Re: [TLS] Ignoring unrecognized extensions

[Martin Rex <[e-mail]>]

Matt McCutchen wrote:
> 
> On Tue, 2010-06-22 at 21:09 +0200, Martin Rex wrote:
> > 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?
> > 
> > My copy of rfc-5746, section 3.6 says this in the second last paragraph:
> > 
> > http://tools.ietf.org/html/rfc5746#section-3.6
> > 
> >    TLS servers implementing this specification MUST ignore any unknown
> >    extensions offered by the client and they MUST accept version numbers
> >    higher than their highest version number and negotiate the highest
> >    common version.  These two requirements reiterate preexisting
> >    requirements in RFC 5246 and are merely stated here in the interest
> >    of forward compatibility.
> 
> I saw that.  My point is that I can't find the preexisting requirements
> in RFC 5246 that are being reiterated.


It contains a similar hint, but in a non-obvious location:

http://tools.ietf.org/html/rfc-5246#page-47


   Note: this extension is not meaningful for TLS versions prior to 1.2.
   Clients MUST NOT offer it if they are offering prior versions.
!  However, even if clients do offer it, the rules specified in [TLSEXT]
!  require servers to ignore extensions they do not understand.


-Martin