request.getHeader("Host") returns wrong host name

[Orpheus66]

We've deployed a website on a server exposed to the internet through
our firewall. IIS handles the pass-off to Resin 2.0.4, and has worked
flawlessly on other applications configured the same way.

The problem is for some reason when we come into the site externally
from a URL like this: http://externaldomain.com/app

Every place in the code that does a request.getHeader("Host") returns
"internalmachinename" instead of "externaldomain.com"

Has anyone seen this before? Because a servlet we use depends on the
fully qualified URL, we need to be able to grab the correct host when
constructing some links.

Thanks in advance.