If you are trying to debug mDNS/Avahi on TrueNAS Core and stumble on this misleading Failed to create client object: Daemon not running, and web search leads nowhere?

$ avahi-resolve -vn obsidian.local
Failed to create client object: Daemon not running

The solution is obscenely simple: in /usr/local/etc/avahi/avahi-daemon.conf turn dbus back on:

[server]
# ...
enable-dbus=yes

and restart the daemon:

$ service avahi-daemon restart
Stopping avahi-daemon.
Starting avahi-daemon.

After that, avahi tools will working:

$ avahi-resolve -vn obsidian.local
Server version: avahi 0.8; Host name: truenas.local
obsidian.local	10.0.50.95