How do apps like Tumblr, Posterous, Wordpress handle custom domain names?

I'm busy doing some research into creating a blogging platform of sorts and can't quite figure out how services handle custom domains. Specifically on the technical side how do they route traffic to an internal mapping (helloworld.tumblr.com) without redirecting?

It seems like Tumblr have you point your A record at a static IP while Wordpress have you use their name s开发者_如何转开发ervers. If anyone is knowledgeable about these things could you shed some light on how they accomplish this and if there are pros/cons to the different methods?

Thanks in advance!

You should look into "mass virtual hosting" for both apache and nginx for more information about this. For example, a quick search on stackoverflow yielded this question:

• For Apache: Dynamically configured virtual hosting