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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):

/_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

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