PuTTY

PuTTY is a free application for secure SSH, Telnet, rlogin, and serial port connections. It enables remote server administration, file transfer via SCP and SFTP, and terminal emulation (xterm, VT102, and others). It supports public/private key authentication, X11 forwarding, and SSH agents. PuTTY runs on Windows and Unix-like systems, offering saved sessions, proxy support, and local flow control. Detailed configuration covers cipher types, compression, and session logging.

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• xterm and VT102 terminal emulation. PuTTY emulates xterm, VT102, VT220, and other terminal types. This ensures text-based remote applications function correctly by interpreting escape sequences, colors, and cursor control. Accurate emulation is required for using editors like vi, managing ncurses menus, or executing interactive scripts on Linux, BSD, or Cisco systems.
• SSH protocol with AES, ChaCha20, and 3DES encryption. The PuTTY SSH implementation supports symmetric ciphers including AES (128, 192, 256 bits), ChaCha20, 3DES, and Blowfish. It also supports Diffie-Hellman key exchange (groups 1..21, ECDH, Curve25519) and MACs (HMAC-SHA1, SHA256, SHA512). Users can select algorithm priority order, strengthening security against man-in-the-middle or downgrade attacks.
• Public key authentication (RSA, DSA, ECDSA, Ed25519). PuTTY generates key pairs using PuTTYgen, supporting RSA (up to 4096 bits), DSA, ECDSA (nistp256/384/521), and Ed25519. The private key is stored in .ppk format, optionally protected with a passphrase. During connection, the server verifies the digital signature, eliminating the need to send passwords across the network.
• Port forwarding (local, remote, and dynamic SSH tunnels). PuTTY redirects TCP traffic through the SSH tunnel. Local mode forwards a local port to a remote destination; remote mode exposes a server port to the client; dynamic mode (SOCKS5) turns the connection into a SOCKS proxy. This function provides secure access to internal network services, databases, or encrypted web browsing.
• X11 forwarding for remote graphical applications. When connecting to a Unix server with X11 forwarding enabled, PuTTY automatically forwards graphical screens from remote applications to the local X server (such as Xming, VcXsrv, or WSLg). This allows running GUI programs (gedit, xclock, Firefox) from a remote machine and displaying their interface on the Windows desktop without additional configuration.
• Serial connection support (RS-232). PuTTY connects directly to serial ports (COM1, COM2, /dev/ttyS0, etc.) with configurable baud rate, data bits, parity, stop bits, and flow control (hardware/software/XON/XOFF). Engineers use this for debugging microcontrollers, configuring network switches, or accessing recovery consoles on embedded systems.
• Telnet and rlogin protocols for legacy systems. Although SSH is more secure, PuTTY retains support for Telnet (port 23) and rlogin (port 513). These protocols are necessary for interacting with older equipment, industrial devices, or mainframes that do not implement encryption. Users can enable Telnet options such as terminal negotiation, local line editing, and control characters.
• Integrated proxy and restrictive network traversal. PuTTY supports multiple proxy types: HTTP, SOCKS 4, SOCKS 5, Telnet, and local proxy (with executable command). It also supports basic and NTLM authentication for corporate proxies. This allows establishing SSH connections even when the network blocks outbound ports (22) or requires passing through intermediate servers.
• SSH compression (zlib) for slow network links. PuTTY compresses data before encryption using zlib (Delay compression) or zlib@openssh.com. Compression reduces traffic volume, accelerating text transfer, logs, or command output on low-bandwidth connections (modem, satellite, 3G). It can be activated in Connection > SSH > Compression.
• Session logging to file (.log, .txt). PuTTY saves all terminal output to a local text file with logging options: all text, only printable text, include timestamps, or automatic rotation when the file reaches a certain size. This function assists in auditing executed commands, debugging network failures, or documenting maintenance sessions.
• Saved sessions with all configuration settings. Users can store connection profiles (name, protocol, port, username, cipher options, proxies, forwards, terminal, colors, keys, and cursor behavior). Sessions are stored in the Windows registry or INI files in portable versions. This avoids re-entering technical parameters each time connecting to different servers.
• PSFTP command for interactive SFTP file transfer. In addition to the pscp utility (simple copy), PuTTY includes PSFTP, an SFTP client with FTP-like commands: ls, cd, get, put, mkdir, rm, rename, chmod. It supports batch sessions (command files) and resumption of partial transfers. This is useful for synchronizing entire directories without mounting remote file systems.
• Plink: command-line interface for automation. Plink (PuTTY Link) is a text-mode utility that executes remote commands or establishes SSH tunnels from scripts, PowerShell, or scheduled tasks. It supports key authentication, redirected standard input, and output without terminal decorations. Example: plink server ls -la. This integrates SSH operations into automated processes.
• Local flow control and keyboard adjustments (Backspace, Delete, Home, End). PuTTY configures special key behavior (Backspace sends ^H or ^?), numeric keypad sequences, function keys (F1-F12), and cursor keys modified by Ctrl/Alt/Shift. It also handles local flow control (XON/XOFF with Ctrl+S/Q) to stop text output on very fast terminals.
• Keepalive connection and network drop detection. PuTTY sends null packets at configurable intervals (seconds) to prevent firewalls or NAT from closing idle SSH sessions. If the network fails, PuTTY detects the disconnection and closes the session, allowing quick reconnection without leaving orphaned processes on the server.

The first version of PuTTY was created by Simon Tatham in 1998, initially as a Telnet client for Windows 95. SSH support (version 1) was added in 1999, followed by SSH-2. The program is written in the C language, using native Windows libraries (Win32 API) and GTK for Unix versions. The core team includes Simon Tatham, Owen Dunn, Jacob Nevins, and Ben Harris, with contributions from dozens of developers. PuTTY has no official "version 1.0" release date but has been considered mature since 2003. Development remains active with stable releases every few months.

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