.TH ARP 3 
.SH NAME
arp \- Internet Address Resolution Protocol
.SH SYNOPSIS
.nf
.B bind -a #a /net/arp
.sp
.B /net/arp/ctl
.B /net/arp/data
.B /net/arp/stats
.fi
.SH DESCRIPTION
The
.I arp
device provides the means by which the kernel resolves
IP addresses
into Ethernet addresses.
A cache is maintained by the
.B arp
device to speed the process.
.PP
The
.B ctl
file controls the ARP cache maintained by the kernel.
The
.B flush
control message invalidates all entries in the cache.
The
.B delete
.I ipaddr
control message invalidates a single cache entry.
All IP addresses passed to the system are in the canonical textual form
described in
.IR ip (2).
The
.B perm
.I ipaddr
control message makes an existing cache entry permanent.
.PP
When the kernel boots,
.B ipconfig
sets up the IP stream and
.B arpd
opens
.B #a/arp/data
(see
.IR ipconfig (8)).
This establishes the ARP cache
and enables
.B arpd
to receive all ARP packets from the network, which it uses to maintain the cache
by writing the results of address resolution requests back into the cache.
The IP stream module uses the cache to translate IP addresses.
.PP
Subsequent opens of the
.B data
file allow the contents of the cache to be examined.
Each cache entry consists of an
IP address, an Ethernet address, and the status of the entry.
Entries may be
invalid, permanent, or temporary.
Permanent entries will never be aged from the cache.
Temporary entries may be replaced by new addresses entered by the ARP server.
.PP
The file
.B stats
reports the cache performance.
.SH SEE ALSO
.IR ip (3),
.IR ipconfig (8)
.SH SOURCE
.B /sys/src/9/port/devarp.c