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 Ethernet interface drivers.

ARP caches Internet-Ethernet address mappings. When an in

terface
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 trans

mitted. 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 trans

mission
attempts. Further demand for this mapping causes ARP re

quest retransmissions, that are ratelimited to one packet per second. The

error is
EHOSTDOWN for a non-responding destination host, and EHOS

TUNREACH for a
non-responding router.

The ARP cache is stored in the system routing table as dy

namically-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. Nor

mally, 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 reach

able through
some other network interface, different from the one the re

quest came in
from. It may be enabled by setting the sysctl(8) MIB vari

able
net.link.ether.inet.proxyall to 1.

MIB Variables

The ARP protocol implements a number of configrable vari

ables 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 un

til it needs

to be refreshed.

maxtries Number of retransmits before host is consid

ered 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 discovered another host on the local network which responds to

mapping
requests for its own Internet address with a different Eth

ernet 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 ref

erenced 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 indi

cate 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 over

write 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)

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

Protocol.

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

Encapsulations.

BSD December 18, 2005

BSD