The Address Resolution Protocol (ARP) is used to dynamically
map between Protocol Addresses (such as IP addresses) and
Local Network Addresses (such as Ethernet addresses).
This implementation maps IP addresses to Ethernet,
or Token Ring addresses.
It is used by all the Ethernet interface drivers.
ARP caches Internet-Ethernet address mappings.
When an interface requests a mapping for an address not in the cache,
ARP queues the message which requires the mapping and broadcasts
a message on the associated network requesting the address mapping.
If a response is provided, the new mapping is cached and any pending
message is transmitted.
ARP will queue at most one packet while waiting for a response to a
only the most recently transmitted packet is kept.
If the target host does not respond after several requests,
the host is considered to be down allowing an error to be returned to
Further demand for this mapping causes ARP request retransmissions, that
are ratelimited to one packet per second.
The error is
for a non-responding destination host, and
for a non-responding router.
The ARP cache is stored in the system routing table as
dynamically-created host routes.
The route to a directly-attached Ethernet network is installed as a
route (one with the
causing routes to individual hosts on that network to be created on
These routes time out periodically (normally 20 minutes after validated;
entries are not validated when not in use).
ARP entries may be added, deleted or changed with the
Manually-added entries may be temporary or permanent,
and may be
in which case the system will respond to ARP requests for that host
as if it were the target of the request.
In the past,
ARP was used to negotiate the use of a trailer encapsulation.
This is no longer supported.
ARP watches passively for hosts impersonating the local host (i.e., a host
which responds to an ARP mapping request for the local hosts address).
Proxy ARP is a feature whereby the local host will respond to requests
for addresses other than itself, with its own address.
Normally, proxy ARP in
is set up on a host-by-host basis using the
utility, by adding an entry for each host inside a given subnet for
which proxying of ARP requests is desired.
feature causes the local host to act as a proxy for
hosts reachable through some other network interface,
different from the one the request came in from.
It may be enabled by setting the