MSC3860: Media Download Redirects (#3860)

* Add media download redirect proposal

* Add urls

* Rename

* Wrap content

* Add note about MSC proposal and implementation work

* Add missing sections to MSC

* Rework to be opt-in via query param

* Fix typos

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Clarify security considerations

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Clarify that unstable prefix should be used

Signed-off-by: Sumner Evans <sumner@beeper.com>

* Add 308 status code for redirects

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Add clarification about no/false redirect param

* Add link to current spec media endpoints

* Add note about additional benefit of not proxying untrusted homeserver media

* Expand issues section to cover possible homeserver abuse

* fix a typo

---------

Signed-off-by: Sumner Evans <sumner@beeper.com>
Co-authored-by: Sumner Evans <sumner@beeper.com>
Co-authored-by: Travis Ralston <travpc@gmail.com>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

proposals/3860-media-download-redirect.md

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+# MSC3860: Media Download Redirects
+
+Currently the media download endpoints must return either a 200 with content or error responses. This
+means the media server instance must stream the data from wherever it is stored, which is likely not
+local to itself. Allowing redirects on these endpoints would  make it possible for the media repo to
+tell clients/servers to pull data direct from the source, e.g. a CDN.
+
+Additionally redirects could be used to avoid downloading media from untrusted homeservers. Currently
+users can make their homeserver download, cache and proxy any matrix mid that I want, including
+bad/illegal content. Allowing for a 307 redirect would permit cautious server operators to not
+store and provide any media that floats in the matrixverse, but just refer to the "original" media.
+
+## Proposal
+
+This MSC proposes that a 307 or 308 redirect code is allowed and followed according to the `Location`
+header. It is possible some clients would already follow these which needs to be confirmed. Specific
+endpoints in question ([current spec link for these](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3downloadservernamemediaid)):
+
++ `/_matrix/media/v3/download/{serverName}/{mediaId}`
++ `/_matrix/media/v3/download/{serverName}/{mediaId}/{fileName}`
++ `/_matrix/media/v3/thumbnail/{serverName}/{mediaId}`
+
+To prevent breaking clients that don't properly follow the redirect response this functionality will
+be enabled by a query string flag `allow_redirect=true`. So specifically in the above cases if a
+client respects redirect responses it can make requests like so to the media endpoints:
+
++ `/_matrix/media/v3/download/{serverName}/{mediaId}?allow_redirect=true`
++ `/_matrix/media/v3/download/{serverName}/{mediaId}/{fileName}?allow_redirect=true`
++ `/_matrix/media/v3/thumbnail/{serverName}/{mediaId}?allow_redirect=true`
+
+In the case where a client wishes not to redirect (either implicitly with no parameter or explicitly
+providing `allow_redirect=false`) the server must continue to serve media directly with no redirect.
+
+## Potential Issues
+
+None for clients, as opt-in functionality this change is 100% backwards compatible.
+
+With redirects it becomes easier to force another homeserver to be your CDN, which isn't great
+(clients could already do that, but not without recompiling).
+
+Redirects also potentially allow changing of media underneath the users as different redirects could
+be returned over time. It is worth noting that a badly behaving media server can already just return
+different content as well.
+
+## Alternatives
+
+None at this time.
+
+## Security Considerations
+
+A media repo could redirect requests to a bad actor, although this would make the primary media
+repo itself a bad actor, thus this does not present any increased security issues.
+
+## Unstable Prefix
+
+Until this functionality has landed in the spec, the `allow_redirect` query
+parameter should be prefixed with `com.beeper.msc3860.`:
+
+```
+?com.beeper.msc3860.allow_redirect=true
+```