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

     arpwatch — keep 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]

     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.

     The -C flag (default) 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

     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 (00:00:5e:00:01: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

     The -Z flag uses zero padded ethernet addresses in arp.dat, e.g.

     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
          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.

     /usr/local/arpwatch  default directory
     arp.dat              default 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.

                                 1 December 2019