pty.go

package sshego

/*
See blog: https://blog.gopheracademy.com/go-and-ssh/

Code from https://github.com/jpillora/go-and-ssh
licensed under:

#### MIT License

Copyright (c) 2014 Jaime Pillora <dev@jpillora.com>

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
'Software'), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

// 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

import (
	"context"
	"encoding/binary"
	"fmt"
	"io"
	"log"
	"os/exec"
	"sync"

	"github.com/glycerine/sshego/xendor/github.com/glycerine/xcryptossh"
)

type ConnectionAlert struct {
	PortOne  chan ssh.Channel
	ShutDown chan struct{}
}

func (cfg *SshegoConfig) handleChannels(ctx context.Context, chans <-chan ssh.NewChannel, sshconn ssh.Conn, ca *ConnectionAlert) {
	// Service the incoming Channel channel in go routine
	var shut chan struct{}
	if ca != nil {
		shut = ca.ShutDown
	}
	// avoid shutdown race by getting this early.
	reqStop := cfg.Esshd.Halt.ReqStopChan()
	for {
		select {
		case newChannel, stillOpen := <-chans:
			if !stillOpen {
				return
			}
			go cfg.handleChannel(ctx, newChannel, sshconn, ca)
		case <-reqStop:
			return
		case <-shut:
			return
		case <-ctx.Done():
			return
		}
	}
}

func (cfg *SshegoConfig) handleChannel(ctx context.Context, newChannel ssh.NewChannel, sshconn ssh.Conn, ca *ConnectionAlert) {

	// Since we're handling a shell, we expect a
	// channel type of "session". The spec also describes
	// "x11", "direct-tcpip" and "forwarded-tcpip"
	// channel types.
	if newChannel == nil {
		// can happen on shutdown
		return
	}
	t := newChannel.ChannelType()

	if t == "direct-tcpip" {
		handleDirectTcp(ctx, cfg.Halt, newChannel, ca)
	}

	if t != "session" {
		if len(cfg.CustomChannelHandlers) > 0 {
			cb, ok := cfg.CustomChannelHandlers[t]
			if ok {
				go cb(newChannel, sshconn, ca)
				return
			}
		}
		newChannel.Reject(ssh.UnknownChannelType, fmt.Sprintf("unknown channel type: %s", t))
		return
	}

	// t == "session", request to open a shell

	// 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
	}

	// Fire up bash for this session
	bash := exec.Command("bash")

	// 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")
	}

	// Allocate a terminal for this channel
	log.Print("Successful login, creating pty...")
	bashf, err := ptyStart(bash)
	if err != nil {
		log.Printf("Could not start pty (%s)", err)
		close()
		return
	}

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