draft-ietf-dhc-authentication-08.txt   draft-ietf-dhc-authentication-09.txt 
Network Working Group R. Droms, Editor Network Working Group R. Droms, Editor
INTERNET DRAFT Bucknell University INTERNET DRAFT Bucknell University
Obsoletes: draft-ietf-dhc-authentication-07.txt W. Arbaugh, Editor Obsoletes: draft-ietf-dhc-authentication-08.txt W. Arbaugh, Editor
University of Pennsylvania University of Pennsylvania
August 1998 November 1998
Expires February 1999 Expires May 1999
Authentication for DHCP Messages Authentication for DHCP Messages
<draft-ietf-dhc-authentication-08.txt> <draft-ietf-dhc-authentication-09.txt>
Status of this memo Status of this memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
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Abstract Abstract
The Dynamic Host Configuration Protocol (DHCP) [1] provides a The Dynamic Host Configuration Protocol (DHCP) provides a framework
framework for passing configuration information to hosts on a TCP/IP for passing configuration information to hosts on a TCP/IP network.
network. In some situations, network administrators may wish to In some situations, network administrators may wish to constrain the
constrain the allocation of addresses to authorized hosts. allocation of addresses to authorized hosts. Additionally, some
Additionally, some network administrators may wish to provide for network administrators may wish to provide for authentication of the
authentication of the source and contents of DHCP messages. This source and contents of DHCP messages. This document defines a new
document defines a new DHCP option through which authorization DHCP option through which authorization tickets can be easily
tickets can be easily generated and newly attached hosts with proper generated and newly attached hosts with proper authorization can be
authorization can be automatically configured from an authenticated automatically configured from an authenticated DHCP server.
DHCP server.
1. Introduction 1. Introduction
DHCP transports protocol stack configuration parameters from DHCP [1] transports protocol stack configuration parameters from
centrally administered servers to TCP/IP hosts. Among those centrally administered servers to TCP/IP hosts. Among those
parameters are an IP address. DHCP servers can be configured to parameters are an IP address. DHCP servers can be configured to
dynamically allocate addresses from a pool of addresses, eliminating dynamically allocate addresses from a pool of addresses, eliminating
a manual step in configuration of TCP/IP hosts.
DRAFT Authentication for DHCP Messages August 1998 DRAFT Authentication for DHCP Messages August 1998
a manual step in configuration of TCP/IP hosts.
Some network administrators may wish to provide authentication of the Some network administrators may wish to provide authentication of the
source and contents of DHCP messages. For example, clients may be source and contents of DHCP messages. For example, clients may be
subject to denial of service attacks through the use of bogus DHCP subject to denial of service attacks through the use of bogus DHCP
servers, or may simply be misconfigured due to unintentionally servers, or may simply be misconfigured due to unintentionally
instantiated DHCP servers. Network administrators may wish to instantiated DHCP servers. Network administrators may wish to
constrain the allocation of addresses to authorized hosts to avoid constrain the allocation of addresses to authorized hosts to avoid
denial of service attacks in "hostile" environments where the network denial of service attacks in "hostile" environments where the network
medium is not physically secured, such as wireless networks or medium is not physically secured, such as wireless networks or
college residence halls. college residence halls.
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enterprise's perimeter gateways.) Regardless of the gateway enterprise's perimeter gateways.) Regardless of the gateway
configuration, however, the insider and outsider threats are the configuration, however, the insider and outsider threats are the
same. same.
The threat specific to a DHCP client is the possibility of the The threat specific to a DHCP client is the possibility of the
establishment of a "rogue" server with the intent of providing establishment of a "rogue" server with the intent of providing
incorrect configuration information to the client. The motivation for incorrect configuration information to the client. The motivation for
doing so may be to establish a "man in the middle" attack or it may doing so may be to establish a "man in the middle" attack or it may
be for a "denial of service" attack. be for a "denial of service" attack.
There is another threat to DHCP clients from mistakenly or
accidentally configured DHCP servers that answer DHCP client requests
with unintentionally incorrect configuration parameters.
The threat specific to a DHCP server is an invalid client The threat specific to a DHCP server is an invalid client
masquerading as a valid client. The motivation for this may be for masquerading as a valid client. The motivation for this may be for
"theft of service", or to circumvent auditing for any number of "theft of service", or to circumvent auditing for any number of
nefarious purposes. nefarious purposes.
The threat common to both the client and the server is the resource The threat common to both the client and the server is the resource
"denial of service" attack. These attacks typically involve the "denial of service" attack. These attacks typically involve the
DRAFT Authentication for DHCP Messages August 1998
exhaustion of valid addresses, or the exhaustion of CPU or network exhaustion of valid addresses, or the exhaustion of CPU or network
bandwidth. bandwidth.
1.2 Design goals 1.2 Design goals
These are the goals that were used in the development of the These are the goals that were used in the development of the
DRAFT Authentication for DHCP Messages August 1998
authentication protocol, listed in order of importance: authentication protocol, listed in order of importance:
1. Address the threats presented in Section 1.1. 1. Address the threats presented in Section 1.1.
2. Avoid changing the current protocol. 2. Avoid changing the current protocol.
3. Limit state required by the server. 3. Limit state required by the server.
4. Limit complexity (complexity breads design and implementation 4. Limit complexity (complexity breads design and implementation
errors). errors).
1.3 Requirements Terminology 1.3 Requirements Terminology
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o "DHCP server" o "DHCP server"
A DHCP server of "server"is an Internet host that returns A DHCP server of "server"is an Internet host that returns
configuration parameters to DHCP clients. configuration parameters to DHCP clients.
2. Format of the authentication option 2. Format of the authentication option
The following diagram defines the format of the DHCP The following diagram defines the format of the DHCP
authentication option: authentication option:
+----------+----------+----------+ DRAFT Authentication for DHCP Messages August 1998
| Code | Length | Protocol |
+----------+----------+----------+-----------+
| Code | Length | Protocol | Algorithm |
+----------+----------+----------+-----------+--- +----------+----------+----------+-----------+---
| Authentication information ... | Authentication information ...
+----------+----------+----------+-----------+--- +----------+----------+----------+-----------+---
The code for the authentication option is TBD, and the length field The code for the authentication option is TBD, and the length field
contains the length of the protocol and authentication information contains the length of the protocol, algorithm and authentication
fields in octets. The protocol field defines the particular information fields in octets. The protocol field defines the
technique for authentication used in the option. particular technique for authentication used in the option. The
algorithm field defines the specific algorithm with the technique
identified by the protocol field.
This document defines two protocols in sections 3 and 4, encoded with This document defines two protocols in sections 3 and 4, encoded with
protocol field values 0 and 1. Protocol field values 2-254 are protocol field values 0 and 1. Protocol field values 2-254 are
DRAFT Authentication for DHCP Messages August 1998
reserved for future use. Other protocols may be defined according to reserved for future use. Other protocols may be defined according to
the procedure described in section 5. the procedure described in section 5.
3. Protocol 0 3. Protocol 0
If the protocol field is 0, the authentication information field If the protocol field is 0, the authentication information field
holds a simple authentication token: holds a simple authentication token:
+----------+----------+----------+ +----------+----------+----------+----------+
| Code | n+1 | 0 | | Code | n+1 | 0 | 0 |
+----------+----------+----------+-----------+------ +----------+----------+----------+----------+------
| Authentication token (n octets) ... | Authentication token (n octets) ...
+----------+----------+----------+-----------+------ +----------+----------+----------+----------+------
The authentication token is an opaque, unencoded value known to both The authentication token is an opaque, unencoded value known to both
the sender and receiver. The sender inserts the authentication token the sender and receiver. The sender inserts the authentication token
in the DHCP message and the receiver matches the token from the in the DHCP message and the receiver matches the token from the
message to the shared token. If the authentication option is present message to the shared token. If the authentication option is present
and the token from the message does not match the shared token, the and the token from the message does not match the shared token, the
receiver MUST discard the message. receiver MUST discard the message.
Protocol 0 may be used to pass a plain-text password and provides Protocol 0 may be used to pass a plain-text password and provides
only weak entity authentication and no message authentication. This only weak entity authentication and no message authentication. This
protocol is useful for rudimentary protection against, e.g., protocol is useful for rudimentary protection against, e.g.,
inadvertently instantiated DHCP servers. inadvertently instantiated DHCP servers.
DISCUSSION: DISCUSSION:
The intent here is to pass a constant, non-computed token such as The intent here is to pass a constant, non-computed token such as
a plain-text password. Other types of entity authentication using a plain-text password. Other types of entity authentication using
DRAFT Authentication for DHCP Messages August 1998
computed tokens such as Kerberos tickets or one-time passwords computed tokens such as Kerberos tickets or one-time passwords
will be defined as separate protocols. will be defined as separate protocols.
4. Protocol 1 4. Protocol 1
If the protocol field is 1, the message is using the "delayed If the protocol field is 1, the message is using the "delayed
authentication" mechanism. In delayed authentication, the client authentication" mechanism. In delayed authentication, the client
requests authentication in its DHCPDISCOVER message and the server requests authentication in its DHCPDISCOVER message and the server
replies with a DHCPOFFER message that includes authentication replies with a DHCPOFFER message that includes authentication
information information. This authentication information contains an information information. This authentication information contains an
encrypted value generated by the source as a message authentication encrypted value generated by the source as a message authentication
code (MAC) to provide message authentication and entity code (MAC) to provide message authentication and entity
authentication. authentication.
This technique is based on the HMAC protocol [3] using the MD5 hash This document defines the use of a particular technique based on the
HMAC protocol [3] using the MD5 hash [2].
DRAFT Authentication for DHCP Messages August 1998
{2].
4.1 Format 4.1 Format
The format of the authentication request in a DHCPDISCOVER message The format of the authentication request in a DHCPDISCOVER message
for protocol 1 is: for protocol 1 is:
+----------+----------+----------+ +----------+----------+----------+----------+
| Code | 1 | 1 | | Code | 2 | 1 | Algorithm|
+----------+----------+----------+ +----------+----------+----------+----------+
The format of the authentication information for protocol 1 is: The format of the authentication information for protocol 1 is:
+----------+----------+----------+ +----------+----------+----------+----------+
| Code | n | 1 | | Code | n | 1 | Algorithm|
+----------+----------+----------+ +----------+----------+----------+----------+
| secret ID | | secret ID |
+----------+----------+----------+----------+- +----------+----------+----------+----------+-
| counter (8 octets) ... | counter (8 octets) ...
+----------+----------+----------+----------+- +----------+----------+----------+----------+-
| MAC ... | MAC ...
+----------+----------+----------+----------+- +----------+----------+----------+----------+-
The following definitions will be used in the description of the This document defines one technique for use with protocol 1, which is
authentication information for protocol 1: identified by setting the algorithm field to 1. Other techniques
that use different algorithms may be defined by future
specifications. The following definitions will be used in the
description of the authentication information for protocol 1,
algorithm 1:
DRAFT Authentication for DHCP Messages August 1998
K - a secret value shared between the source and destination K - a secret value shared between the source and destination
of the message; each secret has a unique identifier of the message; each secret has a unique identifier
Counter - the value of a 64-bit monotonically increasing counter Counter - the value of a 64-bit monotonically increasing counter
HMAC-MD5 - the MAC generating function as defined by [3] and [2] HMAC-MD5 - the MAC generating function [3, 2].
The sender computes the MAC as described in [3]. The 'secret ID' The sender computes the MAC using the HMAC generation algorithm [3]
field MUST be set to the identifier of the secret used to generate and the MD5 hash function [2]. The entire DHCP message (except as
the MAC. The 'counter' field of the authentication option MUST be noted below), including the DHCP message header and the options
set to the value of a monotonically increasing counter and the 'MAC' field, is used as input to the HMAC-MD5 computation function. The
field of the authentication option MUST be set to all 0s for the 'secret ID' field MUST be set to the identifier of the secret used to
computation of the MAC. Because a DHCP relay agent may alter the generate the MAC. The 'counter' field of the authentication option
values of the 'giaddr' and 'hops' fields in the DHCP message, the 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 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 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 current time of day (e.g., an NTP-format timestamp [4]) can reduce
the danger of replay attacks. the danger of replay attacks.
DISCUSSION: DISCUSSION:
Protocol 1 specifies the use of HMAC-MD5. Use of a different Algorithm 1 specifies the use of HMAC-MD5. Use of a different
DRAFT Authentication for DHCP Messages August 1998
technique, such as HMAC-SHA, will be specified as a separate technique, such as HMAC-SHA, will be specified as a separate
protocol. protocol.
Protocol 1 requires a shared secret key for each client on each Protocol 1 requires a shared secret key for each client on each
DHCP server with which that client may wish to use the DHCP DHCP server with which that client may wish to use the DHCP
protocol. Each secret key has a unique identifier that can be protocol. Each secret key has a unique identifier that can be
used by a receiver to determine which secret was used to generate used by a receiver to determine which secret was used to generate
the MAC in the DHCP message. Therefore, protocol 1 may not scale the MAC in the DHCP message. Therefore, protocol 1 may not scale
well in an architecture in which a DHCP client may connect to well in an architecture in which a DHCP client may connect to
multiple administrative domains. multiple administrative domains.
Note that the meaning of an authentication option can be changed Note that the meaning of an authentication option can be changed
by removing the secret ID, counter and MAC, transforming an by removing the secret ID, counter and MAC, transforming an
authentication option with authentication information into a authentication option with authentication information into a
request for authentication. Therefore, the authentication request request for authentication. Therefore, the authentication request
form of this option can only appear in a DHCPDISCOVER message. form of this option can only appear in a DHCPDISCOVER message.
The secret ID has been increased to 32 bits.
4.2 Message validation 4.2 Message validation
To validate an incoming message, the receiver checks the 'counter' To validate an incoming message, the receiver checks the 'counter'
field and computes the MAC as described in [3]. If 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 field does not contain a value larger than the last value of
'counter' used by the sender, the receiver MUST discard the incoming 'counter' used by the sender, the receiver MUST discard the incoming
DRAFT Authentication for DHCP Messages August 1998
message. The receiver MUST set the 'MAC' field of the authentication message. The receiver MUST set the 'MAC' field of the authentication
option to all 0s for computation of the MAC. Because a DHCP relay 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 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 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 zero for the computation of the MAC. If the MAC computed by the
receiver does not match the MAC contained in the authentication receiver does not match the MAC contained in the authentication
option, the receiver MUST discard the DHCP message. option, the receiver MUST discard the DHCP message.
4.3 Key utilization 4.3 Key utilization
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authenticated DHCP messages. Once the client has been given its key, authenticated DHCP messages. Once the client has been given its key,
it may use that key for all transactions even if the client's 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 configuration changes; e.g., if the client is assigned a new network
address. address.
Each DHCP server must know the keys for all authorized clients. If Each DHCP server must know the keys for all authorized clients. If
all clients use the same key, clients can perform both entity and all clients use the same key, clients can perform both entity and
message authentication for all messages received from servers. message authentication for all messages received from servers.
However, sharing of keys is strongly discouraged as it allows for However, sharing of keys is strongly discouraged as it allows for
unauthorized clients to masquerade as authorized clients by obtaining unauthorized clients to masquerade as authorized clients by obtaining
DRAFT Authentication for DHCP Messages August 1998
a copy of the shared key. Servers will be able to perform message a copy of the shared key. Servers will be able to perform message
authentication. To authenticate the identity of individual clients, authentication. To authenticate the identity of individual clients,
each client must be configured with a unique key. Appendix A each client must be configured with a unique key. Appendix A
describes a technique for key management. describes a technique for key management.
4.4 Client considerations 4.4 Client considerations
This section describes the behavior of a DHCP client using This section describes the behavior of a DHCP client using
authentication protocol 1. authentication protocol 1.
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1. The client includes the authentication request option in its 1. The client includes the authentication request option in its
DHCPDISCOVER message. DHCPDISCOVER message.
DISCUSSION: DISCUSSION:
Is the 'chaddr' field sufficient to identify the client or Is the 'chaddr' field sufficient to identify the client or
should the client be required to include a 'client identifier' should the client be required to include a 'client identifier'
option? option?
2. The client validates any DHCPOFFER messages that include 2. The client validates any DHCPOFFER messages that include
DRAFT Authentication for DHCP Messages August 1998
authentication information using the mechanism specified in authentication information using the mechanism specified in
section 4.2. The client MUST discard any messages which fail to section 4.2. The client MUST discard any messages which fail to
pass validation and MAY log the validation failure. The client pass validation and MAY log the validation failure. The client
selects one DHCPOFFER message as its selected configuration. If selects one DHCPOFFER message as its selected configuration. If
none of the DHCPOFFER messages received by the client include none of the DHCPOFFER messages received by the client include
authentication information, the client MAY choose an authentication information, the client MAY choose an
unauthenticated message as its selected configuration. The client unauthenticated message as its selected configuration. The client
SHOULD be configurable to accept or reject unauthenticated SHOULD be configurable to accept or reject unauthenticated
DHCPOFFER messages. DHCPOFFER messages.
3. The client replies with a DHCPREQUEST message that includes 3. The client replies with a DHCPREQUEST message that includes
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fails to pass validation, the client reverts to INIT state and fails to pass validation, the client reverts to INIT state and
returns to step 1. The client MAY choose to remember which server returns to step 1. The client MAY choose to remember which server
replied with a DHCPACK message that failed to pass validation and replied with a DHCPACK message that failed to pass validation and
discard subsequent messages from that server. discard subsequent messages from that server.
4.4.2 INIT-REBOOT state 4.4.2 INIT-REBOOT state
When in INIT-REBOOT state, the client uses the secret it used in its When in INIT-REBOOT state, the client uses the secret it used in its
DHCPREQUEST message to obtain its current configuration to generate DHCPREQUEST message to obtain its current configuration to generate
authentication information for the DHCPREQUEST message. If client authentication information for the DHCPREQUEST message. If client
DRAFT Authentication for DHCP Messages August 1998
receives no DHCPACK messages or none of the DHCPACK messages pass receives no DHCPACK messages or none of the DHCPACK messages pass
validation, the client reverts to INIT state. validation, the client reverts to INIT state.
4.4.3 RENEWING state 4.4.3 RENEWING state
When in RENEWING state, the client uses the secret it used in its When in RENEWING state, the client uses the secret it used in its
initial DHCPREQUEST message to obtain its current configuration to initial DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. If generate authentication information for the DHCPREQUEST message. If
client receives no DHCPACK messages or none of the DHCPACK messages client receives no DHCPACK messages or none of the DHCPACK messages
pass validation, the client behaves as if it had not received a pass validation, the client behaves as if it had not received a
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When in REBINDING state, the client uses the secret it used in its When in REBINDING state, the client uses the secret it used in its
initial DHCPREQUEST message to obtain its current configuration to initial DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. If generate authentication information for the DHCPREQUEST message. If
client receives no DHCPACK messages or none of the DHCPACK messages client receives no DHCPACK messages or none of the DHCPACK messages
pass validation, the client behaves as if it had not received a pass validation, the client behaves as if it had not received a
DHCPACK message in section 4.4.5 of the DHCP specification [1]. DHCPACK message in section 4.4.5 of the DHCP specification [1].
4.5 Server considerations 4.5 Server considerations
DRAFT Authentication for DHCP Messages August 1998
This section describes the behavior of a server in response to client This section describes the behavior of a server in response to client
messages using authentication protocol 1. messages using authentication protocol 1.
4.5.1 General considerations 4.5.1 General considerations
Each server maintains a list of secrets and identifiers for those Each server maintains a list of secrets and identifiers for those
secrets that it shares with clients and potential clients. This secrets that it shares with clients and potential clients. This
information must be maintained in such a way that the server can: information must be maintained in such a way that the server can:
* Identify an appropriate secret and the identifier for that secret * Identify an appropriate secret and the identifier for that secret
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Each server MUST save the counter from the previous authenticated Each server MUST save the counter from the previous authenticated
message. A server MUST discard any incoming message whose counter is message. A server MUST discard any incoming message whose counter is
not strictly greater than the counter from the previous message to not strictly greater than the counter from the previous message to
avoid replay attacks. avoid replay attacks.
DISCUSSION: DISCUSSION:
The authenticated DHCPREQUEST message from a client in INIT-REBOOT The authenticated DHCPREQUEST message from a client in INIT-REBOOT
state can only be validated by servers that used the same secret state can only be validated by servers that used the same secret
in their DHCPOFFER messages. Other servers will discard the in their DHCPOFFER messages. Other servers will discard the
DRAFT Authentication for DHCP Messages August 1998
DHCPREQUEST messages. Thus, only servers that used the secret DHCPREQUEST messages. Thus, only servers that used the secret
selected by the client will be able to determine that their selected by the client will be able to determine that their
offered configuration information was not selected and the offered offered configuration information was not selected and the offered
network address can be returned to the server's pool of available network address can be returned to the server's pool of available
addresses. The servers that cannot validate the DHCPREQUEST addresses. The servers that cannot validate the DHCPREQUEST
message will eventually return their offered network addresses to message will eventually return their offered network addresses to
their pool of available addresses as described in section 3.1 of their pool of available addresses as described in section 3.1 of
the DHCP specification [1]. the DHCP specification [1].
4.5.2 After receiving a DHCPDISCOVER message 4.5.2 After receiving a DHCPDISCOVER message
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client and use that secret for validating subsequent messages with client and use that secret for validating subsequent messages with
the client. the client.
4.5.3 After receiving a DHCPREQUEST message 4.5.3 After receiving a DHCPREQUEST message
The server uses the secret identified in the message and validates The server uses the secret identified in the message and validates
the message as specified in section 4.2. If the message fails to 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 pass validation or the server does not know the secret identified by
the to log the validation failure. the to log the validation failure.
DRAFT Authentication for DHCP Messages August 1998
If the message passes the validation procedure, the server responds If the message passes the validation procedure, the server responds
as described in the DHCP specification. The server MUST include as described in the DHCP specification. The server MUST include
authentication information generated as specified in section 4.1. authentication information generated as specified in section 4.1.
5. Definition of new authentication protocols 5. IANA Considerations
The author of a new DHCP option will follow these steps to obtain 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: acceptance of the protocol as a part of the DHCP Internet Standard:
1. The author devises the new authentication protocol. 1. The author devises the new authentication protocol and/or
2. The author documents the new authentication protocol, leaving the algorithm.
option code as "To Be Determined" (TBD), as an Internet Draft. 2. The author documents the new technique as an Internet Draft. If
this is a new protocol, the protocol code is left as "To Be
Determined" (TBD); otherwise, the protocol code is the code from
the existing protocol. The algorithm code is left as "TBD".
3. The author submits the Internet Draft for review through the IETF 3. The author submits the Internet Draft for review through the IETF
standards process as defined in "Internet Official Protocol standards process as defined in "Internet Official Protocol
Standards" (STD 1). Standards" (STD 1).
4. The new protocol progresses through the IETF standards process; 4. The new protocol progresses through the IETF standards process;
the specification of the new protocol will be reviewed by the the specification of the new protocol will be reviewed by the
Dynamic Host Configuration Working Group (if that group still Dynamic Host Configuration Working Group (if that group still
exists), or as an Internet Draft not submitted by an IETF working exists), or as an Internet Draft not submitted by an IETF working
group. If the options is accepted as a Standard, the group. If the options is accepted as a Standard, the
specification for the option is published as a separate RFC. specification for the option is published as a separate RFC.
5. At the time of acceptance as an Internet Standard and publication 5. At the time of acceptance as an Internet Standard and publication
as an RFC, IANA assigns a DHCP authentication protocol number to as an RFC, IANA assigns a DHCP authentication protocol number to
the new protocol. the new protocol.
DRAFT Authentication for DHCP Messages August 1998
This procedure for defining new authentication protocols will ensure This procedure for defining new authentication protocols will ensure
that: that:
* allocation of new protocol numbers is coordinated from a single * allocation of new protocol numbers is coordinated from a single
authority, authority,
* new protocols are reviewed for technical correctness and * new protocols are reviewed for technical correctness and
appropriateness, and appropriateness, and
* documentation for new protocols is complete and published. * documentation for new protocols is complete and published.
DISCUSSION: DISCUSSION:
This procedure is patterned after the procedure for acceptance of This procedure is patterned after the procedure for acceptance of
new DHCP options. new DHCP options.
6. References 6. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
Bucknell University, March 1997. Bucknell University, March 1997.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", [2] Rivest, R., "The MD5 Message-Digest Algorithm",
DRAFT Authentication for DHCP Messages August 1998
RFC-1321, April 1992. RFC-1321, April 1992.
[3] Krawczyk H., M. Bellare and R. Canetti, "HMAC: Keyed-Hashing for [3] Krawczyk H., M. Bellare and R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication," RFC-2104, February 1997. Message Authentication," RFC-2104, February 1997.
[4] Mills, D., "Network Time Protocol (Version 3)", RFC-1305, March [4] Mills, D., "Network Time Protocol (Version 3)", RFC-1305, March
1992. 1992.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC-2219, March 1997. Levels," RFC-2219, March 1997.
skipping to change at page 11, line 4 skipping to change at page 11, line 33
basis for this document. The editor appreciates Jeff's and basis for this document. The editor appreciates Jeff's and
Christian's patience in reviewing this document and its earlier Christian's patience in reviewing this document and its earlier
drafts. drafts.
The "delayed authentication" mechanism used in section 4 is due to The "delayed authentication" mechanism used in section 4 is due to
William Arbaugh. The threat model and requirements in sections 1.1 William Arbaugh. The threat model and requirements in sections 1.1
and 1.2 come from Bill's negotiation protocol proposal. The attendees 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 of an interim meeting of the DHC WG held in June, 1998, including
Peter Ford, Kim Kinnear, Glenn Waters, Rob Stevens, Bill Arbaugh, Peter Ford, Kim Kinnear, Glenn Waters, Rob Stevens, Bill Arbaugh,
Baiju Patel, Carl Smith, Thomas Narten, Stewart Kwan, Munil Shah, Baiju Patel, Carl Smith, Thomas Narten, Stewart Kwan, Munil Shah,
DRAFT Authentication for DHCP Messages August 1998
Olafur Gudmundsson, Robert Watson, Ralph Droms, Mike Dooley, Greg Olafur Gudmundsson, Robert Watson, Ralph Droms, Mike Dooley, Greg
Rabil and Arun Kapur, developed the threat model and reviewed several Rabil and Arun Kapur, developed the threat model and reviewed several
alternative proposals. alternative proposals.
Other input from Bill Sommerfield is gratefully acknowledged.
Thanks also to John Wilkins, Ran Atkinson, Shawn Mamros and Thomas Thanks also to John Wilkins, Ran Atkinson, Shawn Mamros and Thomas
Narten for reviewing earlier drafts of this document. Narten for reviewing earlier drafts of this document.
8. Security considerations 8. Security considerations
This document describes authentication and verification mechanisms This document describes authentication and verification mechanisms
for DHCP. for DHCP.
9. Editor's address 9. Editors' addresses
Ralph Droms Ralph Droms
Computer Science Department Computer Science Department
323 Dana Engineering 323 Dana Engineering
Bucknell University Bucknell University
Lewisburg, PA 17837 Lewisburg, PA 17837
DRAFT Authentication for DHCP Messages August 1998
Phone: (717) 524-1145 Phone: (717) 524-1145
EMail: droms@bucknell.edu EMail: droms@bucknell.edu
William Arbaugh William Arbaugh
University of Pennsylvania University of Pennsylvania
Philadelphia, PA Philadelphia, PA
Phone: (410) 465-3432 Phone: (410) 465-3432
EMail: waa@dsl.cis.upenn.edu EMail: waa@dsl.cis.upenn.edu
Expiration 10. Expiration
This document will expire on February 2, 1999. This document will expire on May 31, 1999.
11. Changes from previous revision
* Changed 8 bit protocol number to 16 bit (protocol, algorithm) pair.
* Changed 16 bit secret ID to 32 bits.
* Clarified that entire DHCP message (with certain field excluded) is
used as input to the HMAC-MD5 algorithm.
* Added inadvertently instantiated DHCP servers to the threat model.
* Clarified Appendix A.
DRAFT Authentication for DHCP Messages August 1998 DRAFT Authentication for DHCP Messages August 1998
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved. Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and or assist in its implementation may be prepared, copied, published and
skipping to change at page 13, line 11 skipping to change at page 14, line 11
NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
DRAFT Authentication for DHCP Messages August 1998 DRAFT Authentication for DHCP Messages August 1998
Appendix A - Key Management Technique Appendix A - Key Management Technique
To avoid centralized management of a list of random keys, suppose K To avoid centralized management of a list of random keys, suppose K
for each client is generated from the pair (client identifier, subnet for each client is generated from the pair (client identifier, subnet
address), which must be unique to that client. That is, K = MD5(MK, address), which must be unique to that client. That is, K = MAC(MK,
unique-id), where MK is a secret master key and MD5 is some encoding unique-id), where MK is a secret master key and MAC is a keyed one-
function. way function such as HMAC-MD5.
Without knowledge of the master key MK, an unauthorized client cannot Without knowledge of the master key MK, an unauthorized client cannot
generate its own key K. The server can quickly validate an incoming 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 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 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 dynamically from the client-id in the DHCP message, or can choose to
precompute and cache all of the Ks a priori. precompute and cache all of the Ks a priori.
 End of changes. 

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