check.tls.support is the web service that drives the TLS detection mechanisms on https://tls.support. It is a fork of https://github.com/jmhodges/howsmyssl
This is a Go project.
Determining the client's security is done in client_info.go.
This project requires Go 1.16 (or newer). go build will generate a static
binary called howsmyssl. This repo is go get'able, of course.
It has a fork of the Go crypto/tls library at ./tls/ in order to add a ServerHandshake and expose the ClientHello struct.
It's been useful to me to use justrun to recompile the project while modifying the template. Typical use is simply:
justrun -c "go build && ./howsmyssl" -i howsmyssl . templates/
(Justrun has the benefit of controlling the lifecycle of a process, unlike most other file watch utilities.)