I wasn't sure if this is a ServerFault question or StackOverflow, so I am going to ask it here first.
When using ISAPI Rewrite (Helion Tech), I cannot seem to proxy the root directory http://www.somesite.com/
http://www.somesite.com/subdir/ will work if subdir is on the other server, but just plain old '/' will not work.
Here is a copy of my .htaccess
# Helicon ISAPI_Rewrite configuration file
# Version 3.1.0.69
RewriteEngine on
RewriteBase /
RewriteRule (.+) http://internalsite/$1 [P]
<Helicon>
ProxyConnectTimeout 120
</Helicon>
There is no default document in the internalsite, as it uses mod_rewrite (wordpress).
but just plain old '/' will not work
This may be because you have the RewriteBase cutting off the slash and then (.+) doesn't match as you have no more characters. Please try to fix your config like this:
RewriteEngine on
RewriteBase /
RewriteRule (.*) http://internalsite/$1 [P]