MSC3860: Media Download Redirects

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, eg. a CDN.
+
+## Proposal
+
+This MSC proposes that a 307 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. Specifc
+endpoints in question:
+
++ `/_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`
+
+## Potential Issues
+
+None, as opt-in functionality this change is 100% backwards compatible.
+
+## 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, this does present any increased security issues.
+
+## Unstable Prefix
+
+No need for an unstable prefix for redirects as it stands. If a query string was to be used to
+enable this functionality this could use the following unstable prefix:
+
+```
+?com.beeper.msc3860.allow_redirect=true
+```