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3. SSH (Secure Shell).md

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SSH: Secure Shell

SSH (Secure Shell) is a protocol used to securely connect to remote systems over a network. It provides a secure channel over an unsecured network by encrypting the connection. SSH is commonly used for remote command-line login, file transfers, and secure tunneling.

1. Introduction to SSH

  • Purpose: SSH enables secure remote access to servers and devices, allowing users to execute commands, transfer files, and manage systems over a network.
  • Components:
    • SSH Client: The software that initiates the connection.
    • SSH Server: The software running on the remote machine that accepts and manages the connection.

2. Basic SSH Usage

2.1 Connecting to a Remote Server

  • Command:

    ssh username@hostname_or_ip
    • username: The user account on the remote system.
    • hostname_or_ip: The remote system's hostname or IP address.

  • Example:

    ssh [email protected]
    • Connects to the remote server example.com as the user user.

2.2 Specifying a Port

  • Command:

    ssh -p port_number username@hostname_or_ip
    • -p port_number: Specifies a non-default SSH port (default is 22).

  • Example:

    ssh -p 2222 [email protected]
    • Connects to example.com on port 2222.

2.3 Using SSH Keys for Authentication

  • Generate SSH Key Pair:

    ssh-keygen -t rsa -b 4096 -C "[email protected]"
    • -t rsa: Specifies the RSA key type.
    • -b 4096: Generates a 4096-bit key (more secure).
    • -C "[email protected]": Adds a comment (usually your email) to the key.

  • Copy Public Key to Remote Server:

    ssh-copy-id username@hostname_or_ip
    • Copies your public key to the remote server, allowing key-based authentication.

  • Example:

    ssh-copy-id [email protected]
    • Copies your public key to example.com for the user user.

2.4 SSH Config File

  • Location: ~/.ssh/config

  • Purpose: Simplifies SSH connections by storing configuration options.

  • Example Configuration:

    Host example
    HostName example.com
    User user
    Port 2222
    IdentityFile ~/.ssh/id_rsa
    • Allows you to connect using ssh example instead of specifying the full command each time.

3. File Transfers with SSH

3.1 Using scp (Secure Copy)

  • Command:

    scp source_file username@hostname_or_ip:/remote/directory
    • Copies a file from the local system to the remote system.

  • Example:

    scp file.txt [email protected]:/home/user/
    • Copies file.txt to the /home/user/ directory on example.com.

3.2 Using rsync

  • Command:

    rsync -avz source_directory username@hostname_or_ip:/remote/directory
    • -a: Archive mode, preserves permissions, symlinks, etc.
    • -v: Verbose output.
    • -z: Compresses the data during transfer.

  • Example:

    rsync -avz /local/dir/ [email protected]:/remote/dir/
    • Synchronizes the local directory /local/dir/ with /remote/dir/ on example.com.

4. SSH Tunneling

4.1 Local Port Forwarding

  • Purpose: Forwards a local port to a remote network service.

  • Command:

    ssh -L local_port:remote_host:remote_port username@hostname_or_ip
    • -L local_port:remote_host:remote_port: Specifies local port forwarding.

  • Example:

    ssh -L 8080:localhost:80 [email protected]
    • Forwards local port 8080 to port 80 on example.com.

4.2 Remote Port Forwarding

  • Purpose: Forwards a remote port to a local network service.

  • Command:

    ssh -R remote_port:local_host:local_port username@hostname_or_ip
    • -R remote_port:local_host:local_port: Specifies remote port forwarding.

  • Example:

    ssh -R 8080:localhost:80 [email protected]
    • Forwards port 8080 on example.com to port 80 on the local machine.

4.3 Dynamic Port Forwarding (SOCKS Proxy)

  • Purpose: Creates a SOCKS proxy to tunnel traffic through SSH.

  • Command:

    ssh -D local_port username@hostname_or_ip
    • -D local_port: Specifies dynamic port forwarding.

  • Example:

    ssh -D 1080 [email protected]
    • Sets up a SOCKS proxy on local port 1080.

5. Securing SSH

5.1 Disabling Root Login

  • Edit SSH Configuration: 
    sudo nano /etc/ssh/sshd_config
    • Find and set PermitRootLogin to no.

5.2 Changing the Default Port

  • Edit SSH Configuration: 
    sudo nano /etc/ssh/sshd_config
    • Change Port 22 to a non-standard port (e.g., Port 2222).

5.3 Enabling Firewall Rules

  • Example (Using ufw): 
    sudo ufw allow 2222/tcp
sudo ufw enable
    • Allows traffic on the custom SSH port and enables the firewall.

5.4 Two-Factor Authentication

  • Install and Configure: 
    sudo apt-get install libpam-google-authenticator
    • Follow instructions to set up two-factor authentication (2FA) with Google Authenticator.

Conclusion

SSH is a powerful tool for securely managing remote systems. By mastering SSH and its associated commands, you can effectively manage servers, transfer files securely, and even tunnel traffic through encrypted connections. Ensuring that SSH is properly secured is crucial to maintaining the integrity and security of your systems.

Next: Networking Configuration

Previous: Networking Commands