crypto/tls: provide a way to dynamically handle client hellos

[danp]

I'd like to control whether or not to offer the "h2" protocol during TLS handshake based on the client's requested server name. Currently there's no way to do this with crypto/tls.

Would an optional callback similar to GetCertificate be useful for this and other similar cases?

[danp]

Erlang's ssl module has a similar facility: sni_fun in server-side ssl options, though it's only called when SNI is used. A callback that's used regardless of SNI (as GetCertificate is now) would be more generally useful.

[bradfitz]

To @agl for thoughts. (Not necessarily for implementation)

[danp]

@agl this is something I'd really like to help implement or at least see for 1.8. Any input on this and/or #15707?

I'm roughly thinking of something like tls.Config.GetClientConfig(*tls.ClientHelloInfo) (*tls.Config, error) with more info on tls.ClientHelloInfo, such as ALPN protos. Select items on the returned config would be used for the connection. This hook would be called here or so.

[agl]

It's on my desiderata list for 1.8.

[bradfitz]

See #16066 for a related issue and my comment #16066 (comment) for one idea.

[gopherbot]

CL https://golang.org/cl/30790 mentions this issue.