Simple ssh server example in go.

basic-sshd.go

// A small SSH daemon providing bash sessions
//
// Server:
// cd my/new/dir/
// #generate server keypair
// ssh-keygen -t rsa
// go get -v .
// go run sshd.go
//
// Client:
// ssh foo@localhost -p 2200 #pass=bar

package main

import (
	"encoding/binary"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net"
	"os/exec"
	"sync"
	"syscall"
	"unsafe"

	"github.com/creack/pty"
	"golang.org/x/crypto/ssh"
)

func main() {

	// In the latest version of crypto/ssh (after Go 1.3), the SSH server type has been removed
	// in favour of an SSH connection type. A ssh.ServerConn is created by passing an existing
	// net.Conn and a ssh.ServerConfig to ssh.NewServerConn, in effect, upgrading the net.Conn
	// into an ssh.ServerConn

	config := &ssh.ServerConfig{
		//Define a function to run when a client attempts a password login
		PasswordCallback: func(c ssh.ConnMetadata, pass []byte) (*ssh.Permissions, error) {
			// Should use constant-time compare (or better, salt+hash) in a production setting.
			if c.User() == "foo" && string(pass) == "bar" {
				return nil, nil
			}
			return nil, fmt.Errorf("password rejected for %q", c.User())
		},
		// You may also explicitly allow anonymous client authentication, though anon bash
		// sessions may not be a wise idea
		// NoClientAuth: true,
	}

	// You can generate a keypair with 'ssh-keygen -t rsa'
	privateBytes, err := ioutil.ReadFile("./id_rsa")
	if err != nil {
		log.Fatal("Failed to load private key (./id_rsa)")
	}

	private, err := ssh.ParsePrivateKey(privateBytes)
	if err != nil {
		log.Fatal("Failed to parse private key")
	}

	config.AddHostKey(private)

	// Once a ServerConfig has been configured, connections can be accepted.
	listener, err := net.Listen("tcp", "0.0.0.0:2200")
	if err != nil {
		log.Fatalf("Failed to listen on 2200 (%s)", err)
	}

	// Accept all connections
	log.Print("Listening on 2200...")
	for {
		tcpConn, err := listener.Accept()
		if err != nil {
			log.Printf("Failed to accept incoming connection (%s)", err)
			continue
		}

		go handeConn(tcpConn, config)
	}
}

func handeConn(tcpConn net.Conn, config *ssh.ServerConfig) {
	// Before use, a handshake must be performed on the incoming net.Conn.
	sshConn, chans, reqs, err := ssh.NewServerConn(tcpConn, config)
	if err != nil {
		log.Printf("Failed to handshake (%s)", err)
		return
	}

	log.Printf("New SSH connection from %s (%s)", sshConn.RemoteAddr(), sshConn.ClientVersion())
	// Discard all global out-of-band Requests
	go ssh.DiscardRequests(reqs)
	// Accept all channels
	go handleChannels(chans)

}

func handleChannels(chans <-chan ssh.NewChannel) {
	// Service the incoming Channel channel in go routine
	for newChannel := range chans {
		go handleChannel(newChannel)
	}
}

func handleChannel(newChannel ssh.NewChannel) {
	// Since we're handling a shell, we expect a
	// channel type of "session". The also describes
	// "x11", "direct-tcpip" and "forwarded-tcpip"
	// channel types.
	if t := newChannel.ChannelType(); t != "session" {
		newChannel.Reject(ssh.UnknownChannelType, fmt.Sprintf("unknown channel type: %s", t))
		return
	}

	// At this point, we have the opportunity to reject the client's
	// request for another logical connection
	connection, requests, err := newChannel.Accept()
	if err != nil {
		log.Printf("Could not accept channel (%s)", err)
		return
	}

	npty, ntty, err := pty.Open()
	if err != nil {
		log.Printf("Could not start pty (%s)", err)
	}

	// Fire up bash for this session
	bash := exec.Command("bash")
	bash.Stdout = ntty
	bash.Stdin = ntty
	bash.Stderr = ntty
	bash.SysProcAttr = &syscall.SysProcAttr{
		Setctty: true,
		Setsid:  true, // TODO: Evaluate me?
	}

	// Prepare teardown function
	close := func() {
		connection.Close()
		_, err := bash.Process.Wait()
		if err != nil {
			log.Printf("Failed to exit bash (%s)", err)
		}
		log.Printf("Session closed")
	}

	if err := bash.Start(); err != nil {
		log.Printf("Failed to start bash (%s)", err)
	}

	//pipe session to bash and visa-versa
	var once sync.Once
	go func() {
		io.Copy(connection, npty)
		once.Do(close)
	}()
	go func() {
		io.Copy(npty, connection)
		once.Do(close)
	}()

	// Sessions have out-of-band requests such as "shell", "pty-req" and "env"
	go func() {
		for req := range requests {
			switch req.Type {
			case "shell":
				// We only accept the default shell
				// (i.e. no command in the Payload)
				if len(req.Payload) == 0 {
					req.Reply(true, nil)
				}
			case "pty-req":
				termLen := req.Payload[3]
				w, h := parseDims(req.Payload[termLen+4:])
				SetWinsize(npty.Fd(), w, h)
				// Responding true (OK) here will let the client
				// know we have a pty ready for input
				req.Reply(true, nil)
			case "window-change":
				w, h := parseDims(req.Payload)
				SetWinsize(npty.Fd(), w, h)
			}
		}
	}()
}

// =======================

// parseDims extracts terminal dimensions (width x height) from the provided buffer.
func parseDims(b []byte) (uint32, uint32) {
	w := binary.BigEndian.Uint32(b)
	h := binary.BigEndian.Uint32(b[4:])
	return w, h
}

// ======================

// Winsize stores the Height and Width of a terminal.
type Winsize struct {
	Height uint16
	Width  uint16
	x      uint16 // unused
	y      uint16 // unused
}

// SetWinsize sets the size of the given pty.
func SetWinsize(fd uintptr, w, h uint32) {
	ws := &Winsize{Width: uint16(w), Height: uint16(h)}
	syscall.Syscall(syscall.SYS_IOCTL, fd, uintptr(syscall.TIOCSWINSZ), uintptr(unsafe.Pointer(ws)))
}

// Borrowed from https://github.com/creack/termios/blob/master/win/win.go
// corrected from https://gist.github.com/jpillora/b480fde82bff51a06238

Once entered into bash shell from from client, "exit" is NOT closing the connection. Could you please fix that?

Did it start a session for you and open a bash terminal?

Exit doesn't happen from this code. Exit happens because the terminal dies and there's nothing left to hold the program open.

ioutil is now deprecated. It should be changed to os.ReadFile

Thanks @benschlueter! I've found myself pretty busy over the past few months. If you want to share an updated example, I'd be happy to accept a commit to this gist.