Request for Comments: 6704
Category: Standards Track
Swisscom Schweiz AG
Forcerenew Nonce Authentication
Dynamic Host Configuration Protocol (DHCP) FORCERENEW allows for the reconfiguration of a single host by forcing the DHCP client into a Renew state on a trigger from the DHCP server. In the Forcerenew Nonce Authentication protocol, the server sends a nonce to the client in the initial DHCP ACK that is used for subsequent validation of a FORCERENEW message. This document updates RFC 3203.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6704.
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Table of Contents
1. Introduction ....................................................2 2. Requirements Language ...........................................3 3. Message Authentication ..........................................3 3.1. Forcerenew Nonce Authentication ............................3 3.1.1. Forcerenew Nonce Protocol Capability Option .........4 3.1.2. Forcerenew Nonce Authentication Protocol ............6 3.1.3. Server Considerations for Forcerenew Nonce Authentication ......................................8 3.1.4. Client Considerations for Forcerenew Nonce Authentication ......................................9 4. IANA Considerations ............................................10 5. Security Considerations ........................................10 5.1. Protocol Vulnerabilities ..................................11 6. Acknowledgements ...............................................11 7. Normative References ...........................................11
The DHCP reconfigure extension defined in [RFC3203] 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 a Forcerenew message is always authenticated using procedures as described in [RFC3118]. Authentication for DHCP [RFC3118] is mandatory for FORCERENEW; however, as it is currently defined, [RFC3118] 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 [RFC3118] is more stringent than is required for this use case and has limited adoption of DHCP FORCERENEW. [RFC3315] 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 [RFC3118] to create a new authentication protocol for DHCPv4 FORCERENEW [RFC3203] 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 [RFC3203].
2. Requirements Language
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 [RFC2119].
3. Message Authentication
The Forcerenew message MUST be authenticated using either [RFC3118] or the proposed Forcerenew Nonce Authentication protocol.
3.1. Forcerenew Nonce Authentication
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 a Hashed Message Authentication Code (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 Authentication protocol is used (initiated by the server) only if the client and server are not using the authentication mechanism specified in [RFC3118] and the client and server have negotiated to use the Forcerenew Nonce Authentication protocol.
3.1.1. Forcerenew Nonce Protocol Capability Option
A DHCP client indicates DHCP Forcerenew Nonce Protocol capability by including a FORCERENEW_NONCE_CAPABLE (145) option in DHCP Discover and Request messages sent to the server.
A DHCP server that does not support Forcerenew Nonce Authentication protocol authentication SHOULD ignore the FORCERENEW_NONCE_CAPABLE (145) option. A DHCP server indicates DHCP Forcerenew Nonce Protocol preference by including a FORCERENEW_NONCE_CAPABLE (145) 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.
The FORCERENEW_NONCE_CAPABLE option contains code 145, length n, and a sequence of algorithms the client supports:
Code Len Algorithms +-----+-----+----+----+----+ | 145 | n | A1 | A2 | A3 | .... +-----+-----+----+----+----+
Figure 1: FORCERENEW_NONCE_CAPABLE Option
In this document, Section 3.1.2 defines the Forcerenew Nonce Authentication 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 DHCPOFFER.
Server Client Server (not selected) (selected) v v v | | | | Begins initialization | | | | | _____________/|\____________ | |/DHCPDISCOVER | DHCPDISCOVER \| | w/FORCERENEW- | w/FORCERENEW- | | NONCE-CAPABLE | NONCE-CAPABLE | | | | Determines | Determines configuration | configuration | | | |\ | /| | \__________ | _________/ | | DHCPOFFER \ | /DHCPOFFER | |w/FORCERENEW \ | /w/FORCERENEW| |NONCE-CAPABLE \| /NONCE-CAPABLE| | | | | Collects replies | | | | | Selects configuration | | | | | _____________/|\____________ | |/ DHCPREQUEST | DHCPREQUEST\ | | w/Forcerenew- | w/Forcerenew- | | Nonce-Capable | Nonce-Capable | | | | | | Commits configuration | | | | |Creates 128-bit Forcerenew Nonce | | | | | _____________/| | |/ DHCPACK | | | w/Auth-Proto= | | | Forcerenew- | | | Nonce | | | | |Client stores Forcerenew Nonce | | | | | Initialization complete | | | | . . . . . . | | | | Forcerenew | | | _____________/| | |/ DHCPFORCE | | | w/Auth-Proto= | | | Forcerenew- | | | Digest(HMAC)| | | | | Client checks HMAC digest | | using stored Forcerenew Nonce | | | | | |\____________ | | | DHCPREQUEST\ | | | w/FORCERENEW- | | | NONCE-CAPABLE | | | | | | Commits configuration | | | | |Creates 128-bit Forcerenew Nonce | | | | | _____________/| | |/ DHCPACK | | | w/Auth-Proto= | | | Forcerenew- | | | Nonce | | | | | | | | | | . . . . . . | | | | Graceful shutdown | | | | | |\ ____________ | | | DHCPRELEASE \| | | | | | Discards lease | | | v v v
Figure 2: Timeline Diagram of Messages Exchanged between DHCP Client
and Servers Using the Forcerenew Nonce Authentication Protocol
3.1.2. Forcerenew Nonce Authentication Protocol
The Forcerenew Nonce Authentication protocol makes use of both the DHCP authentication option defined in [RFC3118] reusing the option format and of the Reconfigure Key Authentication Protocol defined in [RFC3315].
The following diagram defines the format of the DHCP authentication option:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Length | Protocol | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RDM | Replay Detection (64 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Replay cont. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Replay cont. | | +-+-+-+-+-+-+-+-+ | | | | Authentication Information | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of the DHCP Authentication Option
The following fields are set in an DHCP authentication option for the Forcerenew Nonce Authentication protocol.
Code: 90 (Authentication) per [RFC3118]
contains the length of the protocol
Replay Detection: 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 as follows:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Value (128 bits) | +-+-+-+-+-+-+-+-+ | . . . . . +-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of the Authentication Information
The type of data in Value field carried in this option:
- Forcerenew nonce Value (used in ACK message)
2 HMAC-MD5 digest of the message (Forcerenew message)
Value: The message authentication code generated by applying MD5 to the DHCP message
3.1.3. Server Considerations for Forcerenew Nonce Authentication
The use of Forcerenew Nonce Authentication protocol is dependent on the client indicating its capability through the FORCERENEW_NONCE_CAPABLE (145) DHCP option in any DHCP Discover or Request messages. The DHCP Discovery or Request message from the client MUST contain the FORCERENEW_NONCE_CAPABLE (145) option if the Forcerenew Nonce Protocol is to be used by the server. The absence of the FORCERENEW_NONCE_CAPABLE (145) option indicates to the server that the Forcerenew Nonce Authentication protocol is not supported; 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 Authentication protocol by including the DHCP FORCERENEW_NONCE_CAPABLE (145) option in the DHCPOFFER. 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 (145) option in the DHCP offer may be used by clients to prefer DHCP servers that are Forcerenew Nonce Authentication protocol capable over those servers that 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 it 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 [RFC3315], using the Forcerenew Nonce for the client. The server computes the HMAC-MD5 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
3.1.4. Client Considerations for Forcerenew Nonce Authentication
A client that supports this mechanism MUST indicate Forcerenew nonce Capability by including the FORCERENEW_NONCE_CAPABLE (145) DHCP option defined in Section 3.1.1 in all DHCP Discover and Request messages. DHCP servers that support Forcerenew nonce Protocol authentication MUST include the FORCERENEW_NONCE_CAPABLE (145) 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 Authentication protocol 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 Authentication protocol, the client MUST discard the message and return to the INIT state.
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 [RFC3315], 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.
4. IANA Considerations
IANA has assigned 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:
FORCERENEW_NONCE_CAPABLE Data length: 1
Forcerenew Nonce Capable
5. Security Considerations
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 [RFC3118] or the mechanism described in this document.
The mechanism in [RFC3315] 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; 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.
5.1. Protocol Vulnerabilities
The mechanism described in this document is vulnerable to a denial- of-service (DoS) 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 DoS 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.
This contribution is based on work by Vitali Vinokour. Major sections of this document use modified text from [RFC3315]. The authors wish to thank Ted Lemon, Matthew Ryan, and Bernie Volz for their support.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [RFC3203] T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP reconfigure extension", RFC 3203, December 2001. [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
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