tcpdump filters (pcap filters, really) for Fun and Profit?

I've been fighting what I'm pretty sure is a silly firewall problem while trying to get Synctrain to work when my iPhone isn't on my WiFi. Long time readers will know that when I am not on a trusted WiFi network my i{Pad,}OS devices automatically VPN into my network so they can tunnel all their traffic through my security apparatus (and multi-layer ad and tracking prevention systems). I'm getting…