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.\" @(#)arp4.4 6.5 (Berkeley) 4/18/94
.\" $FreeBSD$
.\"
.Dd October 7, 2016
.Dt ARP 4
.Os
.Sh NAME
.Nm arp
.Nd Address Resolution Protocol
.Sh SYNOPSIS
.Cd "device ether"
.Sh 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.
.Pp
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.
.Pp
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
.Dq cloning
route (one with the
.Li 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).
.Pp
ARP entries may be added, deleted or changed with the
.Xr arp 8
utility.
Manually-added entries may be temporary or permanent,
and may be
.Dq published ,
in which case the system will respond to ARP requests for that host
as if it were the target of the request.
.Pp
In the past,
ARP was used to negotiate the use of a trailer encapsulation.
This is no longer supported.
.Pp
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).
.Pp
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
.Xr arp 8
utility, by adding an entry for each host inside a given subnet for
which proxying of ARP requests is desired.
However, the
.Dq "proxy all"
feature causes the local host to act as a proxy for
.Em all
hosts reachable through some other network interface,
different from the one the request came in from.
It may be enabled by setting the
.Xr sysctl 8
MIB variable
.Va net.link.ether.inet.proxyall
to 1.
.Sh MIB Variables
The ARP protocol implements a number of configurable variables in
.Va net.link.ether.inet
branch
of the
.Xr sysctl 3
MIB.
.Bl -tag -width "log_arp_permanent_modify"
.It Va 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.
.It Va 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.
.Pp
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.
.It Va log_arp_movements
Log movements of IP addresses from one hardware address to another.
See
.Sx DIAGNOSTICS
below.
Turned on by default.
.It Va log_arp_permanent_modify
Log attempts by a remote host to modify a permanent ARP entry.
See
.Sx DIAGNOSTICS
below.
Turned on by default.
.It Va 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
.Sx DIAGNOSTICS
below.
Turned on by default.
.It Va 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.
.It Va max_age
How long an ARP entry is held in the cache until it needs to be refreshed.
Default is 1200 seconds.
.It Va maxhold
How many packets to hold in the per-entry output queue while the entry
is being resolved.
Default is one packet.
.It Va maxtries
Number of retransmits before a host is considered down and an error is
returned.
Default is 5 tries.
.It Va proxyall
Enables ARP proxying.
Turned off by default.
.It Va wait
Lifetime of an incomplete ARP entry.
Default is 20 seconds.
.El
.Sh DIAGNOSTICS
.Bl -diag
.It "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.
.It "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.
.It "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
.Va net.link.ether.inet.log_arp_movements
is set to 1, which is the system's default behaviour.
.It "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.
.It "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
.Va net.link.ether.inet.log_arp_wrong_iface
is set to 1, which is the system's default behaviour.
.It "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
.Va net.link.ether.inet.log_arp_permanent_modify
is set to 1, which is the system's default behaviour.
.It "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
.Va net.link.ether.inet.allow_multicast
to a positive value.
.El
.Sh SEE ALSO
.Xr inet 4 ,
.Xr route 4 ,
.Xr arp 8 ,
.Xr ifconfig 8 ,
.Xr route 8 ,
.Xr sysctl 8
.Rs
.%A Plummer, D.
.%B "An Ethernet Address Resolution Protocol"
.%T RFC826
.Re
.Rs
.%A Leffler, S.J.
.%A Karels, M.J.
.%B "Trailer Encapsulations"
.%T RFC893
.Re
