systemd-resolved causes/triggers issue with reverse DNS lookup in bzfs

[blast007]

In Fedora 35, and possibly other distributions using systemd-resolved for DNS resolution, there are times when reverse DNS resolution seems to hang and never complete in bzfs. The specific instance I found was a case where there either wasn't a reverse DNS hostname, or the server responsible for providing the reverse DNS wasn't responding. After installing the bind-utils package to acquire the host command, I attempted to resolve the reverse DNS of the player's IP. I received a "connection timed out; no servers could be reached" message after several seconds. When I ran the same command but also passed a DNS server to use, I instead received a SERVFAIL almost immediately.

It's possible we're not handling some result from c-ares where a timeout occurs.

[tomfitzhenry]

This occurred for me with bzfs 2.4.24 and systemd-resolved 251.7 on NixOS. The symptom I observed was that when I connected to bzfs from my LAN, the client hung on the "Press Right Mouse or i to join".

Two independent workarounds:

• Set nameserver in /etc/resolv.conf to a public resolver.
• Set up a port redirection of 5154/{udp,tcp} on your server using socat (for example) so that bzflag sees the connections as coming from localhost. This avoids triggering the same issue.