What is SSH Protocol? & How works?

Explore what SSH Protocol is and how it secures your online communications. Master secure connections effortlessly!

Updated: 28 Apr, 24 by Lisa P 12 Min

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"What is SSH Protocol?" is a question often asked by those delving into the realm of network security and remote server management. SSH, or Secure Shell, is a cryptographic network protocol primarily used for secure access to remote computers. It allows users to execute commands, transfer files, and manage network services securely over an unsecured network. SSH protocol not only encrypts the session, rendering the communication between the server and client secure from eavesdropping, but it also ensures the integrity and confidentiality of data. This article aims to explore the intricacies of SSH, its key functionalities, and why it is considered an essential tool for secure network administration.

SSH (Secure Shell) is a vital network protocol that allows secure communication between devices, even over unsecured networks. It provides powerful features for:

  • Remote Login: Enables encrypted access and command execution on remote servers, essential for system administrators and developers.
  • Secure File Transfer: Offers safe file transfers using protocols like SFTP (SSH File Transfer Protocol) protecting sensitive data.
  • Network Tunneling: Creates secure "tunnels" to route other application traffic, enhancing privacy and bypassing network restrictions.

SSH uses strong encryption methods to safeguard all data exchanged during a session, deterring attackers from intercepting or tampering with your communications. It provides a secure channel over an unsecured network by using a client-server architecture, where the SSH client connects to the SSH server. SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary.

This protocol is an essential tool for managing servers, and network devices, and for executing a wide range of tasks securely, from system updates and installations to monitoring and troubleshooting. SSH replaces insecure legacy protocols such as Telnet, rlogin, and FTP, making it the standard for secure remote server access in today's interconnected world.

The SSH protocol works in the client-server model, where one device is the server and the other acts as a client. The SSH client will establish the connection process and use public-key cryptography to verify the identity of the SSH server. After the connection has been set up, the SSH protocol will use strong symmetric encryption and hashing algorithms to ensure the privacy and integrity of the data that is being transferred between the client and server.

The image below will show the server and client data flow.

In the process of working on the SSH protocol, we will be discussing various steps that take place.

  • Establishing an SSH connection.
  • Session encryption negotiation.
  • User authentication.

Step 1: Establishing an SSH connection

To create an SSH connection, you need to run a command from the client’s machine, as shown below.

ssh [username]@[server_ip_or_hostname]

Once the command runs, a session encryption negotiation process will start.

Step 2: Session encryption negotiation

Once the request has been sent to the server, the server will send the client a set of encryption protocols being supported by the server. The server will then use the public key as the authentication method. The client will match the server’s set of protocols with its own. If the protocol matches, then both machines will agree to establish the connection and start the data transfer.

The client will compare the server's public key to the locally stored private key while establishing the connection for the first time. If both the keys match, the client and server will start the symmetric encryption method for communicating during the SSH session. For this communication to occur, they will start communicating using an asymmetrically encrypted process that works on the Diffie-Hellman (DH) key exchange algorithm for securely creating a cryptographic key over a public network. For generating a key, the machines will be performing the following steps:

  • Both the machines will agree on two numbers which are a modulus and a base number. To prevent brute force key decryption, you must choose a modulus as a prime number of at least 600 digits.
  • The machines will individually choose one number and apply it to the two public numbers equation.
  • The server and the client will then exchange these calculated values.
  • Each machine will perform a calculation based on the result received from the other, as shown below.

After the process, a shared secret key will be generated that will be shared between both devices.

Step 3: User authentication

The most commonly used SSH user authentication methods are passwords and SSH keys. The clients will send the encrypted passwords to the server securely. However, in some scenarios, the passwords are considered a risky authentication method as the password's strength depends on how the user creates the password.

While using the asymmetrically encrypted SSH public-private key pairs is considered a better option than passwords. Once the client decrypts the message, the server will then grant the required system access to the client.

To generate an SSH key pair, you must type ssh-keygen in the terminal. Then, the system will generate and store the keys, as shown below.

SSH was introduced to update the insecure terminal emulation, for instance, login or Telnet. SSH performs the same functions with more flexibility and comfort. Not only this, but it also replaces several file transfer programs. For instance, File Transfer Protocol and Remote Copy.

The primary aim of SSH is to connect the remote host in case of a terminal session. You use the following command to achieve the task:

ssh UserName@SSHserver.example.com

What does this Linux command do?

Well, this command will help the client to connect to the server.

Moving onto the next segment, SSH has various associated uses, which are listed below.

  • By default, you will get the SSH terminal for MAC, Windows, and Unix system.
  • Various kinds of communications can be secured with the help of SSH connections between a remote host and a local machine.
  • It is also used to manage routers, virtualization platforms, server hardware, file transfer applications, and much more.
  • You can also connect to servers and use tools or the terminal to upload and exit.
  • SSH keys come with SSO, or single sign-on, that helps the users to move between the accounts without inserting the password every time

Additionally, it plays an important role in access management and identity management. The SSH traffic is highly encrypted as well. It also assists you with the secure issuance of commands on several remote devices. You can expect secured as well as automated file transfers. Also, be assured of the security management of your network infrastructure components.

In a nutshell, SSH is widely used in scripts and several other software to access data and resources remotely and securely. Now, let us incline our focus on our core topic, that is, the SSH default port.

Secure Shell (SSH) offers a wide range of capabilities that make it an indispensable tool for secure remote administration and data transfer. Some of its key capabilities include:

  1. Secure Remote Access: SSH enables secure remote access to SSH-enabled network systems or devices, allowing users to manage them remotely.
  2. Interactive File Transfer: Users can securely transfer files between local and remote systems in an interactive and encrypted manner.
  3. Automated and Secured File Transfers: SSH supports automated and secure file transfers, making it ideal for tasks like backup systems and configuration management.
  4. Secure Command Execution: SSH allows users to issue commands on remote devices or systems securely. This is particularly useful for automated processes and remote management.
  5. Network Infrastructure Management: Administrators rely on SSH for secure management of network infrastructure components, such as routers and server hardware.

SSH is a versatile protocol that caters to various use cases, from simple remote access to complex automation and management tasks.

SSH is generally secure, but there are potential risks to be aware of:

  1. Brute-Force Attacks: Attackers may attempt to guess passwords. Mitigate this by using strong passwords, key-based authentication, or fail2ban, which automatically bans IP addresses with repeated failed login attempts.
  2. Man-in-the-Middle (MitM) Attacks: Employ the SSH known_hosts file to verify the identity of the remote host and consider using SSH certificates.
  3. Unauthorized Access: Limit SSH access to trusted users and IP ranges through firewall rules or security groups.
  4. Two-Factor Authentication (2FA): Enable 2FA for SSH access. This requires users to provide a second authentication factor (e.g., a one-time code) in addition to their password or SSH key.
  5. Security Tokens: Implement security tokens or hardware-based authentication methods for added security. These physical devices generate secure codes for login.

When it comes to using SSH securely, following best practices is crucial to maintaining the integrity of your network and data. Here are some recommended practices to consider:

  1. Strong Password Policies: If you opt for password authentication, ensure your passwords are complex and regularly updated. Use a combination of upper and lower-case letters, numbers, and special characters. Implement password policies that enforce these requirements.
  2. Key Management: Utilize SSH keys for authentication whenever possible. SSH keys offer stronger security compared to passwords. Ensure that keys are adequately protected with passphrases and stored securely.
  3. Disable Root Login: By default, SSH often allows root login. Disable direct root login and use a regular user account to log in, then switch to the root user using the sudo command when necessary. This reduces the risk of unauthorized access.
  4. Firewall Rules: Employ firewall rules to restrict SSH access to trusted IP addresses or networks. This helps prevent unauthorized access attempts.
  5. Regular Updates: Keep your SSH server and client software up to date to address any security vulnerabilities. Enable automatic updates if available.
  6. Logging and Monitoring: Enable SSH logging to track login attempts and monitor SSH activity. Regularly review logs for suspicious activity that might indicate a security breach.

Common SSH issues can be frustrating, but they're often solvable. Here are some troubleshooting tips:

  1. Connection Refused: Ensure the SSH server is running and reachable. Check firewall rules and network connectivity.
  2. Permission Denied (Publickey): This error often indicates an issue with SSH key authentication. Verify that your public key is properly added to the remote server's authorized_keys file.
  3. Authentication Failures: If you encounter authentication failures, double-check usernames, passwords, or SSH keys. Also, review your server's authentication settings.
  4. Port 22 Blocked: If you're unable to connect, check if port 22 (the default SSH port) is blocked by a firewall. Consider changing the SSH port if needed. 

In conclusion, understanding "What is SSH Protocol" is crucial for anyone involved in managing secure communications over the Internet. SSH offers a robust layer of security that is vital in today’s digital age, where data breaches and cyber threats are increasingly common. By encrypting all transferred data and employing strict authentication measures, SSH ensures that remote server access and data transmissions are protected against unauthorized access and attacks. As technology advances and security becomes more imperative, SSH stands as a fundamental component in safeguarding internet communications.

Lisa P

Lisa P

Hello, everyone, my name is Lisa. I'm a passionate electrical engineering student with a keen interest in technology. I'm fascinated by the intersection of engineering principles and technological advancements, and I'm eager to contribute to the field by applying my knowledge and skills to solve real-world problems.