[users@httpd] Mod_header REMOTE_USER variable null via reverse proxy server - HELP!

[Shabbir bharmal]

I have Apache 2.2 setup using mod_proxy, mod_headers and mod_auth
successfully. I am using Apache as my web server and Apache Tomcat as
my web container where my application    is deployed.

Mod_auth challenges a browser session to enter a username and password
when accessing a page on my tomcat container. The username needs to be
set as a 'UserId' variable to be sent over to a reverse proxy
connection to a specific host where the application resides.

I have tried using the REMOTE_USER variable in a number of ways. To
show a couple of them:

1. RequestHeader set UserId %{REMOTE_USER}e
2. RequestHeader set UserId %{LA-U:REMOTE_USER}e

mod_proxy is used as a reverse proxy to send all browser sessions to
the Tomcat container.

ProxyPass / http://localhost:8080/
ProxyPassReverse / http://localhost:8080/

However, in all case, the UserId comes across as (null). I configured
TCPMON to check if Tomcat is for any reason messing the UserId, but
tcpmon shows that the http header UserId sent from the browser is
(null).

I would really appreciate if someone can point me in the right
direction to get this working. We are going live in a few days and time
is of essence.

What needs to be done to see REMOTE_USER variable,  (for that matter
REMOTE_HOST, REMOTE_IDENT, REMOTE_ADDR etc) OR am I using the wrong
variable and should use something else to capture the username from the
browser. Should anything be changed on the browser to send the username
across to the Apache server?

Thanks in advance.
Bharmal