How can I implement SSH port forwarding in Go?

Simple SSH Port Forwarding in Go

In this article, we'll delve into creating and closing a basic TCP port forward over SSH using Go, specifically for newcomers to Go and statically typed languages. We'll provide a clear and concise solution by breaking down the process into comprehensible steps.

Background and Goal

We aim to replicate the functionality of a terminal command like "ssh -L 9000:localhost:9999 username@server" programmatically using Go. This command establishes a port forward from the local port 9000 to the remote port 9999 on the specified server.

Solution Outline

To achieve this, we'll create a local listener on port 9000 and set up an SSH client connection with the remote server. When the local port receives a request, we'll establish a connection to the remote port and forward the data back and forth.

Code Breakdown

<code class="go">package main

import (
    "io"
    "log"
    "net"
    "golang.org/x/crypto/ssh"
)

var (
    username         = "root"
    password         = "password"
    serverAddrString = "192.168.1.100:22"
    localAddrString  = "localhost:9000"
    remoteAddrString = "localhost:9999"
)

func forward(localConn net.Conn, config *ssh.ClientConfig) {
    sshClientConn, err := ssh.Dial("tcp", serverAddrString, config)
    if err != nil {
        log.Fatalf("ssh.Dial failed: %s", err)
    }
    sshConn, err := sshClientConn.Dial("tcp", remoteAddrString)
    go func() { _, err = io.Copy(sshConn, localConn) }()
    go func() { _, err = io.Copy(localConn, sshConn) }()
}

func main() {
    config := &ssh.ClientConfig{
        User: username,
        Auth: []ssh.AuthMethod{
            ssh.Password(password),
        },
    }
    localListener, err := net.Listen("tcp", localAddrString)
    if err != nil {
        log.Fatalf("net.Listen failed: %v", err)
    }
    for {
        localConn, err := localListener.Accept()
        if err != nil {
            log.Fatalf("listen.Accept failed: %v", err)
        }
        go forward(localConn, config)
    }
}</code>

Explanation

• We create a local listener on port 9000 (localAddrString).
• When a connection is made to the local port, we establish an SSH client connection to the server (sshClientConn).
• We then connect to the remote port on the server (sshConn).
• Separate goroutines are used to copy data from the local connection to the SSH connection and vice versa.
• This allows data to be forwarded between the local and remote ports.

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