matrix-docker-ansible-deploy/docs/configuring-playbook-federation.md

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):

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/host_vars/matrix.<your-domain>/vars.yml):

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.