DHCP Server Identifier Override Suboption

There are many situations where the DHCP relay is involved, and it can easily insert a relay agent option with appropriate suboptions into DHCP DISCOVER messages. Once the lease has been granted, however, future DHCP RENEWAL messages are sent directly to the DHCP Server, as specified in the Server Identifier option. This means that the relay may not see the DHCP RENEWAL messages (depending upon network topology) and thus cannot provide the same relay agent option information in the RENEWAL messages. This new DHCP relay agent suboption, Server Identifier override, allows the relay to tell the DHCP server what value to place into the Server Identifier option . Using this, the relay agent can force RENEWAL messages to come to it instead of the server. The relay may then insert the relay agent option with appropriate suboptions and relay the DHCPREQUEST to the actual server. In this fashion, the DHCP server will be provided with the same relay agent information upon renewals (such as Circuit-ID, Remote-ID, Device Class, etc.) as was provided in the initial DISCOVER message. In effect, this makes a RENEWAL into a REBINDING. This new suboption could also be used by the DHCP relay in order to allow the relay to appear as the actual DHCP server to the client. This has the advantage that the relay can more easily keep up-to-date information about leases granted, etc. In short, this new suboption allows the DHCPv4 relay to function in the same fashion as the DHCPv6 relay currently does.

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 format of the suboption is: Code Len Overriding Server Identifier Address +-----+-----+-----+-----+-----+-----+ | 11 | n | a1 | a2 | a3 | a4 | +-----+-----+-----+-----+-----+-----+ The option length (n) is 4. The octets "a1" through "a4" specify the value that MUST be inserted into the Server Identifier option by the DHCP Server upon reply. DHCP Servers that implement this Relay Suboption MUST use this value, if present, as the value to insert into the Server Identifier option whenever responding to a DHCP Client. If a DHCP Server does not understand/implement this Relay Suboption, it will ignore the Suboption, and thus it will insert it's own appropriate interface address as the Server Identifier address. In this case, the DHCP Relay will not receive RENEW DHCPREQUEST packets from the client. When configuring a DHCP Relay to use this Suboption, the administrator of the Relay should take into account whether or not the DHCP Server to which the packet will be relayed will correctly understand this Suboption. When servicing a DHCPREQUEST packet, the DHCP Server would normally look at the Server Identifier option for verification that the address specified there is one of the addresses associated with the DHCP Server, silently ignoring the DHCPREQUEST if it does not match a configured DHCP Server interface address. If the DHCPREQUEST packet contains a Server Identifier Override Suboption, however, comparison should be made between this suboption and the Server Identifier option. If both the Server Identifier Override Suboption and the Server Identifier Option specify the same address, then the Server should accept the DHCPREQUEST packet for processing, regardless of whether or not the Server Identifier Option matches a DHCP Server interface. The DHCP Relay should fill in the giaddr field when relaying the packet, just as it normally would do. In a situation where the DHCP Relay is configured to forward packets to more than one server, the DHCP Relay SHOULD forward all DHCP packets to all servers. This applies to DHCP RENEW packets as well. The intent is that the DHCP Relay should not need to maintain state information about the DHCP lease. DHCP Relays using this suboption SHOULD also implement and use the DHCPv4 Relay Agent Flags Suboption in order to specify whether the DHCP Relay received the original packet as a broadcast or unicast. The DHCP Server receiving a packet containing the Server Identifier Override Suboption may use this additional information in processing the packet. Note that if the DHCP Relay becomes inaccessible by the DHCP Client or loses network access to the DHCP Server, further DHCP RENEW packets from the DHCP Client may not be properly processed and the DHCP Client's lease may time out.

Message authentication in DHCP for intradomain use where the out-of- band exchange of a shared secret is feasible is defined in . Potential exposures to attack are discussed in Section 7 of the DHCP protocol specification in . The DHCP Relay Agent option depends on a trusted relationship between the DHCP relay agent and the server, as described in Section 5 of RFC 3046. While the introduction of fraudulent relay-agent options can be prevented by a perimeter defense that blocks these options unless the relay agent is trusted, a deeper defense using the authentication option for relay agent options SHOULD be deployed as well. If a rogue DHCP relay were inserted between the client and the server, it could redirect clients to it using this suboption. This would allow such a system to later deny RENEW DHCPREQUEST and thus force clients to discontinue use of their allocated address. It could also allow the rogue relay to change, insert, or delete DHCP options in DHCPACK messages and extend leases beyond what the server has allowed. This interception, however, would need to be done during the initial DISCOVER and OFFER phase since the suboption value SHOULD be ignored by the server during RENEWAL state. DHCP Authentication and/or DHCP Relay Agent option authentication would address this case. (Note that, as is always the case, lack of DHCP Authentication would allow a rogue DHCP relay to change the Server-ID option in the DHCPOFFER and DHCPACK packets without detection. This threat is not new to the Server-ID-Override suboption.) This document does not add any new vulnerabilities that were not already present, except in the case where DHCP authentication is already in place, and DHCP clients require its use. It is suggested that DHCP Authentication and DHCP Relay Agent Option Authentication SHOULD be deployed when this option is used, or protection should be provided against the insertion of rogue DHCP relays and server. This relay sub-option is not intended, by itself, to provide any additional security benefits.

IANA has assigned a suboption number (11) for the Server Identifier Override Suboption from the DHCP Relay Agent Information Option suboption number space.

The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights.