SSH Overview: Concepts, Basic Commands, Port Forwarding, and Remote Operations

What is SSH? SSH (Secure Shell) is a network protocol that provides encrypted remote login between computers, originally designed in 1995 by Tatu Ylonen to replace plaintext communication and now a standard component of Linux systems.

Login principle – The client and server negotiate cryptographic keys, verify host authenticity via a public‑key fingerprint, and establish an encrypted channel for all subsequent traffic.

Basic usage – The typical command syntax is ssh -p 22 user@host . The -p option specifies a non‑default port; if the port is 22 it can be omitted, as can the username when it matches the local user.

Remote login example – Using two CentOS 6.5 VMs (192.168.13.135 and 192.168.13.138), the article shows how to check SSH service with netstat -ntlp | grep ssh and connect with ssh -p 22 [email protected] , handling the first‑time host key confirmation and password entry.

Port forwarding – SSH can forward TCP traffic, useful when firewalls block direct access. The article covers:

Local forwarding (client‑side): ssh -L [local_addr:]local_port:target_ip:target_port user@target_ip . Example: ssh -L 3306:127.0.0.1:3306 [email protected] to access a MySQL instance bound to localhost.

Remote forwarding (server‑side): ssh -R [remote_addr:]remote_port:target_ip:target_port user@remote_ip . Example: forwarding port 80 from a Windows host through an intermediate CentOS machine.

Dynamic forwarding (SOCKS proxy): ssh -D [local_addr:]local_port user@remote_ip , allowing all outgoing traffic to be tunneled through the SSH server.

Remote command execution – One‑liner syntax ssh user@host 'command' enables running commands on a remote host, e.g., ssh [email protected] 'uname -a' or transferring files with tar -cz test | ssh [email protected] 'tar -xz' .

Common issues – The article warns about man‑in‑the‑middle attacks when an attacker substitutes the host key, emphasizing the importance of verifying fingerprints because SSH lacks a built‑in certificate authority.

Conclusion – The guide provides a practical foundation for using SSH in everyday operations, covering login, command execution, and various forwarding techniques, while noting that deeper protocol analysis remains a topic for further study.