A Google/Android/firewall mystery

A few days ago, my Galaxy Nexus (ICS, 4.0.2) stopped syncing my Gmail. I could get to Gmail and fetch new mail, but push mail wasn't working. I could wait hours and not get new messages that were appear just fine on my laptop. I could use other services just fine, such as search, browsing, Twitter, etc. My home WiFi signal was strong; no other device was having problem connecting or browsing.

It also synced properly when I was away from home, either out in the wild or connected to another WiFi access point.

The forums weren't much help, although they did point me in the right direction, kinda sorta.  When sync was working, the signal icon was blue.

When I was connected to my WiFi, it was gray.



It was my WiFi firewall. I'd temporarily set up a VPN connection, closed it, and left the firewall enabled.

I haven't figured which port setting is enabling/disabling the synchronization connection to Google's servers. If anyone has any clues, I'd welcome them