Set the magic variable in auto-pelab.ns and swapin your experiment.
Logs will wind up in /proj/blahblah/exp/blahblah/logs/<node>/<name>.tcpdump
after you run stop-experiment.

I did this with the program agent and duct tape.  If it turns out that
you can already do this with the link tracing framework, then someone in
Oregon is going to whomp me upside the head with a clue-stick and he'll
never let me forget...at least til he forgets in a day or two.