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

       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 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 -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

       new activity
              This ethernet/ip address pair has been used for the first time six
              months or more.

       new station
              This ip address has not used this ethernet address 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.

       bogon  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

       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

       Most error messages are posted using syslog.

4th Berkeley Distribution        8 October 2000                      ARPWATCH(8)