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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SIGNALS | NOTE | EXAMPLES | COLOPHON |
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ARPD(8) System Manager's Manual ARPD(8)
arpd - userspace arp daemon.
Usage: arpd [ -lkh? ] [ -a N ] [ -b dbase ] [ -B number ] [ -f
file ] [-p interval ] [ -n time ] [ -R rate ] [ <INTERFACES> ]
The arpd daemon collects gratuitous ARP information, saving it on
local disk and feeding it to the kernel on demand to avoid
redundant broadcasting due to limited size of the kernel ARP
cache.
-h -? Print help
-l Dump the arpd database to stdout and exit. The output
consists of three columns: the interface index, the IP
address of the interface, and the MAC address of the
interface. Negative entries for dead hosts are also shown,
in this case the MAC address is replaced by the word FAILED
followed by a colon and the most recent time when the fact
that the host is dead was proven.
-f <FILE>
Read and load an arpd database from FILE in a text format
similar to that dumped by option -l. Exit after load,
possibly listing resulting database, if option -l is also
given. If FILE is -, stdin is read to get the ARP table.
-b <DATABASE>
the location of the database file. The default location is
/var/lib/arpd/arpd.db
-a <NUMBER>
With this option, arpd not only passively listens for ARP
packets on the interface, but also sends broadcast queries
itself. NUMBER is the number of such queries to make before
a destination is considered dead. When arpd is started as
kernel helper (i.e. with app_solicit enabled in sysctl or
even with option -k) without this option and still did not
learn enough information, you can observe 1 second gaps in
service. Not fatal, but not good.
-k Suppress sending broadcast queries by the kernel. This
option only makes sense together with option -a.
-n <TIME>
Specifies the timeout of the negative cache. When
resolution fails, arpd suppresses further attempts to
resolve for this period. This option only makes sense
together with option '-k'. This timeout should not be too
much longer than the boot time of a typical host not
supporting gratuitous ARP. Default value is 60 seconds.
-p <TIME>
The time to wait in seconds between polling attempts to the
kernel ARP table. TIME may be a floating point number. The
default value is 30.
-R <RATE>
Maximal steady rate of broadcasts sent by arpd in packets
per second. Default value is 1.
-B <NUMBER>
The number of broadcasts sent by arpd back to back. Default
value is 3. Together with the -R option, this option
ensures that the number of ARP queries that are broadcast
does not exceed B+R*T over any interval of time T.
<INTERFACES> is a list of names of networking interfaces to watch.
If no interfaces are given, arpd monitors all the interfaces. In
this case arpd does not adjust sysctl parameters, it is assumed
that the user does this himself after arpd is started.
When arpd receives a SIGINT or SIGTERM signal, it exits
gracefully, syncing the database and restoring adjusted sysctl
parameters. On a SIGHUP it syncs the database to disk. With
SIGUSR1 it sends some statistics to syslog. The effect of any
other signals is undefined. In particular, they may corrupt the
database and leave the sysctl parameters in an unpredictable
state.
In order for arpd to be able to serve as ARP resolver, the kernel
must be compiled with the option CONFIG_ARPD and, in the case when
interface list in not given on command line, variable app_solicit
on interfaces of interest should be in /proc/sys/net/ipv4/neigh/*.
If this is not made arpd still collects gratuitous ARP information
in its database.
arpd -b /var/tmp/arpd.db
Start arpd to collect gratuitous ARP, but not messing with
kernel functionality.
killall arpd ; arpd -l -b /var/tmp/arpd.db
Look at result after some time.
arpd -b /var/tmp/arpd.db -a 1 eth0 eth1
Enable kernel helper, leaving leading role to kernel.
arpd -b /var/tmp/arpd.db -a 3 -k eth0 eth1
Completely replace kernel resolution on interfaces eth0 and
eth1. In this case the kernel still does unicast probing to
validate entries, but all the broadcast activity is
suppressed and made under authority of arpd.
This is the mode in which arpd normally is supposed to work. It is
not the default to prevent occasional enabling of too aggressive a
mode.
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
28 June, 2007 ARPD(8)
Pages that refer to this page: arp(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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