Switch page in reaction to calling URL

I need a plugin which reacts to the URL from which i call my blog.

It can be reached through http://www.domain.tld and blog.domain.tld.
The adress entered in the options is blog.domain.tld.

Now i need a plugin or hack which redirects http://www.domain.tld to blog.domain.tld/site.

And when i click on the link to /site it should be a redirect to http://www.domain.tld.

Is there any solution for this without fiddling around with the apache conf files?