ifplugd.conf is the configuration file for ifplugd. It is a shell script that is sourced by the init script starting the daemon. It shall be used to set environment variables which are interpreted by the init script:
OPTIONS
INTERFACES<1>
Specifies the ethernet interfaces to monitor. It has to contain a space seperated list of network interfaces names. Most users will probably use "eth0" here, however you may add additional interfaces for monitoring more than one device. A special value is supported as well: "auto" will enable a more or less working auto detection of available network devices. This won't make you happy when using network module auto loading, since it cannot detect currently unloaded network devices.
1>
ARGS<1>
Additional command line arguments for ifplugd invocation. See ifplugd(8) <1> for further information.
1>
1>
ARGS_<1>1>iface<1>
If specified for an interface this variable takes precedence over ARGS. This may be useful if more than one network device is present.