How to implement custom domain feature like posterous?

here is how posterous works,

I make a A record of DNS of my domain (blog.mydomainname.com) to posterous IP address 67.207.139.81 , in the manage panel of posterous , i put my custom domain (blog.meetingly.com). Then i can reach my posterous blog http://mydomainname.posterous.com from http://blog.mydomainname.com url.

How posterous implements this? Is this a DNS level thing or web server level setting?

Thanks a lot!

From serverfault Weiwei

• It's just a virtual host for your domain configured in Posterous's web server.

  From joschi

• The A record in DNS is what tells the internet to direct the HTTP requests (or other traffic) to the Posterous web server.

  Most likely the Posterous web server uses the "virtual hosts" feature in Apache (or another web server program) to serve up your content, although it is quite possible that the Posterous folks have written some custom code to do this.

  Weiwei : thanks, i understand the first part "vThe A record in DNS is what tells the internet to direct the HTTP requests (or other traffic) to the Posterous web server." but not very sure how second part being done
  From tomjedrz