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ARP(4)			 BSD Kernel Interfaces Manual			ARP(4)

NAME
     arp — Address Resolution Protocol

SYNOPSIS
     device ether

DESCRIPTION
     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, ARCnet, or Token Ring addresses.	 It is used by all the Ether‐
     net interface drivers.

     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 allowing an error to be returned to transmission
     attempts.	Further demand for this mapping causes ARP request retransmis‐
     sions, that are ratelimited to one packet per second.  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).

     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).

     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 FreeBSD is set up on a host-by-host basis using the arp(8) util‐
     ity, by adding an entry for each host inside a given subnet for which
     proxying of ARP requests is desired.  However, the “proxy all” feature
     causes the local host to act as a proxy for all hosts reachable through
     some other network interface, different from the one the request came in
     from.  It may be enabled by setting the sysctl(8) MIB variable
     net.link.ether.inet.proxyall to 1.

MIB Variables
     The ARP protocol implements a number of configrable variables in
     net.link.ether.inet branch of the sysctl(3) MIB.

     max_age	   How long an ARP entry is held in the cache until it needs
		   to be refreshed.

     maxtries	   Number of retransmits before host is considered down and
		   error is returned.

     useloopback   If an ARP entry is added for local address, force the traf‐
		   fic to go through the loopback interface.

     proxyall	   Enables ARP proxying for all hosts on net.

DIAGNOSTICS
     arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d!	 ARP has dis‐
     covered 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 Inter‐
     net address.

     arp: link 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 on %s
     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.  This message can only be issued if the sysctl
     net.link.ether.inet.log_arp_movements is set to 1, which is the system's
     default behaviour.

     arpresolve: can't allocate llinfo for %d.%d.%d.%d	The route for the ref‐
     erenced 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.

     arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1
     Physical connections exist to the same logical IP network on both if0 and
     if1.  It can also occur if an entry already exists in the ARP cache for
     the IP address above, and the cable has been disconnected from if0, then
     reconnected to if1.  This message can only be issued if the sysctl
     net.link.ether.inet.log_arp_wrong_iface is set to 1, which is the sys‐
     tem's default behaviour.

     arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d
     on %s  ARP has received an ARP reply that attempts to overwrite a perma‐
     nent entry in the local ARP table.	 This error will only be logged if the
     sysctl net.link.ether.inet.log_arp_permanent_modify is set to 1, which is
     the system's default behaviour.

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

     Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.

     Leffler, S.J.  and Karels, M.J., "RFC893", Trailer Encapsulations.

BSD				March 28, 2007				   BSD
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