How can I check clientHello in server

[WAY29]

I tried using the utls library to customize the ClientHello. Now, I'm using utls to write a test server to check if the client has successfully customized the ClientHello, but it seems I have no way to do this. One possible approach could be to use a callback in conn.serverHandshake, or have I missed something?

[fedosgad]

You can wrap a TCP listener at the server so TLS bytes can be accessed, extract ClientHello (parsing TLS' TLV structure is quite easy, at least to read only the first message) and use UnmarshalClientHello() or Fingerprinter.FingerprintClientHello() to parse raw bytes to a more usable form.
Take a look at https://github.com/fedosgad/mirror_proxy/blob/master/hijackers/utls.go#L215 and https://github.com/fedosgad/mirror_proxy/blob/master/hijackers/utls.go#L76 - maybe it will help.