Make sure you have loopbackmedium in your kernel, e.g.,

link
	...
	ethermedium
	loopbackmedium
	...

Then just use ipconfig to set it up:


% ip/ipconfig loopback /dev/null 127.1

The /dev/null is not meaningful, it's ignored, ipconfig just
needed a positional parameter there.

Anything sent to 127.1 will loop back at the bottom of the stack.
Is that what you wanted?