Re: [TLS] WGLC for draft-ietf-tls-sni-encryption

[David Fifield <[e-mail]>]

On Wed, Oct 17, 2018 at 07:25:38PM -0700, Eric Rescorla wrote:
>     >> As it is, there are a number of servers which desperately require
>     >> the presence of TLS extension SNI, or will fail TLS handshakes either
>     >> by choking and dropping connections (Microsoft IIS 8.5+) or by
>     >> very unhelpful alerts (several others), and also HTTP/2.0 requires
>     >> unconditional cleartext presence of TLS extension SNI.  Any kind of
>     >> heuristics-based approach for clients to guess whether or not to
>     >> send TLS extension SNI is flawed from the start.  If a network
>     >> middlebox can make a client present a cleartext TLS extension SNI
>     >> by refusing connections without cleartext TLS extension SNI,
>     >> the entire effort becomes pretty useless.
>     >
>     > Yes, clients must not fall back to cleartext SNI in this case.
> 
>     Please give a clear deterministic algorithm how a client can
>     tell apart a server that requires cleartext SNI from a server
>     that does not want cleartext SNI.
> 
> 
> This isn't really the relevant question, as there are many servers
> now which do not require SNI.
> 
> The relevant question is how a client can be made aware that
> a server does support ESNI and can have high confidence that
> it will accept it. Such an algorithm is provided in:
> https://tools.ietf.org/html/draft-ietf-tls-esni-01#section-4
> 
> Note that as explained in detail, it is not necessary that all clients
> send ESNI to that server.


Right -- it's not a matter of a server requiring or not requiring ESNI.
The way I understand it, a server that supports ESNI merely affords
clients an opportunity to better protect themselves -- it is still the
client's responsibility to resist downgrades, and to somehow discover
(through DNS or otherwise) that the server supports ESNI in the first
place. I imagine that most servers that support ESNI will continue to
simultaneously support plaintext SNI connections.