Re: [Curdle] Client-side SSH_MSG_EXT_INFO: Use it or lose it principle!

Peter Gutmann <> Mon, 27 April 2020 05:37 UTC

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From: Peter Gutmann <>
To: denis bider <>, curdle <>, "" <>, Damien Miller <>
Thread-Topic: [Curdle] Client-side SSH_MSG_EXT_INFO: Use it or lose it principle!
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Date: Mon, 27 Apr 2020 05:37:07 +0000
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Subject: Re: [Curdle] Client-side SSH_MSG_EXT_INFO: Use it or lose it principle!
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denis bider <> writes:

>it has come to my attention that at least one SSH server implementation (a)
>advertises support for SSH_MSG_EXT_INFO as defined in RFC 8308, and (b)
>disconnects on actual receipt of an EXT_INFO message from the client.

Not wanting to do a public name-and-shame on this, but could you share the ID
string needed to fingerprint this server?  Looks like a lot of implementations
will need to be able to deal with this...

>This happens when we define a general mechanism, but then the most widely
>used implementations only use certain aspects of it.

That's a more specific version of "an implementation is fully SSH standards-
compliant when it can connect to OpenSSH (client) or Putty can connect to it
(server)".  Those two are the universal benchmark for SSH implementations, for
better or for worse.  TLS dealt with this to some extent by adding a mechanism
bacronym'd as GREASE for sending random information in extensions to detect
implementations that broke on them, perhaps something similar could be done
for SSH.