MSC2499: Fixes for Well-known URIs #2499

aaronraimist · 2020-04-13T23:55:14Z

Intended to address:
https://github.com/matrix-org/matrix-doc/issues/1919
https://github.com/matrix-org/matrix-doc/issues/1896
https://github.com/matrix-org/matrix-doc/issues/2335

If there are any other issues with the client well-known that I've missed let me know so there doesn't need to be a followup MSC.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

KitsuneRal

Generally ok except one small addition, please.

proposals/2499-client-well-known-fixes.md

Signed-off-by: Aaron Raimist <aaron@raim.ist>

proposals/2499-client-well-known-fixes.md

turt2live · 2021-05-05T20:20:48Z

proposals/2499-client-well-known-fixes.md

+1. The maximum size of size of the well-known file is 51200 bytes. A client or server
+requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes.


This isn't a super realistic thing that can be applied by clients: if the server admin has configured the server so badly that a 10gb JSON file is being served then so be it. It's just not in the server admin's interest to attack bandwidth in this way given the costs they would be incurring from their ISP/service provider.

At least for native clients (and servers) it's fairly easy to abort on 51201st byte but FAIL_PROMPT looks a bit dubious here: should the client re-request .well-known or rather just continue (rather than abort) receiving the payload after confirming with the user? And yes, generally there's no incentive to attack in that way of course; but a recommendation to protect receiving sides from such misconfigurations wouldn't hurt, would it? Maybe not as MUST but as SHOULD.

I'm not convinced that this endpoint specifically needs this recommendation though: surely we should be applying this to all endpoints like profiles, sync, etc...

It feels like another MSC's problem.

That's fine, I can leave it to another MSC. The main reason for including it here is because Synapse has already implemented this limit for the server well-known so a well-known bigger than that already isn't going to work with most servers. If it was specced then at least people could expect there is some reasonable limit on the size.

I would be a bit surprised if people expected 1mb json files to work for this use case, honestly. I think it's reasonable for clients/servers to impose their own limits, and iirc Synapse only really limited the JSON file size on server-server because federation has tiny timeouts (typically), so read delays are not ideal.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

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

Signed-off-by: Aaron Raimist <aaron@raim.ist>

proposals/2499-client-well-known-fixes.md

Co-authored-by: Vladimir Panteleev <CyberShadow@users.noreply.github.com>

uhoreg · 2025-06-24T13:55:09Z

proposals/2499-client-well-known-fixes.md

+SHOULD be `application/json` however it should be assumed to be JSON regardless of Content-Type.
+This is consistent with the Server-Server API.
+
+1. Step 3f in the Client-Server well-known flow should be changed to use the modern


Suggested change

1. Step 3f in the Client-Server well-known flow should be changed to use the modern

1. Step 3.6 in the Client-Server well-known flow should be changed to use the modern

The latest rendering of the spec uses numbers for the sub-steps instead of letters

uhoreg · 2025-06-24T13:56:59Z

proposals/2499-client-well-known-fixes.md

+is the suggested replacement.
+
+1. The maximum size of the well-known file is 51200 bytes. A client or server
+requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes.


Suggested change

requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes.

requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes.

This check becomes part of step 3.2.

(Though #2499 (comment) says that this check is supposed to be removed entirely.)

spaetz · 2025-06-29T07:55:12Z

proposals/2499-client-well-known-fixes.md

+
+1. The spec does not mention that redirects should be followed for `/.well-known/matrix/client`
+and does not clearly specify what type of redirects should be followed for `/.well-known/matrix/server`.
+To fix this, the spec should be changed to state that when a well-known URI is requested,


The server-server spec simply states
30x redirects should be followed, however redirection loops should be avoided.
which is short, reasonable but not overcomplicated. Adopting the same for the Client-Server spec sounds reasonable?

aaronraimist changed the title ~~MSCXXXX: Fixes for Client Well-known URI~~ MSC2499: Fixes for Client Well-known URI Apr 13, 2020

aaronraimist force-pushed the fix-client-well-known branch from 78d3a47 to 9270c4c Compare April 13, 2020 23:55

Create MSC with fixes for client well-known

2f19796

Signed-off-by: Aaron Raimist <aaron@raim.ist>

aaronraimist force-pushed the fix-client-well-known branch from 9270c4c to 2f19796 Compare April 13, 2020 23:56

uhoreg added the proposal A matrix spec change proposal label Apr 14, 2020

turt2live added proposal-in-review kind:maintenance MSC which clarifies/updates existing spec labels Apr 14, 2020

turt2live self-requested a review April 27, 2020 23:38

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hex-m mentioned this pull request Jun 9, 2020

Riot doesn't follow 301 responses during homeserver discovery element-hq/element-web#9084

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richvdh self-requested a review April 7, 2021 13:41

richvdh mentioned this pull request Apr 7, 2021

MSC2713: Remove deprecated v1 Identity Service API #2713

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richvdh mentioned this pull request Apr 7, 2021

Remove the v1 API matrix-org/sydent#338

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aaronraimist changed the title ~~MSC2499: Fixes for Client Well-known URI~~ MSC2499: Fixes for Well-known URIs Apr 10, 2021

Clarify redirects that should be followed, add maximum size

9ca1b65

Signed-off-by: Aaron Raimist <aaron@raim.ist>

turt2live self-requested a review May 4, 2021 15:59

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aaronraimist and others added 3 commits May 6, 2021 13:49

Remove change from FAIL_PROMPT to IGNORE

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Signed-off-by: Aaron Raimist <aaron@raim.ist>

Update proposals/2499-client-well-known-fixes.md

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

Deprecate /_matrix/identity/v2

a8b5fbd

Signed-off-by: Aaron Raimist <aaron@raim.ist>

kevincox approved these changes May 12, 2021

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turt2live added the needs-implementation This MSC does not have a qualifying implementation for the SCT to review. The MSC cannot enter FCP. label Jun 8, 2021

turt2live force-pushed the old_master branch from e895827 to dca99ee Compare August 30, 2021 22:34

Merge branch 'main' into fix-client-well-known

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Signed-off-by: Aaron Raimist <aaron@raim.ist>

aaronraimist changed the base branch from old_master to main December 28, 2021 18:15

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aaronraimist mentioned this pull request Oct 27, 2022

MSC2499 integration: make self-check of /.well-known/matrix/client accept redirects spantaleev/matrix-docker-ansible-deploy#2201

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Remove duplicate words

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Co-authored-by: Vladimir Panteleev <CyberShadow@users.noreply.github.com>

uhoreg reviewed Jun 24, 2025

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matrixbot mentioned this pull request Jul 17, 2025

Remove the v1 API element-hq/sydent#338

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		1. The maximum size of size of the well-known file is 51200 bytes. A client or server
		requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes. No newline at end of file

	1. Step 3f in the Client-Server well-known flow should be changed to use the modern
	1. Step 3.6 in the Client-Server well-known flow should be changed to use the modern

	requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes.
	requesting a well-known file MUST abort and FAIL_PROMPT if the response exceeds 51200 bytes.
	This check becomes part of step 3.2.

MSC2499: Fixes for Well-known URIs #2499

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aaronraimist commented Apr 13, 2020 •

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