Add an MSC for unknown endpoints.

proposals/3742-errors-for-unknown-endpoints.md

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+# MSC3742: Standardized error response for unknown endpoints
+
+Matrix does not define how a server should treat unknown endpoints. This makes it
+more difficult than necessary to determine whether an endpoint is responding with
+a legitimate error (e.g. a 404 for an object not being found) or because it does
+not support the endpoint.
+
+This has impacted clients [wishing to support stable features](https://github.com/vector-im/element-web/issues/19738),
+as well as requiring servers to [implement workarounds](https://github.com/matrix-org/synapse/blob/a711ae78a8f8ba406ff122035c8bf096fac9a26c/synapse/federation/federation_client.py#L602-L622)
+based on heuristics.
+
+
+## Proposal
+
+The Client-Server API, Server-Server API, Application Service API, Identity Service API,
+and Push Gateway shall respond with a 400 HTTP error response with an error code
+of `M_UNKNOWN_ENDPOINT`. This is required for all endpoints under the `/_matrix`
+prefix and is recommended for any server-specific endpoints that might be
+implemented (e.g. the `/_synapse` prefix used by Synapse's admin API).
+
+This applies to a server not known the URL or an incorrect method being used for
+that URL, i.e. whenever nothing exists to handle a request.
+
+
+## Potential issues
+
+Servers and clients may still need to rely on heuristics until this is widely
+available, but this proposal should not cause any additional issues.
+
+
+## Alternatives
+
+[MSC3723](https://github.com/matrix-org/matrix-doc/pull/3723) could be an alternative
+to this, but it has the downside of servers needing to track the version state
+of each other server it is interacting with.
+
+It is seen as being more narrowly applicable (only to the Server-Server API),
+while also being more complicated to implement.
+
+A new error code was chosen since the `M_UNRECOGNISED` error code has more specific
+definitions in the Identity Servier API:
+
+> The request contained an unrecognised value, such as an unknown token or medium.
+
+It could be left to the status quo of being unspecified (or as being implied
+by the HTTP specification), but this has already shown to not work, as Synapse,
+Conduit, and Dendrite return different errors for unknown endpoints.
+
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+None (as this is about unknown endpoints!)
+
+## Current status
+
+[Issue #1492](https://github.com/matrix-org/matrix-doc/issues/1492) discusses this
+problem a bit, but does not propose a concrete solution.
+
+### Client-Server API
+
+Tested by querying for `GET /_matrix/client/v4/login`
+
+* Synapse:
+  * < 1.53.0: 404 error with an HTML body
+  * \>= 1.53.0: 400 error with a JSON body
+    ```json
+    {"errcode": "M_UNRECOGNIZED", "error": "Unrecognized request"}
+    ```
+* Dendrite: 404 error with body of `404 page not found`
+* Conduit: 404 error with no body
+
+### Server-Server API
+
+Tested by querying for `GET /_matrix/federation/unknown`.
+
+* Synapse: 400 error with a JSON body
+  ```json
+  {"errcode": "M_UNRECOGNIZED", "error": "Unrecognized request"}
+  ```
+* Dendrite: 404 error with body of `404 page not found`
+* Conduit: 404 error with no body
+
+### Application Service API
+
+Untested
+
+### Identity Service API,
+
+* Sydent: 404 with an HTML body
+
+### Push Gateway
+
+* Sygnal: 404 with an HTML body
+
+## Dependencies
+
+None.