DHC Working Group M. Stapp Internet-Draft Cisco Systems, Inc. Expires:
December 22, 2002May 2, 2003 T. Lemon Nominum, Inc. June 23,November 1, 2002 The Authentication Suboption for the DHCP Relay Agent Option <draft-ietf-dhc-auth-suboption-00.txt><draft-ietf-dhc-auth-suboption-01.txt> Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 22, 2002.May 2, 2003. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract The DHCP Relay Agent Information Option RFC3046(RFC3046) conveys information between a DHCP Relay Agent and a DHCP server. This specification defines a new authentication suboption for that option which supports source entity authentication and data integrity for that option.relayed DHCP messages. The authentication suboption contains a cryptographic signature in a payload derived from the option used in DHCP Authentication RFC3118.(RFC3118). Table of Contents 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Requirements Terminology . . . . . . . . . . . . . . . . . . 3 1.2 DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Suboption Format . . . . . . . . . . . . . . . . . . . . . . 4 4. Replay Detection . . . . . . . . . . . . . . . . . . . . . . 5 5. The Relay Identifier Field . . . . . . . . . . . . . . . . . 5 6. Computing Authentication Information . . . . . . . . . . . . 5 5.16 6.1 The HMAC-MD5 Algorithm . . . . . . . . . . . . . . . . . . . 6 6.7. Procedures for Sending Messages . . . . . . . . . . . . . . 7 6.18 7.1 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 7 6.28 7.2 Packet Preparation . . . . . . . . . . . . . . . . . . . . . 7 6.38 7.3 Signature Computation . . . . . . . . . . . . . . . . . . . 7 6.48 7.4 Sending the Message . . . . . . . . . . . . . . . . . . . . 8 7.8. Procedures for Processing Incoming Messages . . . . . . . . 8 7.18.1 Initial Examination . . . . . . . . . . . . . . . . . . . . 8 7.29 8.2 Replay Detection Check . . . . . . . . . . . . . . . . . . . 8 7.39 8.3 Signature Check . . . . . . . . . . . . . . . . . . . . . . 9 8.9. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 9 8.110 9.1 Receiving Messages from Other Relay Agents . . . . . . . . . 10 9.2 Sending Messages to Servers . . . . . . . . . . . . . . . . 9 8.210 9.3 Receiving Messages from Servers . . . . . . . . . . . . . . 9 9.10 10. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . 9 9.110 10.1 Receiving Messages from Relay Agents . . . . . . . . . . . . 10 9.211 10.2 Sending Reply Messages to Relay Agents . . . . . . . . . . . 10 10.11 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 10 11.11 12. Security Considerations . . . . . . . . . . . . . . . . . . 10 11.111 12.1 Protocol Vulnerabilities . . . . . . . . . . . . . . . . . . 11 12.12 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 1112 References . . . . . . . . . . . . . . . . . . . . . . . . . 1112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 1213 Full Copyright Statement . . . . . . . . . . . . . . . . . . 1314 1. Terminology 1.1 Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. 1.2 DHCP Terminology DISCUSSION: Is there anything that should go here, or do we think that readers will be sufficiently familiar with DHCP? 2. Introduction DHCP (RFC2131) provides IP addresses and configuration information for IPv4 clients. It includes a relay-agent capability, in which processes within the network infrastructure receive broadcast messages from clients and forward them to servers as unicast messages. In network environments like DOCSIS data-over-cable and DSL,xDSL, for example, it has proven useful for the relay agent to add information to the DHCP message before forwarding it, using the relay-agent information option, RFC3046. The kind of information that relays add is often used in the server's decision making about the addresses and configuration parameters that the client should receive. The way that the relay-agent data is used in server decision-making tends to make that data very important, and highlights the importance of the trust relationship between the relay agent and the server. The existing DHCP Authentication specification only covers communication between the DHCP client and server. Because relay-agent information is added after the client has signed its message, the DHCP Authentication specification explictly excludes relay-agent data from that authentication. The goals of this specification are: 1. to define a method that a relay-agent can use to protect the integrity of the data that the relay adds 2. to provide replay protection for that data 3. to leverage the mechanisms that DHCP Authentication specifies 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 suboption, the Authentication suboption. The format of this suboption is very similar to the DHCP Authentication option's format, and the specification of the cryptographic methods and signature computation for the suboption are inherited fromalso similar to that option.option's specification. The Authentication suboption is included by relay agents whothat wish to ensure the integrity of the data they include in the Relay Agent option. These relay agents are configured with the parameters necessary to generate cryptographically strong signatures of the data in the DHCP messages which they forward to DHCP servers. A DHCP server configured to process the Authentication suboption uses the information in the suboption to validate the signature in the suboption, and continues processing the packetrelay-agent information option only if the signature is valid. If the DHCP server sends a response, it includes an Authentication suboption in its response message, signing the data in its message. Relay agents check the signatures in DHCP server responses and decide whether to forward the responses based on the signatures' validity. 3. Suboption Format The format of the Authentication suboption is inherited from the DHCP Authentication option.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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Length | Algorithm | MBZ | RDM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Replay Detection (64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Replay Detection cont. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Relay Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | Authentication Information | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The code for the suboption is TBD. The length field includes the lengths of the algorithm, RDM, and all subsequent suboption fields in octets. The Algorithm field defines the algorithm used to generate the authentication information. Four bits are reserved for future use. These bits SHOULD be set to zero, and MUST be ignored when the suboption is processed. The Replay Detection Method (RDM) field defines the method used to generate the Replay Detection Data. The ReplyReplay Detection field contains a value used to detect replays,replayed messages, interpreted according to the RDM. The Relay Identifier field is used by relays which do not set giaddr, as described in RFC3046, Section 2.1. The Authentication Information field contains the data required to communicate algorithm-specific parameters, as well as the signature. The signature is usually a digest of the data in the DHCP packet computed using the method specified by the Algorithm field. 4. Replay Detection The replay-detection mechanism is based on the notion that a receiver can determine whether or not a message has a valid replay token value. The default RDM, with value 1, specifies that the Replay Detection field contains an increasing counter value. The receiver associates a replay counter with each sender, and rejects any message containing an authentication suboption with a Replay Detection counter value less than the last valid value. DHCP servers MAY identify relays by giaddr value or by other data in the message (e.g. data in other relay-agent suboptions). Relays identify DHCP servers by source IP address. If the message's replay detection value is valid, and the signature is also valid, the receiver updates the its notion of the last valid replay counter value associated with the sender. All implementations MUST support the default RDM. Additional methods may be defined in the future, following the process described in Section 10.11. Receivers SHOULD perform the replay-detection check before validating the signature. The authentication hash calculation is likely to be much more expensive than the replay-detection value check. DISCUSSION: This places a burden on the receiverreceiver to maintain some run-time state (the most-recent valid counter value) for each sender, but the number of members in a DHCP agent-server system is unlikely to be unmanageably large. 5. The Relay Identifier Field The Relay-Agent Information Option specification permits a relay to add a relay-agent option to relayed messages without setting the giaddr field. In this case, the eventual receiver of the message needs a stable identifier to use in order to associate per-sender state such as Key ID and replay-detection counters. A relay which adds a relay-agent information option and sets giaddr MUST NOT set the Relay ID field. A relay which does not set giaddr MAY be configured to place a value in the Relay ID field. If the relay is configured to use the Relay ID field, it MAY be configured with a value to use, or it MAY be configured to generate a value based on some other data, such its MAC or IP addresses. If a relay generates a Relay ID value it SHOULD select a value that it can regenerate reliably, e.g. across reboots. Servers which process an Authentication Suboption SHOULD use the 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 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 is set, the server MAY be configured to maintainuse some run-time state (the most-recent valid counter value) for each sender, but the number of membersother data in a DHCP agent-server system is unlikely to be unmanageably large. 5.the message, or it MAY increment an error counter. 6. Computing Authentication Information The Authentication Information field contains a computed signature, generated by the sender. All algorithms are defined to process the 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 the DHCP message, including the fixed DHCP message header, the DHCP options, and the relay-agent suboption,suboptions, with the following exceptions. The value of the 'hops' field MUST be set to zero, because its value may be changed in transmission. The value of the 'giaddr' field MUST also be set to all-zeroes because it may be modified in networks where one relay agent adds the relay-agent option but another relay sets 'giaddr' (see RFC3046, section 2.1). In addition, because the relay-agent option itself is included in the computation, the 'signature' part of the 'authentication information' field in the Authentication suboption is set to all zeroes. The relay-agent option length, the Authentication suboption length and other Authentication suboption fields are all included in the computation. All implementations MUST support Algorithm 1, the HMAC-MD5 algorithm. Additional algorithms may be defined in the future, following the process described in Section 10. 5.111. 6.1 The HMAC-MD5 Algorithm Algorithm 1 is assigned to the HMAC protocol, using the MD5 hash function. This algorithm requires that a shared secret key be configured at the relay agent and the DHCP server. A 32-bit Key Identifier is associated with each shared key, and this identifier is carried in the first 4 bytes of the Authentication Information field of the Authentication suboption. The HMAC-MD5 computation generates a 16-byte signature, which is placed in the Authentication Information field after the Key ID. The format of the Authentication suboption when Algorithm 1 is used is: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | 34 |0 0 0 0 0 0 0 1| MBZ | RDM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Replay Detection (64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Replay Detection cont. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Relay Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key ID (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | HMAC-MD5 (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The suboption length is 34, the34. The RDM and Replay Detection fields are as specified in Section 4, the4. 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 in computing the signature, as an integer value in network byte-order. The HMAC signature follows the Key ID. 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 network's relays and DHCP servers. The Key ID values are entirely a matter of local configuration; they only need to be locally unique. This specification does not define any semantics or impose any requirements on this algorithm's Key ID values. DISCUSSION: We specify a four-byte Key ID, following the example of the DHCP Authentication RFC. Other authentication protocols, like DNS TSIG, use a key name. A key name is more flexible and potentially more human-readable than a key id. DHCP servers may well be configured to use key names for DNS updates using TSIG, so it might simplify DHCP server configuration if some of the key-management for both protocols could be shared. Should weOn the other hand, it is crucial to minimize the size expansion caused by the introduction of the relay-agent information option. Named keys would require more physical space, and would entail more complex suboption encoding and parsing implementations. These considerations have led us to specify a variable-lengthfixed-length Key NameID instead of a fixed-length Key ID? 6.variable-length key name. 7. Procedures for Sending Messages 6.17.1 Replay Detection 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 value MUST be greater than any previously-sent value. 6.27.2 Packet Preparation The sender sets the 'giaddr' field and the 'hops' field to all zeroes. The sender appends the relay-agent information option to the client's packet, including the Authentication suboption. The sender selects an appropriate Replay Detection value. The sender places its identifier into the Relay ID field, if necessary, or sets the field to all zeroes. The sender sets the suboption length, places the Replay Detection value into the Replay Detection field of the 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 the appropriate value. The sender sets the field which will contain the signature to all zeroes. Other algorithms may specify additional preparation steps. 6.37.3 Signature Computation The sender computes the signature across the entire DHCP message, using the algorithm it has selected. The sender places the result of the computation into the signature field of the Authentication suboption. 6.47.4 Sending the Message The sender restores the values of the 'hops' field's value,and 'giaddr' fields, and sends the message. 7.8. Procedures for Processing Incoming Messages 7.18.1 Initial Examination The receiver examines the message, the value of the giaddr field, and determines whether the packet includes the relay-agent information option. The receiver uses its configuration to determine whether it should expect an Authentication suboption. The receiver MAY be configured to drop incoming messages which do not contain a valid relay agent information option and Authentication suboption. If the receiver determines that the Authentication suboption is 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 validating the message. If the receiver cannot determine which algorithm, key, and RDM to use, or if it does not support the value indicated in the message, it SHOULD be configured todrop the message. Because this situation could indicate a misconfiguration which could deny service to clients, receivers MAY attempt to notify their administrators or log an error message. 7.28.2 Replay Detection Check The receiver examines the RDM field. Receivers MUST discard messages containing RDM values which they do not support. Because this may indicate a misconfiguration at the sender, an attempt SHOULD be made to indicate this condition to the administrator, by incrementing an error counter or writing a log message. If the receiver supports the RDM, it examines the value in the Replay Detection field using the procedures in the RDM and in Section 4. If the Replay value is not valid, the receiver MUST drop the message. DISCUSSION:Note that the receiver must notMUST NOT update its notion of the last valid Replay Detection value for the sender at this point. Until the signature has been checked, the Replay Detection field cannot be trusted. If the receiver trusts the Replay Detection value without checking the signature, a malicious host could send a replayed message with a Replay Detection value that was very high, tricking the receiver into rejectedrejecting legitimate values from the sender. 7.38.3 Signature Check The receiver prepares the packet in order to check the signature. The receiver sets the 'giaddr' and 'hops' fields to zero, and sets the signature field of the Authentication suboption to all zeroes. Using the algorithm and key associated with the sender, the receiver computes a hash of the message. The receiver compares the result of its computation with the value sent by the sender. If the signatures do not match, the receiver MUST drop the message. Otherwise, the receiver updates its notion of the last valid Replay Detection value associated with the sender, and processes the message. 8.9. Relay Agent Behavior DHCP Relay agents are typically configured with the addresses of one or more DHCP servers. A relay agent which implements this suboption requires an algorithm number for each server, as well as appropriate credentials (i.e. keys) to use. Relay implementations SHOULD support configuration which indicates that all relayed messages should include the authentication suboption. This SHOULD be disabled by default. Relays MAY support configuration that indicates that certain destination servers support the authentication suboption, while other servers do not. Relays MAY support configuration of a single algorithm number and key to be used with all DHCP servers, or they MAY support configuration of different algorithms and keys for each server. 8.19.1 Receiving Messages from Other Relay Agents There are network configurations in which one relay agent adds the relay-agent option, and then forwards the DHCP message to another relay. For example, a layer-2 switch might be directly connected to a client, and it might forward messages to an aggregating router, which sets giaddr and then forwards the message to a DHCP server. When a DHCP relay which implements the Authentication suboption receives a message, it MAY use the procedures in Section 8 to verify the source of the message before forwarding it. 9.2 Sending Messages to Servers When the relay agent receives a broadcast packet from a client, it determines which DHCP servers (or other relays) should receive copies of the message. If the relay is configured to include the Authentication suboption, it determines which Algorithm and RDM to use, and then it performs the steps in Section 6. 8.27. 9.3 Receiving Messages from Servers When the relay agent receives a message, it determines from its configuration whether it expects the message to contain a relay-agent information option and an Authentication suboption. The relay MAY be configured to drop response messages that do not contain the Authentication suboption. The relay then follows the procedures in Section 7. 9.8. 10. DHCP Server Behavior DHCP servers may interact with multiple relay agents. Server implementations MAY support configuration that associates the same algorithm and key with all relay agents. Servers MAY support configuration which specifies the algorithm and key to use with each relay agent individually. 9.110.1 Receiving Messages from Relay Agents When a DHCP server which implements the Authentication suboption receives a message, it performs the steps in Section 7. 9.28. 10.2 Sending Reply Messages to Relay Agents When the server has prepared a reply message, it uses the incoming request message and its configuration to determine whether it should include a relay-agent information option and an Authentication suboption. If the server is configured to include the Authentication suboption, it determines which Algorithm and RDM to use, and then performs the steps in Section 6.7. DISCUSSION: This server behavior represents a slight variance from RFC3046, Section 2.2. The Authentication suboption is not echoed back from the server to the relay: the server generates its own suboption. 10.11. IANA Considerations Section 3 defines a new suboption for the DHCP relay-agent option, called the Authentication Suboption. IANA is requested to allocate a new suboption code from the relay-agent option suboption number space. This specification introduces two new number-spaces for the Authentication suboption's 'Algorithm' and 'Replay Detection Method' fields. These number spaces are to be created and maintained by IANA. The Algorithm identifier is a one-byte value. Algorithm value 0 is reserved. Algorithm value 1 is assigned to the HMAC-MD5 signature as defined in Section 220.127.116.11. Additional algorithm values will be allocated and assigned through IETF consensus, as defined in RFC 2434. 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 as defined in Section 4. Additional RDM values will be allocated and assigned through IETF consensus, as defined in RFC 2434. 11.12. Security Considerations This specification describes a protocol to add source authentication and message integrity protection to the messages between DHCP relay agents and DHCP servers. The use of this protocol imposes a new computational burden on relay agents and servers, because they must perform cryptographic hash calculations when they send and receive messages. This burden may add latency to DHCP messages exchanges. Because relay agents are involved when clients reboot, periods of very high reboot activity will result in the largest number of messages which have to be signed and verified. During a cable MSO head-end reboot event, for example, the time required for all clients to be served may increase. 11.112.1 Protocol Vulnerabilities Because DHCP is a UDP protocol, messages between relays and servers may be delivered in a different order than the order in which they were generated. The replay-detection mechanism will cause receivers to drop packets which are delivered 'late', leading to client retries. The retry mechanisms which most clients implement should 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 re-ordered. 12.13. Acknowledgements The need for this specification was made clear by comments made by Thomas Narten and John Schnizlein, and the use of the DHCP Authentication option format was suggested by Josh Littlefield, at IETF 53. References  Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001.  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997.  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.  Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April 1992.  Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, May 2000.  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. Authors' Addresses Mark Stapp Cisco Systems, Inc. 250 Apollo Dr. Chelmsford, MA 01824 USA Phone: 978.244.8498 EMail: firstname.lastname@example.org Ted Lemon Nominum, Inc. 950 Charter St. Redwood City, CA 94063 USA EMail: email@example.com Full Copyright Statement Copyright (C) The Internet Society (2002). 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