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Re: [TLS] Deprecating alert levels

mrex@sap.com (Martin Rex) Wed, 19 October 2016 15:58 UTC

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To: Kyle Nekritz <knekritz@fb.com>
Date: Wed, 19 Oct 2016 17:58:45 +0200
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From: mrex@sap.com
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Subject: Re: [TLS] Deprecating alert levels
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Kyle Nekritz wrote:
> 
>> This list is already missing the warning-level "unrecognized_name" alert,
>> and such a change would imply that all new/unrecognized alerts are going
>> to be treated as fatal forever (i.e. that no new warning-level alerts
>> can ever be defined).
> 
> That alert is currently defined as a fatal alert (see section 6.2 in the
> current draft).  RFC 6066 also states "It is NOT RECOMMENDED to send a
> warning-level unrecognized_name(112) alert, because the client's behavior
> in response to warning-level alerts is unpredictable.", which I think
> illustrates the problem. Allowing new non-fatal alerts to be added later
> would require that existing clients ignore unknown warning alerts,
> which I think is somewhat dangerous.

It seems that rfc6066 is not clear enough in explaining the issue
about the situation with the two WELL-DEFINED (but poorly implemented)
variants of the TLS alerts

  (1)  unrecognized_name(112)  level WARNING
  (2)  unrecognized_name(112)  level FATAL

See the *ORIGINAL* specification which created *BOTH* of these alert variants:

https://tools.ietf.org/html/rfc3546#page-10


   If the server understood the client hello extension but does not
   recognize the server name, it SHOULD send an "unrecognized_name"
   alert (which MAY be fatal).


-Martin

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