[BitPirate]

Your observation doesn't contradict the use of firewall rules to accomplish this.

[rockskon]

It's not some ufw rule that normally prevents hosting a service on port 0.

[arghwhat]

That's not what was said. They said that a firewall rule can redirect traffic coming in on port 0 to a running service even when a service cannot bind directly to port 0.

Binding with port 0 as argument for AF_INET binds a random available port, not port 0. This is documented behavior of Linux and likely every other OS implementing a BSD-style socket interface.

Also note that ufw is just a tiny, non-standard wrapper for the much more powerful nftables/iptables interfaces