REMOTE_USER through Apache reverse proxy

This is possible with mod_headers, mod_rewrite, and mod_proxy.

On the proxy, I assume you have your authentication working and setting REMOTE_USER appropriately. If so, then put the value of REMOTE_USER into a Proxy-User header to the backend like this:

RewriteRule .* - [E=PROXY_USER:%{LA-U:REMOTE_USER}] # note mod_rewrite's lookahead option
RequestHeader set Proxy-User %{PROXY_USER}e

Here's what happens:

1. The RewriteRule fires for every request and sets the environment variable PROXY_USER equal to the value of REMOTE_USER, which should have been set already by an auth module.
2. The RequestHeader sets a request header named Proxy-User with the value of PROXY_USER

Now on the backend, you can pull that header value and set REMOTE_USER like this:

RewriteCond %{HTTP:Proxy-user} ^(.*)$
RewriteRule .* - [E=REMOTE_USER:%1]

Here's what happens:

1. The RewriteCondition checks the value of the Proxy-User header to see if it matches the pattern ^.*$ (which it will). The parentheses tells mod_rewrite to store that value in %1.
2. The RewriteRule then sets the environment variable REMOTE_USER with the value in %1.