[gnutls-devel] GnuTLS | DTLS handshake restarted by ClientHello using invalid message sequence numbers (#1233)

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Wed May 12 19:35:53 CEST 2021

Paul commented:

I opened a similar [issue for Scandium](https://github.com/eclipse/californium/issues/1620), which also exhibited this behavior. From the discussions I had there, it appears the DTLS 1.2 RFC may provide some ground for accepting ClientHello messages with increased message sequence numbers based on the [quote](https://www.rfc-editor.org/rfc/rfc6347.html#page-18):

> If a server receives a ClientHello with an invalid cookie, it SHOULD
> treat it the same as a ClientHello with no cookie. This avoids
> race/deadlock conditions if the client somehow gets a bad cookie
> (e.g., because the server changes its cookie signing key).

> Note to implementors: This may result in clients receiving multiple
> HelloVerifyRequest messages with different cookies. Clients SHOULD
> handle this by sending a new ClientHello with a cookie in response to
> the new HelloVerifyRequest.

If this is indeed the reason for accepting such ClientHellos, then I think the issue can be closed. I am leaving this issue open to to ensure the behavior is intended (a dev may close it if that is the case), as it does appear to be a bit of an edge case.

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