ARPWATCH(8)                                                        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

       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 net-
       mask 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  tcp-
       dump(1)  or  pcapture(1)) to read from instead of reading from the net-
       work. 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  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 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

       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/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 Re-
       search 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)

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