Set Dendrite's real_ip_header correctly

roles/matrix-dendrite/defaults/main.yml

@@ -68,6 +68,10 @@ matrix_dendrite_allow_guest_access: false
 
 matrix_dendrite_max_file_size_bytes: 10485760
 
+# Controls which HTTP header (e.g. 'X-Forwarded-For', 'X-Real-IP') to inspect to find the real remote IP address of the client.
+# This is likely required if Dendrite is running behind a reverse proxy server.
+matrix_dendrite_sync_api_real_ip_header: ''
+
 # The tmpfs at /tmp needs to be large enough to handle multiple concurrent file uploads.
 matrix_dendrite_tmp_directory_size_mb: 500
 

roles/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -339,6 +339,7 @@ sync_api:
   # address of the client. This is likely required if Dendrite is running behind
   # a reverse proxy server.
   # real_ip_header: X-Real-IP
+  real_ip_header: {{ matrix_dendrite_sync_api_real_ip_header|to_json }}
 
 # Configuration for the User API.
 user_api: