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

[Martin Rex <[e-mail]>]

Nikos Mavrogiannopoulos wrote:
> 
> Michael D'Errico wrote:
> > I agree with Yngve that a server should send either an empty SNI
> > extension OR an unrecognized_name alert but not both.  However, I
> > disagree that the server SHOULD NOT send a warning alert since that
> > hides information from the client.
> [...]
> >     1 - server understood the SNI and used it to select an appropriate
> >         certificate chain and other parameters
> >     2 - server understood the SNI but did not recognize it as one of
> >         its configured virtual hosts; however, the server is set up
> >         to use a default configuration in that case
> >     3 - server understood the SNI but did not recognize it as one of
> >         its configured virtual hosts; there is no default configuration
> >         available so the handshake can not continue
> >     4 - server does not understand the SNI extension
> > 
> > The way my server reacts to each of these cases is:
> > 
> >     1 - add an empty SNI extension to ServerHello
> >     2 - send a warning unrecognized_name alert
> >     3 - send a fatal unrecognized_name alert
> >     4 - send nothing
> > 
> > Yngve would prefer that nothing is sent in case 2, but then a client
> > can not distinguish it from case 4.
> 
> I believe that his point was, what can the client do anyway? In both
> cases the client just follows the handshake and will be prompted if the
> "default" certificate doesn't match the connected host. Would the client
> need to be warned that he is being connected using the "default"
> certificate rather than any specific?


I actually fail to see the exact purpose of a warning-level alert
"unrecognized_name".  The client is supposed to perform the server endpoint
identification in any case, and that warning should not affect the
outcome of that process at all.

Btw. this warning-level alert could be stripped from the handshake
by an attacker _without_ affecting the handshake message hash and
finished calculations (alerts are not handshake messages and are therefore
invisible to the handshake message hash).  Similarly, an attacker
can insert such a warning-level alert into handshakes without affecting
the handshake message hash.


I think there should be a guidance in the spec what the
receiver of that alert at level warning should do.


   A client that receives a warning-level "unrecognized_name" alert
   SHOULD ignore this alert and continue the TLS handshake.  The purpose
   of a warning-level "unrecognized_name" is for diagnostic purposes,
   if anything.

   A client TLS implementation MAY offer API functionality for the client
   application to determine what warning-level alerts were received
   during a TLS handshake.  When the server endpoint identification
   performed by the client application fails, then a client application
   might want to query whether any warning-level alerts have been
   received during the TLS handshake and mention these in the information
   presented to a user or written to a logfile for diagnostic purposes.


-Martin