When someone uses an unroutable room ID ("Failed to join"), suggest using via server derived from the room ID

[MadLittleMods]

Problem

When someone uses an unroutable room ID, it currently just presents them with an error modal that looks like following and isn't helpful for knowing what's wrong or how to fix it:

Failed to join

MatrixError: [404] No known servers (https://matrix-client.matrix.org/_matrix/client/r0/join/!abc%3Agitter.im)

Potential solution

We could instead suggest a best-guess via server derived from the room ID. Yes, this does mean parsing an opaque room ID (gasps) to give the guess, but the user still needs to do the correct thing in the end.

This not only helps the user get where they want but also teaches them for next time.

Failed to join

The !JsbthXjlaqosxISjgl:gitter.im room isn't known to your homeserver (matrix.org).

You can try using a "via" server to join through which is supplied as the 2nd parameter in the /join slash command: /join !JsbthXjlaqosxISjgl:gitter.im gitter.im

Or if you're using a matrix.to link, you can try adding a ?via=gitter.im query parameter to the URL: https://matrix.to/#/!JsbthXjlaqosxISjgl:gitter.im?via=gitter.im

Alternatively, if you know a room alias (starts with a #) associated with this room, that will always be routable and allow you to join.

Error details (expand/collapse)

We can even include matrix.org as a fallback as that is probably a great server to use as a shot in the dark for things to work out. We want to delight and teach people, not bury them caveats.

Have you considered any alternatives?

Ideally, this via server assumption could just be automatic but I don't think this would hold up against decision makers with a strong room IDs are opaque strings opinions. And also doesn't teach people for a future where we can't assume.

Additional context

Related issues:

• Permalinks: ! ids are unroutable, and # aliases are not permanent. Please solve. element-web#2925
• https://github.com/vector-im/element-web/issues/17740
• Confusing error when trying to join via an invite link element-web#13247
  • Provide a more detailed error message than "No known servers" matrix-org/matrix-react-sdk#6048
• Cannot join room from matrix.to link, "No known servers" element-web#11756
• https://github.com/vector-im/element-web/issues/22720
• https://spec.matrix.org/v1.5/appendices/#routing

---

Mockup HTML/CSS

<div class="mx_Dialog_content" id="mx_Dialog_content" style="
   line-height: 2.2rem;
">
<p>The <code>!JsbthXjlaqosxISjgl:gitter.im</code> room isn't known to your homeserver (<code>matrix.org</code>).</p>
<p>You can try using a "via" server to join through which is supplied as the 2nd parameter in the /join slash command: <code>/join !JsbthXjlaqosxISjgl:gitter.im gitter.im</code></p>
<p>Or if you're using a matrix.to link, you can try adding a <code>?via=gitter.im</code> query parameter to the URL: <a href="https://matrix.to/#/!JsbthXjlaqosxISjgl:gitter.im?via=gitter.im">https://matrix.to/#/!JsbthXjlaqosxISjgl:gitter.im?via=gitter.im</a></p>
<p>Alternatively, if you know a room alias (starts with a <code>#</code>) associated with this room, that will always be routable and allow you to join.</p>

<details>
<summary>Error details</summary>
<pre><code>...</code></pre>
</details>

</div>

#mx_Dialog_content code {
    background-color: #f4f6fa;
    border: 1px solid #e3e8f0;
    border-radius: 4px;
    padding: 2px 4px;
    line-height: 2.2rem;
    margin-right: 2px;
}

[abhijain2003]

that's a better approach to help end-user. can I work on this?

[abhijain2003]

and I think most client won't be able to understand the routing / and all, so

[t3chguy]

We could instead suggest a best-guess via server derived from the room ID. Yes, this does mean parsing an opaque room ID (gasps) to give the guess, but the user still needs to do the correct thing in the end.

If this is a heuristic we want then it should be implemented in the spec and server rather than every client where it'll be inconsistent

As for defaulting to matrix.org, leaking room IDs of private federations would be considered a privacy issue.