check is enforced for related ICMP error messages, with This same check is enforced for related ICMP error messages, with
the intent of mitigating the attack vectors described in the intent of mitigating the attack vectors described in
[NISCC2005] and [I-D.ietf-tcpm-icmp-attacks]. [NISCC2005] and [I-D.ietf-tcpm-icmp-attacks].
The TTL field can be used in a similar way in scenarios in which the The TTL field can be used in a similar way in scenarios in which the
cooperating systems either do not use a default TTL of 255, or are cooperating systems are not in the same network segment (i.e., multi-
not in the same network segment (i.e., multi-hop peering). In that hop peering). In that case, the following check could be enforced:
case, the following check could be enforced:
TTL >= 255 - DeltaHops TTL >= 255 - DeltaHops
This means that the set of hosts from which packets will be accepted This means that the set of hosts from which packets will be accepted
for the protected application will be reduced to those that are no for the protected application will be reduced to those that are no
more than DeltaHops away. While for obvious reasons the level of more than DeltaHops away. While for obvious reasons the level of
protection will be smaller than in the case of directly-connected protection will be smaller than in the case of directly-connected
peers, the use of the TTL field for protecting multi-hop peering peers, the use of the TTL field for protecting multi-hop peering
still reduces the set of hosts that could potentially perform a still reduces the set of hosts that could potentially perform a
number of attacks against the protected application. number of attacks against the protected application.
skipping to change at page 23, line 46 skipping to change at page 28, line 6
This use of the TTL field has been officially documented by the IETF This use of the TTL field has been officially documented by the IETF
under the name "Generalized TTL Security Mechanism" (GTSM) in under the name "Generalized TTL Security Mechanism" (GTSM) in
[RFC5082]. [RFC5082].
Some protocol scrubbers enforce a minimum value for the TTL field of Some protocol scrubbers enforce a minimum value for the TTL field of
the packets they forward. It must be understood that depending on the packets they forward. It must be understood that depending on
the minimum TTL being enforced, and depending on the particular the minimum TTL being enforced, and depending on the particular
network setup, the protocol scrubber may actually help attackers to network setup, the protocol scrubber may actually help attackers to
fool the GTSM, by "raising" the TTL of the attacking packets. fool the GTSM, by "raising" the TTL of the attacking packets.
3.10. Protocol 3.8.7. Limiting spread
The originating host sets the TTL field to a small value (frequently
1, for link-scope services) in order to artifically limit the
(topological) distance the packet is allowed to travel. This is
suggested in Section 4.2.2.9 of RFC 1812 [RFC1812]. Further
discussion of this technique can be found in in RFC 1112 [RFC1112].
3.9. Protocol
The Protocol field indicates the protocol encapsulated in the The Protocol field indicates the protocol encapsulated in the
internet datagram. The Protocol field may not only contain a value internet datagram. The Protocol field may not only contain a value
corresponding to an implemented protocol within the system, but also corresponding to a protocol implemented by the system processing the
a value corresponding to a protocol not implemented, or even a value packet, but also a value corresponding to a protocol not implemented,
not yet assigned by the IANA [IANA2006c]. or even a value not yet assigned by the IANA [IANA2006c].
While in theory there should not be security implications from the While in theory there should not be security implications from the
use of any value in the protocol field, there have been security use of any value in the protocol field, there have been security
issues in the past with systems that had problems when handling issues in the past with systems that had problems when handling
packets with some specific protocol numbers [Cisco2003] [CERT2003]. packets with some specific protocol numbers [Cisco2003] [CERT2003].
3.11. Header Checksum 3.10. Header Checksum
The Header Checksum field is an error detection mechanism meant to The Header Checksum field is an error detection mechanism meant to
detect errors in the IP header. While in principle there should not detect errors in the IP header. While in principle there should not
be security implications arising from this field, it should be noted be security implications arising from this field, it should be noted
that due to non-RFC-compliant implementations, the Header Checksum that due to non-RFC-compliant implementations, the Header Checksum
might be exploited to detect firewalls and/or evade network intrusion might be exploited to detect firewalls and/or evade network intrusion
detection systems (NIDS). detection systems (NIDS).
[Ed3f2002] describes the exploitation of the TCP checksum for [Ed3f2002] describes the exploitation of the TCP checksum for
performing such actions. As there are internet routers known to not performing such actions. As there are internet routers known to not
check the IP Header Checksum, and there might also be middle-boxes check the IP Header Checksum, and there might also be middle-boxes
(NATs, firewalls, etc.) not checking the IP checksum allegedly due to (NATs, firewalls, etc.) not checking the IP checksum allegedly due to
performance reasons, similar malicious activity to the one described performance reasons, similar malicious activity to the one described
in [Ed3f2002] might be performed with the IP checksum. in [Ed3f2002] might be performed with the IP checksum.
3.12. Source Address 3.11. Source Address
The Source Address of an IP datagram identifies the node from which The Source Address of an IP datagram identifies the node from which
the packet originated. the packet originated.
o Strictly speaking, the Source Address of an IP datagram identifies Strictly speaking, the Source Address of an IP datagram identifies
the interface of the sending system from which the packet was the interface of the sending system from which the packet was
sent, (rather than the originating "system"), as in the Internet sent, (rather than the originating "system"), as in the Internet
Architecture there's no concept of "node". Architecture there's no concept of "node address".
Unfortunately, it is trivial to forge the Source Address of an Unfortunately, it is trivial to forge the Source Address of an
Internet datagram. This has been exploited in the past for Internet datagram because of the apparent lack of consistent "egress
performing a variety of DoS (Denial of Service) attacks [NISCC2004] filtering" near the edge of the network. This has been exploited in
[RFC4987] [CERT1996a] [CERT1996b] [CERT1998a], and to impersonate as the past for performing a variety of DoS (Denial of Service) attacks
other systems in scenarios in which authentication was based on the [NISCC2004] [RFC4987] [CERT1996a] [CERT1996b] [CERT1998a], and to
Source Address of the sending system [daemon91996]. impersonate as other systems in scenarios in which authentication was
based on the Source Address of the sending system [daemon91996].
The extent to which these attacks can be successfully performed in The extent to which these attacks can be successfully performed in
the Internet can be reduced through deployment of ingress/egress the Internet can be reduced through deployment of ingress/egress
filtering in the internet routers. [NISCC2006] is a detailed guide filtering in the internet routers. [NISCC2006] is a detailed guide
on ingress and egress filtering. [RFC3704] and [RFC2827] discuss on ingress and egress filtering. [RFC2827] and [RFC3704] discuss
ingress filtering. [GIAC2000] discusses egress filtering. ingress filtering. [GIAC2000] discusses egress filtering.
[SpooferProject] measures the Internet's susceptibility to forged
Source Address IP packets.
o Even when the obvious field on which to perform checks for Even when the obvious field on which to perform checks for
ingress/egress filtering is the Source Address and Destination ingress/egress filtering is the Source Address and Destination
Address fields of the IP header, there are other occurrences of IP Address fields of the IP header, there are other occurrences of IP
addresses on which the same type of checks should be performed. addresses on which the same type of checks should be performed.
One example is the IP addresses contained in the payload of ICMP One example is the IP addresses contained in the payload of ICMP
error messages, as discussed in [I-D.ietf-tcpm-icmp-attacks] and error messages, as discussed in [I-D.ietf-tcpm-icmp-attacks] and
[Gont2006]. [Gont2006].
There are a number of sanity checks that should be performed on the There are a number of sanity checks that should be performed on the
Source Address of an IP datagram. Details can be found in Section Source Address of an IP datagram. Details can be found in Section
4.2 ("Addressing"). 4.2 ("Addressing").
Additionally, there exist freely available tools that allow Additionally, there exist freely available tools that allow
administrators to monitor which IP addresses are used with which MAC administrators to monitor which IP addresses are used with which MAC
addresses [LBNL2006]. This functionality is also included in many addresses [LBNL2006]. This functionality is also included in many
Network Intrusion Detection Systems (NIDS). Network Intrusion Detection Systems (NIDS).
It is also very important to understand that authentication should It is also very important to understand that authentication should
never rely on the Source Address of the communicating systems. never rely solely on the Source Address used by the communicating
systems.
3.13. Destination Address 3.12. Destination Address
The Destination Address of an IP datagram identifies the destination The Destination Address of an IP datagram identifies the destination
host to which the packet is meant to be delivered. host to which the packet is meant to be delivered.
o Strictly speaking, the Destination Address of an IP datagram Strictly speaking, the Destination Address of an IP datagram
identifies the interface of the destination network interface, identifies the interface of the destination network interface,
rather than the destination "system", as in the Internet rather than the destination "system", as in the Internet
Architecture there's no concept of "node". Architecture there's no concept of "node address".
There are a number of sanity checks that should be performed on the There are a number of sanity checks that should be performed on the
Destination Address of an IP datagram. Details can be found in Destination Address of an IP datagram. Details can be found in
Section 4.2 ("Addressing"). Section 4.2 ("Addressing").
3.14. Options 3.13. Options
According to RFC 791, IP options must be implemented by all IP According to RFC 791, IP options must be implemented by all IP
modules, both in hosts and gateways (i.e., end-systems and modules, both in hosts and gateways (i.e., end-systems and
intermediate-systems). intermediate-systems). This means that the general rules for
assembling, parsing, and processing of IP options must be
implemented. RFC 791 defines a set of options that "must be
understood", but this set has been updated by RFC 1122 [RFC1122], RFC
1812 [RFC1812], and other documents. Section 3.13.2 of this document
describes for each option type the current understanding of the
implementation requirements. IP systems are required to ignore
options they do not implement.
There are two cases for the format of an option: There are two cases for the format of an option:
o Case 1: A single byte of option-type. o Case 1: A single byte of option-type.
o Case 2: An option-type byte, an option-length byte, and the actual o Case 2: An option-type byte, an option-length byte, and the actual
option-data bytes. option-data bytes.
In the Case 2, the option-length byte counts the option-type byte and In Case 2, the option-length byte counts the option-type byte and the
the option-length byte, as well as the actual option-data bytes. option-length byte, as well as the actual option-data bytes.
All options except "End of Option List" (Type = 0) and "No Operation" All current and future options except "End of Option List" (Type = 0)
(Type = 1), are of Class 2. and "No Operation" (Type = 1), are of Class 2.
The option-type has three fields: The option-type has three fields:
o 1 bit: copied flag. o 1 bit: copied flag.
o 2 bits: option class. o 2 bits: option class.
o 5 bits: option number. o 5 bits: option number.
This format allows for the creation of new options for the extension
of the Internet Protocol (IP) and their transparent treatment on
intermediate systems that do not "understand" them, under direction
of the first three functional parts.
The copied flag indicates whether this option should be copied to all The copied flag indicates whether this option should be copied to all
fragments in the event the packet carrying it needs to be fragmented: fragments in the event the packet carrying it needs to be fragmented:
o 0 = not copied. o 0 = not copied.
o 1 = copied. o 1 = copied.
The values for the option class are: The values for the option class are:
o 0 = control. o 0 = control.
o 1 = reserved for future use. o 1 = reserved for future use.
o 2 = debugging and measurement. o 2 = debugging and measurement.
o 3 = reserved for future use. o 3 = reserved for future use.
This format allows for the creation of new options for the extension
of the Internet Protocol (IP).
Finally, the option number identifies the syntax of the rest of the Finally, the option number identifies the syntax of the rest of the
option. option.
3.14.1. General issues with IP options [IANA2006b] contains the list of the currently assigned IP option
numbers. It should be noted that IP systems are required to ignore
those options they do not implement.
3.13.1. General issues with IP options
The following subsections discuss security issues that apply to all The following subsections discuss security issues that apply to all
IP options. The proposed checks should be performed in addition to IP options. The proposed checks should be performed in addition to
any option-specific checks proposed in the next sections. any option-specific checks proposed in the next sections.
3.14.1.1. Processing requirements 3.13.1.1. Processing requirements
Router manufacturers tend to do IP option processing on the main Router manufacturers tend to do IP option processing on the main
processor, rather than on line cards. Unless special care is taken, processor, rather than on line cards. Unless special care is taken,
this represents Denial of Service (DoS) risk, as there is potential this represents Denial of Service (DoS) risk, as there is potential
for overwhelming the router with option processing. for overwhelming the router with option processing.
To reduce the impact of these packets on the system performance, a To reduce the impact of these packets on the system performance, a
few counter-measures could be implemented: few countermeasures could be implemented:
o Rate-limit the number of packets with IP options that are o Rate-limit the number of packets with IP options that are
processed by the system. processed by the system.
o Enforce a limit on the maximum number of options to be accepted on o Enforce a limit on the maximum number of options to be accepted on
a given internet datagram. a given internet datagram.
The first check avoids a flow of packets with IP options to overwhelm The first check avoids a flow of packets with IP options to overwhelm
the system in question. The second check avoids packets with the system in question. The second check avoids packets with many IP
multiple IP options to affect the performance of the system. options to affect the performance of the system.
3.14.1.2. Processing of the options by the upper layer protocol 3.13.1.2. Processing of the options by the upper layer protocol
Section 3.2.1.8 of RFC 1122 [RFC1122] states that all the IP options Section 3.2.1.8 of RFC 1122 [RFC1122] states that all the IP options
received in IP datagrams must be passed to the transport layer (or to received in IP datagrams must be passed to the transport layer (or to
ICMP processing when the datagram is an ICMP message). Therefore, ICMP processing when the datagram is an ICMP message). Therefore,
care in option processing must be taken not only at the internet care in option processing must be taken not only at the internet
layer, but also in every protocol module that may end up processing layer, but also in every protocol module that may end up processing
the options included in an IP datagram. the options included in an IP datagram.
3.14.1.3. General sanity checks on IP options 3.13.1.3. General sanity checks on IP options
There are a number of sanity checks that should be performed on IP There are a number of sanity checks that should be performed on IP
options before further option processing is done. They help prevent options before further option processing is done. They help prevent
a number of potential security problems, including buffer overflows. a number of potential security problems, including buffer overflows.
When these checks fail, the packet carrying the option should be When these checks fail, the packet carrying the option should be
dropped, and this event should be logged (e.g., a counter could be dropped, and this event should be logged (e.g., a counter could be
incremented to reflect the packet drop). incremented to reflect the packet drop).
RFC 1122 [RFC1122] recommends to send an ICMP "Parameter Problem" RFC 1122 [RFC1122] recommends to send an ICMP "Parameter Problem"
message to the originating system when a packet is dropped because of message to the originating system when a packet is dropped because of
a invalid value in a field, such as the cases discussed in the an invalid value in a field, such as the cases discussed in the
following subsections. Sending such a message might help in following subsections. Sending such a message might help in
debugging some network problems. However, it would also alert debugging some network problems. However, it would also alert
attackers about the system that is dropping packets because of the attackers about the system that is dropping packets because of the
invalid values in the protocol fields. invalid values in the protocol fields.
We advice that systems default to sending an ICMP "Parameter Problem" We advice that systems default to sending an ICMP "Parameter Problem"
error message when a packet is dropped because of an invalid value in error message when a packet is dropped because of an invalid value in
a protocol field (e.g., as a result of dropping a packet due to the a protocol field (e.g., as a result of dropping a packet due to the
sanity checks described in this section). However, we recommend that sanity checks described in this section). However, we recommend that
systems provide a system-wide toggle that allows an administrator to systems provide a system-wide toggle that allows an administrator to
override the default behavior so that packets can be silently dropped override the default behavior so that packets can be silently dropped
due when an invalid value in a protocol field is encountered. when an invalid value in a protocol field is encountered.
Option length Option length
Section 3.2.1.8 of RFC 1122 explicitly states that the IP layer must
not crash as the result of an option length that is outside the
possible range, and mentions that erroneous option lengths have been
observed to put some IP implementations into infinite loops.
For options that belong to the "Case 2" described in the previous Section 3.2.1.8 of RFC 1122 explicitly states that the IP layer
section, the following check should be performed: must not crash as the result of an option length that is outside
the possible range, and mentions that erroneous option lengths
have been observed to put some IP implementations into infinite
loops.
For options that belong to the "Case 2" described in the previous
section, the following check should be performed:
option-length >= 2 option-length >= 2
o The value "2" accounts for the option-type byte, and the option- The value "2" accounts for the option-type byte, and the
length byte. option-length byte.
This check prevents, among other things, loops in option processing This check prevents, among other things, loops in option
that may arise from incorrect option lengths. processing that may arise from incorrect option lengths.
Additionally, while the option-length byte of IP options of "Case 2" Additionally, while the option-length byte of IP options of
allows for an option length of up to 255 bytes, there is a limit on "Case 2" allows for an option length of up to 255 bytes, there is
legitimate option length imposed by the syntax of the IP header. a limit on legitimate option length imposed by the space available
for options in the IP header.
For all options of "Case 2", the following check should be enforced: For all options of "Case 2", the following check should be
enforced:
option-offset + option-length <= IHL * 4 option-offset + option-length <= IHL * 4
Where option-offset is the offset of the first byte of the option Where option-offset is the offset of the first byte of the option
within the IP header, with the first byte of the IP header being within the IP header, with the first byte of the IP header being
assigned an offset of 0. assigned an offset of 0.
If a packet does not pass these checks, the corresponding packet This check assures that the option does not claim to extend beyond
should be dropped, and this event should be logged (e.g., a counter the IP header. If the packet does not pass this check, it should
could be incremented to reflect the packet drop). be dropped, and this event should be logged (e.g., a counter could
be incremented to reflect the packet drop).
The aforementioned check is meant to detect forged option-length The aforementioned check is meant to detect forged option-length
values that might make an option overlap with the IP payload. This values that might make an option overlap with the IP payload.
would be particularly dangerous for those IP options which request This would be particularly dangerous for those IP options which
the processing systems to write information into the option-data area request the processing systems to write information into the
(such as the Record Route option), as it would allow the generation option-data area (such as the Record Route option), as it would
of overflows. allow the generation of overflows.
Data types Data types
Many IP options use pointer and length fields. Care must be taken as Many IP options use pointer and length fields. Care must be taken
to the data type used for these fields in the implementation. For as to the data type used for these fields in the implementation.
example, if an 8-bit signed data type were used to hold an 8-bit For example, if an 8-bit signed data type were used to hold an
pointer, then, pointer values larger than 128 might mistakenly be 8-bit pointer, then, pointer values larger than 128 might
interpreted as negative numbers, and thus might lead to unpredictable mistakenly be interpreted as negative numbers, and thus might lead
results. to unpredictable results.
3.14.2. Issues with specific options
3.14.2.1. End of Option List (Type = 0) 3.13.2. Issues with specific options
3.13.2.1. End of Option List (Type=0)
This option is used to indicate the "end of options" in those cases This option is used to indicate the "end of options" in those cases
in which the end of options would not coincide with the end of the in which the end of options would not coincide with the end of the
Internet Protocol Header. Internet Protocol Header. Octets in the IP header following the "End
of Option List" are to be regarded as padding (they should set to 0
by the originator and must to be ignored by receiving nodes).
IP systems are required to ignore those options they do not However, an originating node could alternatively fill the remaining
implement. Therefore, even in those cases in which this option is space in the Internet header with No Operation options (see
required, but is missing, IP systems should be able to process the Section 3.13.2.2). The End of Option List option allows slightly
remaining bytes of the IP header without any problems. more efficient parsing on receiving nodes and should be preferred by
packet originators. All IP systems are required to understand both
encodings.
3.14.2.2. No Operation (Type = 1) 3.13.2.2. No Operation (Type=1)
The no-operation option is basically meant to allow the sending The no-operation option is basically meant to allow the sending
system to align subsequent options in, for example, 32-bit system to align subsequent options in, for example, 32-bit
boundaries. boundaries, but it can also be used at the end of the options (se
Section Section 3.13.2.1).
With a single exception (see Section 3.13.2.13 below), this option is
the only IP option defined so far that can occur in multiple
instances in a single IP packet.
This option does not have security implications. This option does not have security implications.
3.14.2.3. Loose Source Record Route (LSRR) (Type = 131) 3.13.2.3. Loose Source and Record Route (LSRR) (Type=131)
This option lets the originating system specify a number of This option lets the originating system specify a number of
intermediate systems a packet must pass through to get to the intermediate systems a packet must pass through to get to the
destination host. Additionally, the route followed by the packet is destination host. Additionally, the route followed by the packet is
recorded in the option. The receiving host (end-system) must use the recorded in the option. The receiving host (end-system) must use the
reverse of the path contained in the received LSRR option. reverse of the path contained in the received LSRR option.
The LSSR option can be of help in debugging some network problems. The LSSR option can be of help in debugging some network problems.
Some ISP (Internet Service Provider) peering agreements require Some ISP (Internet Service Provider) peering agreements require
support for this option in the routers within the peer of the ISP. support for this option in the routers within the peer of the ISP.
skipping to change at page 29, line 45 skipping to change at page 35, line 4
The LSSR option can be of help in debugging some network problems. The LSSR option can be of help in debugging some network problems.
Some ISP (Internet Service Provider) peering agreements require Some ISP (Internet Service Provider) peering agreements require
support for this option in the routers within the peer of the ISP. support for this option in the routers within the peer of the ISP.
The LSRR option has well-known security implications. Among other The LSRR option has well-known security implications. Among other
things, the option can be used to: things, the option can be used to:
o Bypass firewall rules o Bypass firewall rules
o Reach otherwise unreachable internet systems o Reach otherwise unreachable internet systems
o Establish TCP connections in a stealthy way o Establish TCP connections in a stealthy way
o Learn about the topology of a network o Learn about the topology of a network
o Perform bandwidth-exhaustion attacks o Perform bandwidth-exhaustion attacks
Of these attack vectors, the one that has probably received least Of these attack vectors, the one that has probably received least
attention is the use of the LSRR option to perform bandwidth attention is the use of the LSRR option to perform bandwidth
exhaustion attacks. The LSRR option can be used as an amplification exhaustion attacks. The LSRR option can be used as an amplification
method for performing bandwidth-exhaustion attacks, as an attacker method for performing bandwidth-exhaustion attacks, as an attacker
could make a packet bounce multiple times between a number of systems could make a packet bounce multiple times between a number of systems
by carefully crafting an LSRR option. by carefully crafting an LSRR option.
o This is the IPv4-version of the IPv6 amplification attack that was This is the IPv4-version of the IPv6 amplification attack that was
widely publicized in 2007 [Biondi2007]. The only difference is widely publicized in 2007 [Biondi2007]. The only difference is
that the maximum length of the IPv4 header (and hence the LSRR that the maximum length of the IPv4 header (and hence the LSRR
option) limits the amplification factor when compared to the IPv6 option) limits the amplification factor when compared to the IPv6
counter-part. counter-part.
While the LSSR option may be of help in debugging some network While the LSSR option may be of help in debugging some network
problems, its security implications outweigh any legitimate use. problems, its security implications outweigh any legitimate use.
All systems should, by default, drop IP packets that contain an LSRR All systems should, by default, drop IP packets that contain an LSRR
option, and should log this event (e.g., a counter could be option, and should log this event (e.g., a counter could be
skipping to change at page 30, line 39 skipping to change at page 35, line 45
Section 3.3.5 of RFC 1122 [RFC1122] states that a host may be able to Section 3.3.5 of RFC 1122 [RFC1122] states that a host may be able to
act as an intermediate hop in a source route, forwarding a source- act as an intermediate hop in a source route, forwarding a source-
routed datagram to the next specified hop. We strongly discourage routed datagram to the next specified hop. We strongly discourage
host software from forwarding source-routed datagrams. host software from forwarding source-routed datagrams.
If processing of source-routed datagrams is explicitly enabled in a If processing of source-routed datagrams is explicitly enabled in a
system, the following sanity checks should be performed. system, the following sanity checks should be performed.
RFC 791 states that this option should appear, at most, once in a RFC 791 states that this option should appear, at most, once in a
given packet. Thus, if a packet is found to have more than one LSRR given packet. Thus, if a packet contains more than one LSRR option,
option, it should be dropped, and this event should be logged (e.g., it should be dropped, and this event should be logged (e.g., a
a counter could be incremented to reflect the packet drop). counter could be incremented to reflect the packet drop).
Therefore, hosts and routers should discard packets that contain more Additionally, packets containing a combination of LSRR and SSRR
than one LSRR option. Additionally, if a packet were found to have options should be dropped, and this event should be logged (e.g., a
both LSRR and SSRR options, it should be dropped, and this event counter could be incremented to reflect the packet drop).
should be logged (e.g., a counter could be incremented to reflect the
packet drop).
As many other IP options, the LSSR contains a Length field that As all other IP options of "Case 2", the LSSR contains a Length field
indicates the length of the option. Given the format of the option, that indicates the length of the option. Given the format of the
the Length field should be checked to be at least 3 (three): option, the Length field should be checked to have a minimum value of
three and be 3 (3 + n*4):
LSRR.Length >= 3 LSRR.Length % 4 == 3 && LSRR.Length != 0
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
Additionally, the following check should be performed on the Length
field:
LSRR.Offset + LSRR.Length < IHL *4
This check assures that the option does not overlap with the IP
payload (i.e., it does not go past the IP header). If the packet
does not pass this check, it should be dropped, and this event should
be logged (e.g., a counter could be incremented to reflect the packet
drop).
The Pointer is relative to this option. Thus, the minimum legal The Pointer is relative to this option. Thus, the minimum legal
value is 4. Therefore, the following check should be performed. value is 4. Therefore, the following check should be performed.
LSRR.Pointer >= 4 LSRR.Pointer >= 4
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). Additionally, the Pointer field should be reflect the packet drop). Additionally, the Pointer field should be
a multiple of 4. Consequently, the following check should be a multiple of 4. Consequently, the following check should be
performed: performed:
LSRR.Pointer % 4 == 0 LSRR.Pointer % 4 == 0
If a packet does not pass this check, it should be dropped, and this If a packet does not pass this check, it should be dropped, and this
event should be logged (e.g., a counter could be incremented to event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
When a system receives an IP packet with the LSRR route option, it When a system receives an IP packet with the LSRR option passing the
should check whether the source route is empty or not. The option is above checks, it should check whether the source route is empty or
empty if: not. The option is empty if:
LSRR.Pointer > LSRR.Length LSRR.Pointer > LSRR.Length
In that case, routing should be based on the Destination Address In that case, routing should be based on the Destination Address
field, and no further processing should be done on the LSRR option. field, and no further processing should be done on the LSRR option.
o [Microsoft1999] is a security advisory about a vulnerability [Microsoft1999] is a security advisory about a vulnerability
arising from improper validation of the LSRR.Pointer field. arising from improper validation of the LSRR.Pointer field.
If the address in the Destination Address field has been reached, and If the address in the Destination Address field has been reached, and
the option is not empty, the next address in the source route the option is not empty, the next address in the source route
replaces the address in the Destination Address field. replaces the address in the Destination Address field, and the IP
address of the interface that will be used to forward this datagram
The IP address of the interface that will be used to forward this is recorded in its place in the LSRR.Data field. Then, the
datagram should be recorded into the LSRR. However, before writing LSRR.Pointer. is incremented by 4.
in the route data area, the following check should be performed:
LSRR.Length - LSRR.Pointer >= 3
This assures that there will be at least 4 bytes of space in which to
record the IP address. If the packet does not pass this check, it
should be dropped, and this event should be logged (e.g., a counter
could be incremented to reflect the packet drop).
o An offset of "1" corresponds to the option type, that's why the Note that the sanity checks for the LSRR.Length and the
performed check is LSRR.Length - LSRR.Pointer >=3, and not LSRR.Pointer fields described above ensure that if the option is
LSRR.Length - LSRR.Pointer >=4. not empty, there will be (4*n) octets in the option. That is,
there will be at least one IP address to read, and enough room to
record the IP address of the interface that will be used to
forward this datagram.
The LSRR must be copied on fragmentation. This means that if a The LSRR must be copied on fragmentation. This means that if a
packet that carries the LSRR is fragmented, each of the fragments packet that carries the LSRR is fragmented, each of the fragments
will have to go through the list of systems specified in the LSRR will have to go through the list of systems specified in the LSRR
option. option.
3.14.2.4. Strict Source and Record Route (SSRR) (Type = 137) 3.13.2.4. Strict Source and Record Route (SSRR) (Type=137)
This option allows the originating system to specify a number of This option allows the originating system to specify a number of
intermediate systems a packet must pass through to get to the intermediate systems a packet must pass through to get to the
destination host. Additionally, the route followed by the packet is destination host. Additionally, the route followed by the packet is
recorded in the option, and the destination host (end-system) must recorded in the option, and the destination host (end-system) must
use the reverse of the path contained in the received SSRR option. use the reverse of the path contained in the received SSRR option.
This option is similar to the Loose Source and Record Route (LSRR) This option is similar to the Loose Source and Record Route (LSRR)
option, with the only difference that in the case of SSRR, the route option, with the only difference that in the case of SSRR, the route
specified in the option is the exact route the packet must take specified in the option is the exact route the packet must take
(i.e., no other intervening routers are allowed to be in the route). (i.e., no other intervening routers are allowed to be in the route).
The SSSR option can be of help in debugging some network problems. The SSSR option can be of help in debugging some network problems.
Some ISP (Internet Service Provider) peering agreements require Some ISP (Internet Service Provider) peering agreements require
support for this option in the routers within the peer of the ISP. support for this option in the routers within the peer of the ISP.
The SSRR option has the same security implications as the LSRR The SSRR option has the same security implications as the LSRR
option. Please refer to Section Section 3.14.2.3 for a discussion of option. Please refer to Section 3.13.2.3 for a discussion of such
such security implications. security implications.
As with the LSRR, while the SSSR option may be of help in debugging As with the LSRR, while the SSSR option may be of help in debugging
some network problems, its security implications outweigh any some network problems, its security implications outweigh any
legitimate use of it. legitimate use of it.
All systems should, by default, drop IP packets that contain an LSRR All systems should, by default, drop IP packets that contain an SSRR
option, and should log this event (e.g., a counter could be option, and should log this event (e.g., a counter could be
incremented to reflect the packet drop). However, they should incremented to reflect the packet drop). However, they should
provide a system-wide toggle to enable support for this option for provide a system-wide toggle to enable support for this option for
those scenarios in which this option is required. Such system-wide those scenarios in which this option is required. Such system-wide
toggle should default to "off" (or "disable"). toggle should default to "off" (or "disable").
[OpenBSD1998] is a security advisory about an improper [OpenBSD1998] is a security advisory about an improper
implementation of such a system-wide toggle in 4.4BSD kernels. implementation of such a system-wide toggle in 4.4BSD kernels.
In the event processing of the SSRR option were explicitly enabled, In the event processing of the SSRR option were explicitly enabled,
the same sanity checks described for the LSRR option in the same sanity checks described for the LSRR option in
Section 3.14.2.3 should be performed on the SSRR option. Namely, Section 3.13.2.3 should be performed on the SSRR option. Namely,
sanity checks shoudl be performed on the option length (SSRR.Length) sanity checks shoudl be performed on the option length (SSRR.Length)
and the Pointer field (SSRR.Pointer). and the pointer field (SSRR.Pointer).
If the packet passes the aforementioned sanity checks, the receiving If the packet passes the aforementioned sanity checks, the receiving
system should determine whether the Destination Address of the packet system should determine whether the Destination Address of the packet
corresponds to one of its IP addresses. If does not, it should be corresponds to one of its IP addresses. If does not, it should be
dropped, and this event should be logged (e.g., a counter could be dropped, and this event should be logged (e.g., a counter could be
incremented to reflect the packet drop). incremented to reflect the packet drop).
Contrary to the IP Loose Source and Record Route (LSRR) option, Contrary to the IP Loose Source and Record Route (LSRR) option,
the SSRR option does not allow in the route other routers than the SSRR option does not allow in the route other routers than
those contained in the option. If the system implements the weak those contained in the option. If the system implements the weak
skipping to change at page 34, line 10 skipping to change at page 38, line 46
In that case, if the address in the destination field has not been In that case, if the address in the destination field has not been
reached, the packet should be dropped, and this event should be reached, the packet should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
[Microsoft1999] is a security advisory about a vulnerability [Microsoft1999] is a security advisory about a vulnerability
arising from improper validation of the SSRR.Pointer field. arising from improper validation of the SSRR.Pointer field.
If the option is not empty, and the address in the Destination If the option is not empty, and the address in the Destination
Address field has been reached, the next address in the source route Address field has been reached, the next address in the source route
replaces the address in the Destination Address field. This IP replaces the address in the Destination Address field, and the IP
address must be reachable without the use of any intervening router address of the interface that will be used to forward this datagram
(i.e., the address must belong to any of the networks to which the is recorded in its place in the source route (SSRR.Data field).
system is directly attached). If that is not the case, the packet Then, the SSRR.Pointer. is incremented by 4.
should be dropped, and this event should be logged (e.g., a counter
could be incremented to reflect the packet drop).
The IP address of the interface that will be used to forward this
datagram should be recorded into the SSRR. However, before doing
that, the following check should be performed:
SSRR.Length - SSRR.Pointer >=3
An offset of "1" corresponds to the option type, that's why the
performed check is SSRR.Length - SSRR.Pointer >=3, and not
SSRR.Length - SSRR.Pointer >=4.
This assures that there will be at least 4 bytes of space on which to Note that the sanity checks for the SSRR.Length and the
record the IP address. If the packet does not pass this check, it SSRR.Pointer fields described above ensure that if the option is
should be dropped, and this event should be logged (e.g., a counter not empty, there will be (4*n) octets in the option. That is,
could be incremented to reflect the packet drop). there will be at least one IP address to read, and enough room to
record the IP address of the interface that will be used to
forward this datagram.
The SSRR option must be copied on fragmentation. This means that if The SSRR option must be copied on fragmentation. This means that if
a packet that carries the SSRR is fragmented, each of the fragments a packet that carries the SSRR is fragmented, each of the fragments
will have to go through the list of systems specified in the SSRR will have to go through the list of systems specified in the SSRR
option. option.
3.14.2.5. Record Route (Type = 7) 3.13.2.5. Record Route (Type=7)
This option provides a means to record the route that a given packet This option provides a means to record the route that a given packet
follows. follows.
The option begins with an 8-bit option code, which must be equal to The option begins with an 8-bit option code, which is equal to 7.
7. The second byte is the option length, which includes the option- The second byte is the option length, which includes the option-type
type byte, the option-length byte, the pointer byte, and the actual byte, the option-length byte, the pointer byte, and the actual
option-data. The third byte is a pointer into the route data, option-data. The third byte is a pointer into the route data,
indicating the first byte of the area in which to store the next indicating the first byte of the area in which to store the next
route data. The pointer is relative to the option start. route data. The pointer is relative to the option start.
RFC 791 states that this option should appear, at most, once in a RFC 791 states that this option should appear, at most, once in a
given packet. Therefore, if a packet has more than one instance of given packet. Therefore, if a packet has more than one instance of
this option, it should be dropped, and this event should be logged this option, it should be dropped, and this event should be logged
(e.g., a counter could be incremented to reflect the packet drop). (e.g., a counter could be incremented to reflect the packet drop).
The same sanity checks performed for the Length field and the Pointer The same sanity checks performed for the Length field and the Pointer
field of the LSRR and the SSRR options should be performed on the field of the LSRR and the SSRR options should be performed on the
Length field (RR.Length) and the Pointer field (RR.Pointer) of the RR Length field (RR.Length) and the Pointer field (RR.Pointer) of the RR
option. And, as with the LSRR and SSRR options, if the packet does option. As with the LSRR and SSRR options, if the packet does not
not pass these checks it should be dropped, and this event should be pass these checks it should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
When a system receives an IP packet with the Record Route option, it When a system receives an IP packet with the Record Route option that
should check whether there is space in the option to store route passes the above checks, it should check whether there is space in
information. The option is full if: the option to store route information. The option is full if:
RR.Pointer > RR.Length RR.Pointer > RR.Length
If the option is full, the datagram should be forwarded without If the option is full, the datagram should be forwarded without
further processing of this option. If not, the following check further processing of this option.
should be performed before writing any route data into the option:
RR.Pointer - RR.Length >= 3
If the packet does not pass this check, the packet should be
considered in error, and therefore should be dropped, and this event
should be logged (e.g., a counter could be incremented to reflect the
packet drop).
If the option is not full (i.e., RR.Pointer <= RR.Length), but
RR.Pointer - RR.Length < 4, it means that while there's space in
the option, there is not not enough space to store an IP address.
It is fair to assume that such an scenario will only occur when
the packet has been crafted.
If the packet passes this check, the IP address of the interface that If the option is not full (i.e., RR.Pointer <= RR.Length), the IP
will be used to forward this datagram should be recorded into the address of the interface that will be used to forward this datagram
area pointed by the RR.Pointer, and RR.Pointer should then be should be recorded into the area pointed to by the RR.Pointer, and
incremented by 4. RR.Pointer should then incremented by 4.
This option is not copied on fragmentation, and thus appears in the This option is not copied on fragmentation, and thus appears in the
first fragment only. If a fragment other than the one with offset 0 first fragment only. If a fragment other than the one with offset 0
contains the Record Route option, it should be dropped, and this contains the Record Route option, it should be dropped, and this
event should be logged (e.g., a counter could be incremented to event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
3.14.2.6. Stream Identifier (Type = 136) The Record Route option can be exploited to learn about the topology
of a network. However, the limited space in the IP header limits the
usefulness of this option for that purpose.
3.13.2.6. Stream Identifier (Type=136)
The Stream Identifier option originally provided a means for the 16- The Stream Identifier option originally provided a means for the 16-
bit SATNET stream Identifier to be carried through networks that did bit SATNET stream Identifier to be carried through networks that did
not support the stream concept. not support the stream concept.
However, as stated by Section 4.2.2.1 of RFC 1812 [RFC1812], this However, as stated by Section 4.2.2.1 of RFC 1812 [RFC1812], this
option is obsolete. Therefore, it should be ignored by the option is obsolete. Therefore, it must be ignored by the processing
processing systems. systems.
In the case of legacy systems still using this option, the length In the case of legacy systems still using this option, the length
field of the option should be checked to be 4. If the option does field of the option should be checked to be 4. If the option does
not pass this check, it should be dropped, and this event should be not pass this check, it should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
RFC 791 states that this option appears at most once in a given RFC 791 states that this option appears at most once in a given
datagram. Therefore, if a packet contains more than one instance of datagram. Therefore, if a packet contains more than one instance of
this option, it should be dropped, and this event should be logged this option, it should be dropped, and this event should be logged
(e.g., a counter could be incremented to reflect the packet drop). (e.g., a counter could be incremented to reflect the packet drop).
3.14.2.7. Internet Timestamp (Type = 68) 3.13.2.7. Internet Timestamp (Type=68)
This option provides a means for recording the time at which each This option provides a means for recording the time at which each
system processed this datagram. The timestamp option has a number of system processed this datagram. The timestamp option has a number of
security implications. Among them are: security implications. Among them are:
o It allows an attacker to obtain the current time of the systems o It allows an attacker to obtain the current time of the systems
that process the packet, which the attacker may find useful in a that process the packet, which the attacker may find useful in a
number of scenarios. number of scenarios.
o It may be used to map the network topology, in a similar way to o It may be used to map the network topology, in a similar way to
skipping to change at page 37, line 17 skipping to change at page 41, line 32
to store timestamps). Therefore, upon receipt of a packet that to store timestamps). Therefore, upon receipt of a packet that
contains an Internet Timestamp option, the following check should be contains an Internet Timestamp option, the following check should be
performed: performed:
IT.Length >= 4 IT.Length >= 4
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
Additionally, the following check should be performed on the option The Pointer is an index within this option, counting the option type
length field: octet as octet #1. It points to the first byte of the area in which
the next timestamp data should be stored and thus, the minimum legal
value is 5. Since the only change of the Pointer allowed by RFC 791
is incrementing it by 4 or 8, the following checks should be
performed on the Internet Timestamp option, depending on the Flag
value (see below).
IT.Offset + IT.Length < IHL *4 If IT.Flag is equal to 0, the following check should be performed:
This check assures that the option does not overlap with the IP IT.Pointer %4 == 1 && IT.Pointer != 1
payload (i.e., it does not go past the IP header). If the packet
does not pass this check, it should be dropped, and this event should
be logged (e.g., a counter could be incremented to reflect the packet
drop).
The pointer byte points to the first byte of the area in which the If the packet does not pass this check, it should be dropped, and
next timestamp data should be stored. As its value is relative to this event should be logged (e.g., a counter could be incremented to
the beginning of the option, its minimum legal value is 5. reflect the packet drop).
Consequently, the following check should be performed on a packet
that contains the Internet Timestamp option:
IT.Pointer >= 5 Otherwise, the following sanity check should be performed on the
option:
IT.Pointer % 8 == 5
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
The flag field has three possible legal values: The flag field has three possible legal values:
o 0: Record time stamps only, stored in consecutive 32-bit words. o 0: Record time stamps only, stored in consecutive 32-bit words.
o 1: Record each timestamp preceded with the internet address of the o 1: Record each timestamp preceded with the internet address of the
skipping to change at page 38, line 15 skipping to change at page 42, line 29
Therefore the following check should be performed: Therefore the following check should be performed:
IT.Flag == 0 || IT.Flag == 1 || IT.Flag == 3 IT.Flag == 0 || IT.Flag == 1 || IT.Flag == 3
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
The timestamp field is a right-justified 32-bit timestamp in The timestamp field is a right-justified 32-bit timestamp in
milliseconds since UT. If the time is not available in milliseconds, milliseconds since UTC. If the time is not available in
or cannot be provided with respect to UT, then any time may be milliseconds, or cannot be provided with respect to UTC, then any
inserted as a timestamp, provided the high order bit of the timestamp time may be inserted as a timestamp, provided the high order bit of
is set, to indicate this non-standard value. the timestamp is set, to indicate this non-standard value.
According to RFC 791, the initial contents of the timestamp area must According to RFC 791, the initial contents of the timestamp area must
be initialized to zero, or internet address/zero pairs. However, be initialized to zero, or internet address/zero pairs. However,
internet systems should be able to handle non-zero values, possibly internet systems should be able to handle non-zero values, possibly
discarding the offending datagram. discarding the offending datagram.
When an internet system receives a packet with an Internet Timestamp When an internet system receives a packet with an Internet Timestamp
option, it decides whether it should record its timestamp in the option, it decides whether it should record its timestamp in the
option. If it determines that it should, it should then determine option. If it determines that it should, it should then determine
whether the timestamp data area is full, by means of the following whether the timestamp data area is full, by means of the following
skipping to change at page 38, line 42 skipping to change at page 43, line 7
If this condition is true, the timestamp data area is full. If not, If this condition is true, the timestamp data area is full. If not,
there is room in the timestamp data area. there is room in the timestamp data area.
If the timestamp data area is full, the overflow byte should be If the timestamp data area is full, the overflow byte should be
incremented, and the packet should be forwarded without inserting the incremented, and the packet should be forwarded without inserting the
timestamp. If the overflow byte itself overflows, the packet should timestamp. If the overflow byte itself overflows, the packet should
be dropped, and this event should be logged (e.g., a counter could be be dropped, and this event should be logged (e.g., a counter could be
incremented to reflect the packet drop). incremented to reflect the packet drop).
If timestamp data area is not full, then further checks should be If the timestamp data area is not full, then processing continues as
performed before actually inserting any data. follows (note that the above checks on IT.Pointer ensure that there
is room for another entry in the option):
If the IT.Flag byte is 0, the following check should be performed:
IT.Length - IT.Pointer >= 3
If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). If the packet passes this check, there is
room for at least one 32-bit timestamp. The system's 32-bit
timestamp should be inserted at the area pointed by the pointer byte,
and the pointer byte should be incremented by four.
If the IT.Flag byte is 1, then the following check should be
performed:
IT.Length - IT.Pointer >= 7
If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). If the packet does pass this check, it
means there is space in the timestamp data area to store at least one
IP address plus the corresponding 32-bit timestamp. The IP address
of the system should be stored at the area pointed to by the pointer
byte, followed by the 32-bit system timestamp. The pointer byte
should then be incremented by 8.
If the flag byte is 3, then the following check should be performed: o If IT.Flag is 0, then the system's 32-bit timestamp is stored into
the area pointed to by the pointer byte and the pointer byte is
incremented by 4.
IT.Length - IT.Pointer >= 7 o If IT.Flag is 1, then the IP address of the system is stored into
the area pointed to by the pointer byte, followed by the 32-bit
system timestamp, and the pointer byte is incremented by 8.
If the packet does not pass this check, it should be dropped, and o Otherwise (IT.Flag is 3), if the IP address in the first 4 bytes
this event should be logged (e.g., a counter could be incremented to pointed to by IT.Pointer matches one of the IP addresses assigned
reflect the packet drop). If it does, it means there is space in the to an interface of the system, then the system's timestamp is
timestamp data area to store an IP address and store the stored into the area pointed to by IT.Pointer + 4, and the pointer
corresponding 32-bit timestamp. The system's timestamp should be byte is incremented by 8.
stored at the area pointed by IT.Pointer + 4. Then, the pointer byte
should be incremented by 8.
[Kohno2005] describes a technique for fingerprinting devices by [Kohno2005] describes a technique for fingerprinting devices by
measuring the clock skew. It exploits, among other things, the measuring the clock skew. It exploits, among other things, the
timestamps that can be obtained by means of the ICMP timestamp timestamps that can be obtained by means of the ICMP timestamp
request messages [RFC0791]. However, the same fingerprinting method request messages [RFC0791]. However, the same fingerprinting method
could be implemented with the aid of the Internet Timestamp option. could be implemented with the aid of the Internet Timestamp option.
3.14.2.8. Router Alert (Type = 148) 3.13.2.8. Router Alert (Type=148)
The Router Alert option is defined in RFC 2113 [RFC2113]. It has the The Router Alert option is defined in RFC 2113 [RFC2113] and later
semantic "routers should examine this packet more closely". A packet updates to it have been clarified by RFC 5350 [RFC5350]. It contains
that contains a Router Alert option will not go through the router's a 16-bit Value governed by an IANA registry (see [RFC5350]). The
fast-path and will be processed in the router more slowly than if the Router Alert option has the semantic "routers should examine this
option were not set. Therefore, this option may impact the packet more closely, if they participate in the functionality denoted
performance of the systems that handle the packet carrying it. by the Value of the option".
According to the syntax of the option as defined in RFC 2113, the According to the syntax of the option as defined in RFC 2113, the
following check should be enforced: following check should be enforced, if the router supports this
option:
RA.Length == 4 RA.Length == 4
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). Furthermore, the following check should be
performed on the Value field:
RA.Value == 0
If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
As explained in RFC 2113, hosts should ignore this option. A packet that contains a Router Alert option with an option value
corresponding to functionality supported by an active module in the
router will not go through the router's fast-path but will be
processed in the slow path of the router, handing it over for closer
inspection to the modules that has registered the matching option
value. Therefore, this option may impact the performance of the
systems that handle the packet carrying it.
3.14.2.9. Probe MTU (Type =11) As explained in RFC 2113 [RFC2113], hosts should ignore this option.
This option is defined in RFC 1063 [RFC1063], and originally provided 3.13.2.9. Probe MTU (Type=11) (obsolete)
a mechanism to discover the Path-MTU.
This option was defined in RFC 1063 [RFC1063], and originally
provided a mechanism to discover the Path-MTU.
This option is obsolete, and therefore any packet that is received This option is obsolete, and therefore any packet that is received
containing this option should be dropped, and this event should be containing this option should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
3.14.2.10. Reply MTU (Type = 12) 3.13.2.10. Reply MTU (Type=12) (obsolete)
This option is defined in RFC 1063 [RFC1063], and originally provided This option is defined in RFC 1063 [RFC1063], and originally provided
a mechanism to discover the Path-MTU. a mechanism to discover the Path-MTU.
This option is obsolete, and therefore any packet that is received This option is obsolete, and therefore any packet that is received
containing this option should be dropped, and this event should be containing this option should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
3.14.2.11. Traceroute (Type = 82) 3.13.2.11. Traceroute (Type=82)
This option is defined in RFC 1393 [RFC1393], and originally provided This option is defined in RFC 1393 [RFC1393], and originally provided
a mechanism to trace the path to a host. a mechanism to trace the path to a host.
This option is obsolete, and therefore any packet that is received This option is obsolete, and therefore any packet that is received
containing this option should be dropped, and this event should be containing this option should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
3.14.2.12. DoD Basic Security Option (Type = 130) 3.13.2.12. DoD Basic Security Option (Type=130)
This option is used by end-systems and intermediate systems of an This option is used by Multi-Level-Secure (MLS) end-systems and
internet to [RFC1108]: intermediate systems in specific environments to [RFC1108]:
o Transmit from source to destination in a network standard o Transmit from source to destination in a network standard
representation the common security labels required by computer representation the common security labels required by computer
security models, security models,
o Validate the datagram as appropriate for transmission from the o Validate the datagram as appropriate for transmission from the
source and delivery to the destination, and, source and delivery to the destination, and,
o Ensure that the route taken by the datagram is protected to the o Ensure that the route taken by the datagram is protected to the
level required by all protection authorities indicated on the level required by all protection authorities indicated on the
datagram. datagram.
It is specified by RFC 1108 [RFC1108] (which obsoletes RFC 1038 It is specified by RFC 1108 [RFC1108] (which obsoletes RFC 1038
[RFC1038]). [RFC1038]).
o RFC 791 [RFC0791] defined the "Security Option" (Type = 130), RFC 791 [RFC0791] defined the "Security Option" (Type=130), which
which used the same option type as the DoD Basic Security option used the same option type as the DoD Basic Security option
discussed in this section. The "Security Option" specified in RFC discussed in this section. The "Security Option" specified in RFC
791 is considered obsolete by Section 4.2.2.1 of RFC 1812, and 791 is considered obsolete by Section 3.2.1.8 of RFC 1122, and
therefore the discussion in this section is focused on the DoD therefore the discussion in this section is focused on the DoD
Basic Security option specified by RFC 1108 [RFC1108]. Basic Security option specified by RFC 1108 [RFC1108].
Section 4.2.2.1 of RFC 1812 states that routers "SHOULD implement Section 4.2.2.1 of RFC 1812 states that routers "SHOULD implement
this option". this option".
The DoD Basic Security Option is currently implemented in a number of The DoD Basic Security Option is currently implemented in a number of
operating systems (e.g., [IRIX2008], [SELinux2008], [Solaris2008], operating systems (e.g., [IRIX2008], [SELinux2008], [Solaris2008],
and [Cisco2008]), and deployed in a number of high-security networks. and [Cisco2008]), and deployed in a number of high-security networks.
Systems that belong to networks in which this option is in use should
process the DoD Basic Security option contained in each packet as
specified in [RFC1108].
RFC 1108 states that the option should appear at most once in a RFC 1108 states that the option should appear at most once in a
datagram. Therefore, if more than one DoD Basic Security option datagram. Therefore, if more than one DoD Basic Security option
(BSO) appears in a given datagram, the corresponding datagram should (BSO) appears in a given datagram, the corresponding datagram should
be dropped, and this event should be logged (e.g., a counter could be be dropped, and this event should be logged (e.g., a counter could be
incremented to reflect the packet drop). incremented to reflect the packet drop).
RFC 1108 states that the option Length is variable, with a minimum RFC 1108 states that the option Length is variable, with a minimum
option Length of 3 bytes. Therefore, the following check should be option Length of 3 bytes. Therefore, the following check should be
performed: performed:
BSO.Length >= 3 BSO.Length >= 3
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
Systems that belong to networks in which this option is in use should Current deployments of the security options described in this
process the DoD Basic Security option contained in each packet as section and the two subsequent sections have motivated the
specified in [RFC1108].
o Current deployments of the DoD Security Options have motivated the
proposal of a "Common Architecture Label IPv6 Security Option proposal of a "Common Architecture Label IPv6 Security Option
(CALIPSO)" for the IPv6 protocol. [RFC1038]. (CALIPSO)" for the IPv6 protocol. [RFC5570].
3.14.2.13. DoD Extended Security Option (Type = 133) 3.13.2.13. DoD Extended Security Option (Type=133)
This option permits additional security labeling information, beyond This option permits additional security labeling information, beyond
that present in the Basic Security Option (Section 3.13.2.12), to be that present in the Basic Security Option (Section 3.13.2.12), to be
supplied in an IP datagram to meet the needs of registered supplied in an IP datagram to meet the needs of registered
authorities. It is specified by RFC 1108 [RFC1108]. authorities. It is specified by RFC 1108 [RFC1108].
This option may be present only in conjunction with the DoD Basic This option may be present only in conjunction with the DoD Basic
Security option. Therefore, if a packet contains a DoD Extended Security option. Therefore, if a packet contains a DoD Extended
Security option (ESO), but does not contain a DoD Basic Security Security option (ESO), but does not contain a DoD Basic Security
option, it should be dropped, and this event should be logged (e.g., option, it should be dropped, and this event should be logged (e.g.,
a counter could be incremented to reflect the packet drop). It a counter could be incremented to reflect the packet drop). It
should be noted that, unlike the DoD Basic Security option, this should be noted that, unlike the DoD Basic Security option, this
option may appear multiple times in a single IP header. option may appear multiple times in a single IP header.
Systems that belong to networks in which this option is in use,
should process the DoD Extended Security option contained in each
packet as specified in RFC 1108 [RFC1108].
RFC 1108 states that the option Length is variable, with a minimum RFC 1108 states that the option Length is variable, with a minimum
option Length of 3 bytes. Therefore, the following check should be option Length of 3 bytes. Therefore, the following check should be
performed: performed:
ESO.Length >= 3 ESO.Length >= 3
If the packet does not pass this check, it should be dropped, and If the packet does not pass this check, it should be dropped, and
this event should be logged (e.g., a counter could be incremented to this event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
Systems that belong to networks in which this option is in use, 3.13.2.14. Commercial IP Security Option (CIPSO) (Type=134)
should process the DoD Extended Security option contained in each
packet as specified in RFC 1108 [RFC1108].
3.14.2.14. Commercial IP Security Option (CIPSO) (Type = 134)
This option was proposed by the Trusted Systems Interoperability This option was proposed by the Trusted Systems Interoperability
Group (TSIG), with the intent of meeting trusted networking Group (TSIG), with the intent of meeting trusted networking
requirements for the commercial trusted systems market place. It is requirements for the commercial trusted systems market place. It is
specified in [CIPSO1992] and [FIPS1994]. specified in [CIPSO1992] and [FIPS1994].
o The TSIG proposal was taken to the Commercial Internet Security The TSIG proposal was taken to the Commercial Internet Security
Option (CIPSO) Working Group of the IETF [CIPSOWG1994], and an Option (CIPSO) Working Group of the IETF [CIPSOWG1994], and an
Internet-Draft was produced [CIPSO1992]. The Internet-Draft was Internet-Draft was produced [CIPSO1992]. The Internet-Draft was
never published as an RFC, and the proposal was later standardized never published as an RFC, but the proposal was later standardized
by the U.S. National Institute of Standards and Technology (NIST) by the U.S. National Institute of Standards and Technology (NIST)
as "Federal Information Processing Standard Publication 188" as "Federal Information Processing Standard Publication 188"
[FIPS1994]. [FIPS1994].
It is currently implemented in a number of operating systems (e.g., It is currently implemented in a number of operating systems (e.g.,
IRIX [IRIX2008], Security-Enhanced Linux [SELinux2008], and Solaris IRIX [IRIX2008], Security-Enhanced Linux [SELinux2008], and Solaris
[Solaris2008]), and deployed in a number of high-security networks. [Solaris2008]), and deployed in a number of high-security networks.
o [Zakrzewski2002] and [Haddad2004] provide an overview of a Linux [Zakrzewski2002] and [Haddad2004] provide an overview of a Linux
implementation. implementation.
Systems that belong to networks in which this option is in use should
process the CIPSO option contained in each packet as specified in
[CIPSO1992].
According to the option syntax specified in [CIPSO1992] the following According to the option syntax specified in [CIPSO1992] the following
validation check should be performed: validation check should be performed:
CIPSO.Length >= 6 CIPSO.Length >= 6
If a packet does not pass this check, it should be dropped, and this If a packet does not pass this check, it should be dropped, and this
event should be logged (e.g., a counter could be incremented to event should be logged (e.g., a counter could be incremented to
reflect the packet drop). reflect the packet drop).
Systems that belong to networks in which this option is in use should 3.13.2.15. Sender Directed Multi-Destination Delivery (Type=149)
process the CIPSO option contained in each packet as specified in
[CIPSO1992].
3.14.2.15. Sender Directed Multi-Destination Delivery (Type = 149)
This option is defined in RFC 1770 [RFC1770], and originally provided This option is defined in RFC 1770 [RFC1770], and originally provided
unreliable UDP delivery to a set of addresses included in the option. unreliable UDP delivery to a set of addresses included in the option.
This option is obsolete. If a received packet contains this option, This option is obsolete. If a received packet contains this option,
it should be dropped, and this event should be logged (e.g., a it should be dropped, and this event should be logged (e.g., a
counter could be incremented to reflect the packet drop). counter could be incremented to reflect the packet drop).
3.15. TOS
Figure 2 shows the syntax of the Type of Service field, as defined by
RFC 791 [RFC0791], and updated by RFC 1349 [RFC1349]. We provide a
discussion of this obsoleted definition, legacy implementations might
still be using these semantics.
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| PRECEDENCE | D | T | R | C | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
Figure 6: Type of Service field
+----------+----------------------------------------------+
| Bits 0-2 | Precedence |
+----------+----------------------------------------------+
| Bit 3 | 0 = Normal Delay, 1 = Low Delay |
+----------+----------------------------------------------+
| Bit 4 | 0 = Normal Throughput, 1 = High Throughput |
+----------+----------------------------------------------+
| Bit 5 | 0 = Normal Reliability, 1 = High Reliability |
+----------+----------------------------------------------+
| Bit 6 | 0 = Normal Cost, 1 = Minimize Monetary Cost |
+----------+----------------------------------------------+
| Bits 7 | Reserved for Future Use (must be zero) |
+----------+----------------------------------------------+
Table 2: TOS bits
+-----+-----------------+
| 111 | Network Control |
+-----+-----------------+
| 110 | Internetwork |
+-----+-----------------+
| 101 | CRITIC/ECP |
+-----+-----------------+
| 100 | Flash Override |
+-----+-----------------+
| 011 | Flash |
+-----+-----------------+
| 010 | Immediate |
+-----+-----------------+
| 001 | Priority |
+-----+-----------------+
| 000 | Routine |
+-----+-----------------+
Table 3: Precedence field
The Type of Service field can be used to affect the way in which the
packet is treated by the systems of a network that process it.
Section 4.2.1 ("Precedence-ordered queue service") and Section 4.2.3
("Weak TOS") of this document describe the security implications of
the Type of Service field in the forwarding of packets.
4. Internet Protocol Mechanisms 4. Internet Protocol Mechanisms
4.1. Fragment reassembly 4.1. Fragment reassembly
To accommodate networks with different Maximum Transmission Units To accommodate networks with different Maximum Transmission Units
(MTUs), the Internet Protocol provides a mechanism for the (MTUs), the Internet Protocol provides a mechanism for the
fragmentation of IP packets by end-systems (hosts) and/or fragmentation of IP packets by end-systems (hosts) and/or
intermediate systems (routers). Reassembly of fragments is performed intermediate systems (routers). Reassembly of fragments is performed
only by the end-systems. only by the end-systems.
o [Cerf1974] provides the rationale for which packet reassembly is [Cerf1974] provides the rationale for why packet reassembly is not
not performed by intermediate systems. performed by intermediate systems.
During the last few decades, IP fragmentation and reassembly has been During the last few decades, IP fragmentation and reassembly has been
exploited in a number of ways, to perform actions such as evading exploited in a number of ways, to perform actions such as evading
Network Intrusion Detection Systems (NIDS), bypassing firewall rules, Network Intrusion Detection Systems (NIDS), bypassing firewall rules,
and performing Denial of Service (DoS) attacks. and performing Denial of Service (DoS) attacks.
o [Bendi1998] and [Humble1998] are examples of the exploitation of [Bendi1998] and [Humble1998] are examples of the exploitation of
these issues for performing Denial of Service (DoS) attacks. these issues for performing Denial of Service (DoS) attacks.
[CERT1997] and [CERT1998b] document these issues. [Anderson2001] [CERT1997] and [CERT1998b] document these issues. [Anderson2001]
is a survey of fragmentation attacks. [US-CERT2001] is an example is a survey of fragmentation attacks. [US-CERT2001] is an example
of the exploitation of IP fragmentation to bypass firewall rules. of the exploitation of IP fragmentation to bypass firewall rules.
[CERT1999] describes the implementation of fragmentation attacks [CERT1999] describes the implementation of fragmentation attacks
in Distributed Denial of Service (DDoS) attack tools. in Distributed Denial of Service (DDoS) attack tools.
The problem with IP fragment reassembly basically has to do with the The problem with IP fragment reassembly basically has to do with the
complexity of the function, in a number of aspects: complexity of the function, in a number of aspects:
o Fragment reassembly is a stateful operation for a stateless o Fragment reassembly is a stateful operation for a stateless
protocol (IP). The IP module at the host performing the protocol (IP). The IP module at the host performing the
reassembly function must allocate memory buffers both for reassembly function must allocate memory buffers both for
temporarily storing the received fragments, and to perform the temporarily storing the received fragments, and to perform the
reassembly function. Attackers can exploit this fact to exhaust reassembly function. Attackers can exploit this fact to exhaust
memory buffers at the system performing the fragment reassembly. memory buffers at the system performing the fragment reassembly.
o The fragmentation and reassembly mechanisms were designed at a o The fragmentation and reassembly mechanisms were designed at a
time in which the available bandwidths were very different from time in which the available bandwidths were very different from
the bandwidths available nowadays. With the current available the bandwidths available nowadays. With the current available
bandwidths, a number of interoperability problems may arise. And bandwidths, a number of interoperability problems may arise, and
these issues may be intentionally exploited by attackers to these issues may be intentionally exploited by attackers to
perform Denial of Service (DoS) attacks. perform Denial of Service (DoS) attacks.
o Fragment reassembly must usually be performed without any o Fragment reassembly must usually be performed without any
knowledge of the properties of the path the fragments follow. knowledge of the properties of the path the fragments follow.
Without this information, hosts cannot make any educated guess on Without this information, hosts cannot make any educated guess on
how long they should wait for missing fragments to arrive. how long they should wait for missing fragments to arrive.
o The fragment reassembly algorithm, as described by the IETF o The fragment reassembly algorithm, as described by the IETF
specifications, is ambiguous, and allows for a number of specifications, is ambiguous, and allows for a number of
skipping to change at page 46, line 30 skipping to change at page 49, line 11
temporarily stored in system memory, until the IP module processes it temporarily stored in system memory, until the IP module processes it
and hands it to the protocol machine that corresponds to the and hands it to the protocol machine that corresponds to the
encapsulated protocol. encapsulated protocol.
In the case of fragmented IP packets, while the IP module may perform In the case of fragmented IP packets, while the IP module may perform
preliminary processing of the IP header (such as checking the header preliminary processing of the IP header (such as checking the header
for errors and processing IP options), fragments must be kept in for errors and processing IP options), fragments must be kept in
system buffers until all fragments are received and are reassembled system buffers until all fragments are received and are reassembled
into a complete internet datagram. into a complete internet datagram.
As mentioned above, the fact that the internet layer will not usually As mentioned above, because the internet layer will not usually have
have information about the characteristics of the path between the information about the characteristics of the path between the system
system and the remote host, no educated guess can be made on the and the remote host, no educated guess can be made on the amount of
amount of time that should be waited for the other fragments to time that should be waited for the other fragments to arrive.
arrive. Therefore, the specifications recommend to wait for a period Therefore, the specifications recommend to wait for a period of time
of time that will be acceptable for virtually all the possible that is acceptable for virtually all the possible network scenarios
network scenarios in which the protocols might operate. in which the protocols might operate. After that time has elapsed,
Specifically, RFC 1122 [RFC1122] states that the reassembly timeout all the received fragments for the corresponding incomplete packet
should be a fixed value between 60 and 120 seconds. If after waiting are discarded.
for that period of time the remaining fragments have not yet arrived,
all the received fragments for the corresponding packet are
discarded.
o The original IP Specification, RFC 791 [RFC0791], states that The original IP Specification, RFC 791 [RFC0791], states that
systems should wait for at least 15 seconds for the missing systems should wait for at least 15 seconds for the missing
fragments to arrive. Systems that follow the "Example Reassembly fragments to arrive. Systems that follow the "Example Reassembly
Procedure" described in Section 3.2 of RFC 791 may end up using a Procedure" described in Section 3.2 of RFC 791 may end up using a
reassembly timer of up to 4.25 minutes, with minimum of 15 reassembly timer of up to 4.25 minutes, with a minimum of 15
seconds. Section 3.3.2 ("Reassembly") of RFC 1122 corrected this seconds. Section 3.3.2 ("Reassembly") of RFC 1122 corrected this
advice, stating that the reassembly timeout should be a fixed advice, stating that the reassembly timeout should be a fixed
value between 60 and 120 seconds. value between 60 and 120 seconds.
However, the longer the system waits for the missing fragments to However, the longer the system waits for the missing fragments to
arrive, the longer the corresponding system resources must be tied to arrive, the longer the corresponding system resources must be tied to
the corresponding packet. The amount of system memory is finite, and the corresponding packet. The amount of system memory is finite, and
even with today's systems, it can still be considered a scarce even with today's systems, it can still be considered a scarce
resource. resource.
An attacker could take advantage of the uncomfortable situation the An attacker could take advantage of the uncomfortable situation the
system performing fragment reassembly is in, by sending forged system performing fragment reassembly is in, by sending forged
fragments that will never reassemble into a complete datagram. That fragments that will never reassemble into a complete datagram. That
is, an attacker would send many different fragments, with different is, an attacker would send many different fragments, with different
IP IDs, without ever sending all the necessary fragments that would IP IDs, without ever sending all the necessary fragments that would
be needed to reassemble them into a full IP datagram. For each of be needed to reassemble them into a full IP datagram. For each of
the fragments, the IP module would allocate resources and tie them to the fragments, the IP module would allocate resources and tie them to
the corresponding fragment, until any the reassembly timer for the the corresponding fragment, until the reassembly timer for the
corresponding packet expires. corresponding packet expires.
There are some implementation strategies which could increase the There are some implementation strategies which could increase the
impact of this attack. For example, upon receipt of a fragment, some impact of this attack. For example, upon receipt of a fragment, some
systems allocate a memory buffer that will be large enough to systems allocate a memory buffer that will be large enough to
reassemble the whole datagram. While this might be beneficial in reassemble the whole datagram. While this might be beneficial in
legitimate cases, this just amplifies the impact of the possible legitimate cases, this just amplifies the impact of the possible
attacks, as a single small fragment could tie up memory buffers for attacks, as a single small fragment could tie up memory buffers for
the size of an extremely large (and unlikely) datagram. The the size of an extremely large (and unlikely) datagram. The
implementation strategy suggested in RFC 815 [RFC0815] leads to such implementation strategy suggested in RFC 815 [RFC0815] leads to such
an implementation. an implementation.
The impact of the aforementioned attack may vary depending on some The impact of the aforementioned attack may vary depending on some
specific implementation details: specific implementation details:
o If the system does not enforce limits on the amount of memory that o If the system does not enforce limits on the amount of memory that
can be allocated by the IP module, then an attacker could tie all can be allocated by the IP module, then an attacker could tie all
system memory to fragments, at which point the system would become system memory to fragments, at which point the system would become
unusable, probably crashing. unusable, perhaps crashing.
o If the system enforces limits on the amount of memory that can be o If the system enforces limits on the amount of memory that can be
allocated by the IP module as a whole, then, when this limit is allocated by the IP module as a whole, then, when this limit is
reached, all further IP packets that arrive would be discarded, reached, all further IP packets that arrive would be discarded,
until some fragments time out and free memory is available again. until some fragments time out and free memory is available again.
o If the system enforces limits on the amount memory that can be o If the system enforces limits on the amount memory that can be
allocated for the reassembly of fragments (in addition to allocated for the reassembly of fragments, then, when this limit
enforcing a limit for the IP module as a whole), then, when this is reached, all further fragments that arrive would be discarded,
limit is reached, all further fragments that arrive would be until some fragment(s) time out and free memory is available
discarded, until some fragment(s) time out and free memory is again.
available again.
4.1.1.2. Problems that arise from the length of the IP Identification 4.1.1.2. Problems that arise from the length of the IP Identification
field field
The Internet Protocols are currently being used in environments that The Internet Protocols are currently being used in environments that
are quite different from the ones in which they were conceived. For are quite different from the ones in which they were conceived. For
instance, the availability of bandwidth at the time the Internet instance, the availability of bandwidth at the time the Internet
Protocol was designed was completely different from the availability Protocol was designed was completely different from the availability
of bandwidth in today's networks. of bandwidth in today's networks.
The Identification field is a 16-bit field that is used for the The Identification field is a 16-bit field that is used for the
fragmentation and reassembly function. In the event a datagram gets fragmentation and reassembly function. In the event a datagram gets
fragmented, all the corresponding fragments will share the same fragmented, all the corresponding fragments will share the same
Identification number. Thus, the system receiving the fragments will {Source Address, Destination Address, Protocol, Identification
be able to uniquely identify them as fragments that correspond to the number} four-tuple. Thus, the system receiving the fragments will be
able to uniquely identify them as fragments that correspond to the
same IP datagram. At a given point in time, there must be at most same IP datagram. At a given point in time, there must be at most
only one packet in the network with a given Identification number. only one packet in the network with a given four-tuple. If not, an
If not, an Identification number "collision" might occur, and the Identification number "collision" might occur, and the receiving
receiving system might end up "mixing" fragments that correspond to system might end up "mixing" fragments that correspond to different
different IP datagrams which simply reused the same Identification IP datagrams which simply reused the same Identification number.
number.
For example, sending over a 1 Gbit/s path a continuous stream of
(UDP) packets of roughly 1 kb size that all get fragmented into
two equally sized fragments of 576 octets each (minimum reasesmbly
buffer size) would repeat the IP Identification values within less
than 0.65 seconds (assuming roughly 10% link layer overhead); with
shorter packets that still get fragmented, this figure could
easily drop below 0.4 seconds. With a single IP packet dropped in
this short timeframe, packets would start to be reassembled
wrongly and continuously once in such interval until an error
detection and recovery algorithm at an upper layer lets the
application back out.
For each group of fragments whose Identification numbers "collide", For each group of fragments whose Identification numbers "collide",
the fragment reassembly will lead to corrupted packets. The IP the fragment reassembly will lead to corrupted packets. The IP
payload of the reassembled datagram will be handed to the payload of the reassembled datagram will be handed to the
corresponding upper layer protocol, where the error will (hopefully) corresponding upper layer protocol, where the error will (hopefully)
be detected by some error detecting code (such as the TCP checksum) be detected by some error detecting code (such as the TCP checksum)
and the packet will be discarded. and the packet will be discarded.
An attacker could exploit this fact to intentionally cause a system An attacker could exploit this fact to intentionally cause a system
to discard all or part of the fragmented traffic it gets, thus to discard all or part of the fragmented traffic it gets, thus
performing a Denial of Service attack. Such an attacker would simply performing a Denial-of-Service attack. Such an attacker would simply
establish a flow of fragments with different IP Identification establish a flow of fragments with different IP Identification
numbers, to trash all or part of the IP Identification space. As a numbers, to trash all or part of the IP Identification space. As a
result, the receiving system would use the attacker's fragments for result, the receiving system would use the attacker's fragments for
the reassembly of legitimate datagrams, leading to corrupted packets the reassembly of legitimate datagrams, leading to corrupted packets
which would later (and hopefully) get dropped. which would later (and hopefully) get dropped.
In most cases, use of a long fragment timeout will benefit the In most cases, use of a long fragment timeout will benefit the
attacker, as forged fragments will keep the IP Identification space attacker, as forged fragments will keep the IP Identification space
trashed for a longer period of time. trashed for a longer period of time.
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As IP packets can be duplicated by the network, and each packet may As IP packets can be duplicated by the network, and each packet may
take a different path to get to the destination host, fragments may take a different path to get to the destination host, fragments may
arrive not only out of order and/or duplicated, but also overlapping. arrive not only out of order and/or duplicated, but also overlapping.
This means that the reassembly process can be somewhat complex, with This means that the reassembly process can be somewhat complex, with
the corresponding implementation being not specifically trivial. the corresponding implementation being not specifically trivial.
[Shannon2001] analyzes the causes and attributes of fragment traffic [Shannon2001] analyzes the causes and attributes of fragment traffic
measured in several types of WANs. measured in several types of WANs.
During the years, a number of attacks have exploited bugs in the During the years, a number of attacks have exploited bugs in the
reassembly function of a number of operating systems, producing reassembly function of several operating systems, producing buffer
buffer overflows that have led, in most cases, to a crash of the overflows that have led, in most cases, to a crash of the attacked
attacked system. system.
4.1.1.4. Problems that arise from the ambiguity of the reassembly 4.1.1.4. Problems that arise from the ambiguity of the reassembly
process process
Network Intrusion Detection Systems (NIDSs) typically monitor the Network Intrusion Detection Systems (NIDSs) typically monitor the
traffic on a given network with the intent of identifying traffic traffic on a given network with the intent of identifying traffic
patterns that might indicate network intrusions. patterns that might indicate network intrusions.
In the presence of IP fragments, in order to detect illegitimate In the presence of IP fragments, in order to detect illegitimate
activity at the transport or application layers (i.e., any protocol activity at the transport or application layers (i.e., any protocol
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While the first of the choices might seem to be the most straight- While the first of the choices might seem to be the most straight-
forward, it implies that even when a single small fragment of a given forward, it implies that even when a single small fragment of a given
packet is received, the amount of memory that will be allocated for packet is received, the amount of memory that will be allocated for
that fragment will account for the size of the complete IP datagram, that fragment will account for the size of the complete IP datagram,
thus using more system resources than what is actually needed. thus using more system resources than what is actually needed.
Furthermore, the only situation in which the actual size of the whole Furthermore, the only situation in which the actual size of the whole
datagram will be known is when the last fragment of the packet is datagram will be known is when the last fragment of the packet is
received first, as that is the only packet from which the total size received first, as that is the only packet from which the total size
of the IP datagram can be asserted. Otherwise, memory should be of the IP datagram can be asserted. Otherwise, memory should be
allocated for largest possible packet size (65535 bytes). allocated for the largest possible packet size (65535 bytes).
The IP module should also enforce a limit on the amount of memory The IP module should also enforce a limit on the amount of memory
that can be allocated for IP fragments, as well as a limit on the that can be allocated for IP fragments, as well as a limit on the
number of fragments that at any time will be allowed in the system. number of fragments that at any time will be allowed in the system.
This will basically limit the resources spent on the reassembly This will basically limit the resources spent on the reassembly
process, and prevent an attacker from trashing the whole system process, and prevent an attacker from trashing the whole system
memory. memory.
Furthermore, the IP module should keep a different buffer for IP Furthermore, the IP module should keep a different buffer for IP
fragments than for complete IP datagrams. This will basically fragments than for complete IP datagrams. This will basically
separate the effects of fragment attacks on non-fragmented traffic. separate the effects of fragment attacks on non-fragmented traffic.
Most TCP/IP implementations, such as that in Linux and those in BSD- Most TCP/IP implementations, such as that in Linux and those in BSD-
derived systems, already implement this. derived systems, already implement this.
[Jones2002] contains an analysis about the amount of memory that may [Jones2002] analyzes the amount of memory that may be needed for the
be needed for the fragment reassembly buffer depending on a number of fragment reassembly buffer depending on a number of network
network characteristics. characteristics.
4.1.2.2. Flushing the fragment buffer 4.1.2.2. Flushing the fragment buffer
In the case of those attacks that aim to consume the memory buffers In the case of those attacks that aim to consume the memory buffers
used for fragments, and those that aim to cause a collision of IP used for fragments, and those that aim to cause a collision of IP
Identification numbers, there are a number of counter-measures that Identification numbers, there are a number of countermeasures that
can be implemented. can be implemented.
The IP module should enforce a limit on the amount of memory that can Even with these countermeasures in place, there is still the issue of
be allocated for IP fragments, as well as a limit on the number of what to do when the buffer pool used for IP fragments gets full.
fragments that at any time will be allowed in the system. This will
basically limit the resources spent on the reassembly process, and
prevent an attacker from trashing the whole system memory.
Additionally, the IP module should keep a different buffer for IP
fragments than for complete IP datagrams. This will basically
separate the effects of fragment attacks on non-fragmented traffic.
Most TCP/IP implementations, such as that in Linux and those in BSD-
derived systems, already implement this.
Even with these counter-measures in place, there is still the issue
of what to do when the buffer used for IP fragments get full.
Basically, if the fragment buffer is full, no instance of Basically, if the fragment buffer is full, no instance of
communication that relies on fragmentation will be able to progress. communication that relies on fragmentation will be able to progress.
Unfortunately, there are not many options for reacting to this Unfortunately, there are not many options for reacting to this
situation. If nothing is done, all the instances of communication situation. If nothing is done, all the instances of communication
that rely on fragmentation will experience a denial of service. that rely on fragmentation will experience a denial of service.
Thus, the only thing that can be done is flush all or part of the Thus, the only thing that can be done is flush all or part of the
fragment buffer, on the premise that legitimate traffic will be able fragment buffer, on the premise that legitimate traffic will be able
to make use of the freed buffer space to allow communication flows to to make use of the freed buffer space to allow communication flows to
progress. progress.
There are a number of factors that should be taken into consideration There are a number of factors that should be taken into consideration
when flushing the fragment buffer. First, if a fragment of a given when flushing the fragment buffers. First, if a fragment of a given
packet (i.e., fragment with a given Identification number) is packet (i.e., fragment with a given Identification number) is
flushed, all the other fragments that correspond to the same datagram flushed, all the other fragments that correspond to the same datagram
should be flushed. As in order for a packet to be reassembled all of should be flushed. As in order for a packet to be reassembled all of
its fragments must be received by the system performing the its fragments must be received by the system performing the
reassembly function, flushing only a subset of the fragments of a reassembly function, flushing only a subset of the fragments of a
given packet would keep the corresponding buffers tied to fragments given packet would keep the corresponding buffers tied to fragments
that would never reassemble into a complete datagram. Additionally, that would never reassemble into a complete datagram. Additionally,
care must be taken so that, in the event that subsequent buffer care must be taken so that, in the event that subsequent buffer
flushes need to be performed, it is not always the same set of flushes need to be performed, it is not always the same set of
fragments that get dropped, as such a behavior would probably cause a fragments that get dropped, as such a behavior would probably cause a
selective Denial of Service (DoS) to the traffic flows to which that selective Denial of Service (DoS) to the traffic flows to which that
set of fragments belong. set of fragments belongs.
Many TCP/IP implementations define a threshold for the number of Many TCP/IP implementations define a threshold for the number of
fragments that, when reached, triggers a fragment-buffer flush. Some fragments that, when reached, triggers a fragment-buffer flush. Some
systems flush 1/2 of the fragment buffer when the threshold is systems flush 1/2 of the fragment buffer when the threshold is
reached. As mentioned before, the idea of flushing the buffer is to reached. As mentioned before, the idea of flushing the buffer is to
create some free space in the fragment buffer, on the premise that create some free space in the fragment buffer, on the premise that
this will allow for new and legitimate fragments to be processed by this will allow for new and legitimate fragments to be processed by
the IP module, thus letting communication survive the overwhelming the IP module, thus letting communication survive the overwhelming
situation. On the other hand, the idea of flushing a somewhat large situation. On the other hand, the idea of flushing a somewhat large
portion of the buffer is to avoid flushing always the same set of portion of the buffer is to avoid flushing always the same set of
packets. packets.
4.1.2.3. A more selective fragment buffer flushing strategy 4.1.2.3. A more selective fragment buffer flushing strategy
One of the difficulties in implementing counter-measures for the One of the difficulties in implementing countermeasures for the
fragmentation attacks described in this document is that it is fragmentation attacks described in throughout Section 4.1 is that it
difficult to perform validation checks on the received fragments. is difficult to perform validation checks on the received fragments.
For instance, the fragment on which validity checks could be For instance, the fragment on which validity checks could be
performed, the first fragment, may be not the first fragment to performed, the first fragment, may be not the first fragment to
arrive at the destination host. arrive at the destination host.
Fragments can not only arrive out of order because of packet Fragments can not only arrive out of order because of packet
reordering performed by the network, but also because the system (or reordering performed by the network, but also because the system (or
systems) that fragmented the IP datagram may indeed transmit the systems) that fragmented the IP datagram may indeed transmit the
fragments out of order. A notable example of this is the Linux fragments out of order. A notable example of this is the Linux
TCP/IP stack, which transmits the fragments in reverse order. TCP/IP stack, which transmits the fragments in reverse order.
o This means that we cannot enforce checks on the fragments for This means that we cannot enforce checks on the fragments for which
which we allocate reassembly resources, as the first fragment we we allocate reassembly resources, as the first fragment we receive
receive for a given packet may be some other fragment than the for a given packet may be some other fragment than the first one (the
first one (the one with an Fragment Offset of 0). one with an Fragment Offset of 0).
However, at the point in which we decide to free some space in the However, at the point in which we decide to free some space in the
fragment buffer, some refinements can be done to the flushing policy. fragment buffer, some refinements can be done to the flushing policy.
The first thing we would like to do is to stop different types of The first thing we would like to do is to stop different types of
traffic from interfering with each other. This means, in principle, traffic from interfering with each other. This means, in principle,
that we do not want fragmented UDP traffic to interfere with that we do not want fragmented UDP traffic to interfere with
fragmented TCP traffic. In order to implement this traffic fragmented TCP traffic. In order to implement this traffic
separation for the different protocols, a different fragment buffer separation for the different protocols, a different fragment buffer
would be needed, in principle, for each of the 256 different pool would be needed, in principle, for each of the 256 different
protocols that can be encapsulated in an IP datagram. protocols that can be encapsulated in an IP datagram.
We believe a tradeoff is to implement two separate fragment buffers: We believe a tradeoff is to implement two separate fragment buffers:
one for IP datagrams that encapsulate IPsec packets, and another for one for IP datagrams that encapsulate IPsec packets, and another for
the rest of the traffic. This basically means that traffic not the rest of the traffic. This basically means that traffic not
protected by IPsec will not interfere with those flows of protected by IPsec will not interfere with those flows of
communication that are being protected by IPsec. communication that are being protected by IPsec.
The processing of each of these two different fragment buffers would The processing of each of these two different fragment buffer pools
be completely independent from each other. In the case of the IPsec would be completely independent from each other. In the case of the
fragment buffer, when the buffer needs to be flushed, the following IPsec fragment buffer pool, when the buffers needs to be flushed, the
refined policy could be applied: following refined policy could be applied:
o First, for each packet for which the IPsec header has been o First, for each packet for which the IPsec header has been
received, check that the SPI field of the IPsec header corresponds received, check that the SPI field of the IPsec header corresponds
to an existing IPsec Security Association (SA), and probably also to an existing IPsec Security Association (SA), and probably also
check that the IPsec sequence number is valid. If the check check that the IPsec sequence number is valid. If the check
fails, drop all the fragments that correspond to this packet. fails, drop all the fragments that correspond to this packet.
o Second, if the fragment buffer still needs to be flushed, drop all o Second, if still more fragment buffers need to be flushed, drop
the fragments that correspond to packets for which the full IPsec all the fragments that correspond to packets for which the full
header has not yet been received. The number of packets for which IPsec header has not yet been received. The number of packets for
this flushing is performed depends on the amount of free space which this flushing is performed depends on the amount of free
that needs to be created. space that needs to be created.
o Third, if after flushing packets with invalid IPsec information o Third, if after flushing packets with invalid IPsec information
(First step), and packets on which validation checks could not be (First step), and packets on which validation checks could not be
performed (Second step), there is still not enough space in the performed (Second step), there is still not enough space in the
fragment buffer, drop all the fragments that correspond to packets fragment buffer, drop all the fragments that correspond to packets
that passed the checks of the first step, until the necessary free that passed the checks of the first step, until the necessary free
space is created. space is created.
The rationale behind this policy is that, at the point of flushing The rationale behind this policy is that, at the point of flushing
the fragment buffer, we prefer to keep those packets on which we fragment buffers, we prefer to keep those packets on which we could
could successfully perform a number of validation checks, over those successfully perform a number of validation checks, over those
packets on which those checks failed, or the checks could not even be packets on which those checks failed, or the checks could not even be
performed. performed.
By checking both the IPsec SPI and the IPsec sequence number, it is By checking both the IPsec SPI and the IPsec sequence number, it is
virtually impossible for an attacker that is off-path to perform a virtually impossible for an attacker that is off-path to perform a
Denial of Service attack to communication flows being protected by Denial-of-Service attack to communication flows being protected by
IPsec. IPsec.
Unfortunately, some IP implementations (such as that in Linux Unfortunately, some IP implementations (such as that in Linux
[Linux2006]), when performing fragmentation, send the corresponding [Linux2006]), when performing fragmentation, send the corresponding
fragments in reverse order. In such cases, at the point of flushing fragments in reverse order. In such cases, at the point of flushing
the fragment buffer, legitimate fragments will receive the same the fragment buffer, legitimate fragments will receive the same
treatment as the possible forged fragments. treatment as the possible forged fragments.
This refined flushing policy provides an increased level of This refined flushing policy provides an increased level of
protection against this type of resource exhaustion attack, while not protection against this type of resource exhaustion attack, while not
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to approximately 15 seconds; although further research on this issue to approximately 15 seconds; although further research on this issue
is necessary. is necessary.
It should be noted that in network scenarios of long-delay and high- It should be noted that in network scenarios of long-delay and high-
bandwidth (usually referred to as "Long-Fat Networks"), using a long bandwidth (usually referred to as "Long-Fat Networks"), using a long
fragment timeout would likely increase the probability of collision fragment timeout would likely increase the probability of collision
of IP ID numbers. Therefore, in such scenarios it is highly of IP ID numbers. Therefore, in such scenarios it is highly
desirable to avoid the use of fragmentation with techniques such as desirable to avoid the use of fragmentation with techniques such as
PMTUD [RFC1191] or PLPMTUD [RFC4821]. PMTUD [RFC1191] or PLPMTUD [RFC4821].
4.1.2.5. Counter-measure for some IDS evasion techniques 4.1.2.5. countermeasure for some IDS evasion techniques
[Shankar2003] introduces a technique named "Active Mapping" that [Shankar2003] introduces a technique named "Active Mapping" that
prevents evasion of a NIDS by acquiring sufficient knowledge about prevents evasion of a NIDS by acquiring sufficient knowledge about
the network being monitored, to assess which packets will arrive at the network being monitored, to assess which packets will arrive at
the intended recipient, and how they will be interpreted by it. the intended recipient, and how they will be interpreted by it.
[Novak2005] describes some techniques that are applied by the Snort [Novak2005] describes some techniques that are applied by the Snort
NIDS to avoid evasion. NIDS to avoid evasion.
4.1.2.6. Counter-measure for firewall-rules bypassing 4.1.2.6. countermeasure for firewall-rules bypassing
One of the classical techniques to bypass firewall rules involves One of the classical techniques to bypass firewall rules involves
sending packets in which the header of the encapsulated protocol is sending packets in which the header of the encapsulated protocol is
fragmented. Even when it would be legal (as far as the IETF fragmented. Even when it would be legal (as far as the IETF
specifications are concerned) to receive such a packets, the MTUs of specifications are concerned) to receive such a packets, the MTUs of
the network technologies used in practice are not that small to the network technologies used in practice are not that small to
require the header of the encapsulated protocol to be fragmented. require the header of the encapsulated protocol to be fragmented.
Therefore, the system performing reassembly should drop all packets Therefore, the system performing reassembly should drop all packets
which fragment the upper-layer protocol header, and this event should which fragment the upper-layer protocol header, and this event should
be logged (e.g., a counter could be incremented to reflect the packet be logged (e.g., a counter could be incremented to reflect the packet
drop). The necessary information to perform this check could be drop).
stored by the IP module together with the rest of the upper-layer
protocol information.
Additionally, given that many middle-boxes such as firewalls create Additionally, given that many middle-boxes such as firewalls create
state according to the contents of the first fragment of a given state according to the contents of the first fragment of a given
packet, it is best that, in the event an end-system receives packet, it is best that, in the event an end-system receives
overlapping fragments, it honors the information contained in the overlapping fragments, it honors the information contained in the
fragment that was received first. fragment that was received first.
RFC 1858 [RFC1858] describes the abuse of IP fragmentation to bypass RFC 1858 [RFC1858] describes the abuse of IP fragmentation to bypass
firewall rules. RFC 3128 [RFC3128] corrects some errors in RFC 1858. firewall rules. RFC 3128 [RFC3128] corrects some errors in RFC 1858.
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to police and remark the Type of Service or Differentiated Services to police and remark the Type of Service or Differentiated Services
values, according to the type of service at which each end-system has values, according to the type of service at which each end-system has
been allowed to send packets. been allowed to send packets.
Bullet 4 of Section 5.3.3.3 of RFC 1812 states that routers "MUST NOT Bullet 4 of Section 5.3.3.3 of RFC 1812 states that routers "MUST NOT
change precedence settings on packets it did not originate". change precedence settings on packets it did not originate".
However, given the security implications of the Precedence field, it However, given the security implications of the Precedence field, it
is fair for routers, switches or other middle-boxes, particularly is fair for routers, switches or other middle-boxes, particularly
those in the network edge, to overwrite the Type of Service (or those in the network edge, to overwrite the Type of Service (or
Differentiated Services) field of the packets they are forwarding, Differentiated Services) field of the packets they are forwarding,
according to a configured network policy. according to a configured network policy (this is the specified
behavior for DS domains [RFC2475]).
Section 5.3.3.1 and Section 5.3.6 of RFC 1812 states that if Section 5.3.3.1 and Section 5.3.6 of RFC 1812 states that if
precedence-ordered queue service is implemented and enabled, the precedence-ordered queue service is implemented and enabled, the
router "MUST NOT discard a packet whose precedence is higher than router "MUST NOT discard a packet whose precedence is higher than
that of a packet that is not discarded". While this recommendation that of a packet that is not discarded". While this recommendation
makes sense given the semantics of the Precedence field, it is makes sense given the semantics of the Precedence field, it is
important to note that it would be simple for an attacker to send important to note that it would be simple for an attacker to send
packets with forged high Precedence value to congest some internet packets with forged high Precedence value to congest some internet
router(s), and cause all (or most) traffic with a lower Precedence router(s), and cause all (or most) traffic with a lower Precedence
value to be discarded. value to be discarded.
4.2.2. Weak Type of Service 4.2.2. Weak Type of Service
Section 5.2.4.3 of RFC 1812 describes the algorithm for determining Section 5.2.4.3 of RFC 1812 describes the algorithm for determining
the next-hop address (i.e., the forwarding algorithm). Bullet 3, the next-hop address (i.e., the forwarding algorithm). Bullet 3,
"Weak TOS", addresses the case in which routes contain a "type of "Weak TOS", addresses the case in which routes contain a "type of
service" attribute. It states that in case a packet contains a non- service" attribute. It states that in case a packet contains a non-
default TOS (i.e., 0000), only routes with the same TOS or with the default TOS (i.e., 0000), only routes with the same TOS or with the
default TOS should be considered for forwarding that packet. default TOS should be considered for forwarding that packet.
However, this means that among the longest match routes for a given However, this means that if among the longest match routes for a
in packet are routes with some TOS other than the one contained in given packet are routes with some TOS other than the one contained in
the received packet, and no routes with the default TOS, the the received packet, and no routes with the default TOS, the
corresponding packet would be dropped. This may or may not be a corresponding packet would be dropped. This may or may not be a
desired behavior. desired behavior.
An alternative to this would be to, in the case among the "longest An alternative for the case in which among the "longest match" routes
match" routes there are only routes with non-default type of services there are only routes with non-default type of service which do not
which do not match the TOS contained in the received packet, to use a match the TOS contained in the received packet, would be to use a
route with any other TOS. While this route would most likely not be route with any other TOS. While this route would most likely not be
able to address the type of service requested by packet, it would, at able to address the type of service requested by packet, it would, at
least, provide a "best effort" service. least, provide a "best effort" service.
It must be noted that Section 5.3.2 of RFC 1812 allows for routers It must be noted that Section 5.3.2 of RFC 1812 allows routers to not
for not honoring the TOS field. Therefore, the proposed alternative honor the TOS field. Therefore, the proposed alternative behavior is
behavior is still compliant with the IETF specifications. still compliant with the IETF specifications.
o While officially specified in the RFC series, TOS-based routing is While officially specified in the RFC series, TOS-based routing is
not widely deployed in the Internet. not widely deployed in the Internet.
4.2.3. Address Resolution 4.2.3. Impact of Address Resolution on Buffer Management
In the case of broadcast link-layer technologies, in order for a In the case of broadcast link-layer technologies, in order for a
system to transfer an IP datagram it must usually first map an IP system to transfer an IP datagram it must usually first map an IP
address to the corresponding link-layer address (for example, by address to the corresponding link-layer address (for example, by
means of the ARP protocol [RFC0826]) . This means that while this means of the ARP protocol [RFC0826]) . This means that while this
operation is being performed, the packets that would require such a operation is being performed, the packets that would require such a
mapping would need to be kept in memory. This may happen both in the mapping would need to be kept in memory. This may happen both in the
case of hosts and in the case of routers. case of hosts and in the case of routers.
This situation might be exploited by an attacker, which could send a This situation might be exploited by an attacker, which could send a
large amount of packets to a non-existent host which would supposedly large amount of packets to a non-existent host which would supposedly
be directly connected to the attacked router. While trying to map be directly connected to the attacked router. While trying to map
the corresponding IP address into a link-layer address, the attacked the corresponding IP address into a link-layer address, the attacked
router would keep in memory all the packets that would need to make router would keep in memory all the packets that would need to make
use of that link-layer address. At the point in which the mapping use of that link-layer address. At the point in which the mapping
function times out, depending on the policy implemented by the function times out, depending on the policy implemented by the
attacked router, only the packet that triggered the call to the attacked router, only the packet that triggered the call to the
mapping function might be dropped. In that case, the same operation mapping function might be dropped. In that case, the same operation
would be repeated for every packet destined to the non-existent host. would be repeated for every packet destined to the non-existent host.
Depending on the timeout value for the mapping function, this Depending on the timeout value for the mapping function, this
situation might lead to the router buffers to run out of free space, situation might lead the router to run out of free buffer space, with
with the consequence that incoming legitimate packets would have to the consequence that incoming legitimate packets would have to be
be dropped, or that legitimate packets already stored in the router's dropped, or that legitimate packets already stored in the router's
buffers might get dropped. Both of these situations would lead buffers might get dropped. Both of these situations would lead
either to a complete Denial of Service, or to a degradation of the either to a complete Denial of Service, or to a degradation of the
network service. network service.
One counter-measure to this problem would be to drop, at the point One countermeasure to this problem would be to drop, at the point the
the mapping function times out all the packets destined to the mapping function times out, all the packets destined to the address
address that timed out. In addition, a "negative cache entry" might that timed out. In addition, a "negative cache entry" might be kept
be kept in the module performing the matching function, so that for in the module performing the matching function, so that for some
some amount of time, the mapping function would return an error when amount of time, the mapping function would return an error when the
the IP module requests to perform a mapping for some address for IP module requests to perform a mapping for some address for which
which the mapping has recently timed out. the mapping has recently timed out.
o A common implementation strategy for routers is that when a packet A common implementation strategy for routers is that when a packet
is received that requires an ARP request to be performed before is received that requires an ARP resolution to be performed before
the packet can be forwarded, the packet is dropped and the router the packet can be forwarded, the packet is dropped and the router
is then engaged in the ARP procedure. is then engaged in the ARP procedure.
4.2.4. Dropping packets 4.2.4. Dropping packets
In some scenarios, it may be necessary for a host or router to drop In some scenarios, it may be necessary for a host or router to drop
packets from the output queue. In the event one of such packets packets from the output queue. In the event one of such packets
happens to be an IP fragment, and there were other fragments of the happens to be an IP fragment, and there were other fragments of the
same packet in the queue, those other fragments should also be same packet in the queue, those other fragments should also be
dropped. The rationale for this policy is that it is nonsensical to dropped. The rationale for this policy is that it is nonsensical to
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spend system resources on those other fragments, because, as long as spend system resources on those other fragments, because, as long as
one fragment is missing, it will be impossible for the receiving one fragment is missing, it will be impossible for the receiving
system to reassemble them into a complete IP datagram. system to reassemble them into a complete IP datagram.
Some systems have been known to drop just a subset of fragments of a Some systems have been known to drop just a subset of fragments of a
given datagram, leading to a denial of service condition, as only a given datagram, leading to a denial of service condition, as only a
subset of all the fragments of the packets were actually transferred subset of all the fragments of the packets were actually transferred
to the next hop. to the next hop.
4.3. Addressing 4.3. Addressing
4.3.1. Unreachable addresses 4.3.1. Unreachable addresses
It is important to understand that while there are some addresses It is important to understand that while there are some addresses
that are supposed to be unreachable from the public Internet (such as that are supposed to be unreachable from the public Internet (such as
those described in RFC 1918 [RFC1918], or the "loopback" address), the private IP addresses described in RFC 1918 [RFC1918], or the
there are a number of tricks an attacker can perform to reach those "loopback" address), there are a number of tricks an attacker can
IP addresses that would otherwise be unreachable (e.g., exploit the perform to reach those IP addresses that would otherwise be
LSRR or SSRR IP options). Therefore, when applicable, packet unreachable (e.g., exploit the LSRR or SSRR IP options). Therefore,
filtering should be performed at organizational network boundary to when applicable, packet filtering should be performed at private
assure that those addresses will be unreachable. network boundary to assure that those addresses will be unreachable.
Similarly, link-local unicast addresses [RFC3927] and multicast
addresses with limited scope (link- and site-local addresses) should
not be accessible from outside the proper network boundaries and not
be passed across these boundaries.
[RFC5735] provides a summary of special use IPv4 addresses. [RFC5735] provides a summary of special use IPv4 addresses.
4.3.2. Private address space 4.3.2. Private address space
The Internet Assigned Numbers Authority (IANA) has reserved the The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets: following three blocks of the IP address space for private internets:
o 10.0.0.0 - 10.255.255.255 (10/8 prefix) o 10.0.0.0 - 10.255.255.255 (10/8 prefix)
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[RFC5735] provides a summary of special use IPv4 addresses. [RFC5735] provides a summary of special use IPv4 addresses.
4.3.2. Private address space 4.3.2. Private address space
The Internet Assigned Numbers Authority (IANA) has reserved the The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets: following three blocks of the IP address space for private internets:
o 10.0.0.0 - 10.255.255.255 (10/8 prefix) o 10.0.0.0 - 10.255.255.255 (10/8 prefix)
o 172.16.0.0 - 172.31.255.255 (172.16/12 prefix) o 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
o 192.168.0.0 - 192.168.255.255 (192.168/16 prefix) o 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
Use of these address blocks is described in RFC 1918 [RFC1918]. Use of these address blocks is described in RFC 1918 [RFC1918].
Where applicable, packet filtering should be performed at the Where applicable, packet filtering should be performed at the
organizational perimeter to assure that these addresses are not organizational perimeter to assure that these addresses are not
reachable from outside the enterprise network. reachable from outside the private network where such addresses are
employed.
4.3.3. Class D addresses (224/4 address block) 4.3.3. Former Class D Addresses (224/4 Address Block)
The Class D addresses correspond to the 224/4 address block, and are The former Class D addresses correspond to the 224/4 address block,
used for Internet multicast. Therefore, if a packet is received with and are used for Internet multicast. Therefore, if a packet is
a Class D address as the Source Address, it should be dropped, and received with a "Class D" address as the Source Address, it should be
this event should be logged (e.g., a counter could be incremented to dropped, and this event should be logged (e.g., a counter could be
reflect the packet drop). Additionally, if an IP packet with a incremented to reflect the packet drop). Additionally, if an IP
multicast Destination Address is received for a connection-oriented packet with a multicast Destination Address is received for a
protocol (e.g., TCP), the packet should be dropped, and this event connection-oriented protocol (e.g., TCP), the packet should be
should be logged (e.g., a counter could be incremented to reflect the dropped (see Section 4.3.5), and this event should be logged (e.g., a
packet drop). counter could be incremented to reflect the packet drop).
4.3.4. Class E addresses (240/4 address block) 4.3.4. Former Class E Addresses (240/4 Address Block)
The Class E addresses correspond to the 240/4 address block, and are The former Class E addresses correspond to the 240/4 address block,
currently reserved for experimental use. As a result, a number of and are currently reserved for experimental use. As a result, a
implementations discard packets that contain a Class E address as the number of implementations discard packets that contain a "Class" E
Source Address or Destination Address. address as the Source Address or Destination Address.
However, there exists ongoing work to reclassify the Class E (240/4) However, there exists ongoing work to reclassify the former Class E
address block as usable unicast address spaces [Fuller2008a] (240/4) address block as usable unicast address spaces [Fuller2008a]
[I-D.fuller-240space] [I-D.wilson-class-e]. Therefore, we recommend [I-D.fuller-240space] [I-D.wilson-class-e]. Therefore, we recommend
implementations to accept addresses in the 240/4 block as valid implementations to accept addresses in the 240/4 block as valid
addresses for the Source Address and Destination Address. addresses for the Source Address and Destination Address.
It should be noted that the broadcast address 255.255.255.255 still It should be noted that the broadcast address 255.255.255.255 still
must be treated as indicated in Section 4.3.7 of this document. must be treated as indicated in Section 4.3.7 of this document.
4.3.5. Broadcast and multicast addresses, and connection-oriented 4.3.5. Broadcast/Multicast addresses, and Connection-Oriented Protocols
protocols
For connection-oriented protocols, such as TCP, shared state is For connection-oriented protocols, such as TCP, shared state is
maintained between only two endpoints at a time. Therefore, if an IP maintained between only two endpoints at a time. Therefore, if an IP
packet with a multicast (or broadcast) Destination Address is packet with a multicast (or broadcast) Destination Address is
received for a connection-oriented protocol (e.g., TCP), the packet received for a connection-oriented protocol (e.g., TCP), the packet
should be dropped, and this event should be logged (e.g., a counter should be dropped, and this event should be logged (e.g., a counter
could be incremented to reflect the packet drop). could be incremented to reflect the packet drop).
4.3.6. Broadcast and network addresses 4.3.6. Broadcast and network addresses
Originally, the IETF specifications did not permit IP addresses to Originally, the IETF specifications did not permit IP addresses to
have the value 0 or -1 for any of the Host number, network number, or have the value 0 or -1 (shorthand for all bits set to 1) for any of
subnet number fields, except for the cases indicated in Section the Host number, network number, or subnet number fields, except for
4.3.7. However, this changed fundamentally with the deployment of the cases indicated in Section 4.3.7. However, this changed
Classless Inter-Domain Routing (CIDR) [RFC4632], as with CIDR a fundamentally with the deployment of Classless Inter-Domain Routing
system cannot know a priori what the subnet mask is for a particular (CIDR) [RFC4632], as with CIDR a system cannot know a priori what the
IP address. subnet mask is for a particular IP address.
Many systems now allow administrators to use the values 0 or -1 for Many systems now allow administrators to use the values 0 or -1 for
those fields. Despite that according to the IETF specifications those fields. Despite that according to the original IETF
these addresses are illegal, modern IP implementations should specifications these addresses are illegal, modern IP implementations
consider these addresses to be valid. should consider these addresses to be valid.
4.3.7. Special Internet addresses 4.3.7. Special Internet addresses
RFC 1812 [RFC1812] discusses the use of some special internet RFC 1812 [RFC1812] discusses the use of some special internet
addresses, which is of interest to perform some sanity checks on the addresses, which is of interest to perform some sanity checks on the
Source Address and Destination Address fields of an IP packet. It Source Address and Destination Address fields of an IP packet. It
uses the following notation for an IP address: uses the following notation for an IP address:
{ <Network-prefix>, <Host-number> } { <Network-prefix>, <Host-number> }
where the length of the network prefix is generally implied by the
network mask assigned to the IP interface under consideration.
o RFC 1122 [RFC1122] contained a similar discussion of special RFC 1122 [RFC1122] contained a similar discussion of special
internet addresses, including some of the form { <Network-prefix>, internet addresses, including some of the form { <Network-prefix>,
<Subnet-number>, <Host-number> }. However, as explained in <Subnet-number>, <Host-number> }. However, as explained in
Section 4.2.2.11 of RFC 1812, in a CIDR world, the subnet number Section 4.2.2.11 of RFC 1812, in a CIDR world, the subnet number
is clearly an extension of the network prefix and cannot be is clearly an extension of the network prefix and cannot be
interpreted without the remainder of the prefix. distinguished from the remainder of the prefix.
{0, 0} {0, 0}
This address means "this host on this network". It is meant to be This address means "this host on this network". It is meant to be
used only during the initialization procedure, by which the host used only during the initialization procedure, by which the host
learns its own IP address. learns its own IP address.
If a packet is received with 0.0.0.0 as the Source Address for any If a packet is received with 0.0.0.0 as the Source Address for any
purpose other than bootstrapping, the corresponding packet should be purpose other than bootstrapping, the corresponding packet should be
silently dropped, and this event should be logged (e.g., a counter silently dropped, and this event should be logged (e.g., a counter
skipping to change at page 61, line 22 skipping to change at page 64, line 5
reflect the packet drop). reflect the packet drop).
{-1, -1} {-1, -1}
This address is the local broadcast address. It should not be used This address is the local broadcast address. It should not be used
as a source IP address. If a packet is received with 255.255.255.255 as a source IP address. If a packet is received with 255.255.255.255
as the Source Address, it should be dropped, and this event should be as the Source Address, it should be dropped, and this event should be
logged (e.g., a counter could be incremented to reflect the packet logged (e.g., a counter could be incremented to reflect the packet
drop). drop).
o Some systems, when receiving an ICMP echo request, for example, Some systems, when receiving an ICMP echo request, for example,
will use the Destination Address in the ICMP echo request packet will use the Destination Address in the ICMP echo request packet
as the Source Address of the response they send (in this case, an as the Source Address of the response they send (in this case, an
ICMP echo reply). Thus, when such systems receive a request sent ICMP echo reply). Thus, when such systems receive a request sent
to a broadcast address, the Source Address of the response will to a broadcast address, the Source Address of the response will
contain a broadcast address. This should be considered a bug, contain a broadcast address. This should be considered a bug,
rather than a malicious use of the limited broadcast address. rather than a malicious use of the limited broadcast address.
{Network number, -1} {Network number, -1}
This is the directed broadcast to the specified network. As This is the directed broadcast to the specified network. As
skipping to change at page 61, line 46 skipping to change at page 64, line 29
As noted in Section 4.3.6 of this document, many systems now allow As noted in Section 4.3.6 of this document, many systems now allow
administrators to configure these addresses as unicast addresses for administrators to configure these addresses as unicast addresses for
network interfaces. In such scenarios, routers should forward these network interfaces. In such scenarios, routers should forward these
addresses as if they were traditional unicast addresses. addresses as if they were traditional unicast addresses.
In some scenarios a host may have knowledge about a particular IP In some scenarios a host may have knowledge about a particular IP
address being a network-directed broadcast address, rather than a address being a network-directed broadcast address, rather than a
unicast address (e.g., that IP address is configured on the local unicast address (e.g., that IP address is configured on the local
system as a "broadcast address"). In such scenarios, if a system can system as a "broadcast address"). In such scenarios, if a system can
infer the Source Address of a received packet is a network-directed infer that the Source Address of a received packet is a network-
broadcast address, the packet should be dropped, and this event directed broadcast address, the packet should be dropped, and this
should be logged (e.g., a counter could be incremented to reflect the event should be logged (e.g., a counter could be incremented to
packet drop). reflect the packet drop).
As noted in Section 4.3.6 of this document, with the deployment of As noted in Section 4.3.6 of this document, with the deployment of
CIDR [RFC4632], it may be difficult for a system to infer whether a CIDR [RFC4632], it may be difficult for a system to infer whether a
particular IP address that does not belong to a directly attached particular IP address that does not belong to a directly attached
subnet is a broadcast address. subnet is a broadcast address.
{127, any} {127.0.0.0/8, any}
This is the internal host loopback address. Any packet that arrives This is the internal host loopback address. Any packet that arrives
on any physical interface containing this address as the Source on any physical interface containing this address as the Source
Address, the Destination Address, or as part of a source route Address, the Destination Address, or as part of a source route
(either LSRR or SSRR), should be dropped, and this event should be (either LSRR or SSRR), should be dropped.
logged (e.g., a counter could be incremented to reflect the packet
drop).
For example, packets with a Destination Address in the 127.0.0.0/8 For example, packets with a Destination Address in the 127.0.0.0/8
address block that are received on an interface other than loopback address block that are received on an interface other than loopback
should be silently dropped, and this event should be logged (e.g., a should be silently dropped. Packets received on any interface other
counter could be incremented to reflect the packet drop). Packets than loopback with a Source Address corresponding to the system
received on any interface other than loopback with a Source Address receiving the packet should also be dropped.
corresponding to the system receiving the packet should also be
dropped, and this event should be logged (e.g., a counter could be In all the above cases, when a packet is dropped, this event should
incremented to reflect the packet drop). be logged (e.g., a counter could be incremented to reflect the packet
drop).
5. Security Considerations 5. Security Considerations
This document discusses the security implications of the Internet This document discusses the security implications of the Internet
Protocol (IP), and discusses a number of implementation strategies Protocol (IP) and a number of implementation strategies that help to
that help to mitigate a number of vulnerabilities found in the mitigate a number of vulnerabilities found in the protocol during the
protocol during the last 25 years or so. last 25 years or so.
6. Acknowledgements 6. Acknowledgements
The author would like to thank Alfred Hoenes, Joel Jaeggli, Bruno The author wishes to thank Alfred Hoenes for providing very thorough
Rohee, and Andrew Yourtchenko for providing valuable comments on reviews of earlier versions of this document, thus leading to
earlier versions of this document. numerous improvements.
The author would like to thank Joel Jaeggli, Bruno Rohee, and Andrew
Yourtchenko for providing valuable comments on earlier versions of
this document.
This document was written by Fernando Gont on behalf of the UK CPNI This document was written by Fernando Gont on behalf of the UK CPNI
(United Kingdom's Centre for the Protection of National (United Kingdom's Centre for the Protection of National
Infrastructure), and is heavily based on the "Security Assessment of Infrastructure), and is heavily based on the "Security Assessment of
the Internet Protocol" [CPNI2008] published by the UK Centre for the the Internet Protocol" [CPNI2008] published by the UK CPNI in 2008.
Protection of National Infrastructure (CPNI).
The author would like to thank Randall Atkinson, John Day, Juan The author would like to thank Randall Atkinson, John Day, Juan
Fraschini, Roque Gagliano, Guillermo Gont, Martin Marino, Pekka Fraschini, Roque Gagliano, Guillermo Gont, Martin Marino, Pekka
Savola, and Christos Zoulas for providing valuable comments on Savola, and Christos Zoulas for providing valuable comments on
earlier versions of [CPNI2008], on which this document is based. earlier versions of [CPNI2008], on which this document is based.
The author would like to thank Randall Atkinson and Roque Gagliano, The author would like to thank Randall Atkinson and Roque Gagliano,
who generously answered a number of questions. who generously answered a number of questions.
Finally, the author would like to thank UK CPNI (formerly NISCC) for Finally, the author would like to thank UK CPNI (formerly NISCC) for
skipping to change at page 63, line 31 skipping to change at page 66, line 15
RFC 826, November 1982. RFC 826, November 1982.
[RFC1038] St. Johns, M., "Draft revised IP security option", [RFC1038] St. Johns, M., "Draft revised IP security option",
RFC 1038, January 1988. RFC 1038, January 1988.
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP [RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, July 1988. MTU discovery options", RFC 1063, July 1988.
[RFC1108] Kent, S., "U.S", RFC 1108, November 1991. [RFC1108] Kent, S., "U.S", RFC 1108, November 1991.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990. November 1990.
[RFC1349] Almquist, P., "Type of Service in the Internet Protocol [RFC1349] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, July 1992. Suite", RFC 1349, July 1992.
[RFC1393] Malkin, G., "Traceroute Using an IP Option", RFC 1393, [RFC1393] Malkin, G., "Traceroute Using an IP Option", RFC 1393,
January 1993. January 1993.
[RFC1770] Graff, C., "IPv4 Option for Sender Directed Multi- [RFC1770] Graff, C., "IPv4 Option for Sender Directed Multi-
Destination Delivery", RFC 1770, March 1995. Destination Delivery", RFC 1770, March 1995.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", [RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995. RFC 1812, June 1995.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113, [RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
February 1997. February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998. December 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998. Services", RFC 2475, December 1998.
[RFC2644] Senie, D., "Changing the Default for Directed Broadcasts [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts
in Routers", BCP 34, RFC 2644, August 1999. in Routers", BCP 34, RFC 2644, August 1999.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927, Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005. May 2005.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007. Discovery", RFC 4821, March 2007.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, October 2007.
[RFC5350] Manner, J. and A. McDonald, "IANA Considerations for the
IPv4 and IPv6 Router Alert Options", RFC 5350,
September 2008.
[RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses", [RFC5735] Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
BCP 153, RFC 5735, January 2010. BCP 153, RFC 5735, January 2010.
7.2. Informative References 7.2. Informative References
[Anderson2001] [Anderson2001]
Anderson, J., "An Analysis of Fragmentation Attacks", Anderson, J., "An Analysis of Fragmentation Attacks",
Available at: http://www.ouah.org/fragma.html , 2001. Available at: http://www.ouah.org/fragma.html , 2001.
[Arkin2000]
Arkin, "IP TTL Field Value with ICMP (Oops - Identifying
Windows 2000 again and more)", http://
ofirarkin.files.wordpress.com/2008/11/
ofirarkin2000-06.pdf, 2000.
[Barisani2006] [Barisani2006]
Barisani, A., "FTester - Firewall and IDS testing tool", Barisani, A., "FTester - Firewall and IDS testing tool",
Available at: http://dev.inversepath.com/trac/ftester , Available at: http://dev.inversepath.com/trac/ftester ,
2001. 2001.
[Bellovin1989] [Bellovin1989]
Bellovin, S., "Security Problems in the TCP/IP Protocol Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", Computer Communication Review Vol. 19, No. 2, pp. Suite", Computer Communication Review Vol. 19, No. 2, pp.
32-48, 1989. 32-48, 1989.
[Bellovin2002] [Bellovin2002]
Bellovin, S., "A Technique for Counting NATted Hosts", Bellovin, S., "A Technique for Counting NATted Hosts",
IMW'02 Nov. 6-8, 2002, Marseille, France, 2002. IMW'02 Nov. 6-8, 2002, Marseille, France, 2002.
[Bendi1998] [Bendi1998]
Bendi, "Boink exploit", http://www.insecure.org/sploits/ Bendi, "Bonk exploit", http://www.insecure.org/sploits/
95.NT.fragmentation.bonk.html , 1998. 95.NT.fragmentation.bonk.html , 1998.
[Biondi2007] [Biondi2007]
Biondi, P. and A. Ebalard, "IPv6 Routing Header Security", Biondi, P. and A. Ebalard, "IPv6 Routing Header Security",
CanSecWest 2007 Security Conference http://www.secdev.org/ CanSecWest 2007 Security Conference http://www.secdev.org/
conf/IPv6_RH_security-csw07.pdf, 2007. conf/IPv6_RH_security-csw07.pdf, 2007.
[CERT1996a] [CERT1996a]
CERT, "CERT Advisory CA-1996-01: UDP Port Denial-of- CERT, "CERT Advisory CA-1996-01: UDP Port Denial-of-
Service Attack", Service Attack",
skipping to change at page 65, line 47 skipping to change at page 69, line 22
[CERT1999] [CERT1999]
CERT, "CERT Advisory CA-1999-17: Denial-of-Service Tools", CERT, "CERT Advisory CA-1999-17: Denial-of-Service Tools",
http://www.cert.org/advisories/CA-1999-17.html, 1999. http://www.cert.org/advisories/CA-1999-17.html, 1999.
[CERT2001] [CERT2001]
CERT, "CERT Advisory CA-2001-09: Statistical Weaknesses in CERT, "CERT Advisory CA-2001-09: Statistical Weaknesses in
TCP/IP Initial Sequence Numbers", TCP/IP Initial Sequence Numbers",
http://www.cert.org/advisories/CA-2001-09.html, 2001. http://www.cert.org/advisories/CA-2001-09.html, 2001.
[CERT2003] [CERT2003]
CERT, "CERT Advisory CA-2003-15 Cisco IOS Interface CERT, "CERT Advisory CA-2003-15: Cisco IOS Interface
Blocked by IPv4 Packet", Blocked by IPv4 Packet",
http://www.cert.org/advisories/CA-2003-15.html, 2003. http://www.cert.org/advisories/CA-2003-15.html, 2003.
[CIPSO1992] [CIPSO1992]
CIPSO, "COMMERCIAL IP SECURITY OPTION (CIPSO 2.2)", IETF CIPSO, "COMMERCIAL IP SECURITY OPTION (CIPSO 2.2)", IETF
Internet-Draft (draft-ietf-cipso-ipsecurity-01.txt), work Internet-Draft (draft-ietf-cipso-ipsecurity-01.txt), work
in progress , 1992. in progress , 1992.
[CIPSOWG1994] [CIPSOWG1994]
CIPSOWG, "Commercial Internet Protocol Security Option CIPSOWG, "Commercial Internet Protocol Security Option
skipping to change at page 67, line 28 skipping to change at page 70, line 52
[Gont2006] [Gont2006]
Gont, F., "Advanced ICMP packet filtering", Gont, F., "Advanced ICMP packet filtering",
http://www.gont.com.ar/papers/icmp-filtering.html, 2006. http://www.gont.com.ar/papers/icmp-filtering.html, 2006.
[Haddad2004] [Haddad2004]
Haddad, I. and M. Zakrzewski, "Security Distribution for Haddad, I. and M. Zakrzewski, "Security Distribution for
Linux Clusters", Linux Linux Clusters", Linux
Journal http://www.linuxjournal.com/article/6943, 2004. Journal http://www.linuxjournal.com/article/6943, 2004.
[Humble1998] [Humble1998]
Gont, F., "Nestea exploit", Humble, "Nestea exploit",
http://www.insecure.org/sploits/linux.PalmOS.nestea.html, http://www.insecure.org/sploits/linux.PalmOS.nestea.html,
1998. 1998.
[I-D.fuller-240space] [I-D.fuller-240space]
Fuller, V., "Reclassifying 240/4 as usable unicast address Fuller, V., "Reclassifying 240/4 as usable unicast address
space", draft-fuller-240space-02 (work in progress), space", draft-fuller-240space-02 (work in progress),
March 2008. March 2008.
[I-D.ietf-tcpm-icmp-attacks] [I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP", Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-10 (work in progress), draft-ietf-tcpm-icmp-attacks-12 (work in progress),
January 2010. March 2010.
[I-D.stjohns-sipso]
StJohns, M., "Common Architecture Label IPv6 Security
Option (CALIPSO)", draft-stjohns-sipso-11 (work in
progress), March 2009.
[I-D.templin-mtuassurance] [I-D.templin-mtuassurance]
Templin, F., "Requirements for IP-in-IP Tunnel MTU Templin, F., "Requirements for IP-in-IP Tunnel MTU
Assurance", draft-templin-mtuassurance-02 (work in Assurance", draft-templin-mtuassurance-02 (work in
progress), October 2006. progress), October 2006.
[I-D.wilson-class-e] [I-D.wilson-class-e]
Wilson, P., Michaelson, G., and G. Huston, "Redesignation Wilson, P., Michaelson, G., and G. Huston, "Redesignation
of 240/4 from "Future Use" to "Private Use"", of 240/4 from "Future Use" to "Private Use"",
draft-wilson-class-e-02 (work in progress), draft-wilson-class-e-02 (work in progress),
skipping to change at page 69, line 45 skipping to change at page 73, line 15
[Northcutt2000] [Northcutt2000]
Northcut, S. and Novak, "Network Intrusion Detection - An Northcut, S. and Novak, "Network Intrusion Detection - An
Analyst's Handbook", Second Edition New Riders Publishing, Analyst's Handbook", Second Edition New Riders Publishing,
2000. 2000.
[Novak2005] [Novak2005]
Novak, "Target-Based Fragmentation Reassembly", Novak, "Target-Based Fragmentation Reassembly",
http://www.snort.org/reg/docs/target_based_frag.pdf, http://www.snort.org/reg/docs/target_based_frag.pdf,
2005. 2005.
[OpenBSD-PF]
Sanfilippo, S., "PF: Scrub (Packet Normalization)",
http://www.openbsd.org/faq/pf/scrub.html, 2010.
[OpenBSD1998] [OpenBSD1998]
OpenBSD, "OpenBSD Security Advisory: IP Source Routing OpenBSD, "OpenBSD Security Advisory: IP Source Routing
Problem", Problem",
http://www.openbsd.org/advisories/sourceroute.txt, 1998. http://www.openbsd.org/advisories/sourceroute.txt, 1998.
[Paxson2001] [Paxson2001]
Paxson, V., Handley, M., and C. Kreibich, "Network Paxson, V., Handley, M., and C. Kreibich, "Network
Intrusion Detection: Evasion, Traffic Normalization, and Intrusion Detection: Evasion, Traffic Normalization, and
End-to-End Protocol Semantics", USENIX Conference, 2001, End-to-End Protocol Semantics", USENIX Conference, 2001,
2001. 2001.
skipping to change at page 70, line 21 skipping to change at page 73, line 43
http://www.aciri.org/vern/Ptacek-Newsham-Evasion-98.ps, http://www.aciri.org/vern/Ptacek-Newsham-Evasion-98.ps,
1998. 1998.
[RFC0815] Clark, D., "IP datagram reassembly algorithms", RFC 815, [RFC0815] Clark, D., "IP datagram reassembly algorithms", RFC 815,
July 1982. July 1982.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858, Considerations for IP Fragment Filtering", RFC 1858,
October 1995. October 1995.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2544] Bradner, S. and J. McQuaid, "Benchmarking Methodology for [RFC2544] Bradner, S. and J. McQuaid, "Benchmarking Methodology for
Network Interconnect Devices", RFC 2544, March 1999. Network Interconnect Devices", RFC 2544, March 1999.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001. via IPv4 Clouds", RFC 3056, February 2001.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny [RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001. Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., [RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, April 2003. (NFS) version 4 Protocol", RFC 3530, April 2003.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the- [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006. Network Tunneling", RFC 4459, April 2006.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007. Errors at High Data Rates", RFC 4963, July 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007. Mitigations", RFC 4987, August 2007.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. [RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
Pignataro, "The Generalized TTL Security Mechanism Architecture", RFC 5559, June 2009.
(GTSM)", RFC 5082, October 2007.
[RFC5570] StJohns, M., Atkinson, R., and G. Thomas, "Common
Architecture Label IPv6 Security Option (CALIPSO)",
RFC 5570, July 2009.
[RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
Nodes", RFC 5670, November 2009.
[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
Encoding and Transport of Pre-Congestion Information",
RFC 5696, November 2009.
[SELinux2008] [SELinux2008]
Security Enhanced Linux, "http://www.nsa.gov/selinux/". Security Enhanced Linux, "http://www.nsa.gov/selinux/".
[Sanfilippo1998a] [Sanfilippo1998a]
Sanfilippo, S., "about the ip header id", Post to Bugtraq Sanfilippo, S., "about the ip header id", Post to Bugtraq
mailing-list, Mon Dec 14 mailing-list, Mon Dec 14
1998 http://www.kyuzz.org/antirez/papers/ipid.html, 1998. 1998 http://www.kyuzz.org/antirez/papers/ipid.html, 1998.
[Sanfilippo1998b] [Sanfilippo1998b]
skipping to change at page 72, line 14 skipping to change at page 75, line 26
[Solaris2008] [Solaris2008]
Solaris Trusted Extensions - Labeled Security for Absolute Solaris Trusted Extensions - Labeled Security for Absolute
Protection, "http://www.sun.com/software/solaris/ds/ Protection, "http://www.sun.com/software/solaris/ds/
trusted_extensions.jsp#3", 2008. trusted_extensions.jsp#3", 2008.
[Song1999] [Song1999]
Song, D., "Frag router tool", Song, D., "Frag router tool",
http://www.anzen.com/research/nidsbench/. http://www.anzen.com/research/nidsbench/.
[SpooferProject]
MIT ANA, "The MIT ANA Spoofer project",
http://spoofer.csail.mit.edu/index.php, 2010.
[US-CERT2001] [US-CERT2001]
US-CERT, "US-CERT Vulnerability Note VU#446689: Check US-CERT, "US-CERT Vulnerability Note VU#446689: Check
Point FireWall-1 allows fragmented packets through Point FireWall-1 allows fragmented packets through
firewall if Fast Mode is enabled", firewall if Fast Mode is enabled",
http://www.kb.cert.org/vuls/id/446689, 2001. http://www.kb.cert.org/vuls/id/446689, 2001.
[US-CERT2002] [US-CERT2002]
US-CERT, "US-CERT Vulnerability Note VU#310387: Cisco IOS US-CERT, "US-CERT Vulnerability Note VU#310387: Cisco IOS
discloses fragments of previous packets when Express discloses fragments of previous packets when Express
Forwarding is enabled", Forwarding is enabled",
http://www.kb.cert.org/vuls/id/310387, 2002. http://www.kb.cert.org/vuls/id/310387, 2002.
[Watson2004] [Watson2004]
Watson, P., "Slipping in the Window: TCP Reset Attacks", Watson, P., "Slipping in the Window: TCP Reset Attacks",
2004 CanSecWest Conference , 2004. CanSecWest Conference , 2004.
[Zakrzewski2002] [Zakrzewski2002]
Zakrzewski, M. and I. Haddad, "Linux Distributed Security Zakrzewski, M. and I. Haddad, "Linux Distributed Security
Module", http://www.linuxjournal.com/article/6215, 2002. Module", http://www.linuxjournal.com/article/6215, 2002.
[daemon91996] [daemon91996]
daemon9, route, and infinity, "IP-spoofing Demystified daemon9, route, and infinity, "IP-spoofing Demystified
(Trust-Relationship Exploitation)", Phrack Magazine, (Trust-Relationship Exploitation)", Phrack Magazine,
Volume Seven, Issue Forty-Eight, File 14 of Volume Seven, Issue Forty-Eight, File 14 of 18 http://
18 http://www.phrack.org/phrack/48/P48-14 , 1988. www.phrack.org/issues.html?issue=48&id=14&mode=txt, 1988.
Appendix A. Advice and guidance to vendors Appendix A. Advice and guidance to vendors
Vendors are urged to contact CPNI (vulteam@cpni.gsi.gov.uk) if they Vendors are urged to contact CPNI (vulteam@cpni.gsi.gov.uk) if they
think they may be affected by the issues described in this document. think they may be affected by the issues described in this document.
As the lead coordination center for these issues, CPNI is well placed As the lead coordination center for these issues, CPNI is well placed
to give advice and guidance as required. to give advice and guidance as required.
CPNI works extensively with government departments and agencies, CPNI works extensively with government departments and agencies,
commercial organizations and the academic community to research commercial organizations and the academic community to research
vulnerabilities and potential threats to IT systems especially where vulnerabilities and potential threats to IT systems especially where
they may have an impact on Critical National Infrastructure's (CNI). they may have an impact on Critical National Infrastructure's (CNI).
Other ways to contact CPNI, plus CPNI's PGP public key, are available Other ways to contact CPNI, plus CPNI's PGP public key, are available
at http://www.cpni.gov.uk . at http://www.cpni.gov.uk .
Appendix B. Changes from previous versions of the draft (to be removed Appendix B. Changes from previous versions of the draft
by the RFC Editor before publishing this document as an
RFC)
B.1. Changes from draft-ietf-opsec-ip-security-01 This whole appendix should be removed by the RFC Editor before
publishing this document as an RFC.
B.1. Changes from draft-ietf-opsec-ip-security-02
o Addresses a very thorough review by Alfred Hoenes (sent off-list)
o Miscellaneous edits (centers expressions, fills missing graphics
with ASCII-art, etc.)
B.2. Changes from draft-ietf-opsec-ip-security-01
o Addresses rest of the feedback received from Andrew Yourtchenko o Addresses rest of the feedback received from Andrew Yourtchenko
(http://www.ietf.org/mail-archive/web/opsec/current/msg00417.html) (http://www.ietf.org/mail-archive/web/opsec/current/msg00417.html)
o Addresses a very thorough review by Alfred Hoenes (sent off-list) o Addresses a very thorough review by Alfred Hoenes (sent off-list)
o Addresses feedback submitted by Joel Jaeggli (off-list) o Addresses feedback submitted by Joel Jaeggli (off-list)
o Addresses feedback submitted (off-list) by Bruno Rohee. o Addresses feedback submitted (off-list) by Bruno Rohee.
o Miscellaneous edits (centers expressions, fills missing graphics o Miscellaneous edits (centers expressions, fills missing graphics
with ASCII-art, etc.) with ASCII-art, etc.)
B.2. Changes from draft-ietf-opsec-ip-security-00 B.3. Changes from draft-ietf-opsec-ip-security-00
o Addresses part of the feedback received from Andrew Yourtchenko o Addresses part of the feedback received from Andrew Yourtchenko
(http://www.ietf.org/mail-archive/web/opsec/current/msg00417.html) (http://www.ietf.org/mail-archive/web/opsec/current/msg00417.html)
B.3. Changes from draft-gont-opsec-ip-security-01 B.4. Changes from draft-gont-opsec-ip-security-01
o Draft resubmitted as draft-ietf, as a result of wg consensus on o Draft resubmitted as draft-ietf, as a result of wg consensus on
adopting the document as an opsec wg item. adopting the document as an opsec wg item.
B.4. Changes from draft-gont-opsec-ip-security-00 B.5. Changes from draft-gont-opsec-ip-security-00
o Fixed author's affiliation. o Fixed author's affiliation.
o Added Figure 5. o Added Figure 6.
o Fixed a few typos. o Fixed a few typos.
o (no technical changes) o (no technical changes)
Author's Address Author's Address
Fernando Gont Fernando Gont
UK Centre for the Protection of National Infrastructure UK Centre for the Protection of National Infrastructure
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