Network Working Group M. Stapp

Network Working Group M. Stapp
Request for Comments: 4030 Cisco Systems, Inc.
Category: Standards Track T. Lemon
Nominum, Inc.
March 2005

The Authentication Suboption for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Option

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

The Dynamic Host Configuration Protocol (DHCP) Relay Agent
Information Option (RFC 3046) conveys information between a DHCP
Relay Agent and a DHCP server. This specification defines an
authentication suboption for that option, containing a keyed hash in
its payload. The suboption supports data integrity and replay
protection for relayed DHCP messages.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Terminology . . . . . . . . . . . . . . . . . . 3
3. DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . 4
4. Suboption Format . . . . . . . . . . . . . . . . . . . . . . 4
5. Replay Detection . . . . . . . . . . . . . . . . . . . . . . 5
6. The Relay Identifier Field . . . . . . . . . . . . . . . . . 5
7. Computing Authentication Information . . . . . . . . . . . . 6
7.1. The HMAC-SHA1 Algorithm . . . . . . . . . . . . . . . 6
8. Procedures for Sending Messages . . . . . . . . . . . . . . 7
8.1. Replay Detection . . . . . . . . . . . . . . . . . . . 7
8.2. Packet Preparation . . . . . . . . . . . . . . . . . . 8
8.3. Checksum Computation . . . . . . . . . . . . . . . . . 8
8.4. Sending the Message . . . . . . . . . . . . . . . . . 8
9. Procedures for Processing Incoming Messages . . . . . . . . 8
9.1. Initial Examination . . . . . . . . . . . . . . . . . 8
9.2. Replay Detection Check . . . . . . . . . . . . . . . . 9
9.3. Testing the Checksum . . . . . . . . . . . . . . . . . 9
10. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . 9
10.1. Receiving Messages from Other Relay Agents . . . . . . 10
10.2. Sending Messages to Servers . . . . . . . . . . . . . 10
10.3. Receiving Messages from Servers . . . . . . . . . . . 10
11. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . 10
11.1. Receiving Messages from Relay Agents . . . . . . . . . 10
11.2. Sending Reply Messages to Relay Agents . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11
13. Security Considerations . . . . . . . . . . . . . . . . . . 11
13.1. The Key ID Field . . . . . . . . . . . . . . . . . . . 12
13.2. Protocol Vulnerabilities . . . . . . . . . . . . . . . 12
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
15.1. Normative References . . . . . . . . . . . . . . . . . 13
15.2. Informative References . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

DHCP (RFC 2131 [6]) provides IP addresses and configuration
information for IPv4 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 such as
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, by using the relay-agent information option (RFC 3046
[1]). The kind of information that relays add is often used in the

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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 it highlights the importance of the trust
relationship between the relay agent and the server.

The existing DHCP Authentication specification (RFC 3118) [9] only
covers communication between the DHCP client and server. Because
relay-agent information is added after the client has sent its
message, the DHCP Authentication specification explicitly excludes
relay-agent data from that authentication.

The goal of this specification is to define methods that a relay
agent can use to

1. protect the integrity of relayed DHCP messages,
2. provide replay protection for those messages, and
3. leverage existing mechanisms, such as DHCP Authentication.

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 format of the DHCP Authentication option, and the
specification of its cryptographic methods and hash computation is
also similar.

The Authentication suboption is included by relay agents that seek to
ensure the integrity of the data they include in the Relay Agent
option. These relay agents are configured with the parameters
necessary for generating cryptographic checksums of the data in the
DHCP messages that they forward to DHCP servers. A DHCP server
configured to process the Authentication suboption uses the
information in the suboption to verify the checksum in the suboption
and continues processing the relay agent information option only if
the checksum is valid. If the DHCP server sends a response, it
includes an Authentication suboption in its response message. Relay
agents test the checksums in DHCP server responses to decide whether
to forward the responses.

2. Requirements Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2].

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3. DHCP Terminology

This document uses the terms "DHCP server" (or "server") and "DHCP
client" (or "client") as defined in RFC 2131 [6]. The term "DHCP
relay agent" refers to a "BOOTP relay agent" as defined in RFC 2131.

4. 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 8. The length field includes the
lengths of the algorithm, the 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 NOT be used 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, which are interpreted according to the RDM.

The Relay Identifier field is used by relay agents that do not set
giaddr, as described in RFC 3046 [1], section 2.1.

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The Authentication Information field contains the data required to
communicate algorithm-specific parameters, as well as the checksum.
The checksum is usually a digest of the data in the DHCP packet
computed by using the method specified by the Algorithm field.

5. Replay Detection

The replay-detection mechanism is designed on the notion that a
receiver can determine whether 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 or equal to 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, and the checksum are valid, the receiver
updates 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 12.

Receivers SHOULD perform the replay-detection check before testing
the checksum. The keyed 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.

6. The Relay Identifier Field

The Relay Agent Information Option [1] 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

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value based on some other data, such as 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 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 sender. If giaddr is not set, the server SHOULD use the
Relay ID field if it is nonzero. 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.

7. Computing Authentication Information

The Authentication Information field contains a keyed hash 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 a
hash 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 zero 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 is itself included
in the computation, the 'authentication information' field in the
Authentication suboption is set to all zeros. 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-SHA1
algorithm. Additional algorithms may be defined in the future,
following the process described in section 12.

7.1. The HMAC-SHA1 Algorithm

Algorithm 1 is assigned to the HMAC [3] protocol by using the SHA-1
[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-SHA1 computation generates
a 20-byte hash value, which is placed in the Authentication
Information field after the Key ID.

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When Algorithm 1 is used, the format of the Authentication suboption
is as follows:

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 | 38 |0 0 0 0 0 0 0 1| MBZ | RDM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replay Detection (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replay Detection cont. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Relay Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC-SHA1 (160 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The suboption length is 38. The RDM and Replay Detection fields are
as specified in section 5. The Relay ID field is set as specified in
section 6. The Key ID is set by the sender to the ID of the key used
in computing the checksum, as an integer value in network byte order.
The HMAC result 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 have to be unique locally.
This specification does not define any semantics or impose any
requirements on this algorithm's Key ID values.

8. Procedures for Sending Messages

8.1. 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.

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8.2. Packet Preparation

The sender sets the 'giaddr' field and the 'hops' field to all zeros.
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 zeros. 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-SHA1, it sets the Key ID field to
the appropriate value. The sender sets the field that will contain
the checksum to all zeros. Other algorithms may specify additional
preparation steps.

8.3. Checksum Computation

The sender computes the checksum across the entire DHCP message,
using the algorithm it has selected. The sender places the result of
the computation into the Authentication Information field of the
Authentication suboption.

8.4. Sending the Message

The sender restores the values of the 'hops' and 'giaddr' fields and
sends the message.

9. Procedures for Processing Incoming Messages

9.1. Initial Examination

The receiver examines the message for 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
MUST support a configuration that allows it to drop incoming messages
that 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 drop the message. Because this
situation could indicate a misconfiguration that could deny service
to clients, receivers MAY attempt to notify their administrators or
to log an error message.

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9.2. 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 by using the
procedures in the RDM and in section 5. If the Replay value is not
valid, the receiver MUST drop the message.

Note that at this point the receiver MUST NOT update its notion of
the last valid Replay Detection value for the sender. Until the
checksum has been tested, the Replay Detection field cannot be
trusted. If the receiver trusts the Replay Detection value without
testing the checksum, 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.

9.3. Testing the Checksum

The receiver prepares the packet in order to test the checksum by
setting the 'giaddr' and 'hops' fields to zero, and by setting the
Authentication Information field of the suboption to all zeros.
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. If the checksums 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.

10. 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
credentials (i.e., keys). Relay implementations SHOULD support a
configuration that 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 and that other servers do not. Relay
agents 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.

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10.1. 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 agent. 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 that implements the
Authentication suboption receives a message, it MAY use the
procedures in section 9 to verify the source of the message before
forwarding it.

10.2. 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 8.

10.3. 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 9.

11. DHCP Server Behavior

DHCP servers may interact with multiple relay agents. Server
implementations MAY support a configuration that associates the same
algorithm and key with all relay agents. Servers MAY support a
configuration that specifies the algorithm and key to use with each
relay agent individually.

11.1. Receiving Messages from Relay Agents

When a DHCP server that implements the Authentication suboption
receives a message, it performs the steps in section 9.

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11.2. 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 8.

DISCUSSION:
This server behavior represents a slight variance from RFC 3046
[1], section 2.2. The Authentication suboption is not echoed
back from the server to the relay; the server generates its own
suboption.

12. IANA Considerations

Section 4 defines a new suboption for the DHCP relay agent option
called the Authentication Suboption. IANA has allocated 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 have been created and will be maintained
by IANA.

The Algorithm identifier is a one-byte value. The Algorithm value 0
is reserved. The Algorithm value 1 is assigned to the HMAC-SHA1
keyed hash, as defined in section 7.1. 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. The RDM value 0 is reserved.
The RDM value 1 is assigned to the use of a monotonically increasing
counter value, as defined in section 5. Additional RDM values will
be allocated and assigned through IETF consensus, as defined in RFC
2434 [5].

13. Security Considerations

This specification describes a protocol that adds 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 message exchanges. Because relay agents are

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involved when clients reboot, periods of very high reboot activity
will result in the largest number of messages that have to be
processed. During a cable MSO head-end reboot event, for example,
the time required for all clients to be served may increase.

13.1. The Key ID Field

The Authentication suboption contains a four-byte Key ID, following
the example of the DHCP Authentication RFC. Other authentication
protocols, such as 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. These
considerations have led us to specify a fixed-length Key ID instead
of a variable-length key name.

13.2. Protocol Vulnerabilities

Because DHCP is a UDP protocol, messages between relays and servers
may be delivered in an order different from that in which they were
generated. The replay-detection mechanism will cause receivers to
drop packets that are delivered 'late', leading to client retries.
The retry mechanisms that 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.

The DHC WG has developed two documents describing authentication of
DHCP relay agent options to accommodate the requirements of different
deployment scenarios: this document and "Authentication of Relay
Agent Options Using IPsec" [11]. As we note in section 11, the
Authentication suboption can be used without pairwise keys between
each relay and each DHCP server. In deployments where IPsec is
readily available and pairwise keys can be managed efficiently, the
use of IPsec as described in that document may be appropriate. If
IPsec is not available or there are multiple relay agents for which
multiple keys must be managed, the protocol described in this
document may be appropriate. As is the case whenever two
alternatives are available, local network administration can choose
whichever is more appropriate. Because the relay agents and the DHCP

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server are all in the same administrative domain, the appropriate
mechanism can be configured on all interoperating DHCP server
elements.

14. 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.

15. References

15.1. Normative References

[1] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.

[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[3] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.

[4] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.

[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

15.2. Informative References

[6] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.

[7] Croft, W. 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 3rd, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.

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[11] Droms, R., "Authentication of Relay Agent Options Using IPsec",
Work in Progress, February 2004.

Authors' Addresses

Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA

Phone: 978.936.0000
EMail: [email protected]

Ted Lemon
Nominum, Inc.
950 Charter St.
Redwood City, CA 94063
USA

EMail: [email protected]

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Full Copyright Statement

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