Federation outbound proxy (#15773)

Allow configuring the set of workers to proxy outbound federation traffic through (`outbound_federation_restricted_to`).

This is useful when you have a worker setup with `federation_sender` instances responsible for sending outbound federation requests and want to make sure *all* outbound federation traffic goes through those instances. Before this change, the generic workers would still contact federation themselves for things like profile lookups, backfill, etc. This PR allows you to set more strict access controls/firewall for all workers and only allow the `federation_sender`'s to contact the outside world.

The original code is from @erikjohnston's branches which I've gotten in-shape to merge.

2 files changed, 44 insertions, 7 deletions

diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index 26d7c7900c..89a92c4682 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -3930,13 +3930,14 @@ federation_sender_instances:
 ---
 ### `instance_map`
 
-When using workers this should be a map from [`worker_name`](#worker_name) to the
-HTTP replication listener of the worker, if configured, and to the main process.
-Each worker declared under [`stream_writers`](../../workers.md#stream-writers) needs
-a HTTP replication listener, and that listener should be included in the `instance_map`.
-The main process also needs an entry on the `instance_map`, and it should be listed under
-`main` **if even one other worker exists**. Ensure the port matches with what is declared 
-inside the `listener` block for a `replication` listener.
+When using workers this should be a map from [`worker_name`](#worker_name) to the HTTP
+replication listener of the worker, if configured, and to the main process. Each worker
+declared under [`stream_writers`](../../workers.md#stream-writers) and
+[`outbound_federation_restricted_to`](#outbound_federation_restricted_to) needs a HTTP replication listener, and that
+listener should be included in the `instance_map`. The main process also needs an entry
+on the `instance_map`, and it should be listed under `main` **if even one other worker
+exists**. Ensure the port matches with what is declared inside the `listener` block for
+a `replication` listener.
 
 
 Example configuration:
@@ -3966,6 +3967,22 @@ stream_writers:
   typing: worker1
 ```
 ---
+### `outbound_federation_restricted_to`
+
+When using workers, you can restrict outbound federation traffic to only go through a
+specific subset of workers. Any worker specified here must also be in the
+[`instance_map`](#instance_map).
+
+```yaml
+outbound_federation_restricted_to:
+  - federation_sender1
+  - federation_sender2
+```
+
+Also see the [worker
+documentation](../../workers.md#restrict-outbound-federation-traffic-to-a-specific-set-of-workers)
+for more info.
+---
 ### `run_background_tasks_on`
 
 The [worker](../../workers.md#background-tasks) that is used to run
diff --git a/docs/workers.md b/docs/workers.md
index 735128762a..303e0f0e7a 100644
--- a/docs/workers.md
+++ b/docs/workers.md
@@ -531,6 +531,26 @@ the stream writer for the `presence` stream:
 
     ^/_matrix/client/(api/v1|r0|v3|unstable)/presence/
 
+#### Restrict outbound federation traffic to a specific set of workers
+
+The `outbound_federation_restricted_to` configuration is useful to make sure outbound
+federation traffic only goes through a specified subset of workers. This allows you to
+set more strict access controls (like a firewall) for all workers and only allow the
+`federation_sender`'s to contact the outside world.
+
+```yaml
+instance_map:
+    main:
+        host: localhost
+        port: 8030
+    federation_sender1:
+        host: localhost
+        port: 8034
+
+outbound_federation_restricted_to:
+  - federation_sender1
+```
+
 #### Background tasks
 
 There is also support for moving background tasks to a separate