Client Listen

Introduction

This is the client.Listen example from Go’s documentation. As is it doesn’t run because they leave some values unassigned. For example in line 12 they define a variable to define a single known host and in line 18 they set up the check, but never assign anything to it. Either we need to assign a key, or we need to tell the client to ignore keys. Also they are only requesting a login with password (line 15) with dummy, hard coded, values.

The Interesting Part

What this code does is kind of interesting.

• line 21 - connect to a server such as sshd.
• line 28 - Request that the remote server opens a socket on port 8080.
• line 35 - On the client side, connect the remote listen port to a simple web server.

If someone connects to 8080 on the remote server, our client server will be connected to them so we ca serve then a “Hello world!” message. This is similar to the -R switch on ssh except that our server gets to directly service the connections.

 1package main
 2
 3import (
 4	"fmt"
 5	"log"
 6	"net/http"
 7
 8	"golang.org/x/crypto/ssh"
 9)
10
11func main() {
12	var hostKey ssh.PublicKey
13	config := &ssh.ClientConfig{
14		User: "username",
15		Auth: []ssh.AuthMethod{
16			ssh.Password("password"),
17		},
18		HostKeyCallback: ssh.FixedHostKey(hostKey),
19	}
20	// Dial your ssh server.
21	conn, err := ssh.Dial("tcp", "localhost:22", config)
22	if err != nil {
23		log.Fatal("unable to connect: ", err)
24	}
25	defer conn.Close()
26
27	// Request the remote side to open port 8080 on all interfaces.
28	l, err := conn.Listen("tcp", "0.0.0.0:8080")
29	if err != nil {
30		log.Fatal("unable to register tcp forward: ", err)
31	}
32	defer l.Close()
33
34	// Serve HTTP with your SSH server acting as a reverse proxy.
35	http.Serve(l, http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
36		fmt.Fprintf(resp, "Hello world!\n")
37	}))
38}