matrix: non-federated rooms

pages/matrix.markdown

@@ -28,6 +28,7 @@ I also have a [txt with a list of all my accounts](/txt/matrix.txt) which [has S
   - [Can I see who is in any specific room without being there?](#can-i-see-who-is-in-any-specific-room-without-being-there)
   - [What are state resets?](#what-are-state-resets)
     - [How about DAG splits?](#how-about-dag-splits)
+  - [Can I have a non-federated room?](#can-i-have-a-non-federated-room)
   - [What are these idlekicks for inactivity, why are they for?](#what-are-these-idlekicks-for-inactivity-why-are-they-for)
     - [But the relaybots look so ugly](#but-the-relaybots-look-so-ugly)
   - [I am told that I should Matrixify my IRC channel, what does that mean?](#i-am-told-that-i-should-matrixify-my-irc-channel-what-does-that-mean)
@@ -145,6 +146,39 @@ People understanding state resolution (which by the way don't include me)
 disagree on the exact cause only agreeing that it's difficult to fix. From
 what is told to me, I understand it to be tracked [in the same Synapse issue #8629](https://github.com/matrix-org/synapse/issues/8629).
 
+### Can I have a non-federated room?
+
+Yes, there are two methods.
+
+1. During room creation, Element Web offers an option to have a non-federated
+   room. That will permanently prevent any other homeserver from joining and
+   to change that a manual room upgrade is required.
+1. What I recommend instead is setting a server ACL, so if necessary it can be
+   changed later. This may be helpful when migrating to another domain (which
+   Matrix doesn't support) or cooperation with another entity with their own
+   homeserver or anything.
+
+The second method begins with the usual `/devtools`, explore room state, `Send
+custom state event`, enter type as `m.room.server_acl` and contents:
+
+```json
+{
+  "allow": ["example.org"],
+  "allow_ip_literals": false,
+  "deny": []
+}
+```
+
+Now assuming all homeservers in the room implement ACL, only `example.org`
+users can join the room.
+
+For futher reading about ACL:
+
+- [matrix.org: Moderation in Matrix, Banning servers from rooms (Server ACLs)](https://matrix.org/docs/guides/moderation#banning-servers-from-rooms-server-acls)
+  - [[TODO: release our server-ACL enforcing scripts]](https://github.com/matrix-org/matrix.org/issues/557)
+- [Matrix Specification on ACL](https://spec.matrix.org/latest/client-server-api/#server-access-control-lists-acls-for-rooms)
+- [matrix-org/matrix-spec#928: Consider handling server ACLs through event auth rules rather than at the network level #928](https://github.com/matrix-org/matrix-spec/issues/928)
+
 ### What are these idlekicks for inactivity, why are they for?
 
 Some Matrix rooms decide to connect their channel to IRC maintaining the same users on both sides, which can be heavy for the IRC network depending on bridge type of which there are three "major" variants: