SSH: The Ultimate Beginner’s Tutorial

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Okay, here’s a comprehensive article on SSH, designed for beginners, spanning approximately 5000 words. I’ve broken it down into logical sections with clear headings and subheadings to make it easily digestible.

SSH: The Ultimate Beginner’s Tutorial

Table of Contents

  1. Introduction: What is SSH and Why Should You Care?

    • 1.1. The Problem SSH Solves: Insecure Network Communication
    • 1.2. What SSH Is: A Secure Shell
    • 1.3. Key Benefits of Using SSH
    • 1.4. Common Use Cases for SSH
    • 1.5. A Note on Terminology: SSH, OpenSSH, Clients, and Servers

  2. Understanding the Basics: How SSH Works

    • 2.1. Client-Server Architecture
    • 2.2. The SSH Protocol: A Layered Approach
      • 2.2.1. Transport Layer: Establishing a Secure Connection
      • 2.2.2. User Authentication Layer: Verifying Identity
      • 2.2.3. Connection Protocol Layer: Multiplexing Channels
    • 2.3. Cryptography in SSH: Keeping Secrets Secret
      • 2.3.1. Symmetric-Key Encryption
      • 2.3.2. Asymmetric-Key Encryption (Public-Key Cryptography)
      • 2.3.3. Hashing
      • 2.3.4. Key Exchange Algorithms (Diffie-Hellman)
    • 2.4. The SSH Handshake: A Step-by-Step Walkthrough

  3. Setting Up SSH: Your First Connection

    • 3.1. Prerequisites: What You’ll Need
    • 3.2. Installing an SSH Client
      • 3.2.1. Windows: PuTTY, MobaXterm, Windows Subsystem for Linux (WSL)
      • 3.2.2. macOS: Terminal (Built-in)
      • 3.2.3. Linux: Terminal (Built-in, OpenSSH Client Usually Pre-installed)
    • 3.3. Installing an SSH Server (Optional – For Receiving Connections)
      • 3.3.1. Linux (Ubuntu/Debian): sudo apt install openssh-server
      • 3.3.2. Linux (CentOS/RHEL/Fedora): sudo yum install openssh-server
      • 3.3.3. macOS: Enabling Remote Login (System Preferences)
      • 3.3.4. Windows (Advanced): OpenSSH Server Feature
    • 3.4. Making Your First Connection: ssh username@hostname
      • 3.4.1. Understanding the Command
      • 3.4.2. The First-Time Host Key Verification
      • 3.4.3. Password Authentication
    • 3.5. Basic SSH Commands: Navigating the Remote System
      • 3.5.1. pwd: Print Working Directory
      • 3.5.2. ls: List Files and Directories
      • 3.5.3. cd: Change Directory
      • 3.5.4. mkdir: Make Directory
      • 3.5.5. rm: Remove Files and Directories
      • 3.5.6. cp: Copy Files and Directories
      • 3.5.7. mv: Move/Rename Files and Directories
      • 3.5.8. cat, less, more: Viewing File Contents
      • 3.5.9. nano, vim, emacs: Text Editors
      • 3.5.10. exit: Closing the SSH Connection

  4. SSH Key-Based Authentication: A More Secure Approach

    • 4.1. Why Use Keys Instead of Passwords?
    • 4.2. Generating an SSH Key Pair: ssh-keygen
      • 4.2.1. Key Types: RSA, DSA, ECDSA, Ed25519
      • 4.2.2. Choosing a Key Size (Bit Length)
      • 4.2.3. Setting a Passphrase (Optional, but Highly Recommended)
      • 4.2.4. Understanding the Output: id_rsa and id_rsa.pub
    • 4.3. Copying Your Public Key to the Server: ssh-copy-id (and Manual Methods)
    • 4.4. Connecting with Your SSH Key
    • 4.5. Managing Multiple SSH Keys
    • 4.6 Disabling password authentication

  5. Advanced SSH Concepts and Techniques

    • 5.1. SSH Configuration File (~/.ssh/config)
      • 5.1.1. Creating and Editing the Config File
      • 5.1.2. Defining Host Aliases
      • 5.1.3. Specifying Usernames
      • 5.1.4. Setting Port Numbers
      • 5.1.5. Using Different Keys for Different Hosts
      • 5.1.6. Other Useful Configuration Options
    • 5.2. SSH Tunneling (Port Forwarding)
      • 5.2.1. Local Port Forwarding: Accessing Remote Services Locally
      • 5.2.2. Remote Port Forwarding: Exposing Local Services Remotely
      • 5.2.3. Dynamic Port Forwarding (SOCKS Proxy)
    • 5.3. SSH Agent: Managing Your SSH Keys
      • 5.3.1. Starting the SSH Agent
      • 5.3.2. Adding Keys to the Agent: ssh-add
      • 5.3.3. Listing Loaded Keys: ssh-add -l
    • 5.4. Running Commands Remotely Without a Full Shell
      • 5.4.1. ssh user@host 'command'
    • 5.5. Copying Files with SSH: scp and sftp
      • 5.5.1. scp: Secure Copy Protocol
      • 5.5.2. sftp: Secure FTP
    • 5.6. Multiplexing SSH Connections
    • 5.7. Using SSH with Git
    • 5.8. X11 Forwarding (Running Graphical Applications Remotely)
    • 5.9 Jump Hosts (Bastion Hosts)

  6. SSH Security Best Practices

    • 6.1. Use Strong Passphrases for Your SSH Keys
    • 6.2. Disable Root Login
    • 6.3. Change the Default SSH Port (22)
    • 6.4. Use Key-Based Authentication and Disable Password Authentication
    • 6.5. Limit User Access with AllowUsers and DenyUsers
    • 6.6. Keep Your SSH Software Updated
    • 6.7. Use a Firewall
    • 6.8. Monitor SSH Logs
    • 6.9. Consider Two-Factor Authentication (2FA)
    • 6.10. Use Fail2Ban or Similar Intrusion Prevention Systems

  7. Troubleshooting Common SSH Issues

    • 7.1. Connection Refused
    • 7.2. Permission Denied (Publickey, Password)
    • 7.3. Network Error: Connection Timed Out
    • 7.4. Host Key Verification Failed
    • 7.5. Agent Connection Refused
    • 7.6. Problems with SSH Keys
    • 7.7. Performance Issues

  8. Conclusion: Your Journey with SSH Begins

1. Introduction: What is SSH and Why Should You Care?

1.1. The Problem SSH Solves: Insecure Network Communication

Before the widespread adoption of SSH, communicating with remote servers over a network often involved protocols like Telnet and FTP. These protocols, while functional, had a major flaw: they transmitted data, including usernames and passwords, in plain text. This meant that anyone with access to the network traffic (e.g., someone on the same Wi-Fi network, an internet service provider, or a malicious actor) could easily intercept and read this sensitive information. This was a huge security risk.

1.2. What SSH Is: A Secure Shell

SSH, which stands for Secure Shell, is a network protocol that provides a secure way to access and manage remote computers over an unsecured network. It achieves this security through strong encryption and authentication mechanisms. Think of it as a heavily armored tunnel through which your data travels, protected from prying eyes.

1.3. Key Benefits of Using SSH

  • Security: This is the primary benefit. SSH encrypts all communication between your computer (the client) and the remote computer (the server), protecting your data from eavesdropping.
  • Authentication: SSH provides robust methods to verify the identity of both the client and the server, preventing man-in-the-middle attacks.
  • Data Integrity: SSH ensures that the data transmitted hasn’t been tampered with during transit.
  • Versatility: SSH is not just for logging into a remote shell. It can be used for file transfer, port forwarding, and running remote commands.
  • Widely Supported: SSH is a standard protocol supported on virtually all Unix-like operating systems (Linux, macOS, BSD) and is increasingly available on Windows.
  • Automation: SSH can be easily integrated into scripts and automation tools, making it ideal for managing large numbers of servers.

1.4. Common Use Cases for SSH

  • Remote Server Administration: This is the most common use. System administrators use SSH to log into servers, manage files, install software, configure services, and perform other administrative tasks.
  • Secure File Transfer: SSH provides secure alternatives to FTP, such as SCP (Secure Copy) and SFTP (Secure FTP), for transferring files between computers.
  • Remote Command Execution: You can run commands on a remote server without needing to log in interactively.
  • Port Forwarding (Tunneling): This allows you to securely access services running on a remote network as if they were running on your local machine, or vice-versa. This is useful for accessing databases, web servers, or other services behind firewalls.
  • Version Control (Git): SSH is often used to securely connect to Git repositories hosted on platforms like GitHub, GitLab, and Bitbucket.
  • Cloud Computing: SSH is the primary method for accessing and managing virtual machines in cloud environments like AWS, Google Cloud, and Azure.
  • Network Device Management: Routers, switches and other network devices often use SSH for secure remote configuration.

1.5. A Note on Terminology: SSH, OpenSSH, Clients, and Servers

  • SSH: This refers to the protocol itself, the set of rules that define how secure communication should happen.
  • OpenSSH: This is the most widely used implementation of the SSH protocol. It’s a suite of tools that includes the ssh client, the sshd server, scp, sftp, ssh-keygen, and other utilities. When people say “SSH,” they often mean OpenSSH.
  • SSH Client: This is the software on your local machine that you use to initiate a connection to a remote server. Examples include PuTTY (Windows), Terminal (macOS and Linux), and MobaXterm (Windows).
  • SSH Server: This is the software running on the remote machine that listens for incoming SSH connections. The most common SSH server is sshd (the OpenSSH daemon).

2. Understanding the Basics: How SSH Works

2.1. Client-Server Architecture

SSH follows a client-server architecture. This means there are two main components:

  • The SSH Client: This is the program you run on your local computer to initiate the connection. It’s responsible for encrypting your requests, sending them to the server, and decrypting the server’s responses.
  • The SSH Server: This is the program running on the remote computer that you want to access. It listens for incoming connection requests, authenticates clients, decrypts their requests, executes commands, and encrypts the results before sending them back.

The client initiates the connection, and the server responds. The server is always listening for incoming connections on a specific port (by default, port 22).

2.2. The SSH Protocol: A Layered Approach

The SSH protocol is structured in layers, similar to the OSI model for networking. This layered approach makes the protocol more modular and easier to maintain. The three main layers are:

  • 2.2.1. Transport Layer: Establishing a Secure Connection

    This is the foundation of SSH security. It’s responsible for:

    • Key Exchange: The client and server use a key exchange algorithm (like Diffie-Hellman) to securely agree on a shared secret key without actually transmitting the key itself over the network. This shared secret is used for symmetric-key encryption.
    • Encryption: All data transmitted between the client and server is encrypted using a symmetric-key algorithm (like AES or ChaCha20) with the shared secret key.
    • Data Integrity: Message Authentication Codes (MACs) are used to ensure that the data hasn’t been tampered with during transit. A MAC is a cryptographic checksum that is calculated based on the data and the shared secret key.
    • Server Authentication: The server presents its public key to the client. The client verifies this key against a list of known hosts (usually stored in ~/.ssh/known_hosts) to ensure it’s connecting to the correct server and not an imposter.

  • 2.2.2. User Authentication Layer: Verifying Identity

    Once the secure connection is established, the client needs to prove its identity to the server. This layer handles the authentication process. Common authentication methods include:

    • Password Authentication: The client sends the user’s password to the server (encrypted, of course). The server checks the password against its user database. This is the simplest but least secure method.
    • Public Key Authentication (Key-Based Authentication): The client uses its private key to digitally sign a challenge sent by the server. The server uses the client’s corresponding public key (which must be pre-configured on the server) to verify the signature. This is much more secure than password authentication.
    • Other Methods: SSH also supports other authentication methods like Kerberos and GSSAPI, but these are less common in typical beginner scenarios.

  • 2.2.3. Connection Protocol Layer: Multiplexing Channels

    This layer sits on top of the authenticated, secure connection. It allows multiple logical channels to be established over a single SSH connection. This is useful for:

    • Interactive Shell Sessions: This is the standard terminal session you get when you log in with SSH.
    • Remote Command Execution: You can run a single command on the remote server without starting an interactive shell.
    • Port Forwarding (Tunneling): This allows you to create secure tunnels for other applications.
    • File Transfer (SCP/SFTP): These use separate channels within the SSH connection.

2.3. Cryptography in SSH: Keeping Secrets Secret

Cryptography is at the heart of SSH’s security. Here’s a breakdown of the key cryptographic concepts used:

  • 2.3.1. Symmetric-Key Encryption

    In symmetric-key encryption, the same key is used for both encryption and decryption. Think of it like a lockbox with a single key. Both the sender and receiver need to have a copy of this key. SSH uses symmetric-key encryption for the bulk of the data transfer because it’s very fast. Common symmetric-key algorithms used in SSH include:

    • AES (Advanced Encryption Standard): The most widely used symmetric-key algorithm. It’s considered very secure and is available in different key sizes (128-bit, 192-bit, 256-bit).
    • ChaCha20: A newer algorithm that’s gaining popularity, especially in environments where hardware acceleration for AES is not available.

  • 2.3.2. Asymmetric-Key Encryption (Public-Key Cryptography)

    Asymmetric-key encryption uses a pair of keys: a public key and a private key. These keys are mathematically related, but it’s computationally infeasible to derive the private key from the public key.

    • Public Key: This key can be freely shared with anyone. It’s used to encrypt data or verify a digital signature.
    • Private Key: This key must be kept secret. It’s used to decrypt data that was encrypted with the corresponding public key, or to create a digital signature.

    SSH uses asymmetric-key encryption for:

    • Key Exchange: To securely establish the shared secret key used for symmetric-key encryption.
    • Server Authentication: The server proves its identity by using its private key to sign a challenge, which the client verifies with the server’s public key.
    • User Authentication (Key-Based Authentication): The client proves its identity by using its private key to sign a challenge, which the server verifies using the client’s public key.

    Common asymmetric-key algorithms used in SSH include:

    • RSA (Rivest-Shamir-Adleman): The most widely used asymmetric-key algorithm.
    • DSA (Digital Signature Algorithm): Primarily used for digital signatures.
    • ECDSA (Elliptic Curve Digital Signature Algorithm): A more modern algorithm that offers stronger security with smaller key sizes compared to RSA.
    • Ed25519: A specific type of elliptic curve algorithm that’s considered very secure and efficient.

  • 2.3.3. Hashing

    Hashing is a one-way function that takes an input (of any size) and produces a fixed-size output (called a hash or digest). It’s computationally infeasible to reverse the process (i.e., find the original input from the hash). Hashing is used in SSH for:

    • Data Integrity: Hashing is used to create Message Authentication Codes (MACs) to ensure that data hasn’t been altered.
    • Password Storage: SSH servers typically don’t store passwords directly. Instead, they store a hash of the password. When a user tries to authenticate, the server hashes the entered password and compares it to the stored hash.

    Common hashing algorithms used in SSH include:

    • SHA-2 (Secure Hash Algorithm 2): A family of hash functions (SHA-256, SHA-384, SHA-512) that are widely used and considered secure.
    • SHA-1 Is considered cryptographically broken and should no longer be used.

  • 2.3.4. Key Exchange Algorithms (Diffie-Hellman)

    The Diffie-Hellman key exchange is a crucial part of how SSH establishes a secure connection. It allows the client and server to agree on a shared secret key without ever transmitting the key itself over the network. Here’s a simplified explanation:

    1. Public Parameters: The client and server agree on two public numbers: a prime number (p) and a generator (g). These are not secret.
    2. Private Secrets:
      • The client chooses a random secret number (a).
      • The server chooses a random secret number (b).
    3. Public Calculations:
      • The client calculates A = g^a mod p and sends A to the server.
      • The server calculates B = g^b mod p and sends B to the client.
    4. Shared Secret:
      • The client calculates s = B^a mod p.
      • The server calculates s = A^b mod p.

    Amazingly, both the client and server end up with the same value for s, which becomes their shared secret key. An eavesdropper who sees p, g, A, and B cannot easily calculate s without knowing either a or b. This is the magic of Diffie-Hellman.

2.4. The SSH Handshake: A Step-by-Step Walkthrough

The SSH handshake is the process that the client and server go through to establish a secure connection. Here’s a simplified overview:

  1. TCP Connection: The client initiates a TCP connection to the server on the SSH port (usually port 22).
  2. Version Exchange: The client and server exchange their SSH protocol version strings. They negotiate the highest mutually supported version.
  3. Algorithm Negotiation: The client and server exchange lists of supported algorithms for:
    • Key Exchange (e.g., Diffie-Hellman)
    • Server Host Key (e.g., RSA, Ed25519)
    • Encryption (e.g., AES, ChaCha20)
    • MAC (e.g., HMAC-SHA256)
    • Compression (e.g., zlib)
      They agree on a common set of algorithms to use.
  4. Key Exchange: The client and server perform the key exchange (e.g., Diffie-Hellman) to establish a shared secret key.
  5. Server Authentication: The server sends its public host key to the client. The client verifies this key:
    • First-Time Connection: The client displays the key fingerprint and asks the user to confirm that it’s correct. If the user accepts, the key is added to the ~/.ssh/known_hosts file.
    • Subsequent Connections: The client checks if the server’s public key matches the one stored in ~/.ssh/known_hosts. If it doesn’t match, the client issues a warning (because this could indicate a man-in-the-middle attack).
  6. Encryption Enabled: From this point on, all communication is encrypted using the shared secret key and the negotiated encryption algorithm.
  7. User Authentication: The client authenticates to the server using the chosen method (e.g., password or public key).
  8. Session Establishment: Once authentication is successful, the connection protocol layer is established, and the client can start an interactive shell, run commands, or use other SSH features.

3. Setting Up SSH: Your First Connection

3.1. Prerequisites: What You’ll Need

  • A Remote Server: You need a computer running an SSH server that you have permission to access. This could be:
    • A virtual machine in the cloud (AWS, Google Cloud, Azure, etc.)
    • A physical server in a data center
    • A Raspberry Pi on your local network
    • Another computer on your local network (if you’ve set up an SSH server on it)
  • Network Connectivity: Your local computer needs to be able to reach the remote server over the network (either the internet or a local network).
  • An SSH Client: You need an SSH client installed on your local computer.
  • User Credentials: You need a username and either a password or an SSH key pair to authenticate to the remote server.

3.2. Installing an SSH Client

  • 3.2.1. Windows:

    • PuTTY: A very popular and free SSH client for Windows. Download it from the official website (https://www.putty.org/). PuTTY is a standalone executable; you don’t need to install it.
    • MobaXterm: A more feature-rich terminal emulator for Windows that includes an SSH client, X11 server, and many other tools. Download it from https://mobaxterm.mobatek.net/.
    • Windows Subsystem for Linux (WSL): WSL allows you to run a Linux distribution (like Ubuntu) directly within Windows. Once you’ve installed WSL, you can use the built-in Linux SSH client. See Microsoft’s documentation for instructions on installing WSL.
    • Git Bash: If you install Git for Windows, it includes a Bash shell environment with an SSH client.

  • 3.2.2. macOS:

    • Terminal: macOS comes with a built-in terminal application (located in /Applications/Utilities/). The ssh command is available by default.

  • 3.2.3. Linux:

    • Terminal: Linux distributions also come with a built-in terminal. The ssh command is almost always pre-installed as part of the OpenSSH client package. If it’s not installed, you can usually install it using your distribution’s package manager (e.g., apt install openssh-client on Debian/Ubuntu, yum install openssh-client on CentOS/RHEL).

3.3. Installing an SSH Server (Optional – For Receiving Connections)

This section is only necessary if you want to receive SSH connections on a computer. If you’re just connecting to a remote server, you don’t need to install an SSH server on your local machine.

  • 3.3.1. Linux (Ubuntu/Debian):

    bash
    sudo apt update
    sudo apt install openssh-server
    sudo systemctl status ssh # Check if the service is running

  • 3.3.2. Linux (CentOS/RHEL/Fedora):

    bash
    sudo yum install openssh-server
    sudo systemctl enable sshd # Enable the service to start on boot
    sudo systemctl start sshd
    sudo systemctl status sshd # Check if the service is running

  • 3.3.3. macOS:

    1. Open System Preferences.
    2. Click on Sharing.
    3. Check the box next to Remote Login. This enables the SSH server.

  • 3.3.4. Windows (Advanced): OpenSSH Server Feature

    Windows 10 and 11 include an optional OpenSSH server feature. This is not enabled by default and requires some configuration. It’s generally easier to use WSL or a third-party SSH server on Windows. However, if you want to use the built-in feature:

    1. Open Settings (Windows key + I).
    2. Go to Apps -> Optional features.
    3. Click Add a feature.
    4. Find OpenSSH Server and click Install.
    5. You may need to configure the firewall to allow incoming connections on port 22.
    6. Start the sshd service.

3.4. Making Your First Connection: ssh username@hostname

The basic syntax for connecting to a remote server using SSH is:

bash
ssh username@hostname
Or
bash
ssh username@ip_address

  • 3.4.1. Understanding the Command

    • ssh: This is the command to invoke the SSH client.
    • username: This is your username on the remote server.
    • hostname: This is the hostname or IP address of the remote server. A hostname is a human-readable name (like example.com), while an IP address is a numerical address (like 192.168.1.100).

    Example:

    bash
    ssh [email protected]

    This command attempts to connect to the server myserver.com as the user jdoe.

    bash
    ssh [email protected]

    This command attempts to connect to the server with the IP address 192.168.1.100 as the user jdoe.

  • 3.4.2. The First-Time Host Key Verification

    The very first time you connect to a new SSH server, you’ll see a message like this:

    The authenticity of host 'myserver.com (192.168.1.100)' can't be established.
    ECDSA key fingerprint is SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.
    Are you sure you want to continue connecting (yes/no/[fingerprint])?

    This is a crucial security step. The SSH client is asking you to verify the fingerprint of the server’s public key. This fingerprint is a unique identifier for the server’s key. The purpose of this check is to prevent man-in-the-middle attacks.

    • How to Verify: Ideally, you should compare the fingerprint displayed by the SSH client with a known, trusted source. This might be:
      • Information provided by your server administrator.
      • A configuration management system.
      • Physically accessing the server and obtaining the fingerprint directly.
    • What to Do:
      • If the fingerprint matches the trusted source, type yes and press Enter. The server’s public key will be added to your ~/.ssh/known_hosts file, and you won’t be prompted again for this server (unless the key changes).
      • If the fingerprint doesn’t match, or you can’t verify it, do not connect. This could indicate a security problem. Contact your server administrator.
      • If you type no the connection will be aborted.

  • 3.4.3. Password Authentication

    After verifying the host key (on the first connection), you’ll be prompted for your password:

    [email protected]'s password:

    Enter your password for the jdoe user on the myserver.com server. The password will not be displayed on the screen as you type. If the password is correct, you’ll be logged in, and you’ll see the remote server’s command prompt.

3.5. Basic SSH Commands: Navigating the Remote System

Once you’re logged in via SSH, you’re interacting with the remote server’s command-line interface (shell). Here are some essential commands to get you started:

  • 3.5.1. pwd: Print Working Directory

    bash
    pwd

    This command shows you the current directory you’re in on the remote server.

  • 3.5.2. ls: List Files and Directories

    bash
    ls
    ls -l # Long listing (more details)
    ls -a # Show hidden files (files starting with .)
    ls -la # Combine -l and -a

    This command lists the files and directories in the current directory.

  • 3.5.3. cd: Change Directory

    bash
    cd /path/to/directory # Change to a specific directory
    cd .. # Go up one level
    cd ~ # Go to your home directory
    cd # Also goes to your home directory

    This command changes the current directory.

  • 3.5.4. mkdir: Make Directory

    bash
    mkdir new_directory

    This command creates a new directory.

  • 3.5.5. rm: Remove Files and Directories

    bash
    rm filename # Remove a file
    rm -r directory_name # Remove a directory and its contents (recursive)

    Be extremely careful with rm, especially with rm -r! There is no “undelete” command. Deleted files are gone.

  • 3.5.6. cp: Copy Files and Directories

    bash
    cp source_file destination_file # Copy a file
    cp -r source_directory destination_directory # Copy a directory (recursive)

    This command copies files and directories.

  • 3.5.7. mv: Move/Rename Files and Directories

    bash
    mv old_name new_name # Rename a file or directory
    mv file_name directory_name # Move a file to a directory

    This command moves or renames files and directories.

  • 3.5.8. cat, less, more: Viewing File Contents

    bash
    cat filename # Display the entire contents of a file
    less filename # View file contents one page at a time (use arrow keys, spacebar, q to quit)
    more filename # Similar to less, but with fewer features

    These commands are used to view the contents of text files.

  • 3.5.9. nano, vim, emacs: Text Editors

    bash
    nano filename # A simple, beginner-friendly text editor
    vim filename # A powerful, modal text editor (steeper learning curve)
    emacs filename # Another powerful, extensible text editor
    You will need to use one of these to modify files on a remote server.

  • 3.5.10. exit: Closing the SSH Connection

    bash
    exit

    This command closes the SSH connection and returns you to your local computer’s command prompt. You can also use logout.

4. SSH Key-Based Authentication: A More Secure Approach

4.1. Why Use Keys Instead of Passwords?

Key-based authentication is significantly more secure than password authentication for several reasons:

  • Brute-Force Resistance: Passwords can be guessed or cracked using brute-force attacks (trying many different passwords). SSH keys are much longer and more complex, making brute-force attacks computationally infeasible.
  • Keyloggers: If a keylogger is installed on your computer, it can record your password as you type it. SSH keys are not typed in, so they’re not vulnerable to keyloggers.
  • Phishing: Phishing attacks trick users into entering their passwords on fake websites. SSH keys are not entered on websites, so they’re not vulnerable to phishing.
  • Password Reuse: Many people reuse the same password across multiple accounts. If one account is compromised, all accounts using that password are at risk. SSH keys are typically unique to each server.
  • Automation: Key based authentication is easily automated.

4.2. Generating an SSH Key Pair: ssh-keygen

The ssh-keygen command is used to generate an SSH key pair (a private key and a public key).

bash
ssh-keygen -t <key_type> -b <key_size> -C "comment"

  • 4.2.1. Key Types: RSA, DSA, ECDSA, Ed25519

    • -t <key_type>: Specifies the type of key to generate. Common options are:
      • rsa: The most common type. Generally a good choice.
      • dsa: An older algorithm. Generally not recommended for

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