server.go

package main

import (
	"io/ioutil"
	"log"
	"net"
	"os"

	"golang.org/x/crypto/ssh"
)

var (
	hostPrivateKeySigner ssh.Signer
)

func init() {
	keyPath := "./host_key"
	if os.Getenv("HOST_KEY") != "" {
		keyPath = os.Getenv("HOST_KEY")
	}

	hostPrivateKey, err := ioutil.ReadFile(keyPath)
	if err != nil {
		panic(err)
	}

	hostPrivateKeySigner, err = ssh.ParsePrivateKey(hostPrivateKey)
	if err != nil {
		panic(err)
	}
}

func keyAuth(conn ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
	log.Println(conn.RemoteAddr(), "authenticate with", key.Type())
	return nil, nil
}

func main() {
	config := ssh.ServerConfig{
		PublicKeyCallback: keyAuth,
	}
	config.AddHostKey(hostPrivateKeySigner)

	port := "2222"
	if os.Getenv("PORT") != "" {
		port = os.Getenv("PORT")
	}
	socket, err := net.Listen("tcp", ":"+port)
	if err != nil {
		panic(err)
	}

	for {
		conn, err := socket.Accept()
		if err != nil {
			panic(err)
		}

		// From a standard TCP connection to an encrypted SSH connection
		sshConn, _, _, err := ssh.NewServerConn(conn, &config)
		if err != nil {
			panic(err)
		}

		log.Println("Connection from", sshConn.RemoteAddr())
		sshConn.Close()
	}
}