Forcerenew Nonce Authentication

The DHCP Reconfigure Extension defined in is a useful mechanism allowing dynamic reconfiguration of a single host triggered by the DHCP server. Its application is currently limited by a requirement that FORCERENEW message is always authenticated using procedures as described in . Authentication for DHCP is mandatory for FORCERENEW, however as it is currently defined requires distribution of constant token or shared-secret out-of-band to DHCP clients. The motivation for making authentication mandatory in DHCP FORCERENEW was to prevent an off-network attacker from taking advantage of DHCP FORCERENEW to accurately predict the timing of a DHCP renewal. Without DHCP FORCERENEW, DHCP renewal timing is under the control of the client, and an off-network attacker has no way of predicting when it will happen, since it doesn't have access to the exchange between the DHCP client and DHCP server. However, the requirement to use the DHCP authentication described in is more stringent than is required for this use case, and has limited adoption of DHCP FORCERENEW. defines an authentication protocol using a nonce to prevent off-network attackers from successfully causing clients to renew. Since the off-network attacker doesn't have access to the nonce, it can't trick the client into renewing at a time of its choosing. This document defines extensions to Authentication for DHCPv4 Messages to create a new authentication protocol for DHCPv4 FORCERENEW messages; this method does not require out-of-band key distribution to DHCP clients. The Forcerenew Nonce is exchanged between server and client on initial DHCP ACK and is used for verification of any subsequent FORCERENEW message. This document updates

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 .

The FORCERENEW message MUST be authenticated using either or the proposed Forcerenew Nonce Authentication protocol.

The Forcerenew nonce authentication protocol provides protection against misconfiguration of a client caused by a FORCERENEW message sent by a malicious DHCP server. In this protocol, a DHCP server sends a Forcerenew nonce to the client in the initial exchange of DHCP messages. The client records the Forcerenew nonce for use in authenticating subsequent Forcerenew messages from that server. The server then includes an HMAC computed from the Forcerenew nonce in subsequent FORCERENEW messages. Both the Forcerenew nonce sent from the server to the client and the HMAC in subsequent FORCERENEW messages are carried as the Authentication information in a DHCP Authentication option. The format of the Authentication information is defined in the following section. The Forcerenew nonce protocol is used (initiated by the server) only if the client and server are not using the authentication mechanism specified in and the client and server have negotiated to use the Forcerenew Nonce Authentication protocol.

A DHCP client indicates DHCP Forcerenew Nonce Protocol capability by including a FORCERENEW_NONCE_CAPABLE(<TBD>) option in DHCP Discover and Request messages sent to the server. A DHCP server that does not support Forcerenew Nonce Protocol authentication SHOULD ignore the FORCERENEW_NONCE_CAPABLE(<TBD>) option. A DHCP server indicates DHCP Forcerenew Nonce Protocol preference by including a FORCERENEW_NONCE_CAPABLE(<TBD>) option in any DHCP Offer messages sent to the client. A DHCP client MUST NOT send DHCP messages with authentication options where the protocol value is Forcerenew Nonce Authentication(<TBD>). The FORCERENEW_NONCE_CAPABLE option contains code <TDB>, length n and a sequence of algorithms the client supports:

This document, in section , defines the Forcerenew Nonce Protocol for algorithm equal to 1 and type equal to 2; future documents will specify the other values for algorithm !=1 and type !=2, allowing a different algorithm to be used with shorter/longer values. The client would indicate that it supports the functionality by inserting the FORCERENEW_NONCE_CAPABLE option in the DHCP Discover and Request messages. If the server supports Forcerenew nonce authentication and requires Forcerenew nonce authentication, it will insert the FORCERENEW_NONCE_CAPABLE option in the DHCP Offer message.

The Forcerenew Nonce Protocol makes use of both the DHCP authentication option defined in re-using the option format and of the Reconfigure Key Authentication Protocol defined in . The following diagram defines the format of the DHCP authentication option:

The following fields are set in an DHCP authentication option for the Forcerenew Nonce Authentication Protocol: code 90 length field contains the length of the protocol protocol 3 algorithm 1 Replay Detection field is per the Replay Detection Method (RDM) Replay Detection Method (RDM) 0 Authentication Information: specified below The format of the Authentication information for the Forcerenew Nonce Authentication Protocol is:

Type of data in Value field carried in this option: 1 Forcerenew nonce Value (used in ACK message) 2 HMAC-MD5 digest of the message (FORCERENEW message) Data as defined by field

The use of Forcerenew Nonce Protocol is dependent on the client indicating its capability through the FORCERENEW_NONCE_CAPABLE(<TBD>) DHCP option in any DHCP Discover or Request messages. The DHCP Discovery or Request message from the client MUST contain the FORCERENEW_NONCE_CAPABLE(<TBD>) option if the Forcerenew Nonce Protocol is to be used by the server. The absence of the FORCERENEW_NONCE_CAPABLE(<TBD>) option indicates to the server that the Forcerenew Nonce Authentication protocol is not supported and thus the server MUST NOT include a Forcerenew Nonce Protocol Authentication option in the DHCP Ack. The server indicates its support of the Forcerenew Nonce Protocol authentication by including the DHCP FORCERENEW_NONCE_CAPABLE(<TBD>) option in the DHCP Offer message. The server SHOULD NOT include this option unless the client has indicated its capability in a DHCP Discovery message . The presence of the FORCERENEW_NONCE_CAPABLE(<TDB>) option in the DHCP offer may be used by clients to prefer Forcerenew nonce Protocol authentication-capable DHCP servers over those servers which do not support such capability. If a capable server receives a DISCOVER or REQUEST (any type) that indicates the client is capable, and the server has no previous nonce recorded, it MUST generate a nonce and include it in the ACK. The server selects a Forcerenew nonce for a client only during Request/Ack message exchange. The server records the Forcerenew nonce and transmits that nonce to the client in an Authentication option in the DHCP Ack message. The server SHOULD NOT include the nonce in an ACK when responding to a renew unless a new nonce was generated. This minimizes the number of times the nonce is sent over the wire. If the server to which the DHCP Request message was sent at time T1 has not responded, the client enters the REBINDING state and attempts to contact any server. The new Server receiving the DHCP message MUST generate a new nonce. The Forcerenew nonce is 128 bits long, and MUST be a cryptographically strong random or pseudo-random number that cannot easily be predicted. The nonce is embedded as a 128-bit value of the Authentication information where type is set to 1 (Forcerenew nonce Value). To provide authentication for a Forcerenew message, the server selects a replay detection value according to the RDM selected by the server, and computes an HMAC-MD5 of the Forcerenew message, based on the procedure specified in section 21.5 of , using the Forcerenew nonce for the client. The server computes the HMAC-MD5, based on the procedure specified in section 21.5 of , over the entire DHCP Forcerenew message, including the Authentication option; the HMAC-MD5 field in the Authentication option is set to zero for the HMAC-MD5 computation. The server includes the HMAC-MD5 in the authentication information field in an Authentication option included in the Forcerenew message sent to the client with type set to 2 (HMAC-MD5 digest).

A client that supports this mechanism MUST indicate Forcerenew nonce Capability by including the FORCERENEW_NONCE_CAPABLE(<TBD>) DHCP option defined in in all DHCP Discover and Request messages. DHCP servers that support Forcerenew nonce Protocol authentication MUST include the FORCERENEW_NONCE_CAPABLE(<TBD>) DHCP option in all DHCP Offers, allowing the client to use this capability in selecting DHCP servers should multiple Offers arrive. The client MUST validate the DHCP Ack message contains a Forcerenew Nonce in a DHCP authentication option. If the server has indicated capability for Forcerenew Nonce Protocol authentication in the DHCP OFFER and the subsequent ACK received by the client while in the selecting state omits a valid DHCP authentication option for the Forcerenew Nonce Protocol, the client MUST discard the message and return to the INIT stat The client MUST record the Forcerenew Nonce from any valid ACK it receives, if the ACK contains one. To authenticate a Forcerenew message, the client computes an HMAC-MD5, based on the procedure specified in section 21.5 of , over the DHCP FORCERENEW message (after setting the HMAC-MD5 field in the Authentication option to zero), using the Forcerenew Nonce received from the server. If this computed HMAC-MD5 matches the value in the Authentication option, the client accepts the FORCERENEW message.

Comments are solicited and should be addressed to the DHC WG mailing list (dhcwg@ietf.org) and/or the authors. This contribution is based on work by Vitali Vinokour. Major sections of this draft use modified text from [RFC3315]. The authors wish to thank Ted Lemon, Matthew Ryan and Bernie Volz for their support.

This document requests IANA to assign the following new DHCPv4 option code from the registry "BOOTP Vendor Extensions and DHCP Options" maintained at http://www.iana.org/assignments/bootp-dhcp-parameters: Tag: TBD Name: FORCERENEW_NONCE_CAPABALE Data lenght: 1 Description: Forcerenew Nonce Capable Reference: this document

As in some network environments FORCERENEW can be used to snoop and spoof traffic, the FORCERENEW message MUST be authenticated using the procedures as described in or the mechanism described in this document. The mechanism in for DHCPv6, which this document mirrors for DHCPv4, uses a nonce to prevent an off-link attacker from successfully triggering a renewal on a client by sending DHCPFORCERENEW; since the attacker is off-link, it doesn't have the nonce, and can't force a renewal. An on-link attacker can always simply watch the DHCP renewal message go out and respond to it, so this mechanism is useless for preventing on-link attacks, and hence the security of the nonce in the case of on-link attacks isn't relevant. Therefore HMAC-MD5 is by definition adequate for the purpose, and there is no need for an extensible HMAC mechanism. FORCERENEW messages failing the authentication should be silently discarded by the client.

The mechanism described in this document is vulnerable to a denial of service attack through flooding a client with bogus FORCERENEW messages. The calculations involved in authenticating the bogus FORECERENEW messages may overwhelm the device on which the client is running. The mechanism described provides protection against the use of a FORCERENEW message by a malicious DHCP server to mount a denial of service or man-in-the-middle attack on a client. This protocol can be compromised by an attacker that can intercept the initial message in which the DHCP server sends the nonce to the client.