arpwatch - keep track of ethernet/ip address pairings
arpwatch [ -dN ] [ -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
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 -f flag is used to set the ethernet/ip address database filename. The default is
The -i flag is used to override the default interface.
The -n flag specifies additional local networks. This can be useful to avoid "bogon" warn-
ings 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 rea-
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 sup-
pressed, but logging via syslog is still done as usual. (This can be useful during ini-
tial 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
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 sup-
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 sys-
The mac ethernet address of the host is a broadcast address.
The ip address of the host is a broadcast address.
bogon 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.
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
/usr/operator/arpwatch - default directory
arp.dat - ethernet/ip address database
ethercodes.dat - 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, Universi-
ty of California, Berkeley, CA.
The current version is available via anonymous ftp:
Please send bug reports to email@example.com.
Attempts are made to suppress DECnet flip flops but they aren't always successful.
Most error messages are posted using syslog.
4th Berkeley Distribution 8 October 2000 ARPWATCH(8)