ARP(4) FreeBSD 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 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 net.link.ether.inet.maxhold packets
while waiting for a response to a mapping request; only the most recently
``transmitted'' packets are 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 per-interface link-level table.
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 16 packets.
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.
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.
FreeBSD 13.1-RELEASE-p6 October 7, 2016 FreeBSD 13.1-RELEASE-p6
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