arp(4) Address Resolution Protocol


device ether


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 Ethernet interface drivers.

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 allowing an error to be returned to transmission attempts. Further demand for this mapping causes ARP request retransmissions, that are ratelimited to one packet per second. The error is Er EHOSTDOWN for a non-responding destination host, and Er 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).

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).

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 Fx is set up on a host-by-host basis using the arp(8) utility, 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 to 1.

MIB Variables

The ARP protocol implements a number of configurable variables in branch of the sysctl(3) MIB.

Should the kernel install ARP entries with multicast bit set in the hardware address. Installing such entries is RFC 1812 violation, but some prorietary load balancing techniques require routers on network to do so. Turned off by default.
Should the kernel log movements of IP addresses from one hardware address to an other. See Sx DIAGNOSTICS below. Turned on by default.
Should the kernel log attempts of remote host on network to modify a permanent ARP entry. See Sx DIAGNOSTICS below. Turned on by default.
Should the kernel log attempts to insert an ARP entry on an interface when the IP network the address belongs to is connected to an other interface. See Sx DIAGNOSTICS below. Turned on by default.
Limit number of remotely triggered logging events to a configured value per second. Default is 1 log message per second.
How long an ARP entry is held in the cache until it needs to be refreshed. Default is 1200 seconds.
How many packets hold in the per-entry output queue while the entry is being resolved. Default is one packet.
Number of retransmits before host is considered down and error is returned. Default is 5 tries.
Enables ARP proxying for all hosts on net. Turned off by default.
If an ARP entry is added for local address, force the traffic to go through the loopback interface. Turned on by default.
Lifetime of an incomplete ARP entry. Default is 20 seconds.


"arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s!"
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: 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 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 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.
"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 is set to 1, which is the system'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 permanent entry in the local ARP table. This error will only be logged if the sysctl is set to 1, which is the system's default behaviour.
"arp: %x:%x:%x:%x:%x:%x is multicast"
Kernel refused to install an entry with multicast hardware address. If you really want such addresses being installed, set the sysctl to a positive value.