It is looking like an issue with the firewall on the Mac. But I couldn't find any incantation that fixes it. And from my tests, it looks like it is blocking the "outbound" traffic, which is even weirder. Will continue playing with it, but we may need to reach the dev forums then.

We found what the issue is (many thanks to rrgeorge), but don't have an easy solution for it yet. The good news is that on your side, it looks like it should be ZeroTier the one solving it too, and once they do, you should be able to run mosh-server normally: mobile-shell/mosh#1254 (comment)

That is very good news. Thank you for the effort. I will be patiently waiting for the update.

@carloscabanero Any news on this issue?

Starting from the workaround. As this is an issue with protections over the local network, the easy fix is to connect over a network like Tailscale. Then Tailscale is responsible to request access and all connections made through it will go through.

What we found is that due to the way the Local Network protection works, there always needs to be a parent process hanging that is responsible for the Local Network permission. The main problem with fixing mosh-server on its own is the ssh -> mosh-server flow. SSH doesn't need any special network permissions, but when it disconnects after starting mosh-server, mosh-server will be "orphaned".

We tried a variety of things like trying to have mosh-server request permission on its own, etc... but none of them are trivial. The new macOS doesn't have any new ways to handle this (like manually adding a command to the "firewall" list), and from what I discussed with engineers at WWDC, they were not sure that would be something they wanted to add.

Another workaround is to start mosh as an agent somehow. We are considering something like this for a Blink version on the Mac. But this is all on paper at the moment.