Answer / jerald

Proxy ARP (RFC 1027) is a way to make a machine physically
located on one network appear to be logically part of a
different physical network connected to the same
router/firewall. Typically it allows us to hide a machine
with a public IP address on a private network behind a
router, and still have the machine appear to be on the
public network "in front of" the router. The
router "proxys" ARP requests and all network traffic to and
from the hidden machine to make this fiction possible.

Consider a router with two interface cards, one connected
to a public network PUBNET and one connected to a private
network PRIVNET. We want to hide a server machine on the
PRIVNET network but have it accessible from the PUBNET
network. The IP address of the server machine lies in the
PUBNET network, even though we are placing the machine on
the PRIVNET network behind the router.

By enabling proxy ARP on the router, any machine on the
PUBNET network that issues an ARP "who has" request for the
server's MAC address will get a proxy ARP reply from the
router containing the router's MAC address. This tells
machines on the PUBNET network that they should be sending
packets destined for the server via the router. The router
forwards the packets from the machines on the PUBNET
network to the server on the PRIVNET network.

Similarly, when the server on the PRIVNET network issues
a "who has" request for any machines on the PUBNET network,
the router provides its own MAC address via proxy ARP. This
tells the server to send packets for machines on the PUBNET
network via the router. The router forwards the packets
from the server on the PRIVNET network to the machines on
the PUBNET network.

The proxy ARP provided by the router allows the server on
the PRIVNETnetwork to appear to be on the PUBNET network.
It lets the router pass ARP requests and other network
packets in both directions between the server machine and
the PUBNET network, making the server machine appear to be
connected to the PUBNET network even though it is on the
PRIVNET network hidden behind the router.

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