arpd - userspace arp daemon.
Usage: arpd [ -lkh? ] [ -a N ] [ -b dbase ] [ -B number ] [ -f file ] [-p
interval ] [ -n time ] [ -R rate ] [ <INTERFACES> ]
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
- 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
- -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.
- 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
- -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
- arpd -b /var/tmp/arpd.db
- Start arpd to collect gratuitous ARP, but not messing with
- 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.