Advertising Rendezvous services in Linux

I'm already running NoCat to give people an opening splash screen. But when they close it (or if their browser is killing pop-up windows for them) then they have no easy way to get back to my local content.

This is one reason why I love multicast DNS service advertisements (and Rendezvous, in particular). My Linux Jukebox and Wiki are now announcing themselves like an old-time barker at the county fair. Wireless users at the cafe across the street (or anywhere within a block or so) can find my local services any time just by looking at available rendezvous sites. As if that weren't enough, my streamer is even advertising itself as a DAAP stream, so iTunes 4 users can see that it's available from inside iTunes itself.

First, get a multicast DNS advertiser. I found Apple's own Posix implementation to be more than adequate. (You can download it after a free registration on Apple's site.) The application you're after is mDNSProxyResponderPosix, in the mDNSPosix/ directory. It built cleanly and without so much as a warning under Linux 2.4.20. Once it's built, install it somewhere handy (like /usr/local/bin, for example.) There are a couple of other multicast DNS projects floating around (like mdnsd), but Apple's example code seemed to work easiest out of any of the responders I tried.

Next, figure out what services you want to advertise. They don't even have to be local services; the Proxy server will obligingly hand out whatever IP addresses you care to throw at it, local or not. I'm advertising my local Wiki, the Jukebox, and the NoCat website, just for fun.

This is all well and good for the jukebox, but what if you need to go to a particular URL? For example, to get to my wiki, you need to go to http://florian.local/wiki, not just http://florian.local/. This is where the optional text field at the end comes in. Safari accepts a path= argument in this field that gets appended to the URL line. For example:

And there you have it. In case you're curious, SWN-BelmontEast is the ESSID of my node on SeattleWireless.

Incidentally, if you're using VirtualHosts in Apache, you'll have to tell apache to respond to the name you're advertising (florian.local in the above example.) This is done easily with the ServerAlias directive from within your <VirtualHost> stanza in your httpd.conf:

Finally, since we have music available, it would be nice to advertise directly to iTunes' sharing feature. Since iTunes is simply expecting advertisements to the daap service (as opposed to http), this is a piece of cake:

Of course, mDNSProxyResponderPosix isn't nearly as efficient as it could be. After all, you're starting a separate instance for each service you want to advertise, and there isn't even a simple configuration file. But keep in mind that this application is just example code from Apple, and in due time more sophisticated multicast DNS advertisement tools will come along. The full source (as well as the complete spec, and some other great documents) are available, just waiting for someone to write the killer Linux / BSD / Windows rendezvous app.

Oh, and if you're trying to get to some of the above urls (like http://florian.rob.swn/) but can't resolve .swn, you should try it from anywhere on SeattleWireless. Funny, but the net looks very different (and much more fun!) from behind a Community Wireless Network...

Are you using multicast DNS service advertisements in a creative way?
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Showing messages 1 through 9 of 9.

Rendezvous Chat
2003-11-07 17:47:01 anonymous2 [Reply | View]

any windows and rendezvous compatible chat clients? iChat on OSX is nice but my parents are anti-mac, and jabber is too difficult to maintain.

mDNSProxyResponder and OpenBSD
2003-07-08 12:24:26 rlotz [Reply | View]

mDNSProxyResponder works in OpenBSD (tested with 3.2), if you enable multicast.

I've posted more details.

why not use mDNSResponder instead of mdNSProxyResponder?
2003-05-09 16:07:42 anonymous2 [Reply | View]

mDNSResponder has a -d flag to autodaemonize, and can also read in a configuration file of hosts/services to advertise as opposed to the one host/service per instance problem mDNSProxyResponder has.

or is there something funky with your setup that requires it? (ie nocat being a captive portal)

why not use mDNSResponder instead of mdNSProxyResponder?
2003-05-09 16:35:51 rflicken [Reply | View]

mDNSResponder won't let you advertise arbitrary IP addresses. It will only advertise local services, and in my case, I'm running the advertiser on a separate box from my web server.

But if you are advertising services on the same machine that they are served from, mDNSResponderPosix would do the trick nicely.

why not use mDNSResponder instead of mdNSProxyResponder?
2003-05-12 08:01:42 anonymous2 [Reply | View]

I'm trying to write a redhat init script to turn the responder on and off. The daemonized version, how can I look up the PID to kill it (and therefore allow the script to respond to a "service rendezvous stop" command)?

Stopping the service
2003-05-12 08:41:20 rflicken [Reply | View]

The quickest way is to just run killall mDNSProxyResponder to kill the job by name. You could also do something like kill `ps ax|grep mDNSProxyResponder|grep -v grep|awk '{print $1}'` (although the first one is much simpler...)

Stopping the service
2003-05-14 00:52:21 anonymous2 [Reply | View]

The proper way to write an init script is to use the init.d functions in /etc/rc.d/init.d/functions by calling them at the start of the script. See other init scripts for an example.

These functions handle all pid details for you, and more. Find more info on them at:

http://www.linuxbase.org/spec/refspecs/LSB_1.3.0/gLSB/gLSB/iniscrptfunc.html

Steven Boger
sboger@hotmail.com

Thanks!
2003-05-08 14:42:29 mnot [Reply | View]

This is very helpful! One question - I can't find the proper details for advertising netatalk shares. The protocol is obviously _afpovertcp._local, but how does one indicate a particular share on that server?

Thanks again.

Finding TXT data for AFP
2003-05-08 15:03:59 rflicken [Reply | View]

Well, I'm pretty sure the protocol would be _afpovertcp._tcp. and the port number is 548.

Maybe you could do a packet dump of an actual AFP session, and see what's in any multicast dns TXT records?

Showing messages 1 through 9 of 9.