Description:
In /etc/rc.d/network in ifup(), ifdown(), and possibly other functions, the script looks for a variable with the name of the interface (e.g. eth0) to determine if it is handled by dhcpcd or not. From the usage it looks like this variable is supposed to contain the arguments to be passed to ifconfig.
However, in rc.conf and /etc/conf.d/{wireless,bonding,bridges,dhcpcd} this variable is never asked for or set by default, so the script fails and any call to "/etc/rc.d/network ifup eth0" or the like just prints usage instructions for ifconfig. This is how ifplugd (and possibly other packages) attempts to bring the specified interface up after the cable is plugged in, so without any changes that script is broken.

The quick fix is to add a line like "eth0=dhcp" to /etc/rc.conf, but the script really should get the information from the combination of $interfaces and $address

Additional info:
* package version(s): 2011.06.4-1 (and previous)

Steps to reproduce:
$ sudo /etc/rc.d/network ifup eth0
up: error fetching interface information: Device not found
<output of ifconfig>