arpwatch (8) - Linux Manuals

arpwatch: keep track of ethernet/ip address pairings


arpwatch - keep track of ethernet/ip address pairings


arpwatch [ -dNp ] [ -f datafile ] [ -i interface ]

          [ -n net[/width ]] [ -r file ] [ -u username ] [ -e username ] [ -s username ]


Arpwatch keeps track for 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.

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 -p flag disables promiscuous mode.

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

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 -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.

If -u flag is used, arpwatch drops root privileges and changes user ID to username and group ID to that of the primary group of username. This is recommended for security reasons.

If the -e flag is used, arpwatch sends e-mail messages to username rather than the default (root). If a single `-' character is given for the username, sending of e-mail is suppressed, but logging via syslog is still done as usual. (This can be useful during initial runs, to collect data without being flooded with messages about new stations.)

If the -s flag is used, arpwatch sends e-mail messages with username as the return address, rather than the default (root).

Note that an empty arp.dat file must be created before the first time you run arpwatch. Also, the default directory (where arp.dat is stored) must be owned by username if -u flag is used.


Here's a quick list of the report messages generated by arpwatch(1) (and arpsnmp(1)):
new activity
This ethernet/ip address pair has been used for the first time six months or more.
new station
The ethernet address has not been seen before.
flip flop
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.)
changed ethernet address
The host switched to a new ethernet address.


Here are some of the syslog messages; note that messages that are reported are also sysloged.
ethernet broadcast
The mac ethernet address of the host is a broadcast address.
ip broadcast
The ip address of the host is a broadcast address.
The source ip address is not local to the local subnet.
ethernet broadcast
The source mac or arp ethernet address was all ones or all zeros.
ethernet mismatch
The source mac ethernet address didn't match the address inside the arp packet.
reused old ethernet address
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.)
suppressed DECnet flip flop
A "flip flop" report was suppressed because one of the two addresses was a DECnet address.


/var/lib/arpwatch - default directory
arp.dat - ethernet/ip address database
ethercodes.dat - vendor ethernet block list


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 arpwatch [at]

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

Most error messages are posted using syslog.


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