I’m frequently firing up tcpdump on my mac, but then having to restart it with the correct interface if I happened to be on a wireless network (which I frequently am on the mac). I finally got tired of that, so I whipped up a quick script that determines which interface is up and consequently starts tcpdump on that interface.  Allow me to cat /usr/bin/stcpdump for you:

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#!/bin/sh INT=`ifconfig | grep -A3 "en.:" | grep -B2 inet\ | head -n 1 | cut -f1 -d:` sudo tcpdump -i $INT $@

Much better - saves me a big of grief. ;o)