NAME
arp —
Address Resolution
Protocol
SYNOPSIS
#include <netinet/if_ether.h>
DESCRIPTION
The Address Resolution Protocol (ARP) is a protocol used to dynamically map
between Internet host addresses and Ethernet addresses. It is used by all the
Ethernet interface drivers. It is not specific to Internet protocols or to
Ethernet, but this implementation currently 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 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 mapping request; 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 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.
Each ARP cache entry is stored in a network interface which a response of ARP
comes in. ARP cache entires 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
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 encapsulation. This
is no longer supported.
DIAGNOSTICS
ARP implements Address Conflict Detection. When an address is first added to the
host, it is marked tentative and ARP probes the network to discover if another
host has the address. If another host replies with the same address, then the
local address is marked duplicate and the host will not use it. Otherwise the
tentative mark is removed and the host can start using the address.
ARP will defend the host's active address when a conflicting message is
received. However, if another conflicting message for the address is found
within a 10 second period, then the address is marked duplicate and the host
will stop using it.
For some systems such as a router or a server, it is desirable never to give up
an assigned address. This can be achieved by setting the
sysctl(7) variable
net.inet.ip_dad_count
to 0.
In all of the above cases, ARP will log diagnostic messages which include the
hardware address of the conflicting host.
SEE ALSO
inet(4),
route(4),
arp(8),
ifconfig(8),
route(8)
Plummer, D., RFC
826, An Ethernet Address Resolution Protocol.
Leffler, S.J. and
Karels, M.J., RFC 893,
Trailer Encapsulations.
Cheshire, S., RFC
5227, IPv4 Address Conflict Detection.
HISTORY
Since
NetBSD 8.0, the ARP cache was not stored in the
routing table.
Address Conflict Detection was added in
NetBSD
8.0.