Publishing additional services with avahi

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Yes, I’m writing a custom module that, when enabled, set the avahi-daemon.preStart property.

I was expecting something like the following fragment to work, however avahi-daemon does not peek up the custom.service file

let 
     customXML = pkgs.writeText "custom.xml" ''
     <?xml>
         <service-group>
              <name replace-wildcards="no">Custom Service</name>
              <service>
              <type>...</type>
              <port>...</port>
              </service>
         </service-group>'';
in
{
    # other stuff removed since unrelated

    config = mkIf program.custom.enable {
         systemd.services.avahi-daemon.preStart = ''
                mkdir -p /etc/avahi/services
                ln -sf ${customXML} /etc/avahi/services/custom.service
         '';
}

The preStart fragment in my module is appended to the default mkdir -p /var/run/avahi-daemon.