2432-revised-alias-publishing.md

MSC2432: Updated semantics for publishing room aliases

This MSC offers an alternative to MSC2260.

Background

The m.room.aliases state event exists to list the available aliases for a given room. This serves two purposes:

• It allows existing members of the room to discover alternative aliases, which may be useful for them to pass this knowledge on to others trying to join.

• Secondarily, it helps to educate users about how Matrix works by illustrating multiple aliases per room and giving a perception of the size of the network.

However, it has problems:

• Any user in the entire ecosystem can create aliases for rooms, which are then unilaterally added to m.room.aliases, and room admins are unable to remove them. This is an abuse vector (#625).

• For various reasons, the m.room.aliases event tends to get out of sync with the actual aliases (https://github.com/matrix-org/matrix-doc/issues/2262).

Proposal

We propose that that room moderators should be able to manually curate a list of "official" aliases for their room, instead of matrix servers automatically publishing lists of all room aliases into the room state. No particular guarantees are offered that this alias list is entirely accurate: it becomes room moderators' responsibility to keep it so.

Meanwhile, the aliases that map to a given room on a given server become the ultimate responsibility of the administrators of that server. We give them tools to inspect the alias list and clean it up when necessary, in addition to the current tools which allow restriction of who can create aliases in the first place.

A detailed list of proposed modifications to the Matrix spec follows:

• m.room.aliases loses any special meaning within the spec. In particular:

  • Clients should no longer format it specially in room timelines.

  • Clients and servers should no longer consider m.room.aliases when calculating the display name for a room.

    (Note: servers follow the room display-name algorithm when calculating room names for certain types of push notification.)

  • A future room version will remove the special authorization rules and redaction rules.

• m.room.canonical_alias is extended to include a new alt_aliases property. This, if present, should be a list of alternative aliases for the room. An example event might look like:

  {
    "content": {
      "alias": "#somewhere:localhost",
      "alt_aliases": [
        "#somewhere:overthere.com",
        "#somewhereelse:example.com"
      ]
    },
    "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
    "state_key": "",
    "type": "m.room.canonical_alias"
  }

  It is valid for alt_aliases to be non-empty even if alias is absent or empty. This means that no alias has been picked out as the 'main' alias.

  (Note: although the spec currently claims that alias is mandatory, Synapse generates m.room.canonical_alias events with no alias property when the main alias is deleted. This change would legitimise that behaviour.)

  (For clarity: it is not proposed that the alt_aliases be considered when calculating the displayname for a room.)

• PUT /_matrix/client/r0/rooms/{roomId}/state/{eventType}/{stateKey} is extended to recommend that servers validate any new aliases added to m.room.canonical_alias by checking that it is a valid alias according to the syntax, and by looking up the alias and and that it corresponds to the expected room ID.

  (Note: Synapse currently implements this check on the main alias, though this is unspecced.)

  The following error codes are specified:

  • HTTP 400, with errcode: M_INVALID_PARAMETER if an attempt is made to add an entry which is not a well-formed alias (examples: too long, doesn't start with #, doesn't contain a :).

  • HTTP 400, with errcode: M_BAD_ALIAS if an added alias does not point at the given room (either because the alias doesn't exist, because it can't be resolved due to an unreachable server, or because the alias points at a different room).

• Currently, PUT /_matrix/client/r0/directory/room/{roomAlias} attempts to send updated m.room.aliases events on the caller's behalf. (This is implemented in Synapse but does not appear to be explicitly specced.) This functionality should be removed.

• Currently, DELETE /_matrix/client/r0/directory/room/{roomAlias}, attempts to send updated m.room.aliases and/or m.room.canonical_alias events on the caller's behalf, removing any aliases which have been deleted. (Again, this is implemented in Synapse but does not appear to be explicitly specced.) The m.room.aliases functionality should be removed, and the m.room.canonical_alias functionality should be extended to cover alt_aliases.

  The behaviour if the calling user has permission to delete the alias but does not have permission to send m.room.canonical_alias events in the room (for example, by virtue of being a "server administrator", or by being the user that created the alias) is implementation-defined. It is recommended that in this case, the alias is deleted anyway, and a successful response is returned to the client.

• A new api endpoint, GET /_matrix/client/r0/rooms/{roomId}/aliases is added, which returns the list of aliases currently defined on the local server for the given room. The response looks like:

  {
    "aliases": [
      "#somewhere:example.com",
      "#somewhereelse:example.com",
      "#special_alias:example.com"
    ]
  }

  This API can be called by any current member of the room (calls from other users result in M_FORBIDDEN). For rooms with history_visibility set to world_readable, it can also be called by users outside the room.

  Servers might also choose to allow access to other users such as server administrators.

• GET /_matrix/client/r0/publicRooms (and the POST variant) no longer return aliases as part of PublicRoomsChunk. Clients do not appear to make use of this field, and canonical_alias is maintained to provide similar information.

Various APIs are currently subject to implementation-defined access restrictions. No change to the specification is introduced in this regard (implementations will continue to be free to impose their own restrictions). Nevertheless as part of this MSC it is useful to consider some proposed changes to Synapse's implementation:

• No change: PUT /_matrix/client/r0/directory/room/{roomAlias}: Synapse only allows access to current members of the room, and also exposes some configuration options which allow restriction of which users are allowed to create aliases in general.

• DELETE /_matrix/client/r0/directory/room/{roomAlias}: in this case, currently Synapse restricts its use to the user that created the alias, and server admins.

  It is proposed to extend this to local users who have a power-level sufficient to send an m.room.canonical_alias event in the room that the alias currently points to.

• PUT /_matrix/client/r0/directory/list/room/{roomId} and the corresponding unspecced DELETE api (both of which set the visibility of a room in the public directory): currently Synapse restricts their use to server admins and local users who have a PL sufficient to send an m.room.aliases event in the room (ignoring the special auth rules). This will be changed to check against the PL required to send an m.room.canonical_alias event.

It is envisaged that Matrix clients will then change their "Room Settings" user interface to display the aliases from m.room.canonical_alias instead of those in m.room.aliases, as well as giving moderators the ability to update that list. Clients might also wish to use the new GET /_matrix/client/r0/rooms/{roomId}/aliases endpoint to obtain and display the currently-available local aliases, though given that this list may be subject to abuse, it should probably not be shown by default.

Future work

This work isn't considered part of this MSC, but rather a potential extension for the future.

• It may be useful to be able to query remote servers for their alias list. This could be done by extending GET /_matrix/client/r0/rooms/{roomId}/aliases to take a server_name parameter, and defining an API in the server_server spec which will expose the requested information, subject to the calling homeserver having at least one user with a right to see it.

• Similarly, room moderators may wish to be able to delete aliases on a remote server for their room. We could envisage a federation API which allows such a request to be made, subject to the calling homeserver having at least one moderator in the room.

Potential issues

The biggest problem with this proposal is that existing clients, which rely on m.room.aliases in one way or another, will lose functionality. In particular, they may not know about aliases that exist, or they may look at outdated m.room.aliases events that list aliases that no longer exist. However, since m.room.aliases is best-effort anyway, these are both problems that exist to some extent today.

Alternatives

We considered continuing to use m.room.aliases to advertise room aliases instead of m.room.canonical_alias, but the significant changes in semantics made that seem inappropriate.

We also considered using separate state events for each advertised alias, rather than listing them all in one event. This might increase the number of aliases which can be advertised, and help to reduce races when editing the list. However, the 64KB limit of an event still allows room for hundreds of aliases of any sane length, and we don't expect the list to be changing frequently enough for races to be a practical concern. Ultimately the added complexity seemed redundant.

A previous suggestion was MSC2260, which proposed keeping m.room.aliases largely as-is, but giving room moderators tools to control who can send them via room power-levels. We dismissed it for the reasons set out at #2260 (comment).

Security considerations

None currently identified.

Unstable prefix

While this feature is in development, the following names will be in use:

Proposed final name	Name while in development
GET /_matrix/client/r0/rooms/{roomId}/aliases	GET /_matrix/client/unstable/org.matrix.msc2432/rooms/{roomId}/aliases

Servers will indicate support for the new endpoint via a non-empty value for feature flag org.matrix.msc2432 in unstable_features in the response to GET /_matrix/client/versions.