DHC Working Group M. Stapp
Internet-Draft Cisco Systems, Inc.
Expires: December 22, 2002 T. Lemon
Nominum, Inc.
June 23, 2002
The Authentication Suboption for the DHCP Relay Agent Option
<draft-ietf-dhc-auth-suboption-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The DHCP Relay Agent Information Option RFC3046[1] 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. The authentication suboption contains a payload derived
from the option used in DHCP Authentication RFC3118[2].
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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. Computing Authentication Information . . . . . . . . . . . . 5
5.1 The HMAC-MD5 Algorithm . . . . . . . . . . . . . . . . . . . 6
6. Procedures for Sending Messages . . . . . . . . . . . . . . 7
6.1 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 7
6.2 Packet Preparation . . . . . . . . . . . . . . . . . . . . . 7
6.3 Signature Computation . . . . . . . . . . . . . . . . . . . 7
6.4 Sending the Message . . . . . . . . . . . . . . . . . . . . 8
7. Procedures for Processing Incoming Messages . . . . . . . . 8
7.1 Initial Examination . . . . . . . . . . . . . . . . . . . . 8
7.2 Replay Detection Check . . . . . . . . . . . . . . . . . . . 8
7.3 Signature Check . . . . . . . . . . . . . . . . . . . . . . 9
8. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 9
8.1 Sending Messages to Servers . . . . . . . . . . . . . . . . 9
8.2 Receiving Messages from Servers . . . . . . . . . . . . . . 9
9. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . 9
9.1 Receiving Messages from Relay Agents . . . . . . . . . . . . 10
9.2 Sending Reply Messages to Relay Agents . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 10
11. Security Considerations . . . . . . . . . . . . . . . . . . 10
11.1 Protocol Vulnerabilities . . . . . . . . . . . . . . . . . . 11
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . 13
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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[3].
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[4]) 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, 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[1]. 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[2] 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
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signature computation for the suboption are inherited from that
option.
The Authentication suboption is included by relay agents who 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 packet 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. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| 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.
The Replay Detection Method (RDM) field defines the method used to
generate the Replay Detection Data.
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The Reply Detection field contains a value used to detect replays,
interpreted according to the RDM.
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.
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 receiver 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. 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, 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
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'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[1], 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.
5.1 The HMAC-MD5 Algorithm
Algorithm 1 is assigned to the HMAC[5] protocol, using the MD5[6]
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. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC-MD5 (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The suboption length is 34, the RDM and Replay Detection fields are
as specified in Section 4, the Key ID is set by the sender to the ID
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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[7], 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 we
specify a variable-length Key Name instead of a fixed-length Key
ID?
6. Procedures for Sending Messages
6.1 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.
6.2 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
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.
6.3 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.
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6.4 Sending the Message
The sender restores the 'hops' field's value, and sends the message.
7. Procedures for Processing Incoming Messages
7.1 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 to drop 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.
7.2 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 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 rejected legitimate values from
the sender.
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7.3 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. 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.
8.1 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.
8.2 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. 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
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configuration which specifies the algorithm and key to use with each
relay agent individually.
9.1 Receiving Messages from Relay Agents
When a DHCP server which implements the Authentication suboption
receives a message, it performs the steps in Section 7.
9.2 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.
DISCUSSION:
This server behavior represents a slight variance from
RFC3046[1], Section 2.2. The Authentication suboption is not
echoed back from the server to the relay: the server generates
its own suboption.
10. 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 5.1. Additional algorithm values will be
allocated and assigned through IETF consensus, as defined in RFC
2434[8].
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[8].
11. Security Considerations
This specification describes a protocol to add source authentication
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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.
11.1 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. 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
[1] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[2] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[4] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[5] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[6] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
1992.
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[7] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[8] 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: mjs@cisco.com
Ted Lemon
Nominum, Inc.
950 Charter St.
Redwood City, CA 94063
USA
EMail: mellon@nominum.com
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