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Man Pages


Manual Reference Pages  - ARP (4)

NAME

arp - Address Resolution Protocol

CONTENTS

Synopsis
Description
MIB Variables
Diagnostics
See Also

SYNOPSIS


.Cd 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 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 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).

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 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.
prune_intvl How frequently the ARP cache is cleaned from expired entries.
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 traffic 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 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 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 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 net.link.ether.inet.log_arp_wrong_iface 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 antry in the local ARP table. This error will be only 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)
.Rs RFC826
.Re
.Rs RFC893
.Re
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December 18, 2005 ARP (4)

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