arpd man page on ElementaryOS

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ARPD(8)								       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
	      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	 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
	      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.

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)
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