It was not until Internet security became an issue that the Secure Shell (SSH) protocol was developed to secure the Internet through encryption. Until then, rlogin and telnet protocols were used by many system administrators to access Unix or Linux shell remotely. Because of its versatility and security Microsoft Windows allows access Unix/Linux systems using the SSH by utilizing the PuTTY utility tool. The PuTTY utility tool is also used to remotely access Unix/Linux systems using the Telnet protocol, though it offers minimum security compared to SSH.

Multi-user operating systems, including Microsoft Windows, Unix, and Linux, normally present command interface for users, which enable them to input their commands executed by computers. Interfaces enable users to remotely login to other computers without having to be physically present in front of other computers. The SSH and telnet protocols make this process possible. Users run a _client_ to connect to a _server_, which helps in transfer information from a client to the server.

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Obtaining PuTTY

Microsoft does not develop Windows with PuTTY utility installed, but makes it easier for users to install and use it by themselves. PuTTY executable program can be easily downloaded from the PuTTY download page. It is important to choose the PuTTY executable program that is compatible with the version of Windows that will run it.

PuTTY Utility Tool

The PuTTY utility tool is a versatile tool used by Microsoft Windows to remotely access Unix or Linux computer systems in a secure manner. It supports many protocols including:

  • Raw, which is usually used to debug computer networks;
  • Serial, which is used to establish a serial connection in a network. It is usually used to serially connect computers in lieu of an Ethernet;
  • Rlogin, which is a Unix unencrypted protocol used to remotely login to a network. It uses port 513;
  • Secure Shell (SSH) – is an encrypted protocol used to remotely login to a network. It uses port 22;
  • Telnet (telecommunication network) is unencrypted protocol used to remotely login to a network. It uses port 23.

SSH and Telnet protocols can only be used to access Unix/Linux systems if the client has an account with the server. The Internet service provider (ISP) provides a client with a shell account, and the client utilizes a Telnet-accessed bulletin board, MUD, or talker.

PuTTY Features

Copying and Pasting Text

A PuTTY session may contain text information, which needs to be typed. PuTTY allows such texts to be copied and pasted instead of typing. It utilizes a mouse to perform this. Click the left button of a mouse in the terminal window, drag to select text on the clipboard, release the left-clicked mouse button to automatically copy the text to clipboard. To paste whatever was copied to the clipboard into a session right-click the mouse button. Double click the left button of a mouse to select a whole word, double-click, hold down the second click, drag the mouse to select many words, triple-click, and hold down the last click to select a sequence of lines.

Scrolling the Screen

Sometimes, while reading texts on the screen, some may scroll off the top of the terminal. PuTTY provides a scroll bar on the right side of the window to scroll and view it. In addition, pressing Shift+Page Up and Shift+Page Down moves to page contents up and down respectively.

System Menu

System menu contains ‘minimize’, ‘size’, ‘close’, and ‘move’ options, which are used by left-clicking the PuTTY window’s top left corner or right-clicking the title bar. Other features found in PuTTY’s system menu include: ‘event log’, starting new sessions, changing session settings, copy all to clipboard, clear and reset the terminal, and full screen mode.

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To log various events during a session, select ‘event log’ from the system menu – a small pop-up window that allows significant events to be copied at the beginning, in the middle, and in the end of sessions. To start a new session, select ‘new session’ on the system menu. This will start a completely new session of PuTTY and reconfigure the configuration box normally. To start a new session with same options as current ones, including connecting to the same server using the same protocol, select ‘duplicate session’. To access previously stored sessions, select ‘saved sessions’.

PuTTY allows adjustment of many properties of present sessions. To do this, select ‘change settings’ from the system menu. A configuration box that allows changing various parameters such as font, color, and events that occur when various keys are pressed, is displayed. To copy all information on the terminal screen and get back to the clipboard at the same time use the ‘copy all to clipboard’ feature.

All texts that have been scrolled and kept on top of the screen can be cleared using PuTTY’s ‘clear scrollback’ feature on the system menu. This is usually useful when one views private information and does not want a casual user to look over his/her shoulder. To reset information in a terminal window, use the ‘reset terminal’ option on the system menu. Sometimes the title bar can be distracting to users. In this case, selecting the ‘full screen’ mode option will expand the PuTTY window to cover the whole screen and hide its title bar. In this case, system menu can be accessed by left-clicking the top left corner of the window.

Creating a Log File of Sessions

At times, users may wish to create a log of all information that appears on the screen. This can be done using the ‘logging’ section in the configuration box. To do this, select ‘change settings’ feature from system menu and move directly to the logging panel. This enables one to input a name for the log file and select a mode for logging. Click ‘apply’ to enable the log to start. To stop logging, get back to the logging panel, choose the ‘logging turned off completely’ option. PuTTY safely closes the log file and it can safely be read by users.

Character Set Configuration Altering

Sometimes users may find that PuTTY wrongly interprets characters sent by the server. To fix this problem click ‘change settings’ and in the ‘translation’ tab select the desired character set.

The PuTTY Command Line

The PuTTY command line enables users to perform various tasks by writing their commands. To launch a session without necessarily having to pass through the configuration window, one can perform any of the following: connect to a server named ‘host’ using the following syntax (PuTTY User Manual, n.d.):

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putty.exe [-telnet] [user@]host


putty.exe elnet://host[:port]/

Accessing Unix/Linux Systems Using the PuTTY Utility Tool

On starting the PuTTY utility tool, a dialog box that enables control of all PuTTY’s functionalities appears. Few parameters have to be configured for effective functioning of the utility. In the ‘host name’ box, write the host name of the server to connect to. Then select a protocol that will enable login to the server from the options labeled ‘protocol’. At this moment, select the ‘Telnet login protocol’. The remaining _Raw_ is not normally used for interactive sessions, but for remotely debugging Internet services of other networks.

When Telnet protocol is selected, the ‘port’ number changes to 23 – the standard port number for running Telnet services. This happens because servers always provide various network logins on different port numbers. Once the ‘host name’, ‘protocol’, and ‘port’ fields are set, the system becomes ready for connection. Press the button labeled ‘open’ at the bottom of the dialogue box and the PuTTY will connect to the server.

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Since the SSH protocol is not used, skip verifying the Host Key interface. The Host Key feature is a SSH protocol feature that protects a system against sending your login information to a different computer, known as _spoofing_.  Without the spoofing, an attacker can obtain login information of a client and use it to access the server as if he was the owner.

After establishing a connection, the system requires login information (user name and password) to access the Unix/Linux system. This information is normally given by system administrator. Enter username and password to access the system. In case of incorrect username and password, Telnet protocol provides several tries of entering usernames and passwords. Always check that Caps Lock is disabled as many Unix systems treat lowercase and uppercase characters as different when verifying passwords. 

Servers act differently after accessing them. Some may give login information or send message following a command prompt that allows users to enter commands to be executed while others may not. After finishing with the server, log out using logout command of the server. The PuTTY window closes after the server finishes logging out.  


Cross-accessing through various operating system programs has been made easy with the availability of many utility programs. The PuTTY, one of these utility programs, enables access to a Unix/Linux system from Microsoft Windows operating system using the Telnet protocol.

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