forward by sub domain

[Alex Egg]

How can I make nginx forward any request for svn.mydomain.com to localhost:3000?

This is my current upstream snippet:

upstream mongrel {
    server 0.0.0.0:8000;
    server 0.0.0.0:8001;
    server 0.0.0.0:8002;
    # forward svn.mydomain.com to 0.0.0.0:3000 here
  }