ARP(4) BSD Kernel Interfaces Manual ARP(4)NAMEarp — Address Resolution Protocol
The Address Resolution Protocol (ARP) is a protocol used to dynamically
map between Internet host addresses and 10Mb/s Ethernet addresses. It is
used by all the 10Mb/s Ethernet interface drivers. It is not specific to
Internet protocols or to 10Mb/s Ethernet, but this implementation cur‐
rently supports only that combination.
ARP caches Internet-Ethernet address mappings. When an interface
requests a mapping for an address not in the cache, ARP queues the mes‐
sage which requires the mapping and broadcasts a message on the associ‐
ated 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 mapping
request; only the most recently ``transmitted'' packet is kept. If the
target host does not respond after several requests, the host is consid‐
ered to be down for a short period (normally 20 seconds), allowing an
error to be returned to transmission attempts during this interval. The
error is EHOSTDOWN for a non-responding destination host, and
EHOSTUNREACH for a non-responding router.
The ARP cache is stored in the system routing table as dynamically-cre‐
ated host routes. The route to a directly-attached Ethernet network is
installed as a “cloning” route (one with the RTF_CLONING flag set), caus‐
ing routes to individual hosts on that network to be created on demand.
These routes time out periodically (normally 20 minutes after validated;
entries are not validated when not in use). An entry for a host which is
not responding is a “reject” route (one with the RTF_REJECT flag set).
ARP entries may be added, deleted or changed with the arp(8) utility.
Manually-added entries may be temporary or permanent, and may be
“published”, 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 encapsula‐
tion. 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 host's address).
duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x.
ARP has discovered another host on the local network which responds to
mapping requests for its own Internet address with a different Ethernet
address, generally indicating that two hosts are attempting to use the
same Internet address.
SEE ALSOinet(4), route(4), arp(8), ifconfig(8), route(8)
Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.
Leffler, S.J. and Karels, M.J., "RFC893", Trailer Encapsulations.
4th Berkeley Distribution April 18, 1994 4th Berkeley Distribution