Allow to limit federation on a user level

[schiessle]

It would be great if the admin of a homeserver could limit/configure the federation capabilities on a per user basis.

Possible Use Cases:

1. In a school you might want to allow teachers and maybe pupils from the secondary school to federate with people outside of the school. Pupils from the primary school should only be able to interact with people from the same school/homeserver. In a second step it could be even more fine-grained, for example teachers have no federation limits at all, pupils from the secondary school can federate with a list of "trusted homeservers", e.g. other schools you collaborate with, primary school pupils have federation disabled and can only chat with pupils on the same home server.

2. Another usecase might be a private homeserver for a single family. The parents want to use the full capabilities of Matrix and the federation features while for the children it should be a save place where they can only communicate with the family members (no federation)

[DMRobertson]

I think this is tricky to do per-user because federation happens at the room level between entire homeservers. It would require some careful thought.

Some very rough and quick ideas:

• It might be possible to hack this server-side by omitting events sent by other HSes in /sync responses for nominated users' client---though we'd probably have to exclude certain classes of state events (power levels, memberships, ...)
• At present one can mark a room as permanently unfederatable. Perhaps a Synapse module could make it so that only "privileged" users (e.g. teachers) can create rooms which are federatable, and enforce that "limited" users (e.g. pupils) can only create unfederatable rooms.

[schiessle]

For rooms we already have the settings to create rooms, only accessible for people on the same homeserver:

Therefore I think such a setting on user level should be possible.

If federation is disabled for a user, than:

• only people from the same server can find the user (without the federation capabilities) and invite them to a 1:1 room
• other users can only invite users (without the federation capabilities) to group rooms if the rooms are limited to the homeserver
• the user (without the federation capabilities) can only find and invite people on the same homeserver to a 1:1 chat
• the user (without the federation capabilities) can only find and join rooms which are limited to the homeserver

Or do I miss something?