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 broad-
       casting 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

       -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

       -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

       -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 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
              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  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.

       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 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)
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