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docs/configuring-playbook-federation.md

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+# Controlling Matrix federation (optional)
+
+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/host_vars/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).
+
+
+## Exposing the room directory over federation
+
+By default, your server's public rooms directory is not exposed to other servers via federation.
+
+If you wish to expose it, add this to your configuration file (`inventory/host_vars/matrix.<your-domain>/vars.yml`):
+
+```yaml
+matrix_synapse_allow_public_rooms_over_federation: true
+```
+
+
+## Disabling federation
+
+To completely disable federation, isolating your server from the rest of the Matrix network, add this to your configuration file (`inventory/host_vars/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.
+
+**Disabling federation does not necessarily disable the federation port** (`8448`). Services like [Dimension](configuring-playbook-dimension.md) and [ma1sd](configuring-playbook-ma1sd.md) normally rely on `openid` APIs exposed on that port. Even if you disable federation and only if necessary, we may still be exposing the federation port and serving the `openid` APIs there. To override this and completely disable Synapse's federation port use:
+
+```yaml
+# This stops the federation port on the Synapse side (normally `matrix-synapse:8048` on the container network).
+matrix_synapse_federation_port_enabled: false
+
+# This removes the `8448` virtual host from the matrix-nginx-proxy reverse-proxy server.
+matrix_nginx_proxy_proxy_matrix_federation_api_enabled: false
+```