draft-ietf-dhc-authentication-09.txt   draft-ietf-dhc-authentication-10.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-08.txt W. Arbaugh, Editor Obsoletes: draft-ietf-dhc-authentication-10.txt W. Arbaugh, Editor
University of Pennsylvania University of Pennsylvania
November 1998 June 1999
Expires May 1999 Expires December 1999
Authentication for DHCP Messages Authentication for DHCP Messages
<draft-ietf-dhc-authentication-09.txt> <draft-ietf-dhc-authentication-11.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 and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet- Drafts as reference time. It is inappropriate to use Internet- Drafts as reference
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Abstract Abstract
The Dynamic Host Configuration Protocol (DHCP) provides a framework The Dynamic Host Configuration Protocol (DHCP) provides a framework
for passing configuration information to hosts on a TCP/IP network. for passing configuration information to hosts on a TCP/IP network.
In some situations, network administrators may wish to constrain the In some situations, network administrators may wish to constrain the
allocation of addresses to authorized hosts. Additionally, some allocation of addresses to authorized hosts. Additionally, some
network administrators may wish to provide for authentication of the network administrators may wish to provide for authentication of the
source and contents of DHCP messages. This document defines a new source and contents of DHCP messages. This document defines a new
DHCP option through which authorization tickets can be easily DHCP option through which authorization tickets can be easily
generated and newly attached hosts with proper authorization can be generated and newly attached hosts with proper authorization can be
automatically configured from an authenticated DHCP server. automatically configured from an authenticated DHCP server.
1. Introduction 1. Introduction
DHCP [1] 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
DRAFT Authentication for DHCP Messages August 1998 DRAFT Authentication for DHCP Messages June 1999
a manual step in configuration of TCP/IP hosts. 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
skipping to change at page 2, line 35 skipping to change at page 2, line 35
authentication mechanism (<draft-ietf-dhc-authentication-06.txt>) authentication mechanism (<draft-ietf-dhc-authentication-06.txt>)
with the "delayed authentication" proposal developed by Bill with the "delayed authentication" proposal developed by Bill
Arbaugh. This draft has been published as a revision to <draft- Arbaugh. This draft has been published as a revision to <draft-
ietf-dhc-authentication-06.txt>. ietf-dhc-authentication-06.txt>.
1.1 DHCP threat model 1.1 DHCP threat model
The threat to DHCP is inherently an insider threat (assuming a The threat to DHCP is inherently an insider threat (assuming a
properly configured network where BOOTP ports are blocked on the properly configured network where BOOTP ports are blocked on the
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 potential attacks by insiders and
same. outsiders are the same.
The threat specific to a DHCP client is the possibility of the The attack 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 There is another threat to DHCP clients from mistakenly or
accidentally configured DHCP servers that answer DHCP client requests accidentally configured DHCP servers that answer DHCP client requests
with unintentionally incorrect configuration parameters. 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" (DoS) attack. These attacks typically involve the
DRAFT Authentication for DHCP Messages August 1998 DRAFT Authentication for DHCP Messages June 1999
exhaustion of valid addresses, or the exhaustion of CPU or network exhaustion of valid addresses, or the exhaustion of CPU or network
bandwidth. bandwidth, and are present anytime there is a shared resource. In
current practice, redundancy mitigates DoS attacks the best.
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
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
skipping to change at page 4, line 5 skipping to change at page 4, line 5
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 DRAFT Authentication for DHCP Messages June 1999
+----------+----------+----------+-----------+ +----------+----------+----------+-----------+
| Code | Length | Protocol | Algorithm | | Code | Length | Protocol | Algorithm |
+----------+----------+----------+-----------+--- +----------+----------+----------+-----------+-
| Global Replay Counter ...
+----------+----------+----------+-----------+-
| 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, algorithm and authentication contains the length of the protocol, algorithm and authentication
information fields in octets. The protocol field defines the information fields in octets. The protocol field defines the
particular technique for authentication used in the option. The particular technique for authentication used in the option. The
algorithm field defines the specific algorithm with the technique algorithm field defines the specific algorithm with the technique
identified by the protocol field. identified by the protocol field. The global replay counter field of
the authentication option MUST be set to the value of a monotonically
increasing counter. 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.
This document defines two protocols in sections 3 and 4, encoded with This document defines two protocols in sections 4 and 5, 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
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 6.
3. Protocol 0 3. Interaction with Relay Agents
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 be set to zero for the computation of any hash function over the
message header. Additionally, a relay agent may append the DHCP relay
agent information option 82 [7] as the last option in a message to
servers. If a server finds option 82 included in a received message,
the server MUST compute any hash function as if the option were NOT
included in the message without changing the order of options. If the
server understands option 82 and will echo the option back to the
relay agent, the server MUST not include the option in the
computation of any hash function over the message.
4. 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:
DRAFT Authentication for DHCP Messages June 1999
+----------+----------+----------+----------+ +----------+----------+----------+----------+
| Code | n+1 | 0 | 0 | | Code | n+1 | 0 | 0 |
+----------+----------+----------+----------+------ +----------+----------+----------+----------+-
| Global Replay Counter ...
+----------+----------+----------+----------+-
| 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 only useful for rudimentary protection against
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 5. 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 a
encrypted value generated by the source as a message authentication nonce value generated by the source as a message authentication code
code (MAC) to provide message authentication and entity (MAC) to provide message authentication and entity authentication.
authentication.
This document defines the use of a particular technique based on the This document defines the use of a particular technique based on the
HMAC protocol [3] using the MD5 hash [2]. HMAC protocol [3] using the MD5 hash [2].
4.1 Format DRAFT Authentication for DHCP Messages June 1999
5.1 Management Issues
This protocol does not attempt to address situations where a client
may roam from one administrative domain to another, i.e. interdomain
roaming. This protocol is focused solving the intradomain problem
where the out-of-band exchange of a shared secret is feasible.
5.2 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 | 2 | 1 | Algorithm| | Code | 2 | 1 | Algorithm|
+----------+----------+----------+----------+ +----------+----------+----------+----------+-
| Global Replay Counter ...
+----------+----------+----------+----------+-
The format of the authentication information for protocol 1 is: The format of the authentication information for protocol 1 is:
+----------+----------+----------+----------+ +----------+----------+----------+----------+
| Code | n | 1 | Algorithm| | Code | n | 1 | Algorithm|
+----------+----------+----------+----------+
| secret ID |
+----------+----------+----------+----------+- +----------+----------+----------+----------+-
| counter (8 octets) ... | Global Replay Counter ...
+----------+----------+----------+----------+-
| secret ID |
+----------+----------+----------+----------+- +----------+----------+----------+----------+-
| MAC ... | MAC ...
+----------+----------+----------+----------+- +----------+----------+----------+----------+-
This document defines one technique for use with protocol 1, which is This document defines one technique for use with protocol 1, which is
identified by setting the algorithm field to 1. Other techniques identified by setting the algorithm field to 1. Other techniques
that use different algorithms may be defined by future that use different algorithms may be defined by future
specifications. The following definitions will be used in the specifications, see section 6. The following definitions will be
description of the authentication information for protocol 1, used in the description of the authentication information for
algorithm 1: protocol 1, algorithm 1:
DRAFT Authentication for DHCP Messages August 1998
K - a secret value shared between the source and destination Global Replay Counter - the value of a 64-bit monotonically
of the message; each secret has a unique identifier increasing counter
Counter - the value of a 64-bit monotonically increasing counter K - a secret value shared between the source and
destination of the message; each secret has a
unique identifier
secret ID - the unique identifier for the secret value
used to generate the MAC for this message
HMAC-MD5 - the MAC generating function [3, 2]. HMAC-MD5 - the MAC generating function [3, 2].
DRAFT Authentication for DHCP Messages June 1999
The sender computes the MAC using the HMAC generation algorithm [3] The sender computes the MAC using the HMAC generation algorithm [3]
and the MD5 hash function [2]. The entire DHCP message (except as and the MD5 hash function [2]. The entire DHCP message (except as
noted below), including the DHCP message header and the options noted below), including the DHCP message header and the options
field, is used as input to the HMAC-MD5 computation function. The field, is used as input to the HMAC-MD5 computation function. The
'secret ID' field MUST be set to the identifier of the secret used to 'secret ID' field MUST be set to the identifier of the secret used to
generate the MAC. The 'counter' field of the authentication option generate the MAC.
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: DISCUSSION:
Algorithm 1 specifies the use of HMAC-MD5. Use of a different Algorithm 1 specifies the use of HMAC-MD5. Use of a different
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, and MAC, transforming an authentication
authentication option with authentication information into a option with authentication information into a request for
request for authentication. Therefore, the authentication request authentication. Therefore, the authentication request form of
form of this option can only appear in a DHCPDISCOVER message. this option can only appear in a DHCPDISCOVER message.
The secret ID has been increased to 32 bits.
4.2 Message validation 5.3 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 message. The receiver MUST set the 'MAC' field of the
authentication option to all 0s for computation of the MAC, and
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.
DRAFT Authentication for DHCP Messages August 1998 5.4 Key utilization
message. The receiver MUST set the 'MAC' field of the authentication Each DHCP client has a key, K. The client uses its key to encode
option to all 0s for computation of the MAC. Because a DHCP relay any messages it sends to the server and to authenticate and verify
agent may alter the values of the 'giaddr' and 'hops' fields in the any messages it receives from the server. The client's key SHOULD
DHCP message, the contents of those two fields MUST also be set to be initially distributed to the client through some out-of-band
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 DRAFT Authentication for DHCP Messages June 1999
Each DHCP client has a key, K. The client uses its key to encode any mechanism, and SHOULD be stored locally on the client for use in
messages it sends to the server and to authenticate and verify any all authenticated DHCP messages. Once the client has been given
messages it receives from the server. The client's key must be its key, it SHOULD use that key for all transactions even if the
initially distributed to the client through some out-of-band client's configuration changes; e.g., if the client is assigned a
mechanism, and must be stored locally on the client for use in all new network address.
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 Each DHCP server MUST know, or be able to obtain in a secure
all clients use the same key, clients can perform both entity and manner, the keys for all authorized clients. If all clients use
message authentication for all messages received from servers. the same key, clients can perform both entity and message
However, sharing of keys is strongly discouraged as it allows for authentication for all messages received from servers. However,
unauthorized clients to masquerade as authorized clients by obtaining the sharing of keys is strongly discouraged as it allows for
a copy of the shared key. Servers will be able to perform message unauthorized clients to masquerade as authorized clients by
authentication. To authenticate the identity of individual clients, obtaining a copy of the shared key. To authenticate the identity
each client must be configured with a unique key. Appendix A of individual clients, each client MUST be configured with a
describes a technique for key management. unique key. Appendix A describes a technique for key management.
4.4 Client considerations 5.5 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.
4.4.1 INIT state 5.5.1 INIT state
When in INIT state, the client uses protocol 1 as follows: When in INIT state, the client uses protocol 1 as follows:
1. The client includes the authentication request option in its 1. The client MUST include the authentication request option in
DHCPDISCOVER message. its DHCPDISCOVER message along with option 61 [6] to identify
itself uniquely to the server.
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
DRAFT Authentication for DHCP Messages August 1998
2. The client MUST validate any DHCPOFFER messages that include
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 5.2. The client MUST discard any messages which fail
pass validation and MAY log the validation failure. The client to pass validation and MAY log the validation failure. The
selects one DHCPOFFER message as its selected configuration. If client selects one DHCPOFFER message as its selected
none of the DHCPOFFER messages received by the client include configuration. If none of the DHCPOFFER messages received by
authentication information, the client MAY choose an the client include authentication information, the client MAY
unauthenticated message as its selected configuration. The client choose an unauthenticated message as its selected
SHOULD be configurable to accept or reject unauthenticated configuration. The client SHOULD be configurable to accept or
DHCPOFFER messages. reject unauthenticated DHCPOFFER messages.
3. The client replies with a DHCPREQUEST message that includes 3. The client replies with a DHCPREQUEST message that MUST include
authentication information encoded with the same secret used by authentication information encoded with the same secret used by
the server in the selected DHCPOFFER message. the server in the selected DHCPOFFER message.
4. The client validates the DHCPACK message from the server. The 4. The client MUST validate the DHCPACK message from the server.
client MUST discard the DHCPACK if the message fails to pass The client MUST discard the DHCPACK if the message fails to
validation and MAY log the validation failure. The the DHCPACK pass validation and MAY log the validation failure. If the
fails to pass validation, the client reverts to INIT state and DHCPACK fails to pass validation, the client MUST revert to
returns to step 1. The client MAY choose to remember which server INIT state and returns to step 1. The client MAY choose to
replied with a DHCPACK message that failed to pass validation and remember which server replied with a DHCPACK message that
discard subsequent messages from that server.
4.4.2 INIT-REBOOT state DRAFT Authentication for DHCP Messages June 1999
When in INIT-REBOOT state, the client uses the secret it used in its failed to pass validation and discard subsequent messages from
DHCPREQUEST message to obtain its current configuration to generate that server.
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.
4.4.3 RENEWING state 5.5.2 INIT-REBOOT state
When in INIT-REBOOT state, the client MUST use the secret it used
in its DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message.
The client MAY choose to accept unauthenticated DHCPACK/DHCPNAK
messages if no authenticated messages were received. The client
MUST treat the receipt (or lack thereof) of any DHCPACK/DHCPNAK
messages as specified in RFC 2131, section 3.2.
5.5.3 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.
client receives no DHCPACK messages or none of the DHCPACK messages If client receives no DHCPACK messages or none of the DHCPACK
pass validation, the client behaves as if it had not received a messages pass validation, the client behaves as if it had not
DHCPACK message in section 4.4.5 of the DHCP specification [1]. received a DHCPACK message in section 4.4.5 of the DHCP
specification [1].
4.4.4 REBINDING state 5.5.4 REBINDING state
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.
client receives no DHCPACK messages or none of the DHCPACK messages If client receives no DHCPACK messages or none of the DHCPACK
pass validation, the client behaves as if it had not received a messages pass validation, the client behaves as if it had not
DHCPACK message in section 4.4.5 of the DHCP specification [1]. received a DHCPACK message in section 4.4.5 of the DHCP
specification [1].
4.5 Server considerations 5.5.5 DHCPINFORM message
DRAFT Authentication for DHCP Messages August 1998 Since the client already has some configuration information, the
client may also have established a shared secret value, K, with a
server. Therefore, the client SHOULD use the authentication
request as in a DHCPDISCOVER message when a shared secret value
exists. The client MUST treat any received DHCPACK messages as it
does DHCPOFFER messages, see section 5.5.1.
This section describes the behavior of a server in response to client 5.6 Server considerations
messages using authentication protocol 1.
4.5.1 General considerations This section describes the behavior of a server in response to
client messages using authentication protocol 1.
5.6.1 General considerations
DRAFT Authentication for DHCP Messages June 1999
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
for use with a client that the server may not have previously secret for use with a client that the server may not have
communicated with previously communicated with
* Retrieve the secret and identifier used by a client to which the * Retrieve the secret and identifier used by a client to which the
server has provided previous configuration information server has provided previous configuration information
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
not strictly greater than the counter from the previous message to is not strictly greater than the counter from the previous message
avoid replay attacks. to avoid replay attacks.
DISCUSSION: DISCUSSION:
The authenticated DHCPREQUEST message from a client in INIT-REBOOT The authenticated DHCPREQUEST message from a client in INIT-
state can only be validated by servers that used the same secret REBOOT state can only be validated by servers that used the
in their DHCPOFFER messages. Other servers will discard the same secret in their DHCPOFFER messages. Other servers will
DHCPREQUEST messages. Thus, only servers that used the secret discard the DHCPREQUEST messages. Thus, only servers that used
selected by the client will be able to determine that their the secret selected by the client will be able to determine
offered configuration information was not selected and the offered that their offered configuration information was not selected
network address can be returned to the server's pool of available and the offered network address can be returned to the server's
addresses. The servers that cannot validate the DHCPREQUEST pool of available addresses. The servers that cannot validate
message will eventually return their offered network addresses to the DHCPREQUEST message will eventually return their offered
their pool of available addresses as described in section 3.1 of network addresses to their pool of available addresses as
the DHCP specification [1]. described in section 3.1 of the DHCP specification [1].
4.5.2 After receiving a DHCPDISCOVER message 5.6.2 After receiving a DHCPDISCOVER message
The server selects a secret for the client and includes The server selects a secret for the client and includes
authentication information generated by that secret as specified in authentication information generated by that secret as specified
section 4.1. The server MUST record the secret selected for the in section 4.1. The server MUST record the secret selected for
client and use that secret for validating subsequent messages with the client and use that secret for validating subsequent messages
the client. with the client.
4.5.3 After receiving a DHCPREQUEST message 5.6.3 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
the to log the validation failure. by the 'secret ID' field, the server MUST discard the message and
MAY choose to log the validation failure.
DRAFT Authentication for DHCP Messages August 1998 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
If the message passes the validation procedure, the server responds DRAFT Authentication for DHCP Messages June 1999
as described in the DHCP specification. The server MUST include
authentication information generated as specified in section 4.1.
5. IANA Considerations section 4.1.
5.6.4 After receiving a DHCPINFORM message
The server MAY choose to accept unauthenticated DHCPINFORM
messages, or only accept authenticated DHCPINFORM messages based
on a site policy.
When a client includes the authentication request in a DHCPINFORM
message, the server MUST respond with an authenticated DHCPACK
message. If the server does not have a shared secret value
established with the sender of the DHCPINFORM message, then the
server can either respond with an unauthenticated DHCPACK message,
or a DHCPNACK if the server does not accept unauthenticated
clients.
6. 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 and/or 1. The author devises the new authentication protocol and/or
algorithm. algorithm.
2. The author documents the new technique as an Internet Draft. If 2. The author documents the new technique as an Internet Draft.
this is a new protocol, the protocol code is left as "To Be If this is a new protocol, the protocol code is left as "To Be
Determined" (TBD); otherwise, the protocol code is the code from Determined" (TBD); otherwise, the protocol code is the code
the existing protocol. The algorithm code is left as "TBD". from the existing protocol. The algorithm code is left as
3. The author submits the Internet Draft for review through the IETF "TBD".
standards process as defined in "Internet Official Protocol 3. The author submits the Internet Draft for review through the
Standards" (STD 1). IETF standards process as defined in "Internet Official
Protocol 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
group. If the options is accepted as a Standard, the working 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
as an RFC, IANA assigns a DHCP authentication protocol number to publication as an RFC, IANA assigns a DHCP authentication
the new protocol. protocol number to the new protocol.
This procedure for defining new authentication protocols will ensure This procedure for defining new authentication protocols will
that: ensure 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
DRAFT Authentication for DHCP Messages June 1999
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
new DHCP options. of 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.
[6] Henry, M., "DHCP Option 61 UUID Type Definition,"
<draft-henry-DHCP-opt61-UUID-type-00.txt> (work in
progress, November 1998.
[7] Patrick, M., "DHCP Relay Agent Information Option,"
<draft-ietf-dhc-agent-options-05.txt> (work in progress),
November 1998.
7. Acknowledgments 7. Acknowledgments
Jeff Schiller and Christian Huitema developed this scheme during a Jeff Schiller and Christian Huitema developed this scheme during a
terminal room BOF at the Dallas IETF meeting, December 1995. The terminal room BOF at the Dallas IETF meeting, December 1995. The
editor transcribed the notes from that discussion, which form the editor transcribed the notes from that discussion, which form the
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
and 1.2 come from Bill's negotiation protocol proposal. The attendees 1.1 and 1.2 come from Bill's negotiation protocol proposal. The
of an interim meeting of the DHC WG held in June, 1998, including attendees of an interim meeting of the DHC WG held in June, 1998,
Peter Ford, Kim Kinnear, Glenn Waters, Rob Stevens, Bill Arbaugh, including Peter Ford, Kim Kinnear, Glenn Waters, Rob Stevens, Bill
Baiju Patel, Carl Smith, Thomas Narten, Stewart Kwan, Munil Shah, Arbaugh, Baiju Patel, Carl Smith, Thomas Narten, Stewart Kwan,
Olafur Gudmundsson, Robert Watson, Ralph Droms, Mike Dooley, Greg
Rabil and Arun Kapur, developed the threat model and reviewed several DRAFT Authentication for DHCP Messages June 1999
alternative proposals.
Munil Shah, Olafur Gudmundsson, Robert Watson, Ralph Droms, Mike
Dooley, Greg Rabil and Arun Kapur, developed the threat model and
reviewed several alternative proposals.
Other input from Bill Sommerfield is gratefully acknowledged. 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. Editors' addresses 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
EMail: waa@dsl.cis.upenn.edu EMail: waa@dsl.cis.upenn.edu
10. Expiration 10. Expiration
This document will expire on May 31, 1999. This document will expire on December 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 June 1999
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved. Copyright (C) The Internet Society (1999). 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
distributed, in whole or in part, without restriction of any kind, distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of developing Internet organizations, except as needed for the purpose of developing
skipping to change at page 14, line 5 skipping to change at page 15, line 5
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT
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 June 1999
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 = MAC(MK, address), which must be unique to that client. That is, K = MAC(MK,
unique-id), where MK is a secret master key and MAC is a keyed one- unique-id), where MK is a secret master key and MAC is a keyed one-
way function such as HMAC-MD5. 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
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