159 lines
6.6 KiB
Markdown
159 lines
6.6 KiB
Markdown
# MSC1730: Mechanism for redirecting to an alternative server during login
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## Background/requirements
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This is a proposal for a mechanism for handling the following situation.
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A large, loosely-coupled organisation wants its members to be able to
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communicate with one another via Matrix. The organisation consists of several
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departments which are cooperative but prefer to host their own infrastructure.
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The organisation has an existing single-sign-on system which covers the entire
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organisation, and which they would like their members to use when
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authenticating to the Matrix system.
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## Proposal
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The response to `POST /_matrix/client/r0/login` currently includes the fields
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`user_id`, `access_token`, `device_id`, and the deprecated `home_server`.
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We will add to this the the field `well_known`, which has the same format as
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the [`/.well-known/matrix/client`
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object](https://matrix.org/docs/spec/client_server/r0.4.0.html#get-well-known-matrix-client).
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Servers MAY add this field to the login response if they wish to redirect
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clients to an alternative homeserver after login. Clients SHOULD use the
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provided `well_known` object to reconfigure themselves, optionally validating the
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URLs within.
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Note: a server that redirects all clients to different servers must nonetheless
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consider clients making requests other than `/login`: for example, some clients
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may fail to support redirection. It is acceptable in such a case to return a
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401 response to all non-`/login` requests if the service does not wish to
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support such clients.
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## Application
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Let's imagine for this description that our organisation is the University of
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Canadialand, which is divided into departments including Engineering, History,
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Physics, and so on.
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Central University IT currently host a SAML2-based single-sign-on system, which
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asks users to select their department, and then defers to the departmental
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authentication system to authenticate them. Note that the users do not have a
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globally-unique identifier.
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University IT now sets up a Matrix Homeserver instance, which they host at
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`https://matrix.ac.cdl`. They run a publicity campaign encouraging university
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members to use the service by configuring off-the-shelf Matrix clients to use
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the homeserver at `https://matrix.ac.cdl`. They may also release customised
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clients configured to use that URL by default.
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However, the departments actually want to host their own homeservers; these
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might be at `https://matrix.eng.ac.cdl`, `https://matrix.hist.ac.cdl`, etc. The
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central IT homeserver therefore redirects clients to the departmental
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homeserver after login.
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A complete login flow is as shown in the following sequence diagram:
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Note that this flow is complicated by the out-of-band SAML2 authentication. We
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envisage that a similar technique could also be used for a standard
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username/password authentication, however.
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## Rejected solutions
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Alternative solutions might include:
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### Have all users on one homeserver
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In many situations, it might be more appropriate to have a single homeserver,
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so users' MXids would look like `@user:ac.cdl` instead of
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`@user:eng.ac.cdl`.
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However, there are circumstances where separate homeservers are required:
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* the departments may be only very loosely related
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* the departments may have privacy concerns
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* the dpeartments may be geographically distributed with slow or unreliable
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links to the central system
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* load-balancing may be essential.
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### Tell users the C-S API for their home homeserver
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We could tell Engineering users to configure their clients with
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`https://matrix.eng.ac.cdl`, History users to use `https://matrix.hist.ac.cdl`,
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etc.
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The problems with this are:
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* Each department must issue its own documentation and publicity advising how
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to configure a Matrix client
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* It becomes impractical to distribute preconfigured clients.
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### Proxy all C-S endpoints
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It would be possible for the the central homeserver to proxy all C-S
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interaction, as well as `/login`, directing requests to the right server for
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the user.
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This is unsatisfactory due to the additional latency imposed, the load on the
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central homeserver, and the fact that it makes the central server a single
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point of failure for the entire system.
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### Require clients to perform a .well-known lookup after login
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We could require clients to do a .well-known lookup on the domain of their MXID
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once they have discovered it from the `/login` response.
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This has the following problems:
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* In most cases this `.well-known` lookup will be entirely redundant. It adds
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latency and overhead, and complicates client implementations.
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* It complicates deployment, since each department has to host a `.well-known`
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file at their root domain.
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### Add an alternative redirection mechanism in the login flow
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We could specify that the `/login` response could contain a `redirect` field
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property instead of the existing `user_id`/`access_token`/`device_id`
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properties. The `redirect` property would give the C-S API of the target
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HS. The client would then repeat its `/login` request, and use the specified
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endpoint for all future C-S interaction.
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This approach would complicate client implementations.
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### Modify the single-sign-on flow
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It would be possible to modify the single-sign-on flow to allow an alternative
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homeserver to be specified for the final `m.login.token`-based call to
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`/login` (and subsequent C-S API calls).
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This is discussed in more detail in
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[MSC1731](https://github.com/matrix-org/matrix-doc/blob/rav/proposals/homeserver_in_sso_login/proposals/1731-redirect-in-sso-login.md).
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It has the disadvantage of limiting the solution to SSO logins. The solution
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presented in this proposal also extends to password-based logins.
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### Use a 3pid login flow
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It has been suggested that we could use a login flow based on third-party
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identifiers.
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In the current ecosystem, to do a 3pid login, clients must still be configured
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to send their `/login` request to a particular homeserver, which will then take
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them through an authentication process. We are therefore still left with the
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problem that we need to switch homeservers between login and initial sync.
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An alternative would be for clients to somehow know that they should go through
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the single-sign-on process *before* choosing a homeserver, and for the
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output of the single-sign-on process to indicate the homeserver to use. This
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would require either substantially customised Matrix clients, or substantial
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modifications to the login flow in Matrix, possibly involving authenticating
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against an identity server. The latter is something which could be considered,
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but the scope of the changes required make it impractical in the short/medium
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term.
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