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SSH Penetration Solution for TCD

1. Introduction

Some students in TCD may need to access their desktops in the lab remotely via SSH. However, the network is behind a firewall, which blocks the incoming traffic and requires a penetration method. Serval potential solutions are listed, and there is no perfect solution for SSH penetration so far.

Method Cost Baned SSH Remote Desktop Reliable
Direct SSH Free ✓ ✓ ✗ high
RestDesk Free ✓ ✓ ✓ medium
ToDesk ~25Euro+/Year ✗ ✗ ✓ high
TeamViewer Free ✗ ✗ ✓ low (commercial usage ban)

This document provides a simple solution to access the desktops in the lab remotely using SSH. The core idea is:

  • Reverse SSH tunneling between the desktop in the lab and an ACCESSIBLE server (e.g., Azure).
  • Port forwarding from prot 22 to another port (e.g., 2222) on the server.

2. Prerequisites

  • Make sure you have an ACCESSIBLE server (e.g., Azure, free for TCD students) with a public IP address (or a domain name).
  • Set up an SSH service on your DESKTOP in the lab.

3. Server Configuration

3.1 Create an Azure Virtual Machine (Optional)

Sign in to the Azure portal, click Free service from the 'create a resource' menu, and then click Linux VM. A 1-core VM with 1GB memory is enough for this purpose. Remember to close the diagnosis function for cost-saving. Create VM The final configuration should look like this:

  • 1 core cpu, 1 GB memory
  • 64GB disk
  • SSH public key authentication (recommended)
  • Dynamic public IP address

Azure VM Fee Policy: The student Azure account can get 750 hours of B1s VMs for free every month. So just setup and forget it, if you have only 1 VM.

3.2 Setup Domain Name for Azure VM (Optional)

Band domain name to the public IP address of the VM. The Azure provides a free domain name. You can get it in the 'DNS name' section of the VM. Domain Name

You can also use your own domain name.

3.3 Server Configuration

  • Make sure the SSH service is running on the server.
  • Open the 'gateway ports' of SSH in the server. With Linux server, edit the SSH configuration file /etc/ssh/sshd_config: 
    sudo vim /etc/ssh/sshd_config
    make sure the following line is uncommented: 
    GatewayPorts yes
    Then restart the SSH service: 
    sudo systemctl restart sshd
  • Add firewall rules to allow the traffic on the port you want to forward. For example, if you want to forward port 2222, add the following rule: 
    sudo ufw allow 22/tcp
sudo ufw allow 2222
sudo ufw enable
  • For cloud services users, you may need to add inbound/outbound rules in the portal to allow the traffic on the port you want to forward.

    For Azure VMs, add inbound/outbound rules in the Azure portal to allow the traffic on the port you want to forward. Add Rule

4. Desktop (in your lab) Configuration

4.1 Opening Reverse SSH Tunneling

Open a reverse SSH tunneling from the desktop to the server. Assume your private key is located at ~.ssh/YOUR_KEY.pem, the username is USER, the domain name is DOMAIN_NAME, and the port you want to forward is 2222. Run the following command on the desktop:

ssh -i ~/.ssh/YOUR_KEY.pem -R 0.0.0.0:2222:localhost:22 USER@DOMAIN_NAME

Please note that the option -R 0.0.0.0:2222:localhost:22 means forwarding the port 22 of your desktop to the port 2222 of the server.

If everything goes well, you should login to the server. Please keep the terminal open.

For public key authentication users: You should set your private key to be read-only by the owner.

  • For Linux, run the following command to set the permission:
sudo chmod 600 ~/.ssh/YOUR_KEY.pem
  • For Windows, it may be a little bit tricky according to my experience. You can find many tutorials on the internet.:P

4.2 Port Forwarding Checking

Check if the port forwarding is working. Run the following command on the server:

sudo netstat -tuln | grep 2222

if you see the following output, the port forwarding is working:

tcp        0      0 0.0.0.0:2222         0.0.0.0:*               LISTEN     
tcp6       0      0 :::2222              :::*                    LISTEN

4.3 Access the Desktop Remotely

Now you can access the desktop remotely. Assume the domain name of the Azure server is DOMAIN_NAME, the username of your lab desktop is LOCAL_USER, and the port you want to forward is 2222. Run the following command on your local machine:

ssh -p 2222 LOCAL_USER@DOMAIN_NAME

If connected successfully, you should input the password of your lab desktop and login to the desktop.

If you have set up the public key authentication in your lab desktop, you add `-i' option to specify the private key.

4.4 Automation

  • You can use autossh to keep the tunnel alive.
  • Or write a script to automate the process. For example, create a file tunnel.sh with the following content: 
    #!/bin/bash

# configs
REMOTE_USER="tcd_user"                    # Cloud server user name
REMOTE_HOST="tcd_user.serverlocation.cloudapp.azure.com"       # domain name OR ip
REMOTE_PORT="2222"                   # listen port
LOCAL_PORT="22"                       # ssh port of your desktop in TCD
PRIVATE_KEY="~/.ssh/my_key.pem"  # your private key of the CLOUD SERVER
WAITING_TIME_SECOND="600"

while true; do
    echo "Try SSH to the server..."
    
    # Establish ssh
    ssh -i "$PRIVATE_KEY" -R 0.0.0.0:"$REMOTE_PORT:localhost:$LOCAL_PORT" "$REMOTE_USER@$REMOTE_HOST"
      
    # Waiting and retry
    sleep WAITING_TIME_SECOND
done

You can set the script to start automatically when the desktop boots up.

5. SSH from Lab to Remote Servers

Sometimes you may need to SSH from your lab desktop to other servers (at your home, for example). You can try following the same steps to establish a reverse SSH tunneling from the lab desktop to the server at home. Then you can SSH from the lab desktop to the server at home.

I have not tried this yet. If you have any questions or suggestions, please feel free to contact me. I am happy to help you.