ARP(4)

NAME
arp - Address Resolution Protocol

SYNOPSIS
pseudo-device ether

DESCRIPTION
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 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.

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 “cloning” route (one with the RTF_CLONING flag set), causing 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 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 host's address).

DIAGNOSTICS
arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!: 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.

arp: ether address is broadcast for IP address %d.%d.%d.%d!: ARP requested information for a host, and received an answer indicating that the host's ethernet address is the ethernet broadcast address. This indicates a misconfigured or broken device.

arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x: ARP had a cached value for the ethernet address of the referenced host, but received a reply indicating that the host is at a new address. This can happen normally when host hardware addresses change, or when a mobile node arrives or leaves the local subnet. It can also indicate a problem with proxy ARP.

arpresolve: can't allocate llinfo for %d.%d.%d.%d: The route for the referenced host points to a device upon which ARP is required, but ARP was unable to allocate a routing table entry in which to store the host's MAC address. This usually points to a misconfigured routing table. It can also occur if the kernel cannot allocate memory.

SEE ALSO
inet(4), route(4), arp(8), ifconfig(8), route(8)

Plummer, D., An Ethernet Address Resolution Protocol, RFC826. Leffler, S.J., Karels, M.J., Trailer Encapsulations, RFC893.