service/forum.md

@@ -304,6 +304,17 @@ So the most straightforward fix is simply to serve the forum via `https` on the
 onion service, that way we can leave the `force_https` setting enabled, and the
 CSP headers don't prevent forum pages from loading.
 
+Another element to take into account is that Discourse forces the hostname as a
+security feature. This was identified as an issue specifically affecting forums
+hosted behind .onion services in this [meta.discourse.org forum post][].
+
+While the solution suggested in that forum discussion involves patching
+Discourse, another workaround was added later on in the form of the
+`DISCOURSE_BACKUP_HOSTNAME` container config environment variable. When set to
+the .onion hostname, the forum works under both hostnames without issue.
+
+[meta.discourse.org forum post]: https://meta.discourse.org/t/forcing-hostname-has-broken-support-for-onion-services/102419
+
 ## Directory structure
 
 The purpose of the various directories under `/srv/discourse` is described in