.well-known request comes with a double slash - federation delegation not working as expected

[apfelnutzer]

Steps to reproduce

1. Where are you starting? What can you see?
   I discovered failed Element requests in my Traefik logs showing that the .well-known request of Element X for iOS starts with a double "/":

[25/Feb/2025:14:46:22 +0000] "GET //.well-known/element/element.json HTTP/1.1" 404 1236 "-" "Element X/25.02.0

Moreover Element X ignored federation delegation on my server which I fixed within Traefik. Synapse is hosted on matrix.server.tld, but requests where send to server.tld despite delegation, so I forwarded everything starting with server.tld/_matrix to matrix.server.tld/_matrix ...

Outcome

What did you expect?

Request should just contain one /, .well-known files is loaded/considered respectively and further request are considering delegation.

What happened instead?

Request contains two slashed, .well-known files are ignored.

Your phone model

iPhone 13

Operating system version

iOS 18.3.1

Application version

25.02.0

Homeserver

1.124.0

Will you send logs?

No

[pixlwave]

Hi, thanks for reporting this. The double slash sounds like a bug, however .well-known look-up isn't meant to respect delegation, it is computed from your user ID.

This spec provides more detail about how this works: https://spec.matrix.org/v1.13/client-server-api/#well-known-uri

[apfelnutzer]

Thanks for looking into this! What I meant is, that subsequent request are not considering the federation delegation. I fixed this (until further notice) by forwarding all server.tld/_matrix traffic respectively.

[pixlwave]

The subsequent requests are not meant to do anything different to the first requests. The .well-known is always meant to be found from the last part of your user ID, no matter where the server lives.

Also, fwiw clients have nothing to do with federation - only .well-known/matrix/server is for servers (and federation).

[apfelnutzer]

I made sure, that the .well-known file can by found under server.tld (by Traefik, hosted by nginx). This holds for a server and client file. The client file says:

{
  "m.homeserver": {
    "base_url": "https://matrix.server.tld"
  }
}

Isn't that meant for delegation (user names are @user:server.tld)?

[pixlwave]

Ohhhh wait sorry. So it's doing server.tld//.well-known/element/element.json which is fine (apart from the double slash) and will continue to do this for well-knowns. But then you're saying it's also doing e.g. server.tld/_matrix/client/v3/login instead of matrix.server.tld/_matrix/client/v3/login?