arpd

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

NAME
       arpd - userspace arp daemon.

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

DESCRIPTION
       The arpd daemon collects gratuitous ARP information, saving it on local
       disk  and  feeding it to the kernel on demand to avoid redundant broad-
       casting due to limited size of the kernel ARP cache.

OPTIONS
       -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 in-
              terface, 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 re-
              cent 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 simi-
              lar to that dumped by option -l. Exit after load, possibly list-
              ing  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 pack-
              ets on the interface, but also sends broadcast  queries  itself.
              NUMBER  is  the number of such queries to make before a destina-
              tion 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  pe-
              riod.  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 ker-
              nel  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 ex-
              ceed 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
       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 sta-
       tistics to syslog. The effect of any other  signals  is  undefined.  In
       particular,  they may corrupt the database and leave the sysctl parame-
       ters in an unpredictable state.

NOTE
       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
       arpd -b /var/tmp/arpd.db
              Start arpd to collect gratuitous ARP, but not messing with  ker-
              nel 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 val-
              idate  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.

                                 28 June, 2007                         ARPD(8)