ARPD(8)                    System Manager's Manual                   ARPD(8)

NAME         top

       arpd - userspace arp daemon.

SYNOPSIS         top

       Usage: arpd [ -lkh? ] [ -a N ] [ -b dbase ] [ -B number ] [ -f file ]
       [-p interval ] [ -n time ] [ -R rate ] [ <INTERFACES> ]

DESCRIPTION         top

       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.

OPTIONS         top

       -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

       -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

       -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.

SIGNALS         top

       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.

NOTE         top

       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.

EXAMPLES         top

       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

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
       networking and traffic) project.  Information about the project can
       be found at 
       If you have a bug report for this manual page, send it to,  This page was obtained
       from the project's upstream Git repository 
       on 2017-03-13.  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

                                28 June, 2007                        ARPD(8)