crypto/tls: add Extensions to ClientHelloInfo

[phuslu]

Similar aws/s2n-tls#607, Having access to raw ClientHello can be useful for fingerprinting clients [1] for further analysis.
Plus, With raw ClientHello message, we could also implements SNI Proxy in tls.Config.GetConfigForClient [2] , e.g. tlsrouter [3] more easily.
In openssl this can be done by setting up callback through SSL_CTX_set_msg_callback.
Would be nice to have similar ability for golang crypto.

[1] https://github.com/salesforce/ja3
[2] https://golang.org/pkg/crypto/tls/#Config
[3] https://github.com/google/tcpproxy/tree/master/cmd/tlsrouter

[bcmills]

CC @FiloSottile

[deancn]

mark

[mysticaltech]

That is really needed!

[deancn]

Possibly add a raw byte for TLS?

Just reference:
https://github.com/bfenetworks/bfe/blob/develop/bfe_tls/handshake_messages.go#L27

[elindsey]

JA3 fingerprinting in particular has proven to be quite useful, and we've been running a patched stdlib for a few years in order to support it.

I personally see less need to eg. more easily support SNI proxying - tlsrouter's support is already straightforward, and I'm not sure that's a common enough use case that it'd make sense to adjust the stdlib to better accommodate. Exposing the raw ClientHello would also bring up a discussion about how we want to handle the client random, if it should be included or sanitized.

For fingerprinting use cases, the only thing missing is exposing extensions. That's a small change to the API surface, in line with existing ClientHelloInfo fields, and addresses a concrete need. I'd be happy to contribute a patch if there was agreement in that direction.

cc @FiloSottile this seems to be in your wheelhouse. Do you have any thoughts on the request or how we can move this towards a decision?

[komuw]

I ran into a need for this because I wanted to implement ja3 fingerprinting on my app.

Note that ja3 is now kind of going mainstream with AWS[1], cloudflare[2], fastly[3], among others, supporting it.

Also, it seems like caddy[4] ran into a need for something like this.

1. https://aws.amazon.com/about-aws/whats-new/2022/11/amazon-cloudfront-supports-ja3-fingerprint-headers/
2. https://developers.cloudflare.com/bots/concepts/ja3-fingerprint/
3. https://docs.rs/fastly/latest/fastly/struct.Request.html#method.get_tls_ja3_md5
4. Detect HTTPS interception caddyserver/caddy#1430

PS: @elindsey , is your patch somewhere public that someone like me can have a look?

[phuslu]

As supplementary material, I also implement a high performance nginx module in https://github.com/phuslu/nginx-ssl-fingerprint

[mysticaltech]

This is extremely useful in controlling bot abuses on a website!

[gopherbot]

Change https://go.dev/cl/471396 mentions this issue: crypto/tls: expose extensions presented by client to GetCertificate