WordPress on internal network

Hello, I have wordpress installed on a webserver on an internal network, where a gateway machine redirects web requests. From the outside world, all is well. From the internal network, however, a connection to my blog is refused.

Gateway (and world-visible blog host): http://www.myhost.com
Internal name for webserver machine: web.localdomain

I.e. (made-up URLS), http://www.myhost.com/wordpress works fine from the Internet.
http://web.localdomain/static.html works fine from Intranet

http://web.localdomain/wordpress does not work from Intranet
The URL gets redirected to http://www.myhost.com/wordpress which does not resolve correctly from the gateway itself.

I can imagine fixing this with iptables rules on the gateway or perhaps Apache settings. However, why does wordpress care so much about its URL?

Thanks,
-Jon