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Following is a step by step guide to configure an application for dynamic subdomains. We are trying to set up wildcard subdomain feature similar to the one done by blogspot.com and wordpress.com.

Step 1
In advanced DNS settings of your domain, add an A record as follows
*.domain.com IN A ***.***.***.*** (server IP address)

Step 2
Modify http.conf to set up a serveralias *.domain.com for http://www.domain.com/.
Make sure you have access, or ask your service provider to do this.

Step 3
Add a wildcard entry for your domain in the Zone file for http://www.domain.com/, so that nonexistant subdomains are resolved to same public_html folder.

Now to check this, keep an index.html file in the public_html folder.
Try browsing http://www.domain.com/ and http://anything.domain.com/ . Both should resolve to the same index.html file. If this succeeds, your dynamic subdomains are set up.

To catch the subdomain in RubyOnRails modify application.rb as follows,

Now you have a subdomain catched by your application controller which can be used in all controllers.
For example, MyController makes use of @subdomain and decides what action to take. (it is a simple redirect action in this case)