Headless Gazebo through firewall

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I'm trying to run gzserver on a cloud instance running behind a firewall, and access the simulation with gzclient from outside the firewall.

To make a long story short, I've discovered that while I can set GAZEBO_IP and GAZEBO_MASTER_URI to handle the intial connection, gzserver sends back to gzclient it's IP and some random port(s) for retrieval of additional data.

It's the random additional ports that I'm having trouble with, being behind a firewall. Any idea where in the code I can go to specify a specific port instead of this random mess?