draft-ietf-dnsext-forgery-resilience-10.txt   rfc5452.txt 
DNS Extensions (DNSEXT) A. Hubert Network Working Group A. Hubert
Internet-Draft Netherlabs Computer Consulting BV. Request for Comments: 5452 Netherlabs Computer Consulting BV.
Updates: 2181 (if approved) R. van Mook Updates: 2181 R. van Mook
Intended status: Standards Track Equinix Category: Standards Track Equinix
Expires: June 18, 2009 December 15, 2008 January 2009
Measures for making DNS more resilient against forged answers
draft-ietf-dnsext-forgery-resilience-10.txt
Status of this Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at Measures for Making DNS More Resilient against Forged Answers
http://www.ietf.org/ietf/1id-abstracts.txt.
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http://www.ietf.org/shadow.html.
This Internet-Draft will expire on June 18, 2009. This document specifies an Internet standards track protocol for the
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improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice Copyright Notice
Copyright (c) 2008 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
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to this document.
Abstract Abstract
The current Internet climate poses serious threats to the Domain Name The current Internet climate poses serious threats to the Domain Name
System. In the interim period before the DNS protocol can be secured System. In the interim period before the DNS protocol can be secured
more fully, measures can already be taken to harden the DNS to make more fully, measures can already be taken to harden the DNS to make
'spoofing' a recursing nameserver many orders of magnitude harder. 'spoofing' a recursing nameserver many orders of magnitude harder.
Even a cryptographically secured DNS benefits from having the ability Even a cryptographically secured DNS benefits from having the ability
to discard bogus responses quickly, as this potentially saves large to discard bogus responses quickly, as this potentially saves large
amounts of computation. amounts of computation.
By describing certain behaviour that has previously not been By describing certain behavior that has previously not been
standardised, this document sets out how to make the DNS more standardized, this document sets out how to make the DNS more
resilient against accepting incorrect responses. This document resilient against accepting incorrect responses. This document
updates RFC 2181. updates RFC 2181.
Table of Contents Table of Contents
1. Requirements and definitions . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements and Definitions . . . . . . . . . . . . . . . . . 4
1.2. Key words . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Description of DNS spoofing . . . . . . . . . . . . . . . . . 7 3. Description of DNS Spoofing . . . . . . . . . . . . . . . . . 5
4. Detailed Description of Spoofing Scenarios . . . . . . . . . . 8 4. Detailed Description of Spoofing Scenarios . . . . . . . . . . 6
4.1. Forcing a query . . . . . . . . . . . . . . . . . . . . . 8 4.1. Forcing a Query . . . . . . . . . . . . . . . . . . . . . 6
4.2. Matching the question section . . . . . . . . . . . . . . 9 4.2. Matching the Question Section . . . . . . . . . . . . . . 7
4.3. Matching the ID field . . . . . . . . . . . . . . . . . . 9 4.3. Matching the ID Field . . . . . . . . . . . . . . . . . . 7
4.4. Matching the source address of the authentic response . . 9 4.4. Matching the Source Address of the Authentic Response . . 7
4.5. Matching the destination address and port of the 4.5. Matching the Destination Address and Port of the
authentic response . . . . . . . . . . . . . . . . . . . . 9 Authentic Response . . . . . . . . . . . . . . . . . . . . 8
4.6. Have the response arrive before the authentic response . . 10 4.6. Have the Response Arrive before the Authentic Response . . 8
5. Birthday attacks . . . . . . . . . . . . . . . . . . . . . . . 12 5. Birthday Attacks . . . . . . . . . . . . . . . . . . . . . . . 9
6. Accepting only in-domain records . . . . . . . . . . . . . . . 13 6. Accepting Only In-Domain Records . . . . . . . . . . . . . . . 9
7. Combined difficulty . . . . . . . . . . . . . . . . . . . . . 14 7. Combined Difficulty . . . . . . . . . . . . . . . . . . . . . 10
7.1. Symbols used in calculation . . . . . . . . . . . . . . . 14 7.1. Symbols Used in Calculation . . . . . . . . . . . . . . . 10
7.2. Calculation . . . . . . . . . . . . . . . . . . . . . . . 15 7.2. Calculation . . . . . . . . . . . . . . . . . . . . . . . 11
8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Repetitive spoofing attempts for a single domain name . . 17 8.1. Repetitive Spoofing Attempts for a Single Domain Name . . 13
9. Forgery countermeasures . . . . . . . . . . . . . . . . . . . 18 9. Forgery Countermeasures . . . . . . . . . . . . . . . . . . . 13
9.1. Query matching rules . . . . . . . . . . . . . . . . . . . 18 9.1. Query Matching Rules . . . . . . . . . . . . . . . . . . . 13
9.2. Extending the Q-ID space by using ports and addresses . . 18 9.2. Extending the Q-ID Space by Using Ports and Addresses . . 14
9.2.1. Justification and Discussion . . . . . . . . . . . . . 19 9.2.1. Justification and Discussion . . . . . . . . . . . . . 14
9.3. Spoof detection and countermeasure . . . . . . . . . . . . 19 9.3. Spoof Detection and Countermeasure . . . . . . . . . . . . 15
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20 10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
13.1. Normative References . . . . . . . . . . . . . . . . . . . 24 12.2. Informative References . . . . . . . . . . . . . . . . . . 17
13.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Requirements and definitions
1.1. Definitions
This document uses the following definitions:
Client: typically a 'stub-resolver' on an end-user's computer
Resolver: a nameserver performing recursive service for clients,
also known as a caching server, or a full service resolver
([RFC1123], paragraph 6.1.3.1)
Stub resolver: a very limited Resolver on a client computer, that
leaves the recursing work to a full resolver
Query: a question sent out by a resolver, typically in a UDP
packet
Response: the answer sent back by an authoritative nameserver,
typically in a UDP packet
Third party: any entity other than the resolver or the intended
recipient of a question. The third party may have access to an
arbitrary authoritative nameserver, but has no access to packets
transmitted by the Resolver or authoritative server
Attacker: malicious third party.
Spoof: the activity of attempting to subvert the DNS process by
getting a chosen answer accepted
Authentic response: the correct answer that comes from the right
authoritative server
Target domain name: domain for which the attacker wishes to spoof
in an answer
Fake data: response chosen by the attacker
1.2. Key words
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 [RFC2119].
2. Introduction 1. Introduction
This document describes several common problems in DNS This document describes several common problems in DNS
implementations which, although previously recognized, remain largely implementations, which, although previously recognized, remain
unsolved. Besides briefly recapping these problems, this document largely unsolved. Besides briefly recapping these problems, this
contains rules that, if implemented, make complying resolvers vastly document contains rules that, if implemented, make complying
more resistant to the attacks described. The goal is to make the resolvers vastly more resistant to the attacks described. The goal
existing DNS as secure as possible within the current protocol is to make the existing DNS as secure as possible within the current
boundaries. protocol boundaries.
The words below are aimed at authors of resolvers: it is up to The words below are aimed at authors of resolvers: it is up to
operators to decide which nameserver implementation to use, or which operators to decide which nameserver implementation to use, or which
options to enable. Operational constraints may override the security options to enable. Operational constraints may override the security
concerns described below. However, implementations are expected to concerns described below. However, implementations are expected to
allow an operator to enable functionality described in this document. allow an operator to enable functionality described in this document.
Almost every transaction on the Internet involves the Domain Name Almost every transaction on the Internet involves the Domain Name
System, which is described in [RFC1034], [RFC1035] and beyond. System, which is described in [RFC1034], [RFC1035], and beyond.
Additionally, it has recently become possible to acquire SSL/TLS Additionally, it has recently become possible to acquire Secure
certificates with no other confirmation of identity than the ability Socket Layer/Transport Layer Security (SSL/TLS) certificates with no
to respond to a verification email sent via SMTP ([RFC2821]) - which other confirmation of identity than the ability to respond to a
generally uses DNS for its routing. verification email sent via SMTP ([RFC5321]) -- which generally uses
DNS for its routing.
In other words, any party that (temporarily) controls the Domain Name In other words, any party that (temporarily) controls the Domain Name
System is in a position to reroute most kinds of Internet System is in a position to reroute most kinds of Internet
transactions, including the verification steps in acquiring an SSL/ transactions, including the verification steps in acquiring an SSL/
TLS certificate for a domain. This in turn means that even TLS certificate for a domain. This in turn means that even
transactions protected by SSL/TLS could be diverted. transactions protected by SSL/TLS could be diverted.
It is entirely conceivable that such rerouted traffic could be used It is entirely conceivable that such rerouted traffic could be used
to the disadvantage of Internet users. to the disadvantage of Internet users.
These and other developments have made the security and These and other developments have made the security and
trustworthiness of DNS of renewed importance. Although the DNS trustworthiness of DNS of renewed importance. Although the DNS
community is working hard on finalising and implementing a community is working hard on finalizing and implementing a
cryptographically enhanced DNS protocol, steps should be taken to cryptographically enhanced DNS protocol, steps should be taken to
make sure that the existing use of DNS is as secure as possible make sure that the existing use of DNS is as secure as possible
within the bounds of the relevant standards. within the bounds of the relevant standards.
It should be noted that the most commonly used resolvers currently do It should be noted that the most commonly used resolvers currently do
not perform as well as possible in this respect, making this document not perform as well as possible in this respect, making this document
of urgent importance. of urgent importance.
A thorough analysis of risks facing DNS can be found in [RFC3833]. A thorough analysis of risks facing DNS can be found in [RFC3833].
This document expands on some of the risks mentioned in RFC 3833, This document expands on some of the risks mentioned in RFC 3833,
especially those outlined in the sections on 'ID Guessing and Query especially those outlined in the sections on "ID Guessing and Query
Prediction' and 'Name Chaining'. Furthermore, it emphasises a number Prediction" and "Name Chaining". Furthermore, it emphasizes a number
of existing rules and guidelines embodied in the relevant DNS of existing rules and guidelines embodied in the relevant DNS
protocol specifications. The following also specifies new protocol specifications. The following also specifies new
requirements to make sure the Domain Name System can be relied upon requirements to make sure the Domain Name System can be relied upon
until a more secure protocol has been standardised and deployed. until a more secure protocol has been standardized and deployed.
It should be noted that even when all measures suggested below are It should be noted that even when all measures suggested below are
implemented, protocol users are not protected against third parties implemented, protocol users are not protected against third parties
with the ability to observe, modify or inject packets in the traffic with the ability to observe, modify, or inject packets in the traffic
of a resolver. of a resolver.
For protocol extensions that offer protection against these For protocol extensions that offer protection against these
scenarios, see [RFC4033] and beyond. scenarios, see [RFC4033] and beyond.
3. Description of DNS spoofing 2. Requirements and Definitions
When certain steps are taken it is feasible to 'spoof' the current 2.1. Definitions
This document uses the following definitions:
Client: typically a 'stub-resolver' on an end-user's computer.
Resolver: a nameserver performing recursive service for clients,
also known as a caching server, or a full service resolver
([RFC1123], Section 6.1.3.1).
Stub resolver: a very limited resolver on a client computer, that
leaves the recursing work to a full resolver.
Query: a question sent out by a resolver, typically in a UDP
packet
Response: the answer sent back by an authoritative nameserver,
typically in a UDP packet.
Third party: any entity other than the resolver or the intended
recipient of a question. The third party may have access to an
arbitrary authoritative nameserver, but has no access to packets
transmitted by the resolver or authoritative server.
Attacker: malicious third party.
Spoof: the activity of attempting to subvert the DNS process by
getting a chosen answer accepted.
Authentic response: the correct answer that comes from the right
authoritative server.
Target domain name: domain for which the attacker wishes to spoof
in an answer
Fake data: response chosen by the attacker.
2.2. Key Words
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 [RFC2119].
3. Description of DNS Spoofing
When certain steps are taken, it is feasible to "spoof" the current
deployed majority of resolvers with carefully crafted and timed DNS deployed majority of resolvers with carefully crafted and timed DNS
packets. Once spoofed, a caching server will repeat the data it packets. Once spoofed, a caching server will repeat the data it
wrongfully accepted, and make its clients contact the wrong, and wrongfully accepted, and make its clients contact the wrong, and
possibly malicious, servers. possibly malicious, servers.
To understand how this process works it is important to know what To understand how this process works it is important to know what
makes a resolver accept a response. makes a resolver accept a response.
The following sentence in section 5.3.3 of [RFC1034] presaged the The following sentence in Section 5.3.3 of [RFC1034] presaged the
present problem: present problem:
The resolver should be highly paranoid in its parsing of responses. The resolver should be highly paranoid in its parsing of responses.
It should also check that the response matches the query it sent It should also check that the response matches the query it sent
using the ID field in the response. using the ID field in the response.
DNS data is to be accepted by a resolver if and only if: DNS data is to be accepted by a resolver if and only if:
1. The question section of the reply packet is equivalent to that of 1. The question section of the reply packet is equivalent to that of
a question packet currently waiting for a response a question packet currently waiting for a response.
2. The ID field of the reply packet matches that of the question 2. The ID field of the reply packet matches that of the question
packet packet.
3. The response comes from the same network address the question was 3. The response comes from the same network address to which the
sent to question was sent.
4. The response comes in on the same network address, including port 4. The response comes in on the same network address, including port
number, as the question was sent from number, from which the question was sent.
In general, the first response matching these four conditions is In general, the first response matching these four conditions is
accepted. accepted.
If a third party succeeds in meeting the four conditions before the If a third party succeeds in meeting the four conditions before the
response from the authentic nameserver does so, it is in a position response from the authentic nameserver does so, it is in a position
to feed a resolver fabricated data. When it does so, we dub it an to feed a resolver fabricated data. When it does so, we dub it an
attacker, attempting to spoof in fake data. "attacker", attempting to spoof in fake data.
All conditions mentioned above can theoretically be met by a third All conditions mentioned above can theoretically be met by a third
party, with the difficulty being a function of the resolver party, with the difficulty being a function of the resolver
implementation and zone configuration. implementation and zone configuration.
4. Detailed Description of Spoofing Scenarios 4. Detailed Description of Spoofing Scenarios
The previous paragraph discussed a number of requirements an attacker The previous paragraph discussed a number of requirements an attacker
must match in order to spoof in manipulated (or fake) data. This must match in order to spoof in manipulated (or fake) data. This
section discusses the relative difficulties and how implementation section discusses the relative difficulties and how implementation-
defined choices impact the amount of work an attacker has to perform defined choices impact the amount of work an attacker has to perform
to meet said difficulties. to meet said difficulties.
Some more details can be found in section 2.2 of [RFC3833]. Some more details can be found in Section 2.2 of [RFC3833].
4.1. Forcing a query 4.1. Forcing a Query
Formally, there is no need for a nameserver to perform service except Formally, there is no need for a nameserver to perform service except
for its operator, its customers or more generally its users. for its operator, its customers, or more generally its users.
Recently, open recursing nameservers have been used to amplify denial Recently, open recursing nameservers have been used to amplify
of service attacks. denial-of-service attacks.
Providing full service enables the third party to send the target Providing full service enables the third party to send the target
resolver a query for the domain name it intends to spoof. On resolver a query for the domain name it intends to spoof. On
receiving this query, and not finding the answer in its cache, the receiving this query, and not finding the answer in its cache, the
resolver will transmit queries to relevant authoritative nameservers. resolver will transmit queries to relevant authoritative nameservers.
This opens up a window of opportunity for getting fake answer data This opens up a window of opportunity for getting fake answer data
accepted. accepted.
Queries may however be forced indirectly, for example by inducing a Queries may however be forced indirectly, for example, by inducing a
mail server to perform DNS lookups. mail server to perform DNS lookups.
Some operators restrict access by not recursing for unauthorised IP Some operators restrict access by not recursing for unauthorized IP
addresses, but only respond with data from the cache. This makes addresses, but only respond with data from the cache. This makes
spoofing harder for a third party as it cannot then force the exact spoofing harder for a third party as it cannot then force the exact
moment a question will be asked. It is still possible however to moment a question will be asked. It is still possible however to
determine a time range when this will happen, because nameservers determine a time range when this will happen, because nameservers
helpfully publish the decreasing TTL of entries in the cache, which helpfully publish the decreasing time to live (TTL) of entries in the
indicate from which absolute time onwards a new query could be sent cache, which indicate from which absolute time onwards a new query
to refresh the expired entry. could be sent to refresh the expired entry.
The time to live of the target domain name's RRSets determines how The time to live of the target domain name's RRSets determines how
often a window of opportunity is available, which implies that a often a window of opportunity is available, which implies that a
short TTL makes spoofing far more viable. short TTL makes spoofing far more viable.
Note that the attacker might very well have authorised access to the Note that the attacker might very well have authorized access to the
target resolver by virtue of being a customer or employee of its target resolver by virtue of being a customer or employee of its
operator. In addition, access may be enabled through the use of operator. In addition, access may be enabled through the use of
reflectors as outlined in [RFC5358]. reflectors as outlined in [RFC5358].
4.2. Matching the question section 4.2. Matching the Question Section
DNS packets, both queries and responses, contain a question section. DNS packets, both queries and responses, contain a question section.
Incoming responses should be verified to have a question section that Incoming responses should be verified to have a question section that
is equivalent to that of the outgoing query. is equivalent to that of the outgoing query.
4.3. Matching the ID field 4.3. Matching the ID Field
The DNS ID field is 16 bits wide, meaning that if full use is made of The DNS ID field is 16 bits wide, meaning that if full use is made of
all these bits, and if their contents are truly random, it will all these bits, and if their contents are truly random, it will
require on average 32768 attempts to guess. Anecdotal evidence require on average 32768 attempts to guess. Anecdotal evidence
suggests there are implementations utilising only 14 bits, meaning on suggests there are implementations utilizing only 14 bits, meaning on
average 8192 attempts will suffice. average 8192 attempts will suffice.
Additionally, if the target nameserver can be forced into having Additionally, if the target nameserver can be forced into having
multiple identical queries outstanding, the 'Birthday Attack' multiple identical queries outstanding, the "Birthday Attack"
phenomenon means that any fake data sent by the attacker is matched phenomenon means that any fake data sent by the attacker is matched
against multiple outstanding queries, significantly raising the against multiple outstanding queries, significantly raising the
chance of success. Further details in Section 5. chance of success. Further details in Section 5.
4.4. Matching the source address of the authentic response 4.4. Matching the Source Address of the Authentic Response
It should be noted that meeting this condition entails being able to It should be noted that meeting this condition entails being able to
transmit packets on behalf of the address of the authoritative transmit packets on behalf of the address of the authoritative
nameserver. While two Best Current Practice documents ([RFC2827] and nameserver. While two Best Current Practice documents ([RFC2827] and
[RFC3013] specifically) direct Internet access providers to prevent [RFC3013] specifically) direct Internet access providers to prevent
their customers from assuming IP addresses that are not assigned to their customers from assuming IP addresses that are not assigned to
them, these recommendations are not universally (nor even widely) them, these recommendations are not universally (nor even widely)
implemented. implemented.
Many zones have two or three authoritative nameservers, which make Many zones have two or three authoritative nameservers, which make
matching the source address of the authentic response very likely matching the source address of the authentic response very likely
with even a naive choice having a double digit success rate. with even a naive choice having a double digit success rate.
Most recursing nameservers store relative performance indications of Most recursing nameservers store relative performance indications of
authoritative nameservers, which may make it easier to predict which authoritative nameservers, which may make it easier to predict which
nameserver would originally be queried - the one most likely to nameserver would originally be queried -- the one most likely to
respond the quickest. respond the quickest.
Generally, this condition requires at most two or three attempts Generally, this condition requires at most two or three attempts
before it is matched. before it is matched.
4.5. Matching the destination address and port of the authentic 4.5. Matching the Destination Address and Port of the Authentic
response Response
Note that the destination address of the authentic response is the Note that the destination address of the authentic response is the
source address of the original query. source address of the original query.
The actual address of a recursing nameserver is generally known; the The actual address of a recursing nameserver is generally known; the
port used for asking questions is harder to determine. Most current port used for asking questions is harder to determine. Most current
resolvers pick an arbitrary port at startup (possibly at random) and resolvers pick an arbitrary port at startup (possibly at random) and
use this for all outgoing queries. In quite a number of cases the use this for all outgoing queries. In quite a number of cases, the
source port of outgoing questions is fixed at the traditional DNS source port of outgoing questions is fixed at the traditional DNS
assigned server port number of 53. assigned server port number of 53.
If the source port of the original query is random, but static, any If the source port of the original query is random, but static, any
authoritative nameserver under observation by the attacker can be authoritative nameserver under observation by the attacker can be
used to determine this port. This means that matching this used to determine this port. This means that matching this
conditions often requires no guess work. conditions often requires no guess work.
If multiple ports are used for sending queries, this enlarges the If multiple ports are used for sending queries, this enlarges the
effective ID space by a factor equal to the number of ports used. effective ID space by a factor equal to the number of ports used.
Less common resolving servers choose a random port per outgoing Less common resolving servers choose a random port per outgoing
query. If this strategy is followed, this port number can be query. If this strategy is followed, this port number can be
regarded as an additional ID field, again containing up to 16 bits. regarded as an additional ID field, again containing up to 16 bits.
If the maximum ports range is utilized, on average, around 32256 If the maximum ports range is utilized, on average, around 32256
source ports would have to be tried before matching the source port source ports would have to be tried before matching the source port
of the original query as ports below 1024 may be unavailable for use, of the original query, as ports below 1024 may be unavailable for
leaving 64512 options. use, leaving 64512 options.
It is in general safe for DNS to use ports in the range 1024-49152 It is in general safe for DNS to use ports in the range 1024-49152
even though some of these ports are allocated to other protocols. even though some of these ports are allocated to other protocols.
DNS resolvers will not be able to use any ports that are already in DNS resolvers will not be able to use any ports that are already in
use. If a DNS resolver uses a port it will release that port after a use. If a DNS resolver uses a port, it will release that port after
short time and migrate to a different port. Only in the case of high a short time and migrate to a different port. Only in the case of a
volume resolver is it possible that an application wanting a high-volume resolver is it possible that an application wanting a
particular UDP port suffers a long term block-out. particular UDP port suffers a long term block-out.
It should be noted that a firewall will not prevent the matching of It should be noted that a firewall will not prevent the matching of
this address, as it will accept answers that (appear) to come from this address, as it will accept answers that (appear to) come from
the correct address, offering no additional security. the correct address, offering no additional security.
4.6. Have the response arrive before the authentic response 4.6. Have the Response Arrive before the Authentic Response
Once any packet has matched the previous four conditions (plus Once any packet has matched the previous four conditions (plus
possible additional conditions), no further responses are generally possible additional conditions), no further responses are generally
accepted. accepted.
This means that the third party has a limited time in which to inject This means that the third party has a limited time in which to inject
its spoofed response. For calculations we will assume a window in its spoofed response. For calculations, we will assume a window in
order of at most 100ms (depending on the network distance to the order of at most 100ms (depending on the network distance to the
authentic authoritative nameserver). authentic authoritative nameserver).
This time period can be far longer if the authentic authoritative This time period can be far longer if the authentic authoritative
nameservers are (briefly) overloaded by queries, perhaps by the nameservers are (briefly) overloaded by queries, perhaps by the
attacker. attacker.
5. Birthday attacks 5. Birthday Attacks
The so called birthday paradox implies that a group of 23 people The so-called "birthday paradox" implies that a group of 23 people
suffices to have a more than even chance of having two or more suffices to have a more than even chance of having two or more
members of the group share a birthday. members of the group share a birthday.
An attacker can benefit from this exact phenomenon if it can force An attacker can benefit from this exact phenomenon if it can force
the target resolver to have multiple equivalent (identical QNAME, the target resolver to have multiple equivalent (identical QNAME,
QTYPE and QCLASS) outstanding queries at any one time to the same QTYPE, and QCLASS) outstanding queries at any one time to the same
authoritative server. authoritative server.
Any packet the attacker sends then has a much higher chance of being Any packet the attacker sends then has a much higher chance of being
accepted because it only has to match any of the outstanding queries accepted because it only has to match any of the outstanding queries
for that single domain. Compared to the birthday analogy above, of for that single domain. Compared to the birthday analogy above, of
the group composed of queries and responses, the chance of having any the group composed of queries and responses, the chance of having any
of these share an ID rises quickly. of these share an ID rises quickly.
As long as small numbers of queries are sent out, the chance of As long as small numbers of queries are sent out, the chance of
successfully spoofing a response rises linearly with the number of successfully spoofing a response rises linearly with the number of
outstanding queries for the exact domain and nameserver. outstanding queries for the exact domain and nameserver.
For larger numbers this effect is less pronounced. For larger numbers, this effect is less pronounced.
More details are available in US-CERT [vu-457875]. More details are available in US-CERT [vu-457875].
6. Accepting only in-domain records 6. Accepting Only In-Domain Records
Responses from authoritative nameservers often contain information Responses from authoritative nameservers often contain information
that is not part of the zone for which we deem it authoritative. As that is not part of the zone for which we deem it authoritative. As
an example, a query for the MX record of a domain might get as its an example, a query for the MX record of a domain might get as its
responses a mail exchanger in another domain, and additionally the IP responses a mail exchanger in another domain, and additionally the IP
address of this mail exchanger. address of this mail exchanger.
If accepted uncritically, the resolver stands the chance of accepting If accepted uncritically, the resolver stands the chance of accepting
data from an untrusted source. Care must be taken to only accept data from an untrusted source. Care must be taken to only accept
data if it is known that the originator is authoritative for the data if it is known that the originator is authoritative for the
QNAME or a parent of the QNAME. QNAME or a parent of the QNAME.
One very simple way to achieve this is to only accept data if it is One very simple way to achieve this is to only accept data if it is
part of the domain the query was for. part of the domain for which the query was intended.
7. Combined difficulty 7. Combined Difficulty
Given a known or static destination port, matching ID field, source Given a known or static destination port, matching ID field, the
and destination address requires on average in the order of 2 * 2^15 source and destination address requires on average in the order of 2
= 65000 packets, assuming a zone has 2 authoritative nameservers. * 2^15 = 65000 packets, assuming a zone has 2 authoritative
nameservers.
If the window of opportunity available is around 100ms, as assumed If the window of opportunity available is around 100ms, as assumed
above, an attacker would need to be able to briefly transmit 650000 above, an attacker would need to be able to briefly transmit 650000
packets/s to have a 50% chance to get spoofed data accepted on the packets/s to have a 50% chance to get spoofed data accepted on the
first attempt. first attempt.
A realistic minimal DNS response consists of around 80 bytes, A realistic minimal DNS response consists of around 80 bytes,
including IP headers, making the packet rate above correspond to a including IP headers, making the packet rate above correspond to a
respectable burst of 416Mb/s. respectable burst of 416 Mbit/s.
As of mid-2006, this kind of bandwidth was not common but not scarce As of mid-2006, this kind of bandwidth was not common but not scarce
either, especially among those in a position to control many servers. either, especially among those in a position to control many servers.
These numbers change when a window of a full second is assumed, These numbers change when a window of a full second is assumed,
possibly because the arrival of the authentic response can be possibly because the arrival of the authentic response can be
prevented by overloading the bonafide authoritative hosts with decoy prevented by overloading the bonafide authoritative hosts with decoy
queries. This reduces the needed bandwidth to 42 Mb/s. queries. This reduces the needed bandwidth to 42 Mbit/s.
If in addition the attacker is granted more than a single chance and If, in addition, the attacker is granted more than a single chance
allowed up to 60 minutes of work on a domain with a time to live of and allowed up to 60 minutes of work on a domain with a time to live
300 seconds, a meagre 4Mb/s suffices for a 50% chance at getting fake of 300 seconds, a meager 4 Mbit/s suffices for a 50% chance at
data accepted. Once equipped with a longer time, matching condition getting fake data accepted. Once equipped with a longer time,
1 mentioned above is straightforward - any popular domain will have matching condition 1 mentioned above is straightforward -- any
been queried a number of times within this hour, and given the short popular domain will have been queried a number of times within this
TTL, this would lead to queries to authoritative nameservers, opening hour, and given the short TTL, this would lead to queries to
windows of opportunity. authoritative nameservers, opening windows of opportunity.
7.1. Symbols used in calculation 7.1. Symbols Used in Calculation
Assume the following symbols are used: Assume the following symbols are used:
I: Number distinct IDs available (maximum 65536) I: Number distinct IDs available (maximum 65536)
P: Number of ports used (maximum around 64000 as ports under 1024 P: Number of ports used (maximum around 64000 as ports under 1024 are
are not always available, but often 1) not always available, but often 1)
N: Number of authoritative nameservers for a domain (averages N: Number of authoritative nameservers for a domain (averages around
around 2.5) 2.5)
F: Number of "fake" packets sent by the attacker
F: Number of 'fake' packets sent by the attacker
R: Number of packets sent per second by the attacker R: Number of packets sent per second by the attacker
W: Window of opportunity, in seconds. Bounded by the response W: Window of opportunity, in seconds. Bounded by the response time
time of the authoritative servers (often 0.1s) of the authoritative servers (often 0.1s)
D: Average number of identical outstanding queries of a resolver D: Average number of identical outstanding queries of a resolver
(typically 1, see Section 5) (typically 1, see Section 5)
A: Number of attempts, one for each window of opportunity A: Number of attempts, one for each window of opportunity
7.2. Calculation 7.2. Calculation
The probability of spoofing a resolver is equal to the amount of fake The probability of spoofing a resolver is equal to the amount of fake
packets that arrive within the window of opportunity, divided by the packets that arrive within the window of opportunity, divided by the
skipping to change at page 15, line 35 skipping to change at page 11, line 37
In symbols, if the probability of being spoofed is denoted as P_s: In symbols, if the probability of being spoofed is denoted as P_s:
D * F D * F
P_s = --------- P_s = ---------
N * P * I N * P * I
It is more useful to reason not in terms of aggregate packets but to It is more useful to reason not in terms of aggregate packets but to
convert to packet rate, which can easily be converted to bandwidth if convert to packet rate, which can easily be converted to bandwidth if
needed. needed.
If the Window of opportunity length is 'W' and the attacker can send If the window of opportunity length is 'W' and the attacker can send
'R' packets per second, the number of fake packets 'F' that are 'R' packets per second, the number of fake packets 'F' that are
candidates to be accepted is: candidates to be accepted is:
D * R * W D * R * W
F = R * W -> P_s = --------- F = R * W -> P_s = ---------
N * P * I N * P * I
Finally, to calculate the combined chance 'P_cs' of spoofing over a Finally, to calculate the combined chance 'P_cs' of spoofing over a
chosen time period 'T', it should be realised that the attacker has a chosen time period 'T', it should be realized that the attacker has a
new window of opportunity each time the TTL 'TTL' of the target new window of opportunity each time the TTL 'TTL' of the target
domain expires. This means that the number of attempts 'A' is equal domain expires. This means that the number of attempts 'A' is equal
to 'T / TTL'. to 'T / TTL'.
To calculate the combined chance of at least one success, the To calculate the combined chance of at least one success, the
following formula holds: following formula holds:
(T / TTL) (T / TTL)
A ( D * R * W ) A ( D * R * W )
P_cs = 1 - ( 1 - P_s ) = 1 - ( 1 - --------- ) P_cs = 1 - ( 1 - P_s ) = 1 - ( 1 - --------- )
( N * P * I ) ( N * P * I )
When common numbers (as listed above) for D, W, N, P and I are When common numbers (as listed above) for D, W, N, P, and I are
inserted, this formula reduces to: inserted, this formula reduces to:
(T / TTL) (T / TTL)
( R ) ( R )
P_cs = 1 - ( 1 - ------- ) P_cs = 1 - ( 1 - ------- )
( 1638400 ) ( 1638400 )
From this formula it can be seen that, if the nameserver From this formula, it can be seen that, if the nameserver
implementation is unchanged, only raising the TTL offers protection. implementation is unchanged, only raising the TTL offers protection.
Raising N, the number of authoritative nameservers, is not feasible Raising N, the number of authoritative nameservers, is not feasible
beyond a small number. beyond a small number.
For the degenerate case of a zero-second TTL, a window of opportunity For the degenerate case of a zero-second TTL, a window of opportunity
opens for each query sent, making the effective TTL equal to 'W' opens for each query sent, making the effective TTL equal to 'W'
above, the response time of the authoritative server. above, the response time of the authoritative server.
This last case also holds for spoofing techniques which do not rely This last case also holds for spoofing techniques that do not rely on
on TTL expiry, but use repeated and changing queries. TTL expiry, but use repeated and changing queries.
8. Discussion 8. Discussion
The calculations above indicate the relative ease with which DNS data The calculations above indicate the relative ease with which DNS data
can be spoofed. For example, using the formula derived earlier on an can be spoofed. For example, using the formula derived earlier on an
RRSet with a 3600 second TTL, an attacker sending 7000 fake response RRSet with a 3600 second TTL, an attacker sending 7000 fake response
packets/s (a rate of 4.5Mb/s), stands a 10% chance of spoofing a packets/s (a rate of 4.5 Mbit/s), stands a 10% chance of spoofing a
record in the first 24 hours, which rises to 50% after a week. record in the first 24 hours, which rises to 50% after a week.
For an RRSet with a TTL of 60 seconds, the 10% level is hit after 24 For an RRSet with a TTL of 60 seconds, the 10% level is hit after 24
minutes, 50% after less than 3 hours, 90% after around 9 hours. minutes, 50% after less than 3 hours, 90% after around 9 hours.
For some classes of attacks, the effective TTL is near zero, as noted For some classes of attacks, the effective TTL is near zero, as noted
above. above.
Note that the attacks mentioned above can be detected by watchful Note that the attacks mentioned above can be detected by watchful
server operators - an unexpected incoming stream of 4.5mbit/s of server operators - an unexpected incoming stream of 4.5 Mbit/s of
packets might be noticed. packets might be noticed.
An important assumption however in these calculations is a known or An important assumption however in these calculations is a known or
static destination port of the authentic response. static destination port of the authentic response.
If that port number is unknown and needs to be guessed as well, the If that port number is unknown and needs to be guessed as well, the
problem space expands by a factor of 64000, leading the attacker to problem space expands by a factor of 64000, leading the attacker to
need in excess of 285Gb/s to achieve similar success rates. need in excess of 285Gb/s to achieve similar success rates.
Such bandwidth is not generally available, nor expected to be so in Such bandwidth is not generally available, nor is it expected to be
the foreseeable future. so in the foreseeable future.
Note that some firewalls may need reconfiguring if they are currently Note that some firewalls may need reconfiguring if they are currently
setup to only allow outgoing queries from a single DNS source port. setup to only allow outgoing queries from a single DNS source port.
8.1. Repetitive spoofing attempts for a single domain name 8.1. Repetitive Spoofing Attempts for a Single Domain Name
Techniques are available to use an effectively infinite number of Techniques are available to use an effectively infinite number of
queries to achieve a desired spoofing goal. In the math above, this queries to achieve a desired spoofing goal. In the math above, this
reduces the effective TTL to 0. reduces the effective TTL to 0.
If such techniques are employed, using the same 7000 packets/s rate If such techniques are employed, using the same 7000 packets/s rate
mentioned above, and using 1 source port, the spoofing chance rises mentioned above, and using 1 source port, the spoofing chance rises
to 50% within 7 seconds. to 50% within 7 seconds.
If 64000 ports are used, as recommended in this document, using the If 64000 ports are used, as recommended in this document, using the
same query rate, the 50% level is reached after around 116 hours. same query rate, the 50% level is reached after around 116 hours.
9. Forgery countermeasures 9. Forgery Countermeasures
9.1. Query matching rules 9.1. Query Matching Rules
A resolver implementation MUST match responses to all of the A resolver implementation MUST match responses to all of the
following attributes of the query: following attributes of the query:
o Source address against query destination address o Source address against query destination address
o Destination address against query source address o Destination address against query source address
o Destination port against query source port o Destination port against query source port
o Query ID o Query ID
o Query name o Query name
o Query class and type o Query class and type
before applying DNS trustworthiness rules (see [RFC2181], section before applying DNS trustworthiness rules (see Section 5.4.1 of
5.4.1). [RFC2181]).
A mismatch and the response MUST be considered invalid. A mismatch and the response MUST be considered invalid.
9.2. Extending the Q-ID space by using ports and addresses 9.2. Extending the Q-ID Space by Using Ports and Addresses
Resolver implementations MUST: Resolver implementations MUST:
o Use an unpredictable source port for outgoing queries from the o Use an unpredictable source port for outgoing queries from the
range of available ports (53, or 1024 and above) that is as large range of available ports (53, or 1024 and above) that is as large
as possible and practicable; as possible and practicable;
o Use multiple different source ports simultaneously in case of o Use multiple different source ports simultaneously in case of
multiple outstanding queries; multiple outstanding queries;
o Use an unpredictable query ID for outgoing queries, utilizing the o Use an unpredictable query ID for outgoing queries, utilizing the
full range available (0-65535) full range available (0-65535).
Resolvers that have multiple IP addresses SHOULD use them in an Resolvers that have multiple IP addresses SHOULD use them in an
unpredictable manner for outgoing queries. unpredictable manner for outgoing queries.
Resolver implementations SHOULD provide means to avoid usage of Resolver implementations SHOULD provide means to avoid usage of
certain ports. certain ports.
Resolvers SHOULD favour authoritative nameservers with which a trust Resolvers SHOULD favor authoritative nameservers with which a trust
relation has been established; Stub-resolvers SHOULD be able to use relation has been established; stub-resolvers SHOULD be able to use
TSIG ([RFC2845]) or IPsec ([RFC4301]) when communicating with their Transaction Signature (TSIG) ([RFC2845]) or IPsec ([RFC4301]) when
recursive resolver communicating with their recursive resolver.
In case a cryptographic verification of response validity is In case a cryptographic verification of response validity is
available (TSIG, SIG(0)), resolver implementations MAY waive above available (TSIG, SIG(0)), resolver implementations MAY waive above
rules, and rely on this guarantee instead. rules, and rely on this guarantee instead.
Proper unpredictability can be achieved by employing a high quality Proper unpredictability can be achieved by employing a high quality
(pseudo-)random generator, as described in [RFC4086]. (pseudo-)random generator, as described in [RFC4086].
9.2.1. Justification and Discussion 9.2.1. Justification and Discussion
Since an attacker can force a full DNS resolver to send queries to Since an attacker can force a full DNS resolver to send queries to
the attacker's own name servers, any constant or sequential state the attacker's own nameservers, any constant or sequential state held
held by such a resolver can be measured, and it must not be trivially by such a resolver can be measured, and it must not be trivially easy
easy to reverse engineer the resolver's internal state in a way that to reverse engineer the resolver's internal state in a way that
allows low-cost high-accuracy prediction of future state. allows low-cost, high-accuracy prediction of future state.
A full DNS resolver with only one or a small number of upstream- A full DNS resolver with only one or a small number of upstream-
facing endpoints is effectively using constants for IP source address facing endpoints is effectively using constants for IP source address
and UDP port number, and these are very predictable by potential and UDP port number, and these are very predictable by potential
attackers, and must therefore be avoided. attackers, and must therefore be avoided.
A full DNS resolver that uses a simple increment to get its next DNS A full DNS resolver that uses a simple increment to get its next DNS
query ID is likewise very predictable and so very spoofable. query ID is likewise very predictable and so very spoofable.
Finally, weak random number generators have been shown to expose Finally, weak random number generators have been shown to expose
their internal state, such that an attacker who witnesses several their internal state, such that an attacker who witnesses several
sequential "random" values can easily predict the next ones. A sequential "random" values can easily predict the next ones. A
crypto-strength random number generator is one whose output cannot be crypto-strength random number generator is one whose output cannot be
predicted no matter how many successive values are witnessed. predicted no matter how many successive values are witnessed.
9.3. Spoof detection and countermeasure 9.3. Spoof Detection and Countermeasure
If a resolver detects that an attempt is being made to spoof it, If a resolver detects that an attempt is being made to spoof it,
perhaps by discovering that many packets fail the criteria as perhaps by discovering that many packets fail the criteria as
outlined above, it MAY abandon the UDP query and re-issue it over outlined above, it MAY abandon the UDP query and re-issue it over
TCP. TCP, by the nature of its use of sequence numbers, is far more TCP. TCP, by the nature of its use of sequence numbers, is far more
resilient against forgery by third parties. resilient against forgery by third parties.
10. Security Considerations 10. Security Considerations
This document provides clarification of the DNS specification to This document provides clarification of the DNS specification to
skipping to change at page 20, line 19 skipping to change at page 15, line 33
forged. Recommendations found above should be considered forged. Recommendations found above should be considered
complementary to possible cryptographical enhancements of the domain complementary to possible cryptographical enhancements of the domain
name system, which protect against a larger class of attacks. name system, which protect against a larger class of attacks.
This document recommends the use of UDP source port number This document recommends the use of UDP source port number
randomization to extend the effective DNS transaction ID beyond the randomization to extend the effective DNS transaction ID beyond the
available 16 bits. available 16 bits.
A resolver that does not implement the recommendations outlined above A resolver that does not implement the recommendations outlined above
can easily be forced to accept spoofed responses, which in turn are can easily be forced to accept spoofed responses, which in turn are
passed on to client computers - misdirecting (user) traffic to passed on to client computers -- misdirecting (user) traffic to
possibly malicious entities. possibly malicious entities.
This document directly impacts the security of the Domain Name This document directly impacts the security of the Domain Name
System, implementers are urged to follow its recommendations. System, implementers are urged to follow its recommendations.
Most security considerations can be found in Section 4 and Section 5, Most security considerations can be found in Sections 4 and 5, while
while proposed countermeasures are described in Section 9. proposed countermeasures are described in Section 9.
For brevity's sake, in lieu of repeating the security considerations For brevity's sake, in lieu of repeating the security considerations
references, the reader is referred to these sections. references, the reader is referred to these sections.
Nothing in this document specifies specific algorithms for operators Nothing in this document specifies specific algorithms for operators
to use; it does specify algorithms implementations SHOULD or MUST to use; it does specify algorithms implementations SHOULD or MUST
support. support.
It should be noted that the effects of source port randomization may It should be noted that the effects of source port randomization may
be dramatically reduced by NAT devices which either serialize or be dramatically reduced by NAT devices that either serialize or limit
limit in volume the UDP source ports used by the querying resolver. in volume the UDP source ports used by the querying resolver.
DNS recursive servers sitting behind at NAT or a statefull firewall DNS recursive servers sitting behind at NAT or a statefull firewall
may consume all available NAT translation entries/ports when may consume all available NAT translation entries/ports when
operating under high query load. Port randomization will cause operating under high query load. Port randomization will cause
translation entries to be consumed faster than with fixed query port translation entries to be consumed faster than with fixed query port.
. To avoid this NAT boxes and statefull firewalls can/should purge To avoid this, NAT boxes and statefull firewalls can/should purge
outgoing DNS query translation entries 10-17 seconds after the last outgoing DNS query translation entries 10-17 seconds after the last
outgoing query on that mapping was sent. [RFC4787] compliant devices outgoing query on that mapping was sent. [RFC4787]-compliant devices
need to treat UDP messages with port 53 differently than most other need to treat UDP messages with port 53 differently than most other
UDP protocols. UDP protocols.
To minimize the potential that port/state exhaustion attacks can be To minimize the potential that port/state exhaustion attacks can be
staged from the outside, it is recommended that services which staged from the outside, it is recommended that services that
generate number of DNS queries for each connection, should be rate generate a number of DNS queries for each connection should be rate
limited. This applies in particular to e-mail servers limited. This applies in particular to email servers.
11. IANA Considerations
This document does not make any assignments and has no actions for
IANA.
12. Acknowledgments 11. Acknowledgments
Source port randomisation in DNS was first implemented and possibly Source port randomization in DNS was first implemented and possibly
invented by Dan. J. Bernstein. invented by Dan J. Bernstein.
Although any mistakes remain our own, the authors gratefully Although any mistakes remain our own, the authors gratefully
acknowledge the help and contributions of: acknowledge the help and contributions of:
Stephane Bortzmeyer
Stephane Bortzmeyer, Alfred Hoenes
Peter Koch
Alfred Hoenes, Sean Leach
Norbert Sendetzky
Peter Koch, Paul Vixie
Florian Weimer
Sean Leach, Wouter Wijngaards
Norbert Sendetzky,
Paul Vixie,
Florian Weimer,
Wouter Wijngaards,
Dan Wing Dan Wing
13. References 12. References
13.1. Normative References 12.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and
STD 13, RFC 1034, November 1987. facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, July 1997.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP
Address Spoofing", BCP 38, RFC 2827, May 2000. Source Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B. [RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS Wellington, "Secret Key Transaction Authentication for
(TSIG)", RFC 2845, May 2000. DNS (TSIG)", RFC 2845, May 2000.
[RFC3013] Killalea, T., "Recommended Internet Service Provider [RFC3013] Killalea, T., "Recommended Internet Service Provider
Security Services and Procedures", BCP 46, RFC 3013, Security Services and Procedures", BCP 46, RFC 3013,
November 2000. November 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005. RFC 4033, March 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005. Requirements for Security", BCP 106, RFC 4086,
June 2005.
13.2. Informative References [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application 12.2. Informative References
and Support", STD 3, RFC 1123, October 1989.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain [RFC1123] Braden, R., "Requirements for Internet Hosts -
Name System (DNS)", RFC 3833, August 2004. Application and Support", STD 3, RFC 1123, October 1989.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the
Domain Name System (DNS)", RFC 3833, August 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005. Internet Protocol", RFC 4301, December 2005.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation [RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127, (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007. RFC 4787, January 2007.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358, Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008. October 2008.
[vu-457875] [vu-457875] United States CERT, "Various DNS service implementations
United States CERT, "Various DNS service implementations
generate multiple simultaneous queries for the same generate multiple simultaneous queries for the same
resource record", VU 457875, November 2002. resource record", VU 457875, November 2002.
Authors' Addresses Authors' Addresses
Bert Hubert Bert Hubert
Netherlabs Computer Consulting BV. Netherlabs Computer Consulting BV.
Braillelaan 10 Braillelaan 10
Rijswijk (ZH) 2289 CM Rijswijk (ZH) 2289 CM
The Netherlands The Netherlands
Email: bert.hubert@netherlabs.nl EMail: bert.hubert@netherlabs.nl
Remco van Mook Remco van Mook
Equinix Equinix
Auke Vleerstraat 1 Auke Vleerstraat 1
Enschede 7521 PE Enschede 7521 PE
The Netherlands The Netherlands
Email: remco@eu.equinix.com EMail: remco@eu.equinix.com
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