Re: [TLS] RFC-4366-bis and the unrecognized_name(112) alert

Martin Rex <mrex@sap.com> Mon, 07 June 2010 17:24 UTC

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From: Martin Rex <mrex@sap.com>
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To: mike-list@pobox.com (Michael D'Errico)
Date: Mon, 7 Jun 2010 17:03:05 +0200 (MEST)
In-Reply-To: <4C09C4EC.1070903@pobox.com> from "Michael D'Errico" at Jun 4, 10 08:30:52 pm
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Cc: tls@ietf.org
Subject: Re: [TLS] RFC-4366-bis and the unrecognized_name(112) alert
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Michael D'Errico wrote:
> 
> Joseph Salowey (jsalowey) wrote:
> > 
> > (1) "The ServerNameList MUST NOT contain more than one name of the same
> > name_type. If the server understood the client hello extension, but does
> > not recognize the server name, and it refuses to continue it MUST send a
> > fatal unrecognized_name(112) alert and terminate the handshake.  If the
> > server decides to continue the  handshake, sending a
> > unrecognized_name(112) alert with a warning level is NOT RECOMMENDED,
> > since the  client behavior is unpredictable.  Some clients respond by
> > aborting the handshake while others allow it  to continue to certificate
> > validation, which may fail as a result of a name mismatch. "
> 
> I prefer the above text, but would modify it slightly:
> 
>      If the server decides to continue the handshake, sending an
>      unrecognized_name(112) alert with a warning level is NOT
>      RECOMMENDED at this time due to legacy client software that
>      escalates it to a fatal error.  New software is encouraged
>      to treat warning alerts as informational, and not to abort
>      an otherwise legitimate handshake.


Technically, there is no such thing as an "escalation to a fatal error",
so this exact wording is inappropriate.

If a TLS peer decides to abort the handshake when receiving a warning-level
alert, that TLS peer  SHOULD send a fatal alert before closing the
connection.

When receiving a fatal-level alert, there is nothing else to send back
(the TLS peer will _NOT_ read this anyway, and there may even be unread
handshake message still in the incoming network pipe of the TLS peer
that sent the fatal alert).

-Martin