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MSC3743: Standardized error response for unknown endpoints

Matrix does not define how a server should treat unknown endpoints. This makes it difficult 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, as well as requiring servers to implement workarounds based on heuristics.

Proposal

The Client-Server API, Server-Server API, Application Service API, Identity Service API, and Push Gateway shall respond with the following for all paths under the /_matrix prefix:

404 HTTP error response with an error code of M_UNRECOGNIZED if a request to an unknown path is received.
405 HTTP error response with an error code of M_UNRECOGNIZED if an invalid method for a known path is received.

Both of these can be treated the same from the client making the request and represent that the request cannot be routed to business logic.

Note that the major homeserver implementations have recently settled on using M_UNRECOGNIZED, so it seems reasonable to specify this formally.

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.

A M_UNRECOGNIZED error code with a status of 400 has more specific definition in the Identity Server API:

The request contained an unrecognised value, such as an unknown token or medium.

Alternatives

MSC3723 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.

It could be proposed to only handle the known prefixes under /_matrix:

/_matrix/client/
/_matrix/federation/
/_matrix/media/
/_matrix/keys/
/_matrix/push/
/_matrix/identity/
/_matrix/app/

But it is more future-proof to include the entire /_matrix prefix in case additional services are added to the Matrix ecosystem.

Security considerations

None.

Unstable prefix

None (as this is about unknown endpoints!)

Current status

Issue #1492 discusses this problem a bit, but does not propose a concrete solution. ¹

Homeserver API

Tested by querying for:

Unprefixed: GET /_matrix/unknown
Client-Server: GET /_matrix/client/v4/login,
Server-Server: GET /_matrix/federation/unknown.
Media: GET /_matrix/media/unknown
Keys: GET /_matrix/keys/unknown

Tested incorrect method via:

PUT /_matrix/client/v3/login
PUT /_matrix/federation/v1/version
PUT /_matrix/media/v3/upload
PUT /_matrix/v3/keys/upload

If the below doesn't include differences for an incorrect method, assume it is identical to the behavior of an unknown endpoint.

Synapse:

Unprefixed, and Keys: 404 error with an HTML body
Client-Server:
- < 1.53.0: 404 error with an HTML body
- >= 1.53.0: 400 error with a JSON body ²
```
{"errcode": "M_UNRECOGNIZED", "error": "Unrecognized request"}
```

Server-Server: 400 error with a JSON body

{"errcode": "M_UNRECOGNIZED", "error": "Unrecognized request"}

Media:
- 404 error with an HTML body
- Incorrect method: 400 error with a JSON body:
```
{"errcode": "M_UNRECOGNIZED", "error": "Unrecognized request"}
```

Dendrite:

Unprefixed and Server-Server:
- 404 error with a text body of 404 page not found
- Incorrect method: 405 error with no body
Client-Server:
- < 0.10.0: 404 error with a text body of 404 page not found
- >= 0.10.0: 404 error with a JSON body (but a text/plain content type) ³
```
{"errcode": "M_UNRECOGNIZED", "error": "Unrecognized request"}
```
Media:
- 404 error with a text body of 404 page not found
- Incorrect method: 405 error with a text body of 405 PUT not allowed on this endpoint
Keys: 404 error with a text body of 404 page not found

Conduit:

Unprefied, Client-Server, Server-Server, and Media:
- < 0.4.0: 404 error with no body
- == 0.4.0: 404 error with a JSON body ⁴
```
{"errcode": "M_NOT_FOUND", "error": "M_NOT_FOUND: Unknown or unimplemented route"}
```
- > 0.4.0: 404 error with a JSON body ⁵
```
{"errcode": "M_UNRECOGNIZED", "error": "M_UNRECOGNIZED: Unrecognized request"}
```
- Incorrect method: 405 error with no body
Keys: handled as unknown endpoints as above -- unimplemented?

matrix-media-repo (Media-only):

404 error with a JSON body:

{"errcode": "M_NOT_FOUND", "error": "Not found", "mr_errcode": "M_NOT_FOUND"}

Incorrect method: 405 error with a JSON body:

{"errcode": "M_UNKNOWN", "error": "Method Not Allowed", "mr_errcode": "M_METHOD_NOT_ALLOWED"}

Application Service API

Tested by querying for GET /_matrix/app/unknown

Untested, but anecdotally been told this is poorly handled.

Identity Service API,

Tested by querying for GET /_matrix/identity/unknown.

Sydent:
- 404 with an HTML body
- Incorrect method: 405 with an HTML body

Push Gateway

Tested by querying for GET /_matrix/push/unknown.

Sygnal:
- 404 with an HTML body
- Incorrect method: 405 with an HTML body

Dependencies