add proposal for adding the dest to federation auth headers

Signed-off-by: Jan Christian Grünhage <jcgruenhage@famedly.com>

proposals/3383-fed-auth-destination.md

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+# MSC3383: Include destination in X-Matrix Auth Header
+
+Currently, a federation request can't be validated mid-flight without talking
+to both the originating server and the destination server. The originating
+server needs to be asked for it's key, and the destination server for it's
+`server_name`, because the `Host` header does not necessarily contain the
+`server_name` of the destination server, if delegation via `.well-known` is
+being used.
+
+This hasn't been a problem so far, as the `server_name` of the destination is
+usually known when validating the `Authorization` header, it's the
+`server_name` of the matrix server that's doing the validation. But there's two
+scenarios where this might now be the case: Forward proxies (that act as an API
+gateway for enforcing additional rules), or Matrix Homeservers implementing
+vhosting and have multiple `server_name`s pointing to the same `Host`.
+
+## Proposal
+
+In addition to the currently present `origin`, `key` and `sig` fields, the
+`Authorization` header of the scheme `X-Matrix` used in the Matrix S2S API
+should also include a `destination` field, which contains the `server_name` of
+the Matrix Homeserver that the request is being sent to.
+
+## Potential issues
+
+Server implementations could theoretically be affected by this change,
+depending on how the header is parsed, which would cause failures in verifying
+the authenticity of the requests. This would be fatal, as it would mean that
+federation would stop working. Luckily, from an initial assessment, it seems
+that all major implementations work here, the parsing implementations in
+Synapse, Dendrite, Conduit, Sydent and SyTest looks like it'd gracefully handle
+this addition without any trouble.
+
+## Alternatives
+
+For the forward proxy scenario, it'd be possible to use the
+`/_matrix/key/v2/server` endpoint for fetching the `server_name` when receiving
+a request. After that though, the `server_name` has to be resolved back to a
+`Host`, for making sure that the domain owner of `server_name` actually intends
+for requests for `server_name` to go to the host in `Host`. This is
+unnecessarily complex and prone to error, which is why it'd be better to have
+that included.
+
+For the vhosting scenario, it'd be possible to have a different hostname to
+delegate to for each vhost. That means that wildcard DNS records and
+certificates have to be used though, to make it manageable to allow anyone
+pointing a `server_name` against a certain service. This is a limitation that
+might be problematic in certain setups, which is why I'd be better to not force
+that.
+
+## Security considerations
+
+I can't think of anything required in this section for this MSC, but I'm open
+to input.