Added docs on how to configure the reverse proxy for a subfolder & Documented the usage of X-Forwared-Prefix

Author: jbtronics

deployment/proxies.rst

@@ -159,6 +159,29 @@ enough, as it will only trust the node sitting directly above your application
 ranges of any additional proxy (e.g. `CloudFront IP ranges`_) to the array of
 trusted proxies.
 
+Reverse proxy in a subpath / subfolder
+--------------------------------------
+
+If you a running a Symfony application behind a reverse proxy, where the application is served in a subpath / subfolder,
+you might encounter the problem that Symfony generates incorrect URLs, which ignore the subpath / subfolder of the reverse proxy.
+To fix this, you need to pass the subpath / subfolder route prefix of the reverse proxy to Symfony by
+setting the ``X-Forwarded-Prefix`` header. This is required for Symfony to generate the correct URLs. The header can normally be configured
+in your reverse proxy configuration. Configure ``X-Forwared-Prefix`` as trusted header to be able to use this feature.
+
+The ``X-Forwarded-Prefix`` is used by Symfony to prefix the base URL of request objects, which is
+used to generate absolute paths and URLs in Symfony applications. Without the header, the base URL would be only determined
+based on the configuration of the web server running Symfony, which leads to incorrect pathes/URLs, when the application
+is served under a subpath by a reverse proxy.
+
+For example if your symfony application is directly served under an URL like ``https://symfony.tld/``
+and you would like to use a reverse proxy to serve the application under ``https://public.tld/app/``, you would need
+to set the ``X-Forwarded-Prefix`` header to ``/app/`` in your reverse proxy configuration.
+Without the header, Symfony would generate URLs based on its servers base URL (e.g. ``/my/route``) instead of the correct
+``/app/my/route``, which is required to access the route via the reverse proxy.
+
+The header can be different for each reverse proxy, so that access via different reverse proxies served under different
+subpaths can be handled correctly.
+
 Custom Headers When Using a Reverse Proxy
 -----------------------------------------