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Re: [TLS] Ignoring unrecognized extensions

Michael D'Errico <mike-list@pobox.com> Tue, 22 June 2010 18:23 UTC

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Date: Tue, 22 Jun 2010 11:23:52 -0700
From: Michael D'Errico <mike-list@pobox.com>
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To: Matt McCutchen <matt@mattmccutchen.net>
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Cc: tls@ietf.org
Subject: Re: [TLS] Ignoring unrecognized extensions
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Matt McCutchen wrote:
> 
> I think it is the intent of RFC 5246 that a TLS server, in addition to
> tolerating an "extensions" field in the ClientHello that is nonempty as
> a whole, MUST ignore individual extensions that it does not recognize.
> However, I cannot find this stated anywhere in RFC 5246.  Am I missing
> something?  Would this be worthy of an erratum?

That is indeed how extensions are supposed to work.  If a server does
not recognize a particular extension it ignores it.  The server hello
will not contain a response, so the client then has the option to abort
or to continue the handshake without the use of the behavior defined by
that extension.

I don't know if there's still time to sneak this into RFC 4366++.

Mike

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