Preserve host in example apache config (#9696)

Fixes redirect loop

Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>

UPGRADE.rst

@@ -98,9 +98,12 @@ will log a warning on each received request.
 
 To avoid the warning, administrators using a reverse proxy should ensure that
 the reverse proxy sets `X-Forwarded-Proto` header to `https` or `http` to
-indicate the protocol used by the client. See the `reverse proxy documentation
-<docs/reverse_proxy.md>`_, where the example configurations have been updated to
-show how to set this header.
+indicate the protocol used by the client.
+
+Synapse also requires the `Host` header to be preserved.
+
+See the `reverse proxy documentation <docs/reverse_proxy.md>`_, where the
+example configurations have been updated to show how to set these headers.
 
 (Users of `Caddy <https://caddyserver.com/>`_ are unaffected, since we believe it
 sets `X-Forwarded-Proto` by default.)

docs/reverse_proxy.md

@@ -104,10 +104,11 @@ example.com:8448 {
 ```
 <VirtualHost *:443>
     SSLEngine on
-    ServerName matrix.example.com;
+    ServerName matrix.example.com
 
     RequestHeader set "X-Forwarded-Proto" expr=%{REQUEST_SCHEME}
     AllowEncodedSlashes NoDecode
+    ProxyPreserveHost on
     ProxyPass /_matrix http://127.0.0.1:8008/_matrix nocanon
     ProxyPassReverse /_matrix http://127.0.0.1:8008/_matrix
     ProxyPass /_synapse/client http://127.0.0.1:8008/_synapse/client nocanon
@@ -116,7 +117,7 @@ example.com:8448 {
 
 <VirtualHost *:8448>
     SSLEngine on
-    ServerName example.com;
+    ServerName example.com
 
     RequestHeader set "X-Forwarded-Proto" expr=%{REQUEST_SCHEME}
     AllowEncodedSlashes NoDecode
@@ -135,6 +136,8 @@ example.com:8448 {
 </IfModule>
 ```
 
+**NOTE 3**: Missing `ProxyPreserveHost on` can lead to a redirect loop.
+
 ### HAProxy
 
 ```