I'm trying to understand how web services which have you setup CNAME records on your domain pointing to their servers work.
For example, following the instructions on LaunchRock (http://support.launchrock.com/knowledgebase/articles/36694-creating-your-cname-record-godaddy), I create a CNAME record as
with the value
I understand that folks accessing signup.mysite.com would be redirected to sites.launchrock.com, but I'm trying to understand how sites.launchrock.com is configured so that it detects this incoming request and tailors the webpage appropriately for my landing page.
What software / configuration is present there that handles this properly?
I looked at this How does bit.ly allow anyone to use cname.bit.ly as their CNAME? - but this did not explain how to serve different content to different domains.