Ubuntu Manpage: arp — Address Resolution Protocol

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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 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 FreeBSD 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 configurable variables in net.link.ether.inet branch of the
sysctl(3) MIB.

allow_multicast Install ARP entries with the multicast bit set in the hardware address.
Installing such entries is an RFC 1812 violation, but some proprietary load
balancing techniques require routers to do so. Turned off by default.

garp_rexmit_count Retransmit gratuitous ARP (GARP) packets when an IPv4 address is added to an
interface. A GARP is always transmitted when an IPv4 address is added to an
interface. A non-zero value causes the GARP packet to be retransmitted the
stated number of times. The interval between retransmissions is doubled each
time, so the retransmission intervals are: {1, 2, 4, 8, 16, ...} (seconds).
The default value of zero means only the initial GARP is sent; no additional
GARP packets are retransmitted. The maximum value is sixteen.

The default behavior of a single GARP packet is usually sufficient. However, a
single GARP might be dropped or lost in some circumstances. This is
particularly harmful when a shared address is passed between cluster nodes.
Neighbors on the network link might then work with a stale ARP cache and send
packets destined for that address to the node that previously owned the
address, which might not respond.

log_arp_movements Log movements of IP addresses from one hardware address to another. See
“DIAGNOSTICS” below. Turned on by default.

log_arp_permanent_modify Log attempts by a remote host to modify a permanent ARP entry. See
“DIAGNOSTICS” below. Turned on by default.

log_arp_wrong_iface Log attempts to insert an ARP entry on an interface when the IP network to
which the address belongs is connected to another interface. See “DIAGNOSTICS”
below. Turned on by default.

max_log_per_second Limit the number of remotely triggered logging events to a configured value per
second. Default is 1 log message per second.

max_age How long an ARP entry is held in the cache until it needs to be refreshed.
Default is 1200 seconds.

maxhold How many packets to hold in the per-entry output queue while the entry is being
resolved. Default is one packet.

maxtries Number of retransmits before a host is considered down and an error is
returned. Default is 5 tries.

proxyall Enables ARP proxying. Turned off by default.

wait Lifetime of an incomplete ARP entry. Default is 20 seconds.

DIAGNOSTICS

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 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 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.

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 net.link.ether.inet.allow_multicast to
a positive value.

NAME

SYNOPSIS

DESCRIPTION

MIB Variables

DIAGNOSTICS

SEE ALSO