ALPN client (openssh s_client but in Go)

go.mod

module alpnc

go 1.18

main.go

package main

import (
	"crypto/tls"
	"flag"
	"fmt"
	"io"
	"net"
	"os"
	"strings"
	"time"
)

func main() {
	alpn := flag.String("alpn", "", "Comma-separated list of ALPN protocols (e.g., h2,http/1.1)")
	serverName := flag.String("servername", "", "Override SNI from address")
	verify := flag.Bool("verify", true, "Verify peer?")
	debug := flag.Bool("debug", false, "Debug connection?")
	flag.Parse()

	if flag.NArg() != 1 {
		fmt.Fprintf(os.Stderr, "Usage: %s [-alpn protocols] [-servername name] host:port\n", os.Args[0])
		os.Exit(1)
	}
	addr := flag.Arg(0)
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Invalid address %q: %v\n", addr, err)
		os.Exit(1)
	}

	if len(*serverName) == 0 {
		serverName = &host
	}

	protos := []string{}
	if *alpn != "" {
		protos = strings.Split(*alpn, ",")
	}

	config := &tls.Config{
		ServerName:         *serverName,
		NextProtos:         protos,
		InsecureSkipVerify: !*verify,
	}

	conn, err := tls.DialWithDialer(&net.Dialer{Timeout: 1 * time.Minute}, "tcp", addr, config)
	if err != nil {
		fmt.Fprintf(os.Stderr, "TLS connection failed: %v\n", err)
		os.Exit(1)
	}
	defer conn.Close()

	if *debug {
		state := conn.ConnectionState()

		out := func(s string, args ...any) {
			fmt.Fprintf(os.Stderr, s, args...)
		}

		out("Negotiated Protocol: %s\n", state.NegotiatedProtocol)
		out("Server Name: %s\n", state.ServerName)
		out("Cipher Suite: 0x%x\n", state.CipherSuite)

		// Print certificates
		for i, cert := range state.PeerCertificates {
			out("Certificate %d:\n", i)
			out("  Subject: %s\n", cert.Subject)
			out("  Issuer:  %s\n", cert.Issuer)
			out("  NotBefore: %s\n", cert.NotBefore)
			out("  NotAfter:  %s\n\n", cert.NotAfter)
		}
	}
	
	go func() {
		io.Copy(conn, os.Stdin)
	}()

	io.Copy(os.Stdout, conn)
}

Get an .exe with: GOOS=windows GOARCH=amd64 go build