RFC 1027 (rfc1027) - Page 2 of 8

Using ARP to implement transparent subnet gateways

RFC 1027          ARP and Transparent Subnet Gateways       October 1987


    Therefore a method for hiding the existence of subnets from hosts
    was highly desirable.  Since all the local area networks supported
    ARP, an ARP-based method (commonly known as "Proxy ARP" or the "ARP
    hack") was chosen.  In this memo, whenever the term "subnet" occurs
    the "RFC-950 subnet method" is assumed.

2.  Design

2.1  Basic method

    On a network that supports ARP, when host A (the source) broadcasts
    an ARP request for the network address corresponding to the IP
    address of host B (the target), host B will recognize the IP address
    as its own and will send a point-to-point ARP reply.  Host A keeps
    the IP-to-network-address mapping found in the reply in a local
    cache and uses it for later communication with host B.

    If hosts A and B are on different physical networks, host B will not
    receive the ARP broadcast request from host A and cannot respond to
    it.  However, if the physical network of host A is connected by a
    gateway to the physical network of host B, the gateway will see the
    ARP request from host A.  Assuming that subnet numbers are made to
    correspond to physical networks, the gateway can also tell that the
    request is for a host that is on a different physical network from
    the requesting host.  The gateway can then respond for host B,
    saying that the network address for host B is that of the gateway
    itself.  Host A will see this reply, cache it, and send future IP
    packets for host B to the gateway.  The gateway will forward such
    packets to host B by the usual IP routing mechanisms.  The gateway
    is acting as an agent for host B, which is why this technique is
    called "Proxy ARP"; we will refer to this as a transparent subnet
    gateway or ARP subnet gateway.

    When host B replies to traffic from host A, the same algorithm
    happens in reverse: the gateway connected to the network of host B
    answers the request for the network address of host A, and host B
    then sends IP packets for host A to gateway.  The physical networks
    of host A and B need not be connected to the same gateway. All that
    is necessary is that the networks be reachable from the gateway.

    With this approach, all ARP subnet handling is done in the ARP
    subnet gateways.  No changes to the normal ARP protocol or routing
    need to be made to the source and target hosts.  From the host point
    of view, there are no subnets, and their physical networks are
    simply one big IP network.  If a host has an implementation of
    subnets, its network masks must be set to cover only the IP network
    number, excluding the subnet bits, for the system to work properly.




Carl-Mitchell & Quarterman