What Firewall Port is required for snap package management?

In addition to allowing DNS of course, Ubuntu Core requires TCP/443 connectivity for snap package management to check for updates & install packages. The Ubuntu Core host on 192.168.1.26 is using a port from the Dynamic/Private port range, TCP/35234

Doing a host lookup of host 91.189.92.20 reports the remote IP as Snapcraft's:

20.92.189.91.in-addr.arpa domain name pointer api.snapcraft.io.

Anyhoo, hope this saves others some time-

I've been having the same issue and had to add the following domains to be able to establish http(s) connections:

api.snapcraft.io 
public.apps.ubuntu.com 
snapcraft.io 
fastly.cdn.snapcraft.io

At least, it was working after adding the fourth one.