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13 14 15 RFC 3118
Network Working Group R. Droms, Editor
INTERNET DRAFT Bucknell University
Obsoletes: draft-ietf-dhc-authentication-06.txt August 1998
Expires February 1999
Authentication for DHCP Messages
<draft-ietf-dhc-authentication-07.txt>
Status of this memo
This document is an Internet-Draft. 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
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Abstract
The Dynamic Host Configuration Protocol (DHCP) [1] provides a
framework for passing configuration information to hosts on a TCP/IP
network. In some situations, network administrators may wish to
constrain the allocation of addresses to authorized hosts.
Additionally, some network administrators may wish to provide for
authentication of the source and contents of DHCP messages. This
document defines a new DHCP option through which authorization
tickets can be easily generated and newly attached hosts with proper
authorization can be automatically configured from an authenticated
DHCP server.
1. Introduction
DHCP transports protocol stack configuration parameters from
centrally administered servers to TCP/IP hosts. Among those
parameters are an IP address. DHCP servers can be configured to
dynamically allocate addresses from a pool of addresses, eliminating
a manual step in configuration of TCP/IP hosts.
Some network administrators may wish to provide authentication of the
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source and contents of DHCP messages. For example, clients may be
subject to denial of service attacks through the use of bogus DHCP
servers, or may simply be misconfigured due to unintentionally
instantiated DHCP servers. Network administrators may wish to
constrain the allocation of addresses to authorized hosts to avoid
denial of service attacks in "hostile" environments where the network
medium is not physically secured, such as wireless networks or
college residence halls.
This document defines a technique that can provide both entity
authentication and message authentication.
DISCUSSION:
This draft combines the original Schiller-Huitema-Droms
authentication mechanism (<draft-ietf-dhc-authentication-06.txt>)
with the "delayed authentication" proposal developed by Bill
Arbaugh. This draft has been published as a revision to <draft-
ietf-dhc-authentication-06.txt>.
1.1 DHCP threat model
The threat to DHCP is inherently an insider threat (assuming a
properly configured network where BOOTP ports are blocked on the
enterprise's perimeter gateways.) Regardless of the gateway
configuration, however, the insider and outsider threats are the
same.
The threat specific to a DHCP client is the possibility of the
establishment of a "rogue" server with the intent of providing
incorrect configuration information to the client. The motivation for
doing so may be to establish a "man in the middle" attack or it may
be for a "denial of service" attack.
The threat specific to a DHCP server is an invalid client
masquerading as a valid client. The motivation for this may be for
"theft of service", or to circumvent auditing for any number of
nefarious purposes.
The threat common to both the client and the server is the resource
"denial of service" attack. These attacks typically involve the
exhaustion of valid addresses, or the exhaustion of CPU or network
bandwidth.
1.2 Design goals
These are the goals that were used in the development of the
authentication protocol, listed in order of importance:
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1. Address the threats presented in Section 1.1.
2. Avoid changing the current protocol.
3. Limit state required by the server.
4. Limit complexity (complexity breads design and implementation
errors).
1.3 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 [5].
1.4 DHCP Terminology
This document uses the following terms:
o "DHCP client"
A DHCP client or "client" is an Internet host using DHCP to obtain
configuration parameters such as a network address.
o "DHCP server"
A DHCP server of "server"is an Internet host that returns
configuration parameters to DHCP clients.
2. Format of the authentication option
The following diagram defines the format of the DHCP
authentication option:
+----------+----------+----------+
| Code | Length | Protocol |
+----------+----------+----------+-----------+---
| Authentication information ...
+----------+----------+----------+-----------+---
The code for the authentication option is TBD, and the length field
contains the length of the protocol and authentication information
fields in octets. The protocol field defines the particular
technique for authentication used in the option.
This document defines two protocols in sections 3 and 4, encoded with
protocol field values 0 and 1. Protocol field values 2-254 are
reserved for future use. Other protocols may be defined according to
the procedure described in section 5.
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3. Protocol 0
If the protocol field is 0, the authentication information field
holds a simple authentication token:
+----------+----------+----------+
| Code | n+1 | 0 |
+----------+----------+----------+-----------+------
| Authentication token (n octets) ...
+----------+----------+----------+-----------+------
The authentication token is an opaque, unencoded value known to both
the sender and receiver. The sender inserts the authentication token
in the DHCP message and the receiver matches the token from the
message to the shared token. If the authentication option is present
and the token from the message does not match the shared token, the
receiver MUST discard the message.
Protocol 0 may be used to pass a plain-text password and provides
only weak entity authentication and no message authentication. This
protocol is useful for rudimentary protection against, e.g.,
inadvertently instantiated DHCP servers.
DISCUSSION:
The intent here is to pass a constant, non-computed token such as
a plain-text password. Other types of entity authentication using
computed tokens such as Kerberos tickets or one-time passwords
will be defined as separate protocols.
4. Protocol 1
If the protocol field is 1, the message is using the "delayed
authentication" mechanism. In delayed authentication, the client
requests authentication in its DHCPDISCOVER message and the server
replies with a DHCPOFFER message that includes authentication
information information. This authentication information contains an
encrypted value generated by the source as a message authentication
code (MAC) to provide message authentication and entity
authentication.
This technique is based on the HMAC protocol [3] using the MD5 hash
{2].
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4.1 Format
The format of the authentication request in a DHCPDISCOVER message
for protocol 1 is:
+----------+----------+----------+
| Code | 1 | 1 |
+----------+----------+----------+
The format of the authentication information for protocol 1 is:
+----------+----------+----------+
| Code | n | 1 |
+----------+----------+----------+
| secret ID |
+----------+----------+----------+----------+-
| counter (8 octets) ...
+----------+----------+----------+----------+-
| MAC ...
+----------+----------+----------+----------+-
The following definitions will be used in the description of the
authentication information for protocol 1:
K - a secret value shared between the source and destination
of the message; each secret has a unique identifier
Counter - the value of a 64-bit monotonically increasing counter
HMAC-MD5 - the MAC generating function as defined by [3] and [2]
The sender computes the MAC as described in [3]. The 'secret ID'
field MUST be set to the identifier of the secret used to generate
the MAC. The 'counter' field of the authentication option MUST be
set to the value of a monotonically increasing counter and the 'MAC'
field of the authentication option MUST be set to all 0s for the
computation of the MAC. Because a DHCP relay agent may alter the
values of the 'giaddr' and 'hops' fields in the DHCP message, the
contents of those two fields MUST also be set to zero for the
computation of the message digest. Using a counter value such as the
current time of day (e.g., an NTP-format timestamp [4]) can reduce
the danger of replay attacks.
DISCUSSION:
Protocol 1 specifies the use of HMAC-MD5. Use of a different
technique, such as HMAC-SHA, will be specified as a separate
protocol.
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Protocol 1 requires a shared secret key for each client on each
DHCP server with which that client may wish to use the DHCP
protocol. Each secret key has a unique identifier that can be
used by a receiver to determine which secret was used to generate
the MAC in the DHCP message. Therefore, protocol 1 may not scale
well in an architecture in which a DHCP client may connect to
multiple administrative domains.
Note that the meaning of an authentication option can be changed
by removing the secret ID, counter and MAC, transforming an
authentication option with authentication information into a
request for authentication. Therefore, the authentication request
form of this option can only appear in a DHCPDISCOVER message.
4.2 Message validation
To validate an incoming message, the receiver checks the 'counter'
field and computes the MAC as described in [3]. If the 'counter'
field does not contain a value larger than the last value of
'counter' used by the sender, the receiver MUST discard the incoming
message. The receiver MUST set the 'MAC' field of the authentication
option to all 0s for computation of the MAC. Because a DHCP relay
agent may alter the values of the 'giaddr' and 'hops' fields in the
DHCP message, the contents of those two fields MUST also be set to
zero for the computation of the MAC. If the MAC computed by the
receiver does not match the MAC contained in the authentication
option, the receiver MUST discard the DHCP message.
4.3 Key utilization
Each DHCP client has a key, K. The client uses its key to encode any
messages it sends to the server and to authenticate and verify any
messages it receives from the server. The client's key must be
initially distributed to the client through some out-of-band
mechanism, and must be stored locally on the client for use in all
authenticated DHCP messages. Once the client has been given its key,
it may use that key for all transactions even if the client's
configuration changes; e.g., if the client is assigned a new network
address.
Each DHCP server must know the keys for all authorized clients. If
all clients use the same key, clients can perform both entity and
message authentication for all messages received from servers.
However, sharing of keys is strongly discouraged as it allows for
unauthorized clients to masquerade as authorized clients by obtaining
a copy of the shared key. Servers will be able to perform message
authentication. To authenticate the identity of individual clients,
each client must be configured with a unique key. Appendix A
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describes a technique for key management.
4.4 Client considerations
This section describes the behavior of a DHCP client using
authentication protocol 1.
4.4.1 INIT state
When in INIT state, the client uses protocol 1 as follows:
1. The client includes the authentication request option in its
DHCPDISCOVER message.
DISCUSSION:
Is the 'chaddr' field sufficient to identify the client or
should the client be required to include a 'client identifier'
option?
2. The client validates any DHCPOFFER messages that include
authentication information using the mechanism specified in
section 4.2. The client MUST discard any messages which fail to
pass validation and MAY log the validation failure. The client
selects one DHCPOFFER message as its selected configuration. If
none of the DHCPOFFER messages received by the client include
authentication information, the client MAY choose an
unauthenticated message as its selected configuration. The client
SHOULD be configurable to accept or reject unauthenticated
DHCPOFFER messages.
3. The client replies with a DHCPREQUEST message that includes
authentication information encoded with the same secret used by
the server in the selected DHCPOFFER message.
4. The client validates the DHCPACK message from the server. The
client MUST discard the DHCPACK if the message fails to pass
validation and MAY log the validation failure. The the DHCPACK
fails to pass validation, the client reverts to INIT state and
returns to step 1. The client MAY choose to remember which server
replied with a DHCPACK message that failed to pass validation and
discard subsequent messages from that server.
4.4.2 INIT-REBOOT state
When in INIT-REBOOT state, the client uses the secret it used in its
DHCPREQUEST message to obtain its current configuration to generate
authentication information for the DHCPREQUEST message. If client
receives no DHCPACK messages or none of the DHCPACK messages pass
validation, the client reverts to INIT state.
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4.4.3 RENEWING state
When in RENEWING state, the client uses the secret it used in its
initial DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. If
client receives no DHCPACK messages or none of the DHCPACK messages
pass validation, the client behaves as if it had not received a
DHCPACK message in section 4.4.5 of the DHCP specification [1].
4.4.4 REBINDING state
When in REBINDING state, the client uses the secret it used in its
initial DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. If
client receives no DHCPACK messages or none of the DHCPACK messages
pass validation, the client behaves as if it had not received a
DHCPACK message in section 4.4.5 of the DHCP specification [1].
4.5 Server considerations
This section describes the behavior of a server in response to client
messages using authentication protocol 1.
4.5.1 General considerations
Each server maintains a list of secrets and identifiers for those
secrets that it shares with clients and potential clients. This
information must be maintained in such a way that the server can:
* Identify an appropriate secret and the identifier for that secret
for use with a client that the server may not have previously
communicated with
* Retrieve the secret and identifier used by a client to which the
server has provided previous configuration information
Each server MUST save the counter from the previous authenticated
message. A server MUST discard any incoming message whose counter is
not strictly greater than the counter from the previous message to
avoid replay attacks.
DISCUSSION:
The authenticated DHCPREQUEST message from a client in INIT-REBOOT
state can only be validated by servers that used the same secret
in their DHCPOFFER messages. Other servers will discard the
DHCPREQUEST messages. Thus, only servers that used the secret
selected by the client will be able to determine that their
offered configuration information was not selected and the offered
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network address can be returned to the server's pool of available
addresses. The servers that cannot validate the DHCPREQUEST
message will eventually return their offered network addresses to
their pool of available addresses as described in section 3.1 of
the DHCP specification [1].
4.5.2 After receiving a DHCPDISCOVER message
The server selects a secret for the client and includes
authentication information generated by that secret as specified in
section 4.1. The server MUST record the secret selected for the
client and use that secret for validating subsequent messages with
the client.
4.5.3 After receiving a DHCPREQUEST message
The server uses the secret identified in the message and validates
the message as specified in section 4.2. If the message fails to
pass validation or the server does not know the secret identified by
the to log the validation failure.
If the message passes the validation procedure, the server responds
as described in the DHCP specification. The server MUST include
authentication information generated as specified in section 4.1.
5. Definition of new authentication protocols
The author of a new DHCP option will follow these steps to obtain
acceptance of the protocol as a part of the DHCP Internet Standard:
1. The author devises the new authentication protocol.
2. The author documents the new protocol as an Internet Draft.
3. The author submits the Internet Draft for review through the IETF
standards process as defined in "Internet Official Protocol
Standards" (STD 1). The new protocol will be submitted for
eventual acceptance as an Internet Standard.
4. The new protocol progresses through the IETF standards process;
the new option will be reviewed by the Dynamic Host Configuration
Working Group (if that group still exists), or as an Internet
Draft not submitted by an IETF working group.
This procedure for defining new authentication protocols will ensure
that:
* new options are reviewed for technical correctness and
appropriateness, and
* documentation for new options is complete and published.
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6. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997.
[2] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC-1321, April 1992.
[3] Krawczyk H., M. Bellare and R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication," RFC-2104, February 1997.
[4] Mills, D., "Network Time Protocol (Version 3)", RFC-1305, March
1992.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC-2219, March 1997.
7. Acknowledgments
Jeff Schiller and Christian Huitema developed this scheme during a
terminal room BOF at the Dallas IETF meeting, December 1995. The
editor transcribed the notes from that discussion, which form the
basis for this document. The editor appreciates Jeff's and
Christian's patience in reviewing this document and its earlier
drafts.
The "delayed authentication" mechanism used in section 4 is due to
William Arbaugh. The threat model and requirements in sections 1.1
and 1.2 come from Bill's negotiation protocol proposal. The attendees
of an interim meeting of the DHC WG held in June, 1998, including
Peter Ford, Kim Kinnear, Glenn Waters, Rob Stevens, Bill Arbaugh,
Baiju Patel, Carl Smith, Thomas Narten, Stewart Kwan, Munil Shah,
Olafur Gudmundsson, Robert Watson, Ralph Droms, Mike Dooley, Greg
Rabil and Arun Kapur, developed the threat model and reviewed several
alternative proposals.
Thanks also to John Wilkins, Ran Atkinson, Shawn Mamros and Thomas
Narten for reviewing earlier drafts of this document.
8. Security considerations
This document describes authentication and verification mechanisms
for DHCP.
9. Editor's address
Ralph Droms
Computer Science Department
Droms [Page 10]
DRAFT Authentication for DHCP Messages August 1998
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
EMail: droms@bucknell.edu
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Appendix A - Key Management Technique
To avoid centralized management of a list of random keys, suppose K
for each client is generated from the pair (client identifier, subnet
address), which must be unique to that client. That is, K = MD5(MK,
unique-id), where MK is a secret master key and MD5 is some encoding
function.
Without knowledge of the master key MK, an unauthorized client cannot
generate its own key K. The server can quickly validate an incoming
message from a new client by regenerating K from the client-id. For
known clients, the server can choose to recover the client's K
dynamically from the client-id in the DHCP message, or can choose to
precompute and cache all of the Ks a priori.
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