I am trying to do an invisible (transparent ?) rewrite via .htaccess and the proxy flag. It works with the [R] flag but not with the [P] flag because it just gives a 500 error. The two domains are on separate servers and the mod_proxy module is enabled. Is my .htaccess correct?
Options +FollowSymLinks
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} =one.com
RewriteRule ^(.*)$ http://two.com/$1 [P]
If mod_proxy was compiled as a shared module, it requires that you explicitly load the relevant submodules too. It's possible that on your server mod_proxy_http is not enabled, which leads to an internal server error when mod_proxy tries to handle the request passed to it by mod_rewrite.
mod_proxy itself is enabled like you said and likely working, since if you don't have it enabled, use of the P flag will just generate a 404 response. It's hard to know for sure though without the error log, so you made need to find someone with access to it to see what's going on to help you resolve the problem.
来源：https://stackoverflow.com/questions/3711189/htaccess-rewriting-with-the-p-proxy-flag