Network Working Group M. Stapp
Internet-Draft Cisco Systems, Inc.
Expires: October 10, 2003 T. Lemon
Nominum, Inc.
R. Droms
Cisco Systems, Inc.
April 11, 2003
The Authentication Suboption for the DHCP Relay Agent Option
<draft-ietf-dhc-relay-agent-auth-00.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 October 10, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The DHCP Relay Agent Information Option (RFC 3046) conveys
information between a DHCP relay agent and a DHCP server. This
specification defines two mechanisms for securing the messages
exchanged between a relay agent and a server. The first mechanism
defines a new authentication suboption for the Relay Agent
Information Option that supports source entity authentication and
data integrity for relayed DHCP messages. The authentication
suboption contains a cryptographic signature in a payload derived
Stapp, et al. Expires October 10, 2003 [Page 1]
Internet-Draft Authentication Suboption April 2003
from the option used in DHCP Authentication (RFC 3118). The second
mechanism uses IPsec (RFC 2041) to protect messages exchanged between
relay agents and servers.
Table of Contents
1. Requirements Terminology . . . . . . . . . . . . . . . . . . 3
2. DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Relay Agent Option Authentication Sub-option . . . . . . . . 4
4.1 Suboption Format . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 6
4.3 The Relay Identifier Field . . . . . . . . . . . . . . . . . 6
4.4 Computing Authentication Information . . . . . . . . . . . . 7
4.4.1 The HMAC-MD5 Algorithm . . . . . . . . . . . . . . . . . . . 7
4.5 Procedures for Sending Messages . . . . . . . . . . . . . . 9
4.5.1 Replay Detection . . . . . . . . . . . . . . . . . . . . . . 9
4.5.2 Packet Preparation . . . . . . . . . . . . . . . . . . . . . 9
4.5.3 Signature Computation . . . . . . . . . . . . . . . . . . . 9
4.5.4 Sending the Message . . . . . . . . . . . . . . . . . . . . 9
4.6 Procedures for Processing Incoming Messages . . . . . . . . 9
4.6.1 Initial Examination . . . . . . . . . . . . . . . . . . . . 9
4.6.2 Replay Detection Check . . . . . . . . . . . . . . . . . . . 10
4.6.3 Signature Check . . . . . . . . . . . . . . . . . . . . . . 10
4.7 Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 10
4.7.1 Receiving Messages from Other Relay Agents . . . . . . . . . 11
4.7.2 Sending Messages to Servers . . . . . . . . . . . . . . . . 11
4.7.3 Receiving Messages from Servers . . . . . . . . . . . . . . 11
4.8 DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . 11
4.8.1 Receiving Messages from Relay Agents . . . . . . . . . . . . 12
4.8.2 Sending Reply Messages to Relay Agents . . . . . . . . . . . 12
5. Use of IPsec to secure DHCP messages . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . 13
7.1 Authentication sub-option Protocol Considerations . . . . . 13
7.2 IPsec Considerations . . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . 16
Stapp, et al. Expires October 10, 2003 [Page 2]
Internet-Draft Authentication Suboption April 2003
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
This document uses the terms "DHCP server" (or "server") and "DHCP
client" (or "client") as defined in RFC 2131. The term "DHCP relay
agent" refers to a "BOOTP relay agent" as defined in RFC 2131.
3. Introduction
DHCP (RFC 2131 [6]) provides IP addresses and configuration
information for DHCP clients. It includes a relay agent capability
(RFC 951 [7], RFC 1542 [8]), 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 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, RFC 3046
[2]. The kind of information that a relay agent adds 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 (RFC 3118) [9] only
secures 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 is to define methods that a relay
agent can use to:
1. protect the integrity of the data that the relay adds
2. provide replay protection for that data
3. leverage existing mechanisms such as DHCP Authentication and
IPsec
The first mechanism defined to meet these goals specifies a new relay
agent suboption, the Authentication suboption. The format of this
suboption is very similar to the format of the DHCP Authentication
Stapp, et al. Expires October 10, 2003 [Page 3]
Internet-Draft Authentication Suboption April 2003
option, and the specification of the cryptographic methods and
signature computation for the suboption are also similar to that
option's specification.
The Authentication suboption is included by relay agents that 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 relay 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.
The second mechanism specifies the use of IPsec between relay agents
and servers to autenticate the identity of the source and contents of
messages carrying relay agent options.
4. Relay Agent Option Authentication Sub-option
The Relay Agent Option Authentication Sub-option, described in this
section of the document, provides identity authentication, detection
of modification of message contents and protection against message
replay.
Stapp, et al. Expires October 10, 2003 [Page 4]
Internet-Draft Authentication Suboption April 2003
4.1 Suboption Format
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 Replay Detection field contains a value used to detect replayed
messages, interpreted according to the RDM.
The Relay Identifier field is used by relay agents that do not set
giaddr, as described in RFC 3046 [2], 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.
Stapp, et al. Expires October 10, 2003 [Page 5]
Internet-Draft Authentication Suboption April 2003
4.2 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 relay agents by giaddr value or by other data in the message
(e.g. data in other relay agent suboptions). Relay agents 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 6.
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.
4.3 The Relay Identifier Field
The Relay Agent Information Option [2] specification permits a relay
agent 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 agent that adds a relay agent information option and sets
giaddr MUST NOT set the Relay ID field. A relay agent that does not
set giaddr MAY be configured to place a value in the Relay ID field.
If the relay agent 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 agent generates a Relay ID value it SHOULD select a value that
Stapp, et al. Expires October 10, 2003 [Page 6]
Internet-Draft Authentication Suboption April 2003
it can regenerate reliably, e.g. across reboots.
Servers that 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 use some other
data in the message, or it MAY increment an error counter.
4.4 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 suboptions, with the following
exceptions. The value of the 'hops' field MUST be set to zero for
the computation, because its value may be changed in transmission.
The value of the 'giaddr' field MUST also be set to sero for the
computation because it may be modified in networks where one relay
agent adds the relay agent option but another relay agent sets
'giaddr' (see RFC 3046, 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 6.
4.4.1 The HMAC-MD5 Algorithm
Algorithm 1 is assigned to the HMAC [3] protocol, using the MD5 [4]
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:
Stapp, et al. Expires October 10, 2003 [Page 7]
Internet-Draft Authentication Suboption April 2003
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. The RDM and Replay Detection fields are
as specified in Section 4.2. The Relay ID field is set as specified
in Section 4.3. 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
[10], 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.
On 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.
Stapp, et al. Expires October 10, 2003 [Page 8]
Internet-Draft Authentication Suboption April 2003
These considerations have led us to specify a fixed-length Key ID
instead of a variable-length key name.
4.5 Procedures for Sending Messages
4.5.1 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.
4.5.2 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.
4.5.3 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.
4.5.4 Sending the Message
The sender restores the values of the 'hops' and 'giaddr' fields, and
sends the message.
4.6 Procedures for Processing Incoming Messages
4.6.1 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 that do not contain a valid
Stapp, et al. Expires October 10, 2003 [Page 9]
Internet-Draft Authentication Suboption April 2003
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 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.
4.6.2 Replay Detection Check
The receiver examines the RDM field. Receivers MUST discard messages
containing RDM values that 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 4.2. If 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
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 rejecting legitimate values from the sender.
4.6.3 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.
4.7 Relay Agent Behavior
DHCP Relay agents are typically configured with the addresses of one
or more DHCP servers. A relay agent that implements this suboption
requires an algorithm number for each server, as well as appropriate
Stapp, et al. Expires October 10, 2003 [Page 10]
Internet-Draft Authentication Suboption April 2003
credentials (i.e. keys) to use. Relay implementations SHOULD
support configuration which indicates that all relayed messages
should include the authentication suboption. Use of the
authentication suboption SHOULD be disabled by default. Relay agents
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.
4.7.1 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 4.6 to
verify the source of the message before forwarding it.
4.7.2 Sending Messages to Servers
When the relay agent receives a broadcast packet from a client, it
determines which DHCP servers (or other relay agents) should receive
copies of the message. If the relay agent is configured to include
the Authentication suboption, it determines which Algorithm and RDM
to use, and then it performs the steps in Section 4.5.
4.7.3 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 agent
MAY be configured to drop response messages that do not contain the
Authentication suboption. The relay agent then follows the
procedures in Section 4.6.
4.8 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.
Stapp, et al. Expires October 10, 2003 [Page 11]
Internet-Draft Authentication Suboption April 2003
4.8.1 Receiving Messages from Relay Agents
When a DHCP server which implements the Authentication suboption
receives a message, it performs the steps in Section 4.6.
4.8.2 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 4.5.
DISCUSSION:
This server behavior represents a slight variance from RFC 3046
[2], Section 2.2. The Authentication suboption is not echoed back
from the server to the relay: the server generates its own
suboption.
5. Use of IPsec to secure DHCP messages
Relay agents and servers that exchange messages securely can use
IPsec mechanisms as described in this section. Relay agents and
servers MUST support manual configuration and installation of static
keys. If a client message is relayed through multiple relay agents,
each of the relay agents must have established independent, pairwise
trust relationships. That is, if messages from client C will be
relayed by relay agent A to relay agent B and then to the server,
relay agents A and B must be configured to use IPSec for the messages
they exchange, and relay agent B and the server must be configured to
use IPSec for the messages they exchange.
Relay agents and servers that support secure relay agent to server or
relay agent to relay agent communication, MUST include an IPsec
implementation with the following restrictions:
o The IPsec implementation MUST use ESP
o Packet authentication MUST be applied
o Encryption MAY be applied (i.e., NULL encryption can be used)
Stapp, et al. Expires October 10, 2003 [Page 12]
Internet-Draft Authentication Suboption April 2003
6. IANA Considerations
Section 4.1 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 4.4.1. Additional algorithm values will be
allocated and assigned through IETF consensus, as defined in RFC 2434
[5].
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 4.2. Additional RDM values will
be allocated and assigned through IETF consensus, as defined in RFC
2434 [5].
7. Security Considerations
This specification describes two mechanisms that can be used to
provide authentication and message integrity protection to the
messages between DHCP relay agents and DHCP servers.
The use of the authentication sub-option 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.
7.1 Authentication sub-option Protocol Considerations
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
Stapp, et al. Expires October 10, 2003 [Page 13]
Internet-Draft Authentication Suboption April 2003
do computational work which will be wasted if their messages are re-
ordered.
The authentication sub-option protocol requires configuration of
relay agents and servers with shared secret keys.
7.2 IPsec Considerations
The use of IPsec for securing relay agent options in DHCP messages
requires the existence of an IPsec implementation available to the
relay agents and DHCP servers. It also requires manual configuration
of the participants, including manual distribution of keys.
8. 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.
Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[3] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[4] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
1992.
[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.
Stapp, et al. Expires October 10, 2003 [Page 14]
Internet-Draft Authentication Suboption April 2003
[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
2845, May 2000.
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
Ralph Droms
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, MA 01824
USA
Phone: +1 978 497 4733
EMail: rdroms@cisco.com
Stapp, et al. Expires October 10, 2003 [Page 15]
Internet-Draft Authentication Suboption April 2003
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Stapp, et al. Expires October 10, 2003 [Page 16]
