draft-ietf-dhc-auth-suboption-01.txt   draft-ietf-dhc-auth-suboption-02.txt 
DHC Working Group M. Stapp DHC Working Group M. Stapp
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Expires: May 2, 2003 T. Lemon Expires: April 23, 2004 T. Lemon
Nominum, Inc. Nominum, Inc.
November 1, 2002 October 24, 2003
The Authentication Suboption for the DHCP Relay Agent Option The Authentication Suboption for the DHCP Relay Agent Option
<draft-ietf-dhc-auth-suboption-01.txt> <draft-ietf-dhc-auth-suboption-02.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
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skipping to change at page 1, line 33 skipping to change at page 1, line 33
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This Internet-Draft will expire on May 2, 2003. This Internet-Draft will expire on April 23, 2004.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
The DHCP Relay Agent Information Option (RFC3046[1]) conveys The DHCP Relay Agent Information Option (RFC3046[1]) conveys
information between a DHCP Relay Agent and a DHCP server. This information between a DHCP Relay Agent and a DHCP server. This
specification defines a new authentication suboption for that option specification defines a new authentication suboption for that option
which supports source entity authentication and data integrity for which supports source entity authentication and data integrity for
relayed DHCP messages. The authentication suboption contains a relayed DHCP messages. The authentication suboption contains a
cryptographic signature in a payload derived from the option used in cryptographic signature in a payload derived from the option used in
DHCP Authentication (RFC3118[2]). DHCP Authentication (RFC 3118[9]).
Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Requirements Terminology . . . . . . . . . . . . . . . . . . 3
1.1 Requirements Terminology . . . . . . . . . . . . . . . . . . 3 2. DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . 3
1.2 DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . 3 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Suboption Format . . . . . . . . . . . . . . . . . . . . . . 4
3. Suboption Format . . . . . . . . . . . . . . . . . . . . . . 4 5. Replay Detection . . . . . . . . . . . . . . . . . . . . . . 5
4. Replay Detection . . . . . . . . . . . . . . . . . . . . . . 5 6. The Relay Identifier Field . . . . . . . . . . . . . . . . . 5
5. The Relay Identifier Field . . . . . . . . . . . . . . . . . 5 7. Computing Authentication Information . . . . . . . . . . . . 6
6. Computing Authentication Information . . . . . . . . . . . . 6 7.1 The HMAC-MD5 Algorithm . . . . . . . . . . . . . . . . . . . 6
6.1 The HMAC-MD5 Algorithm . . . . . . . . . . . . . . . . . . . 6 8. Procedures for Sending Messages . . . . . . . . . . . . . . 8
7. Procedures for Sending Messages . . . . . . . . . . . . . . 8 8.1 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 8 8.2 Packet Preparation . . . . . . . . . . . . . . . . . . . . . 8
7.2 Packet Preparation . . . . . . . . . . . . . . . . . . . . . 8 8.3 Signature Computation . . . . . . . . . . . . . . . . . . . 8
7.3 Signature Computation . . . . . . . . . . . . . . . . . . . 8 8.4 Sending the Message . . . . . . . . . . . . . . . . . . . . 8
7.4 Sending the Message . . . . . . . . . . . . . . . . . . . . 8 9. Procedures for Processing Incoming Messages . . . . . . . . 8
8. Procedures for Processing Incoming Messages . . . . . . . . 8 9.1 Initial Examination . . . . . . . . . . . . . . . . . . . . 8
8.1 Initial Examination . . . . . . . . . . . . . . . . . . . . 9 9.2 Replay Detection Check . . . . . . . . . . . . . . . . . . . 9
8.2 Replay Detection Check . . . . . . . . . . . . . . . . . . . 9 9.3 Signature Check . . . . . . . . . . . . . . . . . . . . . . 9
8.3 Signature Check . . . . . . . . . . . . . . . . . . . . . . 9 10. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 9
9. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 10 10.1 Receiving Messages from Other Relay Agents . . . . . . . . . 10
9.1 Receiving Messages from Other Relay Agents . . . . . . . . . 10 10.2 Sending Messages to Servers . . . . . . . . . . . . . . . . 10
9.2 Sending Messages to Servers . . . . . . . . . . . . . . . . 10 10.3 Receiving Messages from Servers . . . . . . . . . . . . . . 10
9.3 Receiving Messages from Servers . . . . . . . . . . . . . . 10 11. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . 10
10. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . 10 11.1 Receiving Messages from Relay Agents . . . . . . . . . . . . 11
10.1 Receiving Messages from Relay Agents . . . . . . . . . . . . 11 11.2 Sending Reply Messages to Relay Agents . . . . . . . . . . . 11
10.2 Sending Reply Messages to Relay Agents . . . . . . . . . . . 11 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11 13. Security Considerations . . . . . . . . . . . . . . . . . . 11
12. Security Considerations . . . . . . . . . . . . . . . . . . 11 13.1 Protocol Vulnerabilities . . . . . . . . . . . . . . . . . . 12
12.1 Protocol Vulnerabilities . . . . . . . . . . . . . . . . . . 12 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . 12
References . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13
Full Copyright Statement . . . . . . . . . . . . . . . . . . 14 Full Copyright Statement . . . . . . . . . . . . . . . . . . 14
1. Terminology 1. Requirements Terminology
1.1 Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119[3]. document are to be interpreted as described in RFC 2119[2].
1.2 DHCP Terminology 2. DHCP Terminology
DISCUSSION: This document uses the terms "DHCP server" (or "server") and "DHCP
Is there anything that should go here, or do we think that client" (or "client") as defined in (RFC 2131[6]. The term "DHCP
readers will be sufficiently familiar with DHCP? relay agent" refers to a "BOOTP relay agent" as defined in RFC 2131.
2. Introduction 3. Introduction
DHCP (RFC2131[4]) provides IP addresses and configuration DHCP (RFC 2131[6]) provides IP addresses and configuration
information for IPv4 clients. It includes a relay-agent capability, information for IPv4 clients. It includes a relay-agent capability
in which processes within the network infrastructure receive (RFC 951[7], RFC 1542[8]), in which processes within the network
broadcast messages from clients and forward them to servers as infrastructure receive broadcast messages from clients and forward
unicast messages. In network environments like DOCSIS them to servers as unicast messages. In network environments like
data-over-cable and xDSL, for example, it has proven useful for the DOCSIS data-over-cable and xDSL, for example, it has proven useful
relay agent to add information to the DHCP message before forwarding for the relay agent to add information to the DHCP message before
it, using the relay-agent information option, RFC3046[1]. The kind forwarding it, using the relay-agent information option, RFC
of information that relays add is often used in the server's 3046[1]. The kind of information that relays add is often used in
decision making about the addresses and configuration parameters the server's decision making about the addresses and configuration
that the client should receive. The way that the relay-agent data is parameters that the client should receive. The way that the
used in server decision-making tends to make that data very relay-agent data is used in server decision-making tends to make
important, and highlights the importance of the trust relationship that data very important, and highlights the importance of the trust
between the relay agent and the server. relationship between the relay agent and the server.
The existing DHCP Authentication[2] specification only covers The existing DHCP Authentication specification (RFC 3118)[9] only
communication between the DHCP client and server. Because covers communication between the DHCP client and server. Because
relay-agent information is added after the client has signed its relay-agent information is added after the client has signed its
message, the DHCP Authentication specification explictly excludes message, the DHCP Authentication specification explictly excludes
relay-agent data from that authentication. relay-agent data from that authentication.
The goals of this specification are: The goal of this specification is to define methods that a relay
1. to define a method that a relay-agent can use to protect the agent can use to:
integrity of the data that the relay adds 1. protect the integrity of the data that the relay adds
2. to provide replay protection for that data 2. provide replay protection for that data
3. to leverage the mechanisms that DHCP Authentication specifies 3. leverage existing mechanisms such as DHCP Authentication
in order to leverage the security review and implementation
code-base of that specification.
In order to meet these goals, we specify a new relay-agent In order to meet these goals, we specify a new relay-agent
suboption, the Authentication suboption. The format of this suboption, the Authentication suboption. The format of this
suboption is very similar to the DHCP Authentication option's suboption is very similar to the format of the DHCP Authentication
format, and the specification of the cryptographic methods and option, and the specification of the cryptographic methods and
signature computation for the suboption are also similar to that signature computation for the suboption are also similar to that
option's specification. option's specification.
The Authentication suboption is included by relay agents that wish The Authentication suboption is included by relay agents that wish
to ensure the integrity of the data they include in the Relay Agent to ensure the integrity of the data they include in the Relay Agent
option. These relay agents are configured with the parameters option. These relay agents are configured with the parameters
necessary to generate cryptographically strong signatures of the necessary to generate cryptographically strong signatures of the
data in the DHCP messages which they forward to DHCP servers. A DHCP data in the DHCP messages which they forward to DHCP servers. A DHCP
server configured to process the Authentication suboption uses the server configured to process the Authentication suboption uses the
information in the suboption to validate the signature in the information in the suboption to validate the signature in the
suboption, and continues processing the relay-agent information suboption, and continues processing the relay agent information
option only if the signature is valid. If the DHCP server sends a option only if the signature is valid. If the DHCP server sends a
response, it includes an Authentication suboption in its response response, it includes an Authentication suboption in its response
message, signing the data in its message. Relay agents check the message, signing the data in its message. Relay agents check the
signatures in DHCP server responses and decide whether to forward signatures in DHCP server responses and decide whether to forward
the responses based on the signatures' validity. the responses based on the signatures' validity.
3. Suboption Format 4. Suboption Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Algorithm | MBZ | RDM | | Code | Length | Algorithm | MBZ | RDM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replay Detection (64 bits) | | Replay Detection (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replay Detection cont. | | Replay Detection cont. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The code for the suboption is TBD. The length field includes the The code for the suboption is TBD. The length field includes the
lengths of the algorithm, RDM, and all subsequent suboption fields lengths of the algorithm, RDM, and all subsequent suboption fields
in octets. in octets.
The Algorithm field defines the algorithm used to generate the The Algorithm field defines the algorithm used to generate the
authentication information. authentication information.
Four bits are reserved for future use. These bits SHOULD be set to Four bits are reserved for future use. These bits SHOULD be set to
zero, and MUST be ignored when the suboption is processed. zero, and MUST be not be used when the suboption is processed.
The Replay Detection Method (RDM) field defines the method used to The Replay Detection Method (RDM) field defines the method used to
generate the Replay Detection Data. generate the Replay Detection Data.
The Replay Detection field contains a value used to detect replayed The Replay Detection field contains a value used to detect replayed
messages, interpreted according to the RDM. messages, interpreted according to the RDM.
The Relay Identifier field is used by relays which do not set The Relay Identifier field is used by relay agents that do not set
giaddr, as described in RFC3046[1], Section 2.1. giaddr, as described in RFC3046[1], Section 2.1.
The Authentication Information field contains the data required to The Authentication Information field contains the data required to
communicate algorithm-specific parameters, as well as the signature. communicate algorithm-specific parameters, as well as the signature.
The signature is usually a digest of the data in the DHCP packet The signature is usually a digest of the data in the DHCP packet
computed using the method specified by the Algorithm field. computed using the method specified by the Algorithm field.
4. Replay Detection 5. Replay Detection
The replay-detection mechanism is based on the notion that a The replay-detection mechanism is based on the notion that a
receiver can determine whether or not a message has a valid replay receiver can determine whether or not a message has a valid replay
token value. The default RDM, with value 1, specifies that the token value. The default RDM, with value 1, specifies that the
Replay Detection field contains an increasing counter value. The Replay Detection field contains an increasing counter value. The
receiver associates a replay counter with each sender, and rejects receiver associates a replay counter with each sender, and rejects
any message containing an authentication suboption with a Replay any message containing an authentication suboption with a Replay
Detection counter value less than the last valid value. DHCP servers Detection counter value less than or equal to the last valid value.
MAY identify relays by giaddr value or by other data in the message DHCP servers MAY identify relay agents by giaddr value or by other
(e.g. data in other relay-agent suboptions). Relays identify DHCP data in the message (e.g. data in other relay agent suboptions).
servers by source IP address. If the message's replay detection Relay agents identify DHCP servers by source IP address. If the
value is valid, and the signature is also valid, the receiver message's replay detection value is valid, and the signature is also
updates the its notion of the last valid replay counter value valid, the receiver updates its notion of the last valid replay
associated with the sender. counter value associated with the sender.
All implementations MUST support the default RDM. Additional methods All implementations MUST support the default RDM. Additional methods
may be defined in the future, following the process described in may be defined in the future, following the process described in
Section 11. Section 12.
Receivers SHOULD perform the replay-detection check before Receivers SHOULD perform the replay-detection check before
validating the signature. The authentication hash calculation is validating the signature. The authentication hash calculation is
likely to be much more expensive than the replay-detection value likely to be much more expensive than the replay-detection value
check. check.
DISCUSSION: DISCUSSION:
This places a burden on the receiver to maintain some run-time This places a burden on the receiver to maintain some run-time
state (the most-recent valid counter value) for each sender, but state (the most-recent valid counter value) for each sender, but
the number of members in a DHCP agent-server system is unlikely the number of members in a DHCP agent-server system is unlikely
to be unmanageably large. to be unmanageably large.
5. The Relay Identifier Field 6. The Relay Identifier Field
The Relay-Agent Information Option[1] specification permits a relay The Relay Agent Information Option[1] specification permits a relay
to add a relay-agent option to relayed messages without setting the agent to add a relay agent option to relayed messages without
giaddr field. In this case, the eventual receiver of the message setting the giaddr field. In this case, the eventual receiver of the
needs a stable identifier to use in order to associate per-sender message needs a stable identifier to use in order to associate
state such as Key ID and replay-detection counters. per-sender state such as Key ID and replay-detection counters.
A relay which adds a relay-agent information option and sets giaddr A relay agent that adds a relay agent information option and sets
MUST NOT set the Relay ID field. A relay which does not set giaddr giaddr MUST NOT set the Relay ID field. A relay agent that does not
MAY be configured to place a value in the Relay ID field. If the set giaddr MAY be configured to place a value in the Relay ID field.
relay is configured to use the Relay ID field, it MAY be configured If the relay agent is configured to use the Relay ID field, it MAY
with a value to use, or it MAY be configured to generate a value be configured with a value to use, or it MAY be configured to
based on some other data, such its MAC or IP addresses. If a relay generate a value based on some other data, such its MAC or IP
generates a Relay ID value it SHOULD select a value that it can addresses. If a relay generates a Relay ID value it SHOULD select a
regenerate reliably, e.g. across reboots. value that it can regenerate reliably, e.g. across reboots.
Servers which process an Authentication Suboption SHOULD use the Servers that process an Authentication Suboption SHOULD use the
giaddr value to identify the sender if the giaddr field is set. giaddr value to identify the sender if the giaddr field is set.
Servers MAY be configured to use some other data in the message to Servers MAY be configured to use some other data in the message to
identify the signer. If giaddr is not set, the server SHOULD use the identify the signer. If giaddr is not set, the server SHOULD use the
Relay ID field if it is non-zero. If neither the giaddr nor the Relay ID field if it is non-zero. If neither the giaddr nor the
Relay ID field is set, the server MAY be configured to use some Relay ID field is set, the server MAY be configured to use some
other data in the message, or it MAY increment an error counter. other data in the message, or it MAY increment an error counter.
6. Computing Authentication Information 7. Computing Authentication Information
The Authentication Information field contains a computed signature, The Authentication Information field contains a computed signature,
generated by the sender. All algorithms are defined to process the generated by the sender. All algorithms are defined to process the
data in the DHCP messages in the same way. The sender and receiver data in the DHCP messages in the same way. The sender and receiver
compute the signature across a buffer containing all of the bytes in compute the signature across a buffer containing all of the bytes in
the DHCP message, including the fixed DHCP message header, the DHCP the DHCP message, including the fixed DHCP message header, the DHCP
options, and the relay-agent suboptions, with the following options, and the relay agent suboptions, with the following
exceptions. The value of the 'hops' field MUST be set to zero, exceptions. The value of the 'hops' field MUST be set to zero for
because its value may be changed in transmission. The value of the the computation, because its value may be changed in transmission.
'giaddr' field MUST also be set to all-zeroes because it may be The value of the 'giaddr' field MUST also be set to zero for the
modified in networks where one relay agent adds the relay-agent computation because it may be modified in networks where one relay
option but another relay sets 'giaddr' (see RFC3046[1], section agent adds the relay agent option but another relay agent sets
2.1). In addition, because the relay-agent option itself is included 'giaddr' (see RFC 3046, section 2.1). In addition, because the relay
in the computation, the 'signature' part of the 'authentication agent option itself is included in the computation, the 'signature'
information' field in the Authentication suboption is set to all part of the 'authentication information' field in the Authentication
zeroes. The relay-agent option length, the Authentication suboption suboption is set to all zeroes. The relay agent option length, the
length and other Authentication suboption fields are all included in Authentication suboption length and other Authentication suboption
the computation. fields are all included in the computation.
All implementations MUST support Algorithm 1, the HMAC-MD5 All implementations MUST support Algorithm 1, the HMAC-MD5
algorithm. Additional algorithms may be defined in the future, algorithm. Additional algorithms may be defined in the future,
following the process described in Section 11. following the process described in Section 12.
6.1 The HMAC-MD5 Algorithm 7.1 The HMAC-MD5 Algorithm
Algorithm 1 is assigned to the HMAC[5] protocol, using the MD5[6] Algorithm 1 is assigned to the HMAC[3] protocol, using the MD5[4]
hash function. This algorithm requires that a shared secret key be hash function. This algorithm requires that a shared secret key be
configured at the relay agent and the DHCP server. A 32-bit Key configured at the relay agent and the DHCP server. A 32-bit Key
Identifier is associated with each shared key, and this identifier Identifier is associated with each shared key, and this identifier
is carried in the first 4 bytes of the Authentication Information is carried in the first 4 bytes of the Authentication Information
field of the Authentication suboption. The HMAC-MD5 computation field of the Authentication suboption. The HMAC-MD5 computation
generates a 16-byte signature, which is placed in the Authentication generates a 16-byte signature, which is placed in the Authentication
Information field after the Key ID. Information field after the Key ID.
The format of the Authentication suboption when Algorithm 1 is used The format of the Authentication suboption when Algorithm 1 is used
is: is:
skipping to change at page 7, line 34 skipping to change at page 7, line 30
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID (32 bits) | | Key ID (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| HMAC-MD5 (128 bits) | | HMAC-MD5 (128 bits) |
| | | |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The suboption length is 34. The RDM and Replay Detection fields are The suboption length is 34. The RDM and Replay Detection fields are
as specified in Section 4. The Relay ID field is set as specified in as specified in Section 5. The Relay ID field is set as specified in
Section 5. The Key ID is set by the sender to the ID of the key used Section 6. The Key ID is set by the sender to the ID of the key used
in computing the signature, as an integer value in network in computing the signature, as an integer value in network
byte-order. The HMAC signature follows the Key ID. byte-order. The HMAC signature follows the Key ID.
The Key ID exists only to allow the sender and receiver to specify a The Key ID exists only to allow the sender and receiver to specify a
shared secret in cases where more than one secret is in use among a shared secret in cases where more than one secret is in use among a
network's relays and DHCP servers. The Key ID values are entirely a network's relays and DHCP servers. The Key ID values are entirely a
matter of local configuration; they only need to be locally unique. matter of local configuration; they only need to be locally unique.
This specification does not define any semantics or impose any This specification does not define any semantics or impose any
requirements on this algorithm's Key ID values. requirements on this algorithm's Key ID values.
DISCUSSION: DISCUSSION:
We specify a four-byte Key ID, following the example of the DHCP We specify a four-byte Key ID, following the example of the DHCP
Authentication RFC. Other authentication protocols, like DNS Authentication RFC. Other authentication protocols, like DNS
TSIG[7], use a key name. A key name is more flexible and TSIG[10], use a key name. A key name is more flexible and
potentially more human-readable than a key id. DHCP servers may potentially more human-readable than a key id. DHCP servers may
well be configured to use key names for DNS updates using TSIG, well be configured to use key names for DNS updates using TSIG,
so it might simplify DHCP server configuration if some of the so it might simplify DHCP server configuration if some of the
key-management for both protocols could be shared. key-management for both protocols could be shared.
On the other hand, it is crucial to minimize the size expansion On the other hand, it is crucial to minimize the size expansion
caused by the introduction of the relay-agent information option. caused by the introduction of the relay agent information option.
Named keys would require more physical space, and would entail Named keys would require more physical space, and would entail
more complex suboption encoding and parsing implementations. more complex suboption encoding and parsing implementations.
These considerations have led us to specify a fixed-length Key ID These considerations have led us to specify a fixed-length Key ID
instead of a variable-length key name. instead of a variable-length key name.
7. Procedures for Sending Messages 8. Procedures for Sending Messages
7.1 Replay Detection 8.1 Replay Detection
The sender obtains a replay-detection counter value to use, based on The sender obtains a replay-detection counter value to use, based on
the RDM it is using. If the sender is using RDM 1, the default RDM, the RDM it is using. If the sender is using RDM 1, the default RDM,
the value MUST be greater than any previously-sent value. the value MUST be greater than any previously-sent value.
7.2 Packet Preparation 8.2 Packet Preparation
The sender sets the 'giaddr' field and the 'hops' field to all The sender sets the 'giaddr' field and the 'hops' field to all
zeroes. The sender appends the relay-agent information option to the zeroes. The sender appends the relay agent information option to the
client's packet, including the Authentication suboption. The sender client's packet, including the Authentication suboption. The sender
selects an appropriate Replay Detection value. The sender places its selects an appropriate Replay Detection value. The sender places its
identifier into the Relay ID field, if necessary, or sets the field identifier into the Relay ID field, if necessary, or sets the field
to all zeroes. The sender sets the suboption length, places the to all zeroes. The sender sets the suboption length, places the
Replay Detection value into the Replay Detection field of the Replay Detection value into the Replay Detection field of the
suboption, and sets the algorithm to the algorithm number that it is suboption, and sets the algorithm to the algorithm number that it is
using. If the sender is using HMAC-MD5, it sets the Key ID field to using. If the sender is using HMAC-MD5, it sets the Key ID field to
the appropriate value. The sender sets the field which will contain the appropriate value. The sender sets the field which will contain
the signature to all zeroes. Other algorithms may specify additional the signature to all zeroes. Other algorithms may specify additional
preparation steps. preparation steps.
7.3 Signature Computation 8.3 Signature Computation
The sender computes the signature across the entire DHCP message, The sender computes the signature across the entire DHCP message,
using the algorithm it has selected. The sender places the result of using the algorithm it has selected. The sender places the result of
the computation into the signature field of the Authentication the computation into the signature field of the Authentication
suboption. suboption.
7.4 Sending the Message 8.4 Sending the Message
The sender restores the values of the 'hops' and 'giaddr' fields, The sender restores the values of the 'hops' and 'giaddr' fields,
and sends the message. and sends the message.
8. Procedures for Processing Incoming Messages 9. Procedures for Processing Incoming Messages
8.1 Initial Examination 9.1 Initial Examination
The receiver examines the message, the value of the giaddr field, The receiver examines the message, the value of the giaddr field,
and determines whether the packet includes the relay-agent and determines whether the packet includes the relay agent
information option. The receiver uses its configuration to determine information option. The receiver uses its configuration to determine
whether it should expect an Authentication suboption. The receiver whether it should expect an Authentication suboption. The receiver
MAY be configured to drop incoming messages which do not contain a MUST support configuration that allows it to drop incoming messages
valid relay agent information option and Authentication suboption. that do not contain a valid relay agent information option and
Authentication suboption.
If the receiver determines that the Authentication suboption is If the receiver determines that the Authentication suboption is
present and that it should process the suboption, it uses the data present and that it should process the suboption, it uses the data
in the message to determine which algorithm, key, and RDM to use in in the message to determine which algorithm, key, and RDM to use in
validating the message. If the receiver cannot determine which validating the message. If the receiver cannot determine which
algorithm, key, and RDM to use, or if it does not support the value algorithm, key, and RDM to use, or if it does not support the value
indicated in the message, it SHOULD drop the message. Because this indicated in the message, it SHOULD drop the message. Because this
situation could indicate a misconfiguration which could deny service situation could indicate a misconfiguration which could deny service
to clients, receivers MAY attempt to notify their administrators or to clients, receivers MAY attempt to notify their administrators or
log an error message. log an error message.
8.2 Replay Detection Check 9.2 Replay Detection Check
The receiver examines the RDM field. Receivers MUST discard The receiver examines the RDM field. Receivers MUST discard
messages containing RDM values which they do not support. Because messages containing RDM values that they do not support. Because
this may indicate a misconfiguration at the sender, an attempt this may indicate a misconfiguration at the sender, an attempt
SHOULD be made to indicate this condition to the administrator, by SHOULD be made to indicate this condition to the administrator, by
incrementing an error counter or writing a log message. If the incrementing an error counter or writing a log message. If the
receiver supports the RDM, it examines the value in the Replay receiver supports the RDM, it examines the value in the Replay
Detection field using the procedures in the RDM and in Section 4. If Detection field using the procedures in the RDM and in Section 5. If
the Replay value is not valid, the receiver MUST drop the message. the Replay value is not valid, the receiver MUST drop the message.
Note that the receiver MUST NOT update its notion of the last valid Note that the receiver MUST NOT update its notion of the last valid
Replay Detection value for the sender at this point. Until the Replay Detection value for the sender at this point. Until the
signature has been checked, the Replay Detection field cannot be signature has been checked, the Replay Detection field cannot be
trusted. If the receiver trusts the Replay Detection value without trusted. If the receiver trusts the Replay Detection value without
checking the signature, a malicious host could send a replayed checking the signature, a malicious host could send a replayed
message with a Replay Detection value that was very high, tricking message with a Replay Detection value that was very high, tricking
the receiver into rejecting legitimate values from the sender. the receiver into rejecting legitimate values from the sender.
8.3 Signature Check 9.3 Signature Check
The receiver prepares the packet in order to check the signature. The receiver prepares the packet in order to check the signature.
The receiver sets the 'giaddr' and 'hops' fields to zero, and sets The receiver sets the 'giaddr' and 'hops' fields to zero, and sets
the signature field of the Authentication suboption to all zeroes. the signature field of the Authentication suboption to all zeroes.
Using the algorithm and key associated with the sender, the receiver Using the algorithm and key associated with the sender, the receiver
computes a hash of the message. The receiver compares the result of computes a hash of the message. The receiver compares the result of
its computation with the value sent by the sender. If the signatures its computation with the value sent by the sender. If the signatures
do not match, the receiver MUST drop the message. Otherwise, the do not match, the receiver MUST drop the message. Otherwise, the
receiver updates its notion of the last valid Replay Detection value receiver updates its notion of the last valid Replay Detection value
associated with the sender, and processes the message. associated with the sender, and processes the message.
9. Relay Agent Behavior 10. Relay Agent Behavior
DHCP Relay agents are typically configured with the addresses of one DHCP Relay agents are typically configured with the addresses of one
or more DHCP servers. A relay agent which implements this suboption or more DHCP servers. A relay agent that implements this suboption
requires an algorithm number for each server, as well as appropriate requires an algorithm number for each server, as well as appropriate
credentials (i.e. keys) to use. Relay implementations SHOULD support credentials (i.e. keys) to use. Relay implementations SHOULD support
configuration which indicates that all relayed messages should configuration which indicates that all relayed messages should
include the authentication suboption. This SHOULD be disabled by include the authentication suboption. Use of the authentication
default. Relays MAY support configuration that indicates that suboption SHOULD be disabled by default. Relay agents MAY support
certain destination servers support the authentication suboption, configuration that indicates that certain destination servers
while other servers do not. Relays MAY support configuration of a support the authentication suboption, while other servers do not.
single algorithm number and key to be used with all DHCP servers, or Relay agents MAY support configuration of a single algorithm number
they MAY support configuration of different algorithms and keys for and key to be used with all DHCP servers, or they MAY support
each server. configuration of different algorithms and keys for each server.
9.1 Receiving Messages from Other Relay Agents 10.1 Receiving Messages from Other Relay Agents
There are network configurations in which one relay agent adds the There are network configurations in which one relay agent adds the
relay-agent option, and then forwards the DHCP message to another relay agent option, and then forwards the DHCP message to another
relay. For example, a layer-2 switch might be directly connected to relay agent. For example, a layer-2 switch might be directly
a client, and it might forward messages to an aggregating router, connected to a client, and it might forward messages to an
which sets giaddr and then forwards the message to a DHCP server. aggregating router, which sets giaddr and then forwards the message
When a DHCP relay which implements the Authentication suboption to a DHCP server. When a DHCP relay which implements the
receives a message, it MAY use the procedures in Section 8 to verify Authentication suboption receives a message, it MAY use the
the source of the message before forwarding it. procedures in Section 9 to verify the source of the message before
forwarding it.
9.2 Sending Messages to Servers 10.2 Sending Messages to Servers
When the relay agent receives a broadcast packet from a client, it When the relay agent receives a broadcast packet from a client, it
determines which DHCP servers (or other relays) should receive determines which DHCP servers (or other relay agents) should receive
copies of the message. If the relay is configured to include the copies of the message. If the relay agent is configured to include
Authentication suboption, it determines which Algorithm and RDM to the Authentication suboption, it determines which Algorithm and RDM
use, and then it performs the steps in Section 7. to use, and then it performs the steps in Section 8.
9.3 Receiving Messages from Servers 10.3 Receiving Messages from Servers
When the relay agent receives a message, it determines from its When the relay agent receives a message, it determines from its
configuration whether it expects the message to contain a configuration whether it expects the message to contain a relay
relay-agent information option and an Authentication suboption. The agent information option and an Authentication suboption. The relay
relay MAY be configured to drop response messages that do not agent MAY be configured to drop response messages that do not
contain the Authentication suboption. The relay then follows the contain the Authentication suboption. The relay agent then follows
procedures in Section 8. the procedures in Section 9.
10. DHCP Server Behavior 11. DHCP Server Behavior
DHCP servers may interact with multiple relay agents. Server DHCP servers may interact with multiple relay agents. Server
implementations MAY support configuration that associates the same implementations MAY support configuration that associates the same
algorithm and key with all relay agents. Servers MAY support algorithm and key with all relay agents. Servers MAY support
configuration which specifies the algorithm and key to use with each configuration which specifies the algorithm and key to use with each
relay agent individually. relay agent individually.
10.1 Receiving Messages from Relay Agents 11.1 Receiving Messages from Relay Agents
When a DHCP server which implements the Authentication suboption When a DHCP server which implements the Authentication suboption
receives a message, it performs the steps in Section 8. receives a message, it performs the steps in Section 9.
10.2 Sending Reply Messages to Relay Agents 11.2 Sending Reply Messages to Relay Agents
When the server has prepared a reply message, it uses the incoming When the server has prepared a reply message, it uses the incoming
request message and its configuration to determine whether it should request message and its configuration to determine whether it should
include a relay-agent information option and an Authentication include a relay agent information option and an Authentication
suboption. If the server is configured to include the Authentication suboption. If the server is configured to include the Authentication
suboption, it determines which Algorithm and RDM to use, and then suboption, it determines which Algorithm and RDM to use, and then
performs the steps in Section 7. performs the steps in Section 8.
DISCUSSION: DISCUSSION:
This server behavior represents a slight variance from This server behavior represents a slight variance from RFC
RFC3046[1], Section 2.2. The Authentication suboption is not 3046[1], Section 2.2. The Authentication suboption is not echoed
echoed back from the server to the relay: the server generates back from the server to the relay: the server generates its own
its own suboption. suboption.
11. IANA Considerations 12. IANA Considerations
Section 3 defines a new suboption for the DHCP relay-agent option, Section 4 defines a new suboption for the DHCP relay agent option,
called the Authentication Suboption. IANA is requested to allocate a called the Authentication Suboption. IANA is requested to allocate a
new suboption code from the relay-agent option suboption number new suboption code from the relay agent option suboption number
space. space.
This specification introduces two new number-spaces for the This specification introduces two new number-spaces for the
Authentication suboption's 'Algorithm' and 'Replay Detection Method' Authentication suboption's 'Algorithm' and 'Replay Detection Method'
fields. These number spaces are to be created and maintained by IANA. fields. These number spaces are to be created and maintained by IANA.
The Algorithm identifier is a one-byte value. Algorithm value 0 is The Algorithm identifier is a one-byte value. Algorithm value 0 is
reserved. Algorithm value 1 is assigned to the HMAC-MD5 signature as reserved. Algorithm value 1 is assigned to the HMAC-MD5 signature as
defined in Section 6.1. Additional algorithm values will be defined in Section 7.1. Additional algorithm values will be
allocated and assigned through IETF consensus, as defined in RFC allocated and assigned through IETF consensus, as defined in RFC
2434[8]. 2434[5].
The RDM identifier is a four-bit value. RDM value 0 is reserved. RDM The RDM identifier is a four-bit value. RDM value 0 is reserved. RDM
value 1 is assigned to the use of a monotonically increasing counter value 1 is assigned to the use of a monotonically increasing counter
value as defined in Section 4. Additional RDM values will be value as defined in Section 5. Additional RDM values will be
allocated and assigned through IETF consensus, as defined in RFC allocated and assigned through IETF consensus, as defined in RFC
2434[8]. 2434[5].
12. Security Considerations 13. Security Considerations
This specification describes a protocol to add source authentication This specification describes a protocol to add source authentication
and message integrity protection to the messages between DHCP relay and message integrity protection to the messages between DHCP relay
agents and DHCP servers. agents and DHCP servers.
The use of this protocol imposes a new computational burden on relay The use of this protocol imposes a new computational burden on relay
agents and servers, because they must perform cryptographic hash agents and servers, because they must perform cryptographic hash
calculations when they send and receive messages. This burden may calculations when they send and receive messages. This burden may
add latency to DHCP messages exchanges. Because relay agents are add latency to DHCP message exchanges. Because relay agents are
involved when clients reboot, periods of very high reboot activity involved when clients reboot, periods of very high reboot activity
will result in the largest number of messages which have to be will result in the largest number of messages which have to be
signed and verified. During a cable MSO head-end reboot event, for signed and verified. During a cable MSO head-end reboot event, for
example, the time required for all clients to be served may increase. example, the time required for all clients to be served may increase.
12.1 Protocol Vulnerabilities 13.1 Protocol Vulnerabilities
Because DHCP is a UDP protocol, messages between relays and servers Because DHCP is a UDP protocol, messages between relays and servers
may be delivered in a different order than the order in which they may be delivered in a different order than the order in which they
were generated. The replay-detection mechanism will cause receivers were generated. The replay-detection mechanism will cause receivers
to drop packets which are delivered 'late', leading to client to drop packets which are delivered 'late', leading to client
retries. The retry mechanisms which most clients implement should retries. The retry mechanisms which most clients implement should
not cause this to be an enormous issue, but it will cause senders to not cause this to be an enormous issue, but it will cause senders to
do computational work which will be wasted if their messages are do computational work which will be wasted if their messages are
re-ordered. re-ordered.
13. Acknowledgements 14. Acknowledgements
The need for this specification was made clear by comments made by The need for this specification was made clear by comments made by
Thomas Narten and John Schnizlein, and the use of the DHCP Thomas Narten and John Schnizlein, and the use of the DHCP
Authentication option format was suggested by Josh Littlefield, at Authentication option format was suggested by Josh Littlefield, at
IETF 53. IETF 53.
References Normative References
[1] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, [1] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001. January 2001.
[2] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
RFC 3118, June 2001.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", RFC 2119, March 1997.
[4] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [3] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
March 1997.
[5] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997. for Message Authentication", RFC 2104, February 1997.
[6] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April [4] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
1992. 1992.
[7] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, [5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
Informative References
[6] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[7] Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[8] Wimer, W., "Clarifications and Extensions for the Bootstrap
Protocol", RFC 1542, October 1993.
[9] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[10] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC "Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000. 2845, May 2000.
[8] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
Authors' Addresses Authors' Addresses
Mark Stapp Mark Stapp
Cisco Systems, Inc. Cisco Systems, Inc.
250 Apollo Dr. 1414 Massachusetts Ave.
Chelmsford, MA 01824 Boxborough, MA 01719
USA USA
Phone: 978.244.8498 Phone: 978.936.1535
EMail: mjs@cisco.com EMail: mjs@cisco.com
Ted Lemon Ted Lemon
Nominum, Inc. Nominum, Inc.
950 Charter St. 950 Charter St.
Redwood City, CA 94063 Redwood City, CA 94063
USA USA
EMail: mellon@nominum.com EMail: mellon@nominum.com
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
 End of changes. 

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