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

   -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.


   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.

                             28 June, 2007                         ARPD(8)


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