The Complete Overview of SSH: From Basics to Advanced

Overview of SSH (Secure Shell) from basics to advanced concepts.

1. Basics:
— SSH is a network protocol that allows secure remote access to systems over an unsecured network.
— It provides encryption and authentication mechanisms to protect data during transmission.
— SSH uses a client-server model, where the client connects to the server and establishes a secure channel for communication.
— By default, SSH operates on TCP port 22.

2. Key Concepts:
— Public-key cryptography: SSH uses public-key cryptography to authenticate and establish secure connections.
— Public key: A key pair consists of a public key and a private key. The public key is stored on the server and the private key is kept securely by the client.
— Authentication: SSH supports different authentication methods, such as password-based authentication and public key authentication.
— SSH daemon: The SSH server process running on the remote system that listens for incoming SSH connections.
— SSH client: The program used to initiate an SSH connection to a remote server.

3. Connecting to a Remote Server:
— To connect to a remote server, you need an SSH client installed on your local machine.
— On Linux and macOS, you can use the built-in OpenSSH client. On Windows, popular clients include PuTTY and OpenSSH for Windows.
— To establish a connection, use the command `ssh username@hostname` in the terminal, where `username` is your username on the remote server and `hostname` is the IP address or domain name of the server.

4. Public Key Authentication:
— Public key authentication is more secure and convenient than password-based authentication.
— Generate an SSH key pair using the `ssh-keygen` command.
— The public key (`id_rsa.pub`) is added to the server’s `~/.ssh/authorized_keys` file, while the private key (`id_rsa`) is kept on the client machine.
— When connecting, the client signs a challenge from the server with its private key, and the server verifies it using the stored public key.

5. Advanced SSH Features:
— SSH tunneling/port forwarding: Allows secure access to remote services by forwarding ports through an SSH connection.
— ProxyJump/JumpHost: Connect to a remote server via an intermediate “jump host” or “bastion host.”
— SSH agent: A program that holds private keys in memory, allowing seamless authentication to multiple servers without re-entering passwords or passphrases.
— SFTP (SSH File Transfer Protocol): Securely transfer files between systems using the SSH protocol.
— X11 forwarding: Forward graphical applications from the server to the client machine.

6. Security Considerations:
— Keep your SSH client and server software up to date to benefit from security patches.
— Disable password-based authentication and rely on public key authentication.
— Configure SSH to listen on non-standard ports to avoid automated scanning.
— Protect your private key with a strong passphrase.
— Use firewall rules to restrict SSH access to trusted networks.

Remember that this is just an overview of SSH, and there are many more advanced topics and configurations available. It’s always recommended to refer to official documentation and resources for detailed instructions and best practices.