Subject: Re: Implementation support for SSH_MSG_UNIMPLEMENTED

From: [email-masked] (Peter Gutmann)
Date: Thu, 24 Jul 2003 13:38:55 +1200

[I've made this a public reply because it contains a point that may be worth
 considering]

>Are there any implementations which do not respond with SSH_MSG_UNIMPLEMENTED
>to unknown packet types during the key exchange phase of the protocol?

I don't, but that can be fixed if it becomes a critical requirement of the
protocol.  The reason I don't is that I always send the minimal amount of info
in error returns for any protocol I do (SSH/SSL/CMP/TSP/RTCS/OCSP/etc), which
has saved me from at least two attacks on SSL and probably attacks on other
protocols as well.  In other words if the protocol requires a certain response
in order to function I'll do it, but if it's merely a nicety for debugging,
I'll send the most generic response I can get away with.

Peter.

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From: "Joel N. Weber II" <[email-masked]>
Date: Thu, 24 Jul 2003 12:16:35 -0400

> I'll send the most generic response I can get away with.

The text of the protocol spec says that sending SSH_MSG_UNIMPLEMENTED
is mandatory, if I recall correctly.  Not implementing this according
to the spec makes interoperability harder when new features get added
later.

SSH_MSG_UNIMPLEMENTED is already fairly generic.