ssh tunnel

Introduction to SSH Tunnels

Secure Shell, or SSH, is used to create a secure channel between a local and remote computer. While SSH is commonly used for secure terminal access and file transfers, it can also be used to create a secure tunnel between computers for forwarding other network connections that are not normally encrypted. SSH tunnels are also useful for allowing outside access to internal network resources.

An SSH tunneling (SSH Port Forwarding) establishes a connection between your local machine and the remote machine via a TCP port. When you configure your local application to use an SSH tunnel you tell it to connect to your local machine at a specified port, rather than the remote machine. The tunnel then carries your traffic securely to the remote machine.

SSH Port Forwarding： - Local Port Forwarding - Remote Port Forwarding - Dynamic Port Forwarding

For both local and remote port forwarding, there are three roles:

• Client
  • Any machine from which you can type ssh to start Port Forwarding
• SSH server
  • Clients can use SSH connections for machine learning
• target server
  • Some machines you want to establish a connection to, usually to open services on this machine to the outside world

Note that both the client and the SSH server can themselves be the target server. It does not mean that only three machines can be used for port forwarding!

常用的 SSH 指令參數

sh -N -L 0.0.0.0:local-port:target-server:target-port username@proxy-server

• -L
  • 建立 Local forwarding
• -N
  • 不要執行任何遠端指令。沒有加這個參數時，建立 Port Forwarding 的同時也會開啟 Remote Shell，讓你可以對 SSH Server 下指令，而這個參數可以讓 Remote Shell 不要打開。
• -f
  • 讓 ssh 指令在背景執行，讓你可以繼續用 Shell 做事情。通常會搭上面的 -N 使用。
• lsof -i -n | egrep '\<ssh\>'
  • 列出目前有的 SSH tunnel

Local Port Forwarding

Local forwarding is used to forward a port from the client machine to the server machine.

Quite a few organizations for all incoming SSH access through a single jump server. The server may be a standard Linux/Unix box, usually with some extra hardening, intrusion detection, and/or logging, or it may be a commercial jump server solution.

To create a local port forward add the -L parameter to the ssh command line.

$ ssh -L 9090:localhost:8080 user@server-ip

$ ssh -L 9090:192.168.1.101:8080 user@server-ip

The 192.168.1.101 (like Target server) here is relative to user@server-ip, so it is the IP address of the server behind the firewall or VPN. Then, you can visit 192.168.1.101:9090

Client -- SSH server -- Target server

• Client
  • 你的電腦
• SSH Server
  • 防火牆後你的機器
  • SSH Destination： user@server-ip
• Target Server
  • 防火牆後的伺服器
  • 192.168.1.101:8080

ssh -L 9090:192.168.1.101:8080 user@server-ip

bind_address

如果你沒有給 bind_address ，預設會 Bind 在 localhost 上。如果你想把 Port 9090 開放給所有人用：

$ ssh -L 0.0.0.0:9090:localhost:8080 user@server-ip

Remote Port Forwarding

ssh -R 9090:localhost:8080 user@server-ip

References

What is a Jump Server?
SSH Tunneling (Port Forwarding) 詳解