docs-matrix-spec/proposals/1730-cs-api-in-login-response.md

MSC1730: Mechanism for redirecting to an alternative server during login

Background/requirements

This is a proposal for a mechanism for handling the following situation.

A large, loosely-coupled organisation wants its members to be able to communicate with one another via Matrix. The organisation consists of several departments which are cooperative but prefer to host their own infrastructure.

The organisation has an existing single-sign-on system which covers the entire organisation, and which they would like their members to use when authenticating to the Matrix system.

Proposal

The response to POST /_matrix/client/r0/login currently includes the fields user_id, access_token, device_id, and the deprecated home_server.

We will add to this the the field well_known, which has the same format as the /.well-known/matrix/client object.

Servers MAY add this field to the login response if they wish to redirect clients to an alternative homeserver after login. Clients SHOULD use the provided well_known object to reconfigure themselves, optionally validating the URLs within.

Application

Let's imagine for this description that our organisation is the University of Canadialand, which is divided into departments including Engineering, History, Physics, and so on.

Central University IT currently host a SAML2-based single-sign-on system, which asks users to select their department, and then defers to the departmental authentication system to authenticate them. Note that the users do not have a globally-unique identifier.

University IT now sets up a Matrix Homeserver instance, which they host at https://matrix.ac.cdl. They run a publicity campaign encouraging university members to use the service by configuring off-the-shelf Matrix clients to use the homeserver at https://matrix.ac.cdl. They may also release customised clients configured to use that URL by default.

However, the departments actually want to host their own homeservers; these might be at https://matrix.eng.ac.cdl, https://matrix.hist.ac.cdl, etc. The central IT homeserver therefore redirects clients to the departmental homeserver after login.

A complete login flow is as shown in the following sequence diagram:

Note that this flow is complicated by the out-of-band SAML2 authentication. We envisage that a similar technique could also be used for a standard username/password authentication, however.

Rejected solutions

Alternative solutions might include:

Have all users on one homeserver

In many situtations, it might be more appropriate to have a single homeserver, so users' MXids would look like @user:ac.cdl instead of @user:eng.ac.cdl.

However, there are circumstances where separate homeservers are required:

• the departments may be only very loosely related
• the departments may have privacy concerns
• the dpeartments may be geographically distributed with slow or unreliable links to the central system
• load-balancing may be essential.

Tell users the C-S API for their home homeserver

We could tell Engineering users to configure their clients with https://matrix.eng.ac.cdl, History users to use https://matrix.hist.ac.cdl, etc.

The problems with this are:

• Each department must issue its own documentation and publicity advising how to configure a Matrix client

• It becomes impractical to distribute preconfigured clients.

Proxy all C-S endpoints

It would be possible for the the central homeserver to proxy all C-S interaction, as well as /login, directing requests to the right server for the user.

This is unsatisfactory due to the additional latency imposed, the load on the central homeserver, and the fact that it makes the central server a single point of failure for the entire system.

Require clients to perform a .well-known lookup after login

We could require clients to do a .well-known lookup on the domain of their MXID once they have discovered it from the /login response.

This has the following problems:

• In most cases this .well-known lookup will be entirely redundant. It adds latency and overhead, and complicates client implementations.

• It complicates deployment, since each department has to host a .well-known file at their root domain.

Add an alternative redirection mechanism in the login flow

We could specify that the /login response could contain a redirect field property instead of the existing user_id/access_token/device_id properties. The redirect property would give the C-S API of the target HS. The client would then repeat its /login request, and use the specified endpoint for all future C-S interaction.

This approach would complicate client implementations.