Apache Conditional Redirect Rule not forwarding header values - apache

Question

Goal:
To perform conditional redirect/forward from Site1 to Site2 based on specific header (HTTP_SM_USER) value of the request (in Site1) and ensure all the current custom header data is forwarded as part of the redirect/forward to Site2. The user can see the url change in the browser window after the redirect.
Flow:
User accesses Site 1 --> External Application Sets few custom headers in addition to default HTTP headers based on certain criteria --> Web-server looks for specific header value and if matches redirect all the headers to Site 2.
Apache Web server Config:
<VirtualHost *.443>
ServerName site1.com
ServerAdmin ashish#test.com
UseCanonicalName on
DocumentRoot /var/www/html
#
# SSL Config comes here
#
ErrorLog /path/to/error/log
TransferLog /path/to/access/log
LogLevel warn
RewriteEngine On
RewriteCond %{HTTP:SM_USER} ^USER1$ [NC]
RewriteRule .* https://site2.com/$1 [R=301, L]
<Location /page1>
.
.
</Location>
.
.
.
</VirtualHost>
Question: The Conditional Redirect happens but the headers are lost. What should I do to ensure the HTTP headers are forwarded as well ? I tried to see into apache mod_proxy but wasn't sure how in this scenario to use it. Experts please help. Any alternative suggestions are welcome too.

Answer 1

Solution:
I finally got it to work (with mod_proxy) after a lot of troubleshooting using the below flags and proxy rules. All Rewrite rules were removed.
ProxyRequests Off
ProxyPreserveHost On
SSLProxyEngine On
ProxyPass "/" "https://site2.com/" Keepalive=On
ProxyPassReverse "/" "https://site2.com/"
Note: User accesses Site 1 -> External app sets headers and apache immediately proxies to Site 2. Headers are now available on Site2.

Answer 2

You can't have the redirect "forward" headers, because it is up to the client what it sends to the next server after it has been issued a redirect. It may choose not to follow the redirect at all! Unlikely but just to make it clear what the situation is.
If you want the URL to change in the browser, then you need to do this on the application side. Have your application handle directing to the next server, and just issue a 200 response with an instruction for it to do so. Then it can send the custom headers as it did with the first request.
If that is not acceptable, then using mod_proxy could work as you said, but it would not change the URL in the browser, because the first server would be proxying the request to the second, so the browser would still see it as being the first.
In summary, you cannot do what you want to do, because HTTP does not work that way, so you are going to have to compromise in some way, either by not changing the URL in the browser, or by updating the application to take care of it.