Go SSH reverse tunnel implementation (SSH -R)

go-ssh-reverse-tunnel.go

/*

Go-Language implementation of an SSH Reverse Tunnel, the equivalent of below SSH command:

   ssh -R 8080:127.0.0.1:8080 [email protected]

which opens a tunnel between the two endpoints and permit to exchange information on this direction:

   server:8080 -----> client:8080

   once authenticated a process on the SSH server can interact with the service answering to port 8080 of the client
   without any NAT rule via firewall

Copyright 2017, Davide Dal Farra
MIT License, http://www.opensource.org/licenses/mit-license.php

*/

package main

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

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

type Endpoint struct {
	Host string
	Port int
}

func (endpoint *Endpoint) String() string {
	return fmt.Sprintf("%s:%d", endpoint.Host, endpoint.Port)
}

// From https://sosedoff.com/2015/05/25/ssh-port-forwarding-with-go.html
// Handle local client connections and tunnel data to the remote server
// Will use io.Copy - http://golang.org/pkg/io/#Copy
func handleClient(client net.Conn, remote net.Conn) {
	defer client.Close()
	chDone := make(chan bool)

	// Start remote -> local data transfer
	go func() {
		_, err := io.Copy(client, remote)
		if err != nil {
			log.Println(fmt.Sprintf("error while copy remote->local: %s", err))
		}
		chDone <- true
	}()

	// Start local -> remote data transfer
	go func() {
		_, err := io.Copy(remote, client)
		if err != nil {
			log.Println(fmt.Sprintf("error while copy local->remote: %s", err))
		}
		chDone <- true
	}()

	<-chDone
}

func publicKeyFile(file string) ssh.AuthMethod {
	buffer, err := ioutil.ReadFile(file)
	if err != nil {
		log.Fatalln(fmt.Sprintf("Cannot read SSH public key file %s", file))
		return nil
	}

	key, err := ssh.ParsePrivateKey(buffer)
	if err != nil {
		log.Fatalln(fmt.Sprintf("Cannot parse SSH public key file %s", file))
		return nil
	}
	return ssh.PublicKeys(key)
}

// local service to be forwarded
var localEndpoint = Endpoint{
	Host: "localhost",
	Port: 8080,
}

// remote SSH server
var serverEndpoint = Endpoint{
	Host: "146.148.22.123",
	Port: 22,
}

// remote forwarding port (on remote SSH server network)
var remoteEndpoint = Endpoint{
	Host: "localhost",
	Port: 8080,
}

func main() {

	// refer to https://godoc.org/golang.org/x/crypto/ssh for other authentication types
	sshConfig := &ssh.ClientConfig{
		// SSH connection username
		User: "operatore",
		Auth: []ssh.AuthMethod{
			// put here your private key path
			publicKeyFile("/home/operatore/.ssh/id_rsa"),
		},
		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
	}

	// Connect to SSH remote server using serverEndpoint
	serverConn, err := ssh.Dial("tcp", serverEndpoint.String(), sshConfig)
	if err != nil {
		log.Fatalln(fmt.Printf("Dial INTO remote server error: %s", err))
	}

	// Listen on remote server port
	listener, err := serverConn.Listen("tcp", remoteEndpoint.String())
	if err != nil {
		log.Fatalln(fmt.Printf("Listen open port ON remote server error: %s", err))
	}
	defer listener.Close()

	// handle incoming connections on reverse forwarded tunnel
	for {
		// Open a (local) connection to localEndpoint whose content will be forwarded so serverEndpoint
		local, err := net.Dial("tcp", localEndpoint.String())
		if err != nil {
			log.Fatalln(fmt.Printf("Dial INTO local service error: %s", err))
		}		
		
		client, err := listener.Accept()
		if err != nil {
			log.Fatalln(err)
		}

		handleClient(client, local)
	}

}

This is an awesome implementation. Works great.
-amit

Thanks for the awesome gist! I improved it quite a bit (parallel connections, logging, retries on errors, start on system startup via systemd, configuration from JSON file, prebuilt binaries): https://github.com/function61/holepunch-client

Hi joonas-fi,
I am new to this and I am trying to build and use your code by following the README. But I am falling short. Could you please update the README on how I can build holepunch client and execute it? Also, if the remote server has the sshd running on a port other than 22, then how can I introduce custom port in the json file?
So far I have done this:

1. Edited a file holepunch.json (similar to holepunch.example.json) and kept it in cmd/holepunch directory.
2. Invoked the command: go build main.go

But I am unable to make this work. Getting errors.
Thanks,
-amit

Sorry, I didn't get a notification here. But we worked out the issue: function61/holepunch-client#10 (in case anyone else has trouble)