Snapcraft firewall port(s)

When I use snapcraft to create a snap, it works if I have my firewall disabled (ufw). If I enable my firewall, snapcraft will just write ‘Launching instance…’ without going any further.

Ideally, I’d prefer not to disable my firewall each time I make a snap. What rule(s) do I need to add to my software firewall (ufw) to allow snapcraft to do its thing?