Add config option to use non-default manhole password and keys (#10643)

docs/manhole.md

@@ -11,7 +11,7 @@ Note that this will give administrative access to synapse to **all users** with
 shell access to the server. It should therefore **not** be enabled in
 environments where untrusted users have shell access.
 
-***
+## Configuring the manhole
 
 To enable it, first uncomment the `manhole` listener configuration in
 `homeserver.yaml`. The configuration is slightly different if you're using docker.
@@ -52,16 +52,37 @@ listeners:
     type: manhole
 ```
 
-#### Accessing synapse manhole
+### Security settings
+
+The following config options are available:
+
+- `username` - The username for the manhole (defaults to `matrix`)
+- `password` - The password for the manhole (defaults to `rabbithole`)
+- `ssh_priv_key` - The path to a private SSH key (defaults to a hardcoded value)
+- `ssh_pub_key` - The path to a public SSH key (defaults to a hardcoded value)
+
+For example:
+
+```yaml
+manhole_settings:
+  username: manhole
+  password: mypassword
+  ssh_priv_key: "/home/synapse/manhole_keys/id_rsa"
+  ssh_pub_key: "/home/synapse/manhole_keys/id_rsa.pub"
+```
+
+
+## Accessing synapse manhole
 
 Then restart synapse, and point an ssh client at port 9000 on localhost, using
-the username `matrix`:
+the username and password configured in `homeserver.yaml` - with the default 
+configuration, this would be:
 
 ```bash
 ssh -p9000 matrix@localhost
 ```
 
-The password is `rabbithole`.
+Then enter the password when prompted (the default is `rabbithole`).
 
 This gives a Python REPL in which `hs` gives access to the
 `synapse.server.HomeServer` object - which in turn gives access to many other

docs/sample_config.yaml

@@ -335,6 +335,24 @@ listeners:
   #  bind_addresses: ['::1', '127.0.0.1']
   #  type: manhole
 
+# Connection settings for the manhole
+#
+manhole_settings:
+  # The username for the manhole. This defaults to 'matrix'.
+  #
+  #username: manhole
+
+  # The password for the manhole. This defaults to 'rabbithole'.
+  #
+  #password: mypassword
+
+  # The private and public SSH key pair used to encrypt the manhole traffic.
+  # If these are left unset, then hardcoded and non-secret keys are used,
+  # which could allow traffic to be intercepted if sent over a public network.
+  #
+  #ssh_priv_key_path: CONFDIR/id_rsa
+  #ssh_pub_key_path: CONFDIR/id_rsa.pub
+
 # Forward extremities can build up in a room due to networking delays between
 # homeservers. Once this happens in a large room, calculation of the state of
 # that room can become quite expensive. To mitigate this, once the number of