What is SSH: secure connection between servers

SSH (Secure Shell) is an encrypted network protocol used to access and administer remote computers securely. The protocol creates an encrypted channel between client and server over port 22, protecting passwords and commands against interception and replacing the old Telnet as the standard for remote access.

• What is SSH?
• Why use SSH instead of Telnet?
• How does SSH work?
• How to start using SSH?
• What are the main features of SSH?
• Where is SSH used in practice?

What is SSH?

SSH (Secure Shell) is an encrypted network protocol that allows accessing, running commands and managing remote machines securely. It establishes an encrypted connection between two devices, ensuring confidentiality and integrity of the transmitted data even over insecure public networks.

The protocol was born in 1995: Finnish researcher Tatu Ylönen created SSH after a password-sniffing attack on his university's network and released the first version as freeware in July 1995. The official document RFC 4251 summarizes the purpose well: "SSH is a protocol for secure remote login and other secure network services over an insecure network."

In practice, SSH works as a protected bridge: you open a terminal, type a command and operate the server as if you were sitting in front of it — only with all the traffic encrypted. That is why it is the foundation of the daily work of anyone administering cloud infrastructure and Linux servers.

Why use SSH instead of Telnet?

SSH replaced Telnet because it solves its biggest problem: the absence of encryption. Telnet transmits login, password and commands in plain text, which allows any attacker on the network to capture this information. SSH encrypts everything end-to-end, making interception useless in practice.

Beyond encryption, SSH offers strong authentication — including public key — ensuring that only authorized users reach the server. This combination of secrecy and authentication is what makes SSH a pillar of information security in production environments.

How does SSH work?

SSH works in the client-server model: a server program (the sshd daemon) listens on port 22 and a client (ssh) initiates the connection. Before any password travels, both sides negotiate a session key and establish an encrypted tunnel, so all the rest of the communication already travels encrypted.

Authentication happens inside this tunnel and can be by password or by key pair. In key authentication, the private key stays on your computer and the public key is installed on the server; the connection is only accepted when both match, dispensing with typing a password. The standardization of SSH-2 by the IETF, published in January 2006 in RFCs 4251 through 4254, consolidated this design and made it the model followed by all modern implementations.

The dominant implementation is OpenSSH, maintained by the OpenBSD project. According to Wikipedia, OpenSSH is by far the most widespread SSH implementation in the world, being the default on Linux, macOS and Windows since 2018 — which explains why the commands in this guide work on virtually any machine.

How to start using SSH?

Getting started with SSH requires only the installed client and the server address. Follow the steps below:

1. Check the installation: most modern systems already ship with SSH. In the terminal, run ssh -V to check the version; if missing, install the openssh-client package through your system's package manager.
2. Connect to the server: use the ssh command followed by the user and the IP address (or domain) of the remote machine. If the server uses a port other than 22, add the -p option.
3. Authenticate: enter the remote user's password when prompted, or configure a key pair to log in without a password. Then press Enter.
4. Explore the server: once connected, run commands, transfer files with scp and perform the necessary administrative tasks as if you were on the local machine.

Example basic connection:

ssh user@ip_address

If you are still sharpening the fundamentals, it is worth reviewing the essential terminal commands for Windows, Mac and Linux, since every SSH session happens on the command line.

What are the main features of SSH?

SSH brings together features that go far beyond remote login. The main ones are:

• Strong encryption: uses robust algorithms to encrypt all communication between client and server.
• Public key authentication: in addition to password, it accepts key pairs, raising security and enabling passwordless login.
• Secure file transfer: the scp (Secure Copy) command and sftp move files between machines inside the encrypted tunnel.
• Port forwarding (tunneling): redirects network connections through SSH, protecting other services.
• Remote command execution: runs scripts and tasks on the server without opening a full interactive session.

Where is SSH used in practice?

SSH appears in nearly every serious infrastructure operation, from deploying applications to versioning code. When you git push to GitHub over SSH, for example, it is this protocol that authenticates and encrypts the transfer. Below, some real-world uses that show its versatility:

• Cloud server administration: administrators connect to remote instances to install software, adjust settings and monitor performance.
• Task automation: companies use SSH for automatic backups, updates and running scheduled scripts on remote servers.
• IoT device access: enthusiasts access a Raspberry Pi remotely to control Internet of Things (IoT) projects from anywhere.
• Creative experiments: someone has already run the classic game Doom and even streamed movies directly in the terminal via SSH, proving the protocol's flexibility.

Here at CodeCrush these scenarios sum up the point well: SSH is the secure gateway to virtually any remote machine, whether a production server or a Raspberry Pi in your home.

Conclusion

Mastering SSH is no longer optional for anyone working with development and infrastructure — it is the difference between operating servers securely and exposing credentials in plain text. If you are going to learn a single remote access tool, start with SSH with key authentication: in a few minutes you eliminate most of the risks and gain a faster workflow, without typing a password on every connection.