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 broadcasting 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
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
/var/lib/arpd/arpd.db
-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
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
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
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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