The nice thing about this, for all of you people who understandably don't get it, is that setting up wildcard dns on OS X is a royal PITA. You can't just modify /etc/hosts, you have to <a href="http://www.macosxhints.com/article.php?story=2005110220352084" rel="nofollow">http://www.macosxhints.com/article.php?story=200511022035208...</a><p>Given all rails developers use a Mac ;) this is useful.
Why is this specific to Rails?<p>The beauty of the web and of the internet protocols it's built on is that you can have multiple implementations that respond to the same interface. DNS doesn't care what language it's pointing to.
If you're developing on Rack-based webapps on MacOS, check out <a href="http://github.com/alloy/passengerpane" rel="nofollow">http://github.com/alloy/passengerpane</a> -- it's a System Preferences panel to automatically sets up local DNS and Apache/Phusion Passenger vhosts for projects.<p>If there's a similar tool for non-MacOS or non-Rack stuff I'd love to hear about it.
Do you need subdomains specificially, or just different addresses that resolve to localhost? If so you could use e.g <a href="http://127.1:3000/" rel="nofollow">http://127.1:3000/</a>, <a href="http://127.2:3000/" rel="nofollow">http://127.2:3000/</a> etc. (or the full 127.0.0.1 127.0.0.2 if your browser doesn't like 127.1).