What is SSH and How Does It Work? An In-Depth Guide

Have you ever needed to securely access computers located elsewhere – say, your office workstation or a remote server hosting your company‘s website? SSH, or Secure Shell, is the go-to solution for securely managing Linux/UNIX systems and infrastructure over the internet.

This comprehensive guide will explain what SSH is, how it establishes encrypted connections between devices, key capabilities, best practices for usage, and additional resources to level up your SSH skills. Whether you‘re a network engineer, DevOps professional, or IT leader overseeing business-critical infrastructure, understanding SSH is essential.

Overview of SSH

SSH, or Secure Shell, is a cryptographic protocol that creates encrypted network connections between two devices – typically a client and a remote server. It can securely:

• Log into remote Linux/UNIX servers for administration or software development
• Transfer files between systems using associated SFTP protocol
• Forward networking ports for tunneling traffic through SSH connections
• Automate remote execution of commands and scripts

First released in 1995, SSH was designed as a secure replacement for Telnet and other insecure remote access protocols where traffic is transmitted plainly over networks. SSH uses strong symmetric encryption algorithms like AES-256 during sessions and public-key cryptography for authentication so that all data exchanged is encrypted and remote hosts can verify client identities.

As per DB-Engines usage statistics, SSH is the 7th most widely used computing protocol globally based on mentions across programming language drivers and connectors. This underscores SSH‘s ubiquity – over 4 billion SSH connections occur daily!

Why has SSH become irreplaceable for remote system management? Some key benefits:

Security – Encrypting traffic and tracing hosts with digital signatures protects against spoofing, snooping attacks.

Automation – SSH keys enable "hands-free", scriptable remote system control without manual input.

Scalability – Managing thousands of Linux VMs, cloud servers and network gear is made possible.

Now let‘s explore exactly how SSH connections work under the hood.

How SSH Authentication Works

SSH employs two main methods for authenticating users – password and public key authentication. First, the SSH client contacts a remote SSH server requesting to establish a connection.

With password authentication, users input the password matching their username/account on the remote system when prompted. However, sending passwords over any medium poses security issues. Brute force attacks that automatically guess passwords are also a concern.

More secure is public key authentication using cryptographically-linked SSH key pairs. The steps are:

1. Generate unique SSH key pair consisting of a public and private key using the ssh-keygen tool.
2. Configure the public key on remote Linux/UNIX servers under ~/.ssh/authorized_keys.
3. The private key remains on client devices like user workstations.

When attempting to connect to configured servers using SSH, the private key digitally signs data packets presented by the SSH server. This mathematically proves you possess the linked private key without transmitting it, allowing the server to authenticate you.

Public key auth avoids sending raw passwords that could be intercepted. Private keys also never leave client devices, unlike passwords which are handed over to remote hosts. However, properly securing local private keys is crucial – anyone gaining access can indefinitely masquerade as you!

Establishing an SSH Connection

With authentication methods covered, let‘s see what happens when connecting to a remote server via SSH using the OpenSSH client:

ssh user@host -p port

Common parameters:

• user – Remote system username
• host – IP or domain name of remote server
• port – TCP port SSH server is listening on (default 22)

1. Client initiates TCP handshake with remote server on destination port, usually 22.
2. Server responds, and SSH handshake begins negotiating encryption algorithms, key exchange mechanisms, and other session parameters.
3. Server and client verify identities – server requests client credentials via supported authentication types.
4. An encrypted tunnel is established between client and server using agreed parameters like ciphers and hashing functions.
5. User can now securely execute shell commands, transfer files using SFTP, forward ports, or conduct other remote computing tasks.

Common encryption protocols used in SSH include AES, Blowfish, 3DES for data encryption and SHA-1, SHA-2 for hashing transported data. SSH sessions utilize a combination of asymmetric public-key encryption to verify identities and generate session keys plus symmetric encryption achieving better performance for bulk data transport.

Now that secure connectivity is ensured between devices, what can you actually accomplish via an SSH connection?

Transferring Data Securely with SFTP

While SSH itself allows executing shell commands on remote systems, the related SFTP protocol facilitates securely transferring files:

sftp user@host

This opens an encrypted SFTP session. Common SFTP commands like get, put, mkdir, rmdir then upload/download files or manipulate remote directories just as if handled locally.

When SSH public key authentication is active, no passwords or passphrase prompts occur – enabling scripted SFTP transfers. For automated file sharing workflows, this avoids manual steps each run.

Here‘s an example SFTP command downloading a remote ZIP archive to local folder:

sftp user@host:/path/document.zip /local/folder

This leverages SSH‘s encrypted transport layer for file operations.

Port Forwarding via SSH Tunnels

SSH also can forward arbitrary networking ports bi-directionally through its secure channels allowing other TCP-based networking traffic to bypass potential vulnerabilities on the local network via a technique called port forwarding.

Local port forwarding configures services on the SSH client host to tunnel through the SSH connection to the remote server – for example, securely connecting a local web browser to an application running on a remote database server.

Remote port forwarding tunnels ports on the remote SSH server down the SSH tunnel back to services running the client local machine. Reverse connections like this can enable accessing local clients behind residential firewalls from VPS instances.

Here‘s an example SSH tunnel connecting a process on client host 192.168.1.10 to a remote database server db.example.com:

ssh -L 8080:db.example.com:80 user@server-hostname

This forwards local port 8080 to port 80 on db.example.com through server-hostname allowing local programs to communicate securely to the database system as if directly connected.

Port forwarding is invaluable for concealing traffic, accessing devices behind firewalls, securING web-facing services, and segmenting networks.

Conclusion

Understanding how SSH connections are established, authenticated, and encrypted along with associated capabilities like SFTP transfers and port forwarding is invaluable knowledge for anyone managing remote infrastructure.

As this guide has shown in detail, SSH unlocks immense power thanks to its ubiquitous deployment on Linux/UNIX systems and strong, tested security foundations enabling all remote traffic to be securely encrypted and programmatically automated.

Hopefully you now feel equipped to better utilize SSH connectivity for remote system administration, cloud infrastructure management, website deployment workflows, and related computing tasks.

To take your SSH skills to the next level, consider researching advanced topics like SSH certificates for mass management, Ansible for configuration automation, creating a VPN over SSH, or setting up Single Sign-On (SSO) using SSH certificates.

What SSH topics are still unclear or would you like to see covered further? What scenarios could you use SSH to address in your own infrastructure? I welcome feedback to continuously improve future articles!