- where the same server is also the OpenVPN server and is using a tap device and
- the mac was using Tunnelblick
though this setup seemed to work for my linux machine it didn't quite do the job on the mac: there the mdns information was only used occasionally (though ethereal aka. wireshark confirmed that it was being transmitted).
currently i think that the mDNSResponder is behaving - at least - strange and a possible work-around seems to be
- install the tun/tap-driver separately from Tunnelblick (so the driver is loaded at boot time and mDNSResponder is not 'surprised' by its late appearance...)
- turn the interface of the internet connection (the one tunnelblick uses to connect to the server - not the tap one...) off and on