SSH

Secure Remote Access

SSH (Secure Shell) is a cryptographic protocol that provides secure command-line access, file transfer, and port forwarding over an untrusted network. It replaced plain-text protocols like Telnet and rsh that transmitted passwords and data in the clear. An SSH connection begins with a key exchange that establishes a shared session key using Diffie-Hellman or ECDH, then authenticates the user via password or (preferably) public-key authentication, and finally provides an encrypted TCPTransmission Control Protocol. A reliable, connection-oriented transport protocol that guarantees ordered, error-checked delivery of data through three-way handshakes, acknowledgments, and retransmission. The foundation of HTTP, SSH, and most internet services. channel for all subsequent communication.

Key-Based Authentication

SSH public-key authentication works by placing your public key in the ~/.ssh/authorized_keys file on the server. When you connect, the server challenges you with a random value encrypted with your public key; your private key decrypts it and proves ownership without transmitting the private key. This is both more secure than passwords (immune to brute-force and phishing) and more convenient with an SSH agent that manages unlocked keys in memory.

Tunneling and Port Forwarding

Beyond remote shell access, SSH supports three tunneling modes. Local forwarding maps a local port through the SSH connection to a remote host (useful for accessing services behind a FirewallA network security device or software that monitors and filters incoming and outgoing traffic based on predefined rules. Firewalls can block traffic by IP address, port number, protocol, or application-layer content.). Remote forwarding exposes a local service through the server to the remote network. Dynamic forwarding creates a SOCKS Proxy ServerAn intermediary server that forwards requests between a client and destination server. Forward proxies anonymize client traffic; reverse proxies sit in front of servers for load balancing, caching, and security. that routes arbitrary TCP traffic through the SSH connection, functioning like a lightweight VPNVirtual Private Network. A technology that creates an encrypted tunnel between a device and a remote server, protecting data in transit and masking the user's real IP address. Used for privacy, security, and accessing restricted networks. for TCP. SSH runs on TCPTransmission Control Protocol. A reliable, connection-oriented transport protocol that guarantees ordered, error-checked delivery of data through three-way handshakes, acknowledgments, and retransmission. The foundation of HTTP, SSH, and most internet services. port 22 by default. Use Open Port Checker to verify SSH port accessibility.