Add support for controlling Matrix federation

docs/configuration-playbook-federation.md

@@ -0,0 +1,28 @@
+# Controlling Matrix federation
+
+By default, your server federates with the whole Matrix network.
+That is, people on your server can communicate with people on any other Matrix server.
+
+
+## Federating only with select servers
+
+To make your server only federate with servers of your choosing, add this to your configuration file (`inventory/matrix.<your-domain>/vars.yml`):
+
+```yaml
+matrix_synapse_federation_domain_whitelist:
+- example.com
+- another.com
+```
+
+If you wish to disable federation, you can do that with an empty list (`[]`), or better yet by completely disabling federation (see below).
+
+
+## Disabling federation
+
+To completely disable federation, isolating your server from the rest of the Matrix network, add this to your configuration file (`inventory/matrix.<your-domain>/vars.yml`):
+
+```yaml
+matrix_synapse_federation_enabled: false
+```
+
+With that, your server's users will only be able to talk among themselves, but not to anyone who is on another server.

docs/configuring-playbook.md

@@ -23,6 +23,8 @@ When you're done with all the configuration you'd like to do, continue with [Ins
 
 ## Other configuration options
 
+- [Controlling Matrix federation](configuration-playbook-federation.md) (optional)
+
 - [Adjusting email-sending settings](configuring-playbook-email.md) (optional)
 
 - [Storing Matrix media files on Amazon S3](configuring-playbook-s3.md) (optional)