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ARPWATCH(8) System Manager's Manual ARPWATCH(8)


arpwatchkeep track of ethernet/ip address pairings


arpwatch [-CdFNpqsvzZ] [-D arpdir] [-f datafile] [-i interface] [-P pidfile] [-w watcher@email] [-W watchee@email] [-n net[/width]] [-x net[/width]] [-r file] [-u username]


arpwatch keeps track of ethernet/ip address pairings. It syslogs activity and reports certain changes via email. arpwatch uses pcap(3) to listen for arp packets on a local ethernet interface.

arp.dat contains three or four fields. First is the ethernet mac address, second is the IPv4 address, and third is the time expressed in seconds since midnight, January 1, 1970. The optional forth field is the short hostname derived from PTR record of the ip address when a mac/ip pair is first seen; it is never updated and was intended to be a breadcrumb to determine the origin of a mac/ip pair.

The -C flag uses compact padded ethernet addresses in arp.dat, e.g. 0:8:e1:1:2:d6.

The -d flag is used enable debugging. This also inhibits forking into the background and emailing the reports. Instead, they are sent to stderr.

The -D flag is used to specify the arpwatch working directory. This defaults to /usr/local/arpwatch.

The -f flag is used to set the ethernet/ip address database filename. The default is arp.dat.

The -F flag is prevents arpwatch from forking causing it to run in the foreground.

The -i flag is used to override the default interface.

The -n flag specifies additional local networks. This can be useful to avoid "bogon" warnings when there is more than one network running on the same wire. If the optional width is not specified, the default netmask for the network's class is used.

The -N flag disables reporting any bogons.

The -p flag disables promiscuous mode.

The -P flag specifies the pidfile.

The -q flag suppresses reports being logged or printed to stderr.

The -r flag is used to specify a savefile (perhaps created by tcpdump(1) or pcapture(1)) to read from instead of reading from the network. In this case arpwatch does not fork.

Note that an empty arp.dat file must be created before the first time you run -arpwatch.

The -s flag suppresses reports sent by email.

The -v flag disables the reporting of VRRP/CARP ethernet prefixes as described in RFC5798 (0:0:5e:0:1:xx).

The -w flag is used to specify the target address for email reports. The default is root.

The -W flag is used specifies the from address for email reports. The default is root.

The -z flag disables reporting changes, helpful in busy DHCP-served networks.

The -Z flag uses zero padded ethernet addresses in arp.dat, e.g. 00:08:e1:01:02:d6.

The -u flag allows to drop root privileges and change to the user ID and group ID to that of the primary group of username.


Here's a quick list of the report messages generated by arpwatch(1) (and arpsnmp(1)):

This ethernet/ip address pair has been used for the first time six months or more.
The ethernet address has not been seen before.
The ethernet address has changed from the most recently seen address to the second most recently seen address. (If either the old or new ethernet address is a DECnet address and it is less than 24 hours, the email version of the report is suppressed.)
The host switched to a new ethernet address.


Here are some of the syslog messages; note that messages that are reported are also syslog'ed.

The mac ethernet address of the host is a broadcast address.
The ip address of the host is a broadcast address.
The source ip address is not local to the local subnet.
The source mac or arp ethernet address was all ones or all zeros.
The source mac ethernet address didn't match the address inside the arp packet.
The ethernet address has changed from the most recently seen address to the third (or greater) least recently seen address. (This is similar to a flip flop.)
A "flip flop" report was suppressed because one of the two addresses was a DECnet address.


Normally arpwatch updates arp.dat once every 15 minutes. The SIGHUP signal causes it to update immediately.


default directory
default ethernet/ip address database
vendor ethernet block list


arpsnmp(8), arp(8), bpf(4), tcpdump(1), pcapture(1), pcap(3)


Craig Leres of the Lawrence Berkeley National Laboratory Network Research Group, University of California, Berkeley, CA.

The current version is available via anonymous ftp:


Please send bug reports to ⟨⟩.

Attempts are made to suppress DECnet flip flops but they aren't always successful.

Most error messages are posted using syslog.

2 December 2023