Virtual Subnet Selection Option

There is a growing use of Virtual Private Network (VPN) configurations. The growth comes from many areas; individual client systems needing to appear to be on the home corporate network even when traveling, ISPs providing extranet connectivity for customer companies, etc. In some of these cases there is a need for the DHCP server to know the VPN (hereafter called a "Virtual Subnet Selector" or "VSS") from which an address, and other resources, should be allocated. If the allocation is being done through a DHCP relay, then a relay sub-option could be included. In some cases, however an IP address is being sought by a DHCP proxy on behalf of a client (which may be assigned the address via a different protocol). In this case, there is a need to include VSS information relating to the client as a DHCP option. A good example might be a dial-in aggregation device where PPP addresses are acquired via DHCP and then given to the remote customer system via IPCP . In a network where such a device is used to aggregate PPP dial-in from multiple companies, each company may be assigned a unique VSS. This memo defines a new DHCP option, the VSS Information option, which allows the DHCP client to specify the VSS Information needed in order to allocate an address. If the receiving DHCP server understands the VSS Information option, this information may be used in conjunction with other information in determining the subnet on which to select an address as well as other information such as DNS server, default router, etc.

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 . This document also uses the following terms: DHCP Client or "Client" is an Internet host using DHCP to obtain configuration parameters such as a network address. A DHCP Server or "Server" is an Internet host that returns configuration parameters to DHCP Clients. A DHCP relay agent is a third-party agent that transfers BOOTP and DHCP messages between clients and servers residing on different subnets, per and . Downstream is the direction from the access concentrator towards the subscriber. Upstream is the direction from the subscriber towards the access concentrator. Information about a VPN necessary to allocate an address to a DHCP client on that VPN and necessary to forward a DHCP reply packet to a DHCP client on that VPN. Virtual private network. A network which appears to the client to be a private network. The VPN-ID is defined by to be a sequence of 7 octets.

The VSS Information option is a DHCP option . The option contains generalized VSS information in one of two formats: NVT ASCII VPN identifier, or RFC2685 VPN-ID .

The format of the option is: Code Len Type VSS Information octets +-----+-----+------+-----+-----+-----+--- | 221 | n | t | v1 | v2 | v3 | ... +-----+-----+------+-----+-----+-----+--- Type: 0 NVT ASCII VPN identifier 1 RFC2685 VPN-ID 2-255 Not Allowed The option minimum length (n) is 2. There are two types of identifiers which can be placed in the VSS Information Option. The first type of identifier which can be placed in the VSS Information Option is an NVT ASCII string. It MUST NOT be terminated with a zero byte. The second type of identifier which can be placed in the VSS Information Option is an RFC2685 VPN-ID , which is typically 7 octets (3 of VPN OUI followed by 4 of VPN index) in length (though it can be any length as far as the VSS Information Option is concerned). If the type field is set to zero (0), it indicates that all following bytes of the option contain a NVT ASCII string. This string MUST NOT be terminated with a zero byte. If the type field is set to one (1), it indicates that all following bytes should be interpreted in agreement with RFC2685 as a VPN Identifier, typically 7 octets. All other values of the type field are invalid as of this memo and VSS options containing any other value than zero (0) or one (1) SHOULD be ignored. Since this option is placed in the packet in order to change the VPN on which an IP address is allocated for a particular DHCP client, one presumes that an allocation on that VPN is necessary for correct operation. If this presumption is correct, then a client which places this option in a packet and doesn't receive it in the returning packet should drop the packet since the IP address that was allocated will not be in the correct VPN. If an IP address that is not on the requested VPN is not required, then the client is free to accept the IP address that is not on the VPN that the was requested. Servers configured to support this option MUST return an identical copy of the option to any client that sends it, regardless of whether or not the client requests the option in a parameter request list. This option provides the DHCP server additional information upon which to make a determination of address to be assigned. The DHCP server, if it is configured to support this option, should use this information in addition to other options included in the DHCPDISCOVER packet in order to assign an IP address for DHCP client. In the event that a Virtual Subnet Selection option and a Virtual Subnet Selection sub-option are both received in a particular DHCP client packet, the information from the Virtual Subnet Selection sub-option MUST be used in preference to the information in the Virtual Subnet Selection option. This reasoning behind this approach is that the relay-agent is almost certainly more trusted than the DHCP client, and therefore information in the relay-agent-information option that conflicts with information in the packet generated by the DHCP client is more likely to be correct. Servers that do not understand this option will allocate an address using their normal algorithms and will not return this option in the DHCPOFFER or DHCPACK. In this case the client should consider discarding the DHCPOFFER or DHCPACK, as mentioned above. Servers that understand this option but are administratively configured to ignore the option MUST ignore the option, use their normal algorithms to allocate an address, and MUST NOT return this option in the DHCPOFFER or DHCPACK such that the client will know that the allocated address is not in the VPN requested and will consider this information in deciding whether or not to accept the DHCPOFFER. In other words, this option MUST NOT appear in a DHCPOFFER or DHCPACK from a server unless it was used by the server in making or updating the address allocation requested.

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 VSS Information option could be used by a client in order to obtain an IP address from a VPN other than the one where it should. Another possible defense would be for the DHCP relay to insert a Relay option containing a VSS Information Relay Sub-option, which would override the DHCP VSS Information option. This option would allow a client to perform a more complete address-pool exhaustion attack since the client would no longer be restricted to attacking address-pools on just its local subnet. Servers that implement the VSS Information option MUST by default disable use of the feature; it must specifically be enabled through configuration. Moreover, a server SHOULD provide the ability to selectively enable use of the feature under restricted conditions, e.g., by enabling use of the option only from explicitly configured client-ids, enabling its use only by clients on a particular subnet, or restricting the VSSs from which addresses may be requested. Implementations should consider using the DHCP Authentication option in order to provide a higher level of security if it is deemed necessary in their environment.

IANA is requested to assign DHCP option number 221 for this option, in accordance with . While the type byte of the Virtual Subnet Selection option defines a number space that could be managed by IANA, expansion of this number space is not anticipated and so creation of a registry of these numbers is not required by this document. In the event that additional values for the type byte are defined in subsequent documents, IANA should at that time create a registry for these type bytes. New values for the type byte may only be defined by IETF Consensus, as described in . Basically, this means that they are defined by RFCs approved by the IESG. Moreover, any changes or additions to the type byte codes MUST be made concurrently in the type byte codes of the VSS Information Option. The type bytes and data formats of the VSS Information Option and VSS Information Relay Sub-option MUST always be identical.

This document is the result of work done within Cisco Systems. Thanks to Kim Kinnear, Mark Stapp, and Jay Kumarasamy for their work on this option definition and the other related work for which this is necessary.