arpwatch does not support a configuration file, but the systemd unit files
shipped with Debian allow to launch arpwatch with different configurations on
each interface

In order to do that, create a file called IFNAME.iface which contains variable
assignments in sh syntax (comments are allowed). You can use the following
variables to influence the invocation for that specific interface only:
* ARGS: overwrite the ARGS from /etc/default/arpwatch
* PCAP_FILTER: overwrite (or set) the pcap filter
* IFACE_ARGS: additional options to be passed to arpwatch

See `man 8 arpwatch` for available arguments. Note that the -f, -i and -u
options are added by the init system and should not used. If you just want to
apply the options specified in /etc/default/arpwatch for an interface you do
not need to create a configuration file.


For example, if you want to send mails about arpwatch events on eth0 to
arpwatch+eth0@example.com, you can generate the configuration file using the
following command:

echo 'IFACE_ARGS="-m arpwatch+eth0@example.com"' > /etc/arpwatch/eth0.iface