arp(7P)arp(7P)NAMEarp - Address Resolution Protocol
ARP is a protocol used to dynamically map between DARPA Internet and
hardware station addresses. It is used by all LAN drivers.
ARP caches Internet-to-hardware station address mappings. When an
interface requests a mapping for an address not in the cache, ARP
queues the message that requires the mapping, and broadcasts a message
on the associated network requesting the address mapping if the encap‐
sulation method has been enabled for the interface. If a response is
provided, the new mapping is cached and any pending message is trans‐
mitted. ARP queues at most one packet while waiting for a mapping
request to be responded to; only the most recently ``transmitted''
packet is kept.
To facilitate communications with systems that do not use ARP, calls
are provided to enter and delete entries in the Internet-to-hardware
station address tables.
Each call takes the same structure as an argument. sets an ARP entry,
gets an ARP entry, and deletes an ARP entry. These calls can be
applied to any socket descriptor s, but only by the super-user. The
The address family for the must be for the it must be The only flag
bits that can be written are and Fibre Channel hosts only support the
flag. causes the entry to be permanent. specifies that the ARP code
should respond to ARP requests for the indicated host coming from other
machines. This allows a host to act as an ARP server, which may be
useful in convincing an ARP-only machine to talk to a non-ARP machine.
ARP watches passively for hosts impersonating the local host (i.e., a
host that responds to an ARP mapping request for the local host's
This message printed on the console screen means that
ARP has discovered another host on the local network that responds
to mapping requests for its own Internet address.
To enable the encapsulation method, use the command (see ifconfig(1M)).
ARP was developed by the University of California, Berkeley.
SEE ALSOifconfig(1M), inet(3N), lan(7), arp(1M).
RFC826, Dave Plummer, Network Information Center, SRI.