Honor upstream forwarded port if available

nginx.tmpl

@@ -24,6 +24,13 @@ map $http_x_forwarded_proto $proxy_x_forwarded_proto {
   ''      $scheme;
 }
 
+# If we receive X-Forwarded-Port, pass it through; otherwise, pass along the
+# server port the client connect to
+map $http_x_forwarded_port $proxy_x_forwarded_port {
+  default $http_x_forwarded_port;
+  ''      $server_port;
+}
+
 # If we receive Upgrade, set Connection to "upgrade"; otherwise, delete any
 # Connection header that may have been passed to this server
 map $http_upgrade $proxy_connection {
@@ -51,7 +58,7 @@ proxy_set_header Connection $proxy_connection;
 proxy_set_header X-Real-IP $remote_addr;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
-proxy_set_header X-Forwarded-Port $server_port;
+proxy_set_header X-Forwarded-Port $proxy_x_forwarded_port;
 
 # Mitigate httpoxy attack (see README for details)
 proxy_set_header Proxy "";