1st Security Agent 4.6 serial key or number

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This chapter describes all the configuration options in PuTTY.

PuTTY is configured using the control panel that comes up before you start a session. Some options can also be changed in the middle of a session, by selecting ‘Change Settings’ from the window menu.

4.1 The Session panel

The Session configuration panel contains the basic options you need to specify in order to open a session at all, and also allows you to save your settings to be reloaded later.

4.1.1 The host name section

The top box on the Session panel, labelled ‘Specify your connection by host name’, contains the details that need to be filled in before PuTTY can open a session at all.

• The ‘Host Name’ box is where you type the name, or the IP address, of the server you want to connect to.
• The ‘Connection type’ radio buttons let you choose what type of connection you want to make: a raw connection, a Telnet connection, an Rlogin connection, an SSH connection, or a connection to a local serial line. (See section 1.2 for a summary of the differences between SSH, Telnet and rlogin; see section 3.6 for an explanation of ‘raw’ connections; see section 3.7 for information about using a serial line.)
• The ‘Port’ box lets you specify which port number on the server to connect to. If you select Telnet, Rlogin, or SSH, this box will be filled in automatically to the usual value, and you will only need to change it if you have an unusual server. If you select Raw mode, you will almost certainly need to fill in the ‘Port’ box yourself.

If you select ‘Serial’ from the ‘Connection type’ radio buttons, the ‘Host Name’ and ‘Port’ boxes are replaced by ‘Serial line’ and ‘Speed’; see section 4.28 for more details of these.

4.1.2 Loading and storing saved sessions

The next part of the Session configuration panel allows you to save your preferred PuTTY options so they will appear automatically the next time you start PuTTY. It also allows you to create saved sessions, which contain a full set of configuration options plus a host name and protocol. A saved session contains all the information PuTTY needs to start exactly the session you want.

• To save your default settings: first set up the settings the way you want them saved. Then come back to the Session panel. Select the ‘Default Settings’ entry in the saved sessions list, with a single click. Then press the ‘Save’ button.

If there is a specific host you want to store the details of how to connect to, you should create a saved session, which will be separate from the Default Settings.

• To save a session: first go through the rest of the configuration box setting up all the options you want. Then come back to the Session panel. Enter a name for the saved session in the ‘Saved Sessions’ input box. (The server name is often a good choice for a saved session name.) Then press the ‘Save’ button. Your saved session name should now appear in the list box.

  You can also save settings in mid-session, from the ‘Change Settings’ dialog. Settings changed since the start of the session will be saved with their current values; as well as settings changed through the dialog, this includes changes in window size, window title changes sent by the server, and so on.

• To reload a saved session: single-click to select the session name in the list box, and then press the ‘Load’ button. Your saved settings should all appear in the configuration panel.
• To modify a saved session: first load it as described above. Then make the changes you want. Come back to the Session panel, and press the ‘Save’ button. The new settings will be saved over the top of the old ones.

  To save the new settings under a different name, you can enter the new name in the ‘Saved Sessions’ box, or single-click to select a session name in the list box to overwrite that session. To save ‘Default Settings’, you must single-click the name before saving.

• To start a saved session immediately: double-click on the session name in the list box.
• To delete a saved session: single-click to select the session name in the list box, and then press the ‘Delete’ button.

Each saved session is independent of the Default Settings configuration. If you change your preferences and update Default Settings, you must also update every saved session separately.

Saved sessions are stored in the Registry, at the location

If you need to store them in a file, you could try the method described in section 4.29.

4.1.3 ‘Close Window on Exit’

Finally in the Session panel, there is an option labelled ‘Close Window on Exit’. This controls whether the PuTTY terminal window disappears as soon as the session inside it terminates. If you are likely to want to copy and paste text out of the session after it has terminated, or restart the session, you should arrange for this option to be off.

‘Close Window On Exit’ has three settings. ‘Always’ means always close the window on exit; ‘Never’ means never close on exit (always leave the window open, but inactive). The third setting, and the default one, is ‘Only on clean exit’. In this mode, a session which terminates normally will cause its window to close, but one which is aborted unexpectedly by network trouble or a confusing message from the server will leave the window up.

4.2 The Logging panel

The Logging configuration panel allows you to save log files of your PuTTY sessions, for debugging, analysis or future reference.

The main option is a radio-button set that specifies whether PuTTY will log anything at all. The options are:

• ‘None’. This is the default option; in this mode PuTTY will not create a log file at all.
• ‘Printable output’. In this mode, a log file will be created and written to, but only printable text will be saved into it. The various terminal control codes that are typically sent down an interactive session alongside the printable text will be omitted. This might be a useful mode if you want to read a log file in a text editor and hope to be able to make sense of it.
• ‘All session output’. In this mode, everything sent by the server into your terminal session is logged. If you view the log file in a text editor, therefore, you may well find it full of strange control characters. This is a particularly useful mode if you are experiencing problems with PuTTY's terminal handling: you can record everything that went to the terminal, so that someone else can replay the session later in slow motion and watch to see what went wrong.
• ‘SSH packets’. In this mode (which is only used by SSH connections), the SSH message packets sent over the encrypted connection are written to the log file (as well as Event Log entries). You might need this to debug a network-level problem, or more likely to send to the PuTTY authors as part of a bug report. BE WARNED that if you log in using a password, the password can appear in the log file; see section 4.2.4 for options that may help to remove sensitive material from the log file before you send it to anyone else.
• ‘SSH packets and raw data’. In this mode, as well as the decrypted packets (as in the previous mode), the raw (encrypted, compressed, etc) packets are also logged. This could be useful to diagnose corruption in transit. (The same caveats as the previous mode apply, of course.)

Note that the non-SSH logging options (‘Printable output’ and ‘All session output’) only work with PuTTY proper; in programs without terminal emulation (such as Plink), they will have no effect, even if enabled via saved settings.

4.2.1 ‘Log file name’

In this edit box you enter the name of the file you want to log the session to. The ‘Browse’ button will let you look around your file system to find the right place to put the file; or if you already know exactly where you want it to go, you can just type a pathname into the edit box.

There are a few special features in this box. If you use the character in the file name box, PuTTY will insert details of the current session in the name of the file it actually opens. The precise replacements it will do are:

• will be replaced by the current year, as four digits.
• will be replaced by the current month, as two digits.
• will be replaced by the current day of the month, as two digits.
• will be replaced by the current time, as six digits (HHMMSS) with no punctuation.
• will be replaced by the host name you are connecting to.
• will be replaced by the port number you are connecting to on the target host.

For example, if you enter the host name , you will end up with files looking like

4.2.2 ‘What to do if the log file already exists’

This control allows you to specify what PuTTY should do if it tries to start writing to a log file and it finds the file already exists. You might want to automatically destroy the existing log file and start a new one with the same name. Alternatively, you might want to open the existing log file and add data to the end of it. Finally (the default option), you might not want to have any automatic behaviour, but to ask the user every time the problem comes up.

4.2.3 ‘Flush log file frequently’

This option allows you to control how frequently logged data is flushed to disc. By default, PuTTY will flush data as soon as it is displayed, so that if you view the log file while a session is still open, it will be up to date; and if the client system crashes, there's a greater chance that the data will be preserved.

However, this can incur a performance penalty. If PuTTY is running slowly with logging enabled, you could try unchecking this option. Be warned that the log file may not always be up to date as a result (although it will of course be flushed when it is closed, for instance at the end of a session).

4.2.4 Options specific to SSH packet logging

These options only apply if SSH packet data is being logged.

The following options allow particularly sensitive portions of unencrypted packets to be automatically left out of the log file. They are only intended to deter casual nosiness; an attacker could glean a lot of useful information from even these obfuscated logs (e.g., length of password).

4.2.4.1 ‘Omit known password fields’

When checked, decrypted password fields are removed from the log of transmitted packets. (This includes any user responses to challenge-response authentication methods such as ‘keyboard-interactive’.) This does not include X11 authentication data if using X11 forwarding.

Note that this will only omit data that PuTTY knows to be a password. However, if you start another login session within your PuTTY session, for instance, any password used will appear in the clear in the packet log. The next option may be of use to protect against this.

This option is enabled by default.

4.2.4.2 ‘Omit session data’

When checked, all decrypted ‘session data’ is omitted; this is defined as data in terminal sessions and in forwarded channels (TCP, X11, and authentication agent). This will usually substantially reduce the size of the resulting log file.

This option is disabled by default.

4.3 The Terminal panel

The Terminal configuration panel allows you to control the behaviour of PuTTY's terminal emulation.

4.3.1 ‘Auto wrap mode initially on’

Auto wrap mode controls what happens when text printed in a PuTTY window reaches the right-hand edge of the window.

With auto wrap mode on, if a long line of text reaches the right-hand edge, it will wrap over on to the next line so you can still see all the text. With auto wrap mode off, the cursor will stay at the right-hand edge of the screen, and all the characters in the line will be printed on top of each other.

If you are running a full-screen application and you occasionally find the screen scrolling up when it looks as if it shouldn't, you could try turning this option off.

Auto wrap mode can be turned on and off by control sequences sent by the server. This configuration option controls the default state, which will be restored when you reset the terminal (see section 3.1.3.6). However, if you modify this option in mid-session using ‘Change Settings’, it will take effect immediately.

4.3.2 ‘DEC Origin Mode initially on’

DEC Origin Mode is a minor option which controls how PuTTY interprets cursor-position control sequences sent by the server.

The server can send a control sequence that restricts the scrolling region of the display. For example, in an editor, the server might reserve a line at the top of the screen and a line at the bottom, and might send a control sequence that causes scrolling operations to affect only the remaining lines.

With DEC Origin Mode on, cursor coordinates are counted from the top of the scrolling region. With it turned off, cursor coordinates are counted from the top of the whole screen regardless of the scrolling region.

It is unlikely you would need to change this option, but if you find a full-screen application is displaying pieces of text in what looks like the wrong part of the screen, you could try turning DEC Origin Mode on to see whether that helps.

DEC Origin Mode can be turned on and off by control sequences sent by the server. This configuration option controls the default state, which will be restored when you reset the terminal (see section 3.1.3.6). However, if you modify this option in mid-session using ‘Change Settings’, it will take effect immediately.

4.3.3 ‘Implicit CR in every LF’

Most servers send two control characters, CR and LF, to start a new line of the screen. The CR character makes the cursor return to the left-hand side of the screen. The LF character makes the cursor move one line down (and might make the screen scroll).

Some servers only send LF, and expect the terminal to move the cursor over to the left automatically. If you come across a server that does this, you will see a stepped effect on the screen, like this:

If this happens to you, try enabling the ‘Implicit CR in every LF’ option, and things might go back to normal:

4.3.4 ‘Implicit LF in every CR’

Most servers send two control characters, CR and LF, to start a new line of the screen. The CR character makes the cursor return to the left-hand side of the screen. The LF character makes the cursor move one line down (and might make the screen scroll).

Some servers only send CR, and so the newly written line is overwritten by the following line. This option causes a line feed so that all lines are displayed.

4.3.5 ‘Use background colour to erase screen’

Not all terminals agree on what colour to turn the screen when the server sends a ‘clear screen’ sequence. Some terminals believe the screen should always be cleared to the default background colour. Others believe the screen should be cleared to whatever the server has selected as a background colour.

There exist applications that expect both kinds of behaviour. Therefore, PuTTY can be configured to do either.

With this option disabled, screen clearing is always done in the default background colour. With this option enabled, it is done in the current background colour.

Background-colour erase can be turned on and off by control sequences sent by the server. This configuration option controls the default state, which will be restored when you reset the terminal (see section 3.1.3.6). However, if you modify this option in mid-session using ‘Change Settings’, it will take effect immediately.

4.3.6 ‘Enable blinking text’

The server can ask PuTTY to display text that blinks on and off. This is very distracting, so PuTTY allows you to turn blinking text off completely.

When blinking text is disabled and the server attempts to make some text blink, PuTTY will instead display the text with a bolded background colour.

Blinking text can be turned on and off by control sequences sent by the server. This configuration option controls the default state, which will be restored when you reset the terminal (see section 3.1.3.6). However, if you modify this option in mid-session using ‘Change Settings’, it will take effect immediately.

4.3.7 ‘Answerback to ^E’

This option controls what PuTTY will send back to the server if the server sends it the ^E enquiry character. Normally it just sends the string ‘PuTTY’.

If you accidentally write the contents of a binary file to your terminal, you will probably find that it contains more than one ^E character, and as a result your next command line will probably read ‘PuTTYPuTTYPuTTY...’ as if you had typed the answerback string multiple times at the keyboard. If you set the answerback string to be empty, this problem should go away, but doing so might cause other problems.

Note that this is not the feature of PuTTY which the server will typically use to determine your terminal type. That feature is the ‘Terminal-type string’ in the Connection panel; see section 4.14.3 for details.

You can include control characters in the answerback string using notation. (Use to get a literal .)

4.3.8 ‘Local echo’

With local echo disabled, characters you type into the PuTTY window are not echoed in the window by PuTTY. They are simply sent to the server. (The server might choose to echo them back to you; this can't be controlled from the PuTTY control panel.)

Some types of session need local echo, and many do not. In its default mode, PuTTY will automatically attempt to deduce whether or not local echo is appropriate for the session you are working in. If you find it has made the wrong decision, you can use this configuration option to override its choice: you can force local echo to be turned on, or force it to be turned off, instead of relying on the automatic detection.

4.3.9 ‘Local line editing’

Normally, every character you type into the PuTTY window is sent immediately to the server the moment you type it.

If you enable local line editing, this changes. PuTTY will let you edit a whole line at a time locally, and the line will only be sent to the server when you press Return. If you make a mistake, you can use the Backspace key to correct it before you press Return, and the server will never see the mistake.

Since it is hard to edit a line locally without being able to see it, local line editing is mostly used in conjunction with local echo (section 4.3.8). This makes it ideal for use in raw mode or when connecting to MUDs or talkers. (Although some more advanced MUDs do occasionally turn local line editing on and turn local echo off, in order to accept a password from the user.)

Some types of session need local line editing, and many do not. In its default mode, PuTTY will automatically attempt to deduce whether or not local line editing is appropriate for the session you are working in. If you find it has made the wrong decision, you can use this configuration option to override its choice: you can force local line editing to be turned on, or force it to be turned off, instead of relying on the automatic detection.

4.3.10 Remote-controlled printing

A lot of VT100-compatible terminals support printing under control of the remote server. PuTTY supports this feature as well, but it is turned off by default.

To enable remote-controlled printing, choose a printer from the ‘Printer to send ANSI printer output to’ drop-down list box. This should allow you to select from all the printers you have installed drivers for on your computer. Alternatively, you can type the network name of a networked printer (for example, ) even if you haven't already installed a driver for it on your own machine.

When the remote server attempts to print some data, PuTTY will send that data to the printer raw - without translating it, attempting to format it, or doing anything else to it. It is up to you to ensure your remote server knows what type of printer it is talking to.

Since PuTTY sends data to the printer raw, it cannot offer options such as portrait versus landscape, print quality, or paper tray selection. All these things would be done by your PC printer driver (which PuTTY bypasses); if you need them done, you will have to find a way to configure your remote server to do them.

To disable remote printing again, choose ‘None (printing disabled)’ from the printer selection list. This is the default state.

4.4 The Keyboard panel

The Keyboard configuration panel allows you to control the behaviour of the keyboard in PuTTY. The correct state for many of these settings depends on what the server to which PuTTY is connecting expects. With a Unix server, this is likely to depend on the or entry it uses, which in turn is likely to be controlled by the ‘Terminal-type string’ setting in the Connection panel; see section 4.14.3 for details. If none of the settings here seems to help, you may find question A.7.15 to be useful.

4.4.1 Changing the action of the Backspace key

Some terminals believe that the Backspace key should send the same thing to the server as Control-H (ASCII code 8). Other terminals believe that the Backspace key should send ASCII code 127 (usually known as Control-?) so that it can be distinguished from Control-H. This option allows you to choose which code PuTTY generates when you press Backspace.

If you are connecting over SSH, PuTTY by default tells the server the value of this option (see section 4.24.2), so you may find that the Backspace key does the right thing either way. Similarly, if you are connecting to a Unix system, you will probably find that the Unix command lets you configure which the server expects to see, so again you might not need to change which one PuTTY generates. On other systems, the server's expectation might be fixed and you might have no choice but to configure PuTTY.

If you do have the choice, we recommend configuring PuTTY to generate Control-? and configuring the server to expect it, because that allows applications such as to use Control-H for help.

(Typing Shift-Backspace will cause PuTTY to send whichever code isn't configured here as the default.)

4.4.2 Changing the action of the Home and End keys

The Unix terminal emulator disagrees with the rest of the world about what character sequences should be sent to the server by the Home and End keys.

, and other terminals, send for the Home key, and for the End key. sends for the Home key and for the End key.

If you find an application on which the Home and End keys aren't working, you could try switching this option to see if it helps.

4.4.3 Changing the action of the function keys and keypad

This option affects the function keys (F1 to F12) and the top row of the numeric keypad.

• In the default mode, labelled , the function keys generate sequences like , and so on. This matches the general behaviour of Digital's terminals.
• In Linux mode, F6 to F12 behave just like the default mode, but F1 to F5 generate through to . This mimics the Linux virtual console.
• In Xterm R6 mode, F5 to F12 behave like the default mode, but F1 to F4 generate through to , which are the sequences produced by the top row of the keypad on Digital's terminals.
• In VT400 mode, all the function keys behave like the default mode, but the actual top row of the numeric keypad generates through to .
• In VT100+ mode, the function keys generate through to
• In SCO mode, the function keys F1 to F12 generate through to . Together with shift, they generate through to . With control they generate through to , and with shift and control together they generate through to .

If you don't know what any of this means, you probably don't need to fiddle with it.

4.4.4 Controlling Application Cursor Keys mode

Application Cursor Keys mode is a way for the server to change the control sequences sent by the arrow keys. In normal mode, the arrow keys send through to . In application mode, they send through to .

Application Cursor Keys mode can be turned on and off by the server, depending on the application. PuTTY allows you to configure the initial state.

You can also disable application cursor keys mode completely, using the ‘Features’ configuration panel; see section 4.6.1.

4.4.5 Controlling Application Keypad mode

Application Keypad mode is a way for the server to change the behaviour of the numeric keypad.

In normal mode, the keypad behaves like a normal Windows keypad: with NumLock on, the number keys generate numbers, and with NumLock off they act like the arrow keys and Home, End etc.

In application mode, all the keypad keys send special control sequences, including Num Lock. Num Lock stops behaving like Num Lock and becomes another function key.

Depending on which version of Windows you run, you may find the Num Lock light still flashes on and off every time you press Num Lock, even when application mode is active and Num Lock is acting like a function key. This is unavoidable.

Application keypad mode can be turned on and off by the server, depending on the application. PuTTY allows you to configure the initial state.

You can also disable application keypad mode completely, using the ‘Features’ configuration panel; see section 4.6.1.

4.4.6 Using NetHack keypad mode

PuTTY has a special mode for playing NetHack. You can enable it by selecting ‘NetHack’ in the ‘Initial state of numeric keypad’ control.

In this mode, the numeric keypad keys 1-9 generate the NetHack movement commands (). The 5 key generates the command (do nothing).

In addition, pressing Shift or Ctrl with the keypad keys generate the Shift- or Ctrl-keys you would expect (e.g. keypad-7 generates ‘’, so Shift-keypad-7 generates ‘’ and Ctrl-keypad-7 generates Ctrl-Y); these commands tell NetHack to keep moving you in the same direction until you encounter something interesting.

For some reason, this feature only works properly when Num Lock is on. We don't know why.

4.4.7 Enabling a DEC-like Compose key

DEC terminals have a Compose key, which provides an easy-to-remember way of typing accented characters. You press Compose and then type two more characters. The two characters are ‘combined’ to produce an accented character. The choices of character are designed to be easy to remember; for example, composing ‘e’ and ‘`’ produces the ‘è’ character.

If your keyboard has a Windows Application key, it acts as a Compose key in PuTTY. Alternatively, if you enable the ‘AltGr acts as Compose key’ option, the AltGr key will become a Compose key.

4.4.8 ‘Control-Alt is different from AltGr’

Some old keyboards do not have an AltGr key, which can make it difficult to type some characters. PuTTY can be configured to treat the key combination Ctrl + Left Alt the same way as the AltGr key.

By default, this checkbox is checked, and the key combination Ctrl + Left Alt does something completely different. PuTTY's usual handling of the left Alt key is to prefix the Escape (Control-) character to whatever character sequence the rest of the keypress would generate. For example, Alt-A generates Escape followed by . So Alt-Ctrl-A would generate Escape, followed by Control-A.

If you uncheck this box, Ctrl-Alt will become a synonym for AltGr, so you can use it to type extra graphic characters if your keyboard has any.

(However, Ctrl-Alt will never act as a Compose key, regardless of the setting of ‘AltGr acts as Compose key’ described in section 4.4.7.)

4.5 The Bell panel

The Bell panel controls the terminal bell feature: the server's ability to cause PuTTY to beep at you.

In the default configuration, when the server sends the character with ASCII code 7 (Control-G), PuTTY will play the Windows Default Beep sound. This is not always what you want the terminal bell feature to do; the Bell panel allows you to configure alternative actions.

4.5.1 ‘Set the style of bell’

This control allows you to select various different actions to occur on a terminal bell:

• Selecting ‘None’ disables the bell completely. In this mode, the server can send as many Control-G characters as it likes and nothing at all will happen.
• ‘Make default system alert sound’ is the default setting. It causes the Windows ‘Default Beep’ sound to be played. To change what this sound is, or to test it if nothing seems to be happening, use the Sound configurer in the Windows Control Panel.
• ‘Visual bell’ is a silent alternative to a beeping computer. In this mode, when the server sends a Control-G, the whole PuTTY window will flash white for a fraction of a second.
• ‘Beep using the PC speaker’ is self-explanatory.
• ‘Play a custom sound file’ allows you to specify a particular sound file to be used by PuTTY alone, or even by a particular individual PuTTY session. This allows you to distinguish your PuTTY beeps from any other beeps on the system. If you select this option, you will also need to enter the name of your sound file in the edit control ‘Custom sound file to play as a bell’.

4.5.2 ‘Taskbar/caption indication on bell’

This feature controls what happens to the PuTTY window's entry in the Windows Taskbar if a bell occurs while the window does not have the input focus.

In the default state (‘Disabled’) nothing unusual happens.

If you select ‘Steady’, then when a bell occurs and the window is not in focus, the window's Taskbar entry and its title bar will change colour to let you know that PuTTY session is asking for your attention. The change of colour will persist until you select the window, so you can leave several PuTTY windows minimised in your terminal, go away from your keyboard, and be sure not to have missed any important beeps when you get back.

‘Flashing’ is even more eye-catching: the Taskbar entry will continuously flash on and off until you select the window.

4.5.3 ‘Control the bell overload behaviour’

A common user error in a terminal session is to accidentally run the Unix command (or equivalent) on an inappropriate file type, such as an executable, image file, or ZIP file. This produces a huge stream of non-text characters sent to the terminal, which typically includes a lot of bell characters. As a result of this the terminal often doesn't stop beeping for ten minutes, and everybody else in the office gets annoyed.

To try to avoid this behaviour, or any other cause of excessive beeping, PuTTY includes a bell overload management feature. In the default configuration, receiving more than five bell characters in a two-second period will cause the overload feature to activate. Once the overload feature is active, further bells will have no effect at all, so the rest of your binary file will be sent to the screen in silence. After a period of five seconds during which no further bells are received, the overload feature will turn itself off again and bells will be re-enabled.

If you want this feature completely disabled, you can turn it off using the checkbox ‘Bell is temporarily disabled when over-used’.

Alternatively, if you like the bell overload feature but don't agree with the settings, you can configure the details: how many bells constitute an overload, how short a time period they have to arrive in to do so, and how much silent time is required before the overload feature will deactivate itself.

Bell overload mode is always deactivated by any keypress in the terminal. This means it can respond to large unexpected streams of data, but does not interfere with ordinary command-line activities that generate beeps (such as filename completion).

4.6 The Features panel

PuTTY's terminal emulation is very highly featured, and can do a lot of things under remote server control. Some of these features can cause problems due to buggy or strangely configured server applications.

The Features configuration panel allows you to disable some of PuTTY's more advanced terminal features, in case they cause trouble.

4.6.1 Disabling application keypad and cursor keys

Application keypad mode (see section 4.4.5) and application cursor keys mode (see section 4.4.4) alter the behaviour of the keypad and cursor keys. Some applications enable these modes but then do not deal correctly with the modified keys. You can force these modes to be permanently disabled no matter what the server tries to do.

4.6.2 Disabling -style mouse reporting

PuTTY allows the server to send control codes that let it take over the mouse and use it for purposes other than copy and paste. Applications which use this feature include the text-mode web browser , the Usenet newsreader version 4, and the file manager (Midnight Commander).

If you find this feature inconvenient, you can disable it using the ‘Disable xterm-style mouse reporting’ control. With this box ticked, the mouse will always do copy and paste in the normal way.

Note that even if the application takes over the mouse, you can still manage PuTTY's copy and paste by holding down the Shift key while you select and paste, unless you have deliberately turned this feature off (see section 4.11.3).

4.6.3 Disabling remote terminal resizing

PuTTY has the ability to change the terminal's size and position in response to commands from the server. If you find PuTTY is doing this unexpectedly or inconveniently, you can tell PuTTY not to respond to those server commands.

4.6.4 Disabling switching to the alternate screen

Many terminals, including PuTTY, support an ‘alternate screen’. This is the same size as the ordinary terminal screen, but separate. Typically a screen-based program such as a text editor might switch the terminal to the alternate screen before starting up. Then at the end of the run, it switches back to the primary screen, and you see the screen contents just as they were before starting the editor.

Some people prefer this not to happen. If you want your editor to run in the same screen as the rest of your terminal activity, you can disable the alternate screen feature completely.

4.6.5 Disabling remote window title changing

PuTTY has the ability to change the window title in response to commands from the server. If you find PuTTY is doing this unexpectedly or inconveniently, you can tell PuTTY not to respond to those server commands.

4.6.6 Response to remote window title querying

PuTTY can optionally provide the xterm service of allowing server applications to find out the local window title. This feature is disabled by default, but you can turn it on if you really want it.

NOTE that this feature is a potential security hazard. If a malicious application can write data to your terminal (for example, if you merely a file owned by someone else on the server machine), it can change your window title (unless you have disabled this as mentioned in section 4.6.5) and then use this service to have the new window title sent back to the server as if typed at the keyboard. This allows an attacker to fake keypresses and potentially cause your server-side applications to do things you didn't want. Therefore this feature is disabled by default, and we recommend you do not set it to ‘Window title’ unless you really know what you are doing.

There are three settings for this option:

‘None’

PuTTY makes no response whatsoever to the relevant escape sequence. This may upset server-side software that is expecting some sort of response.

‘Empty string’

PuTTY makes a well-formed response, but leaves it blank. Thus, server-side software that expects a response is kept happy, but an attacker cannot influence the response string. This is probably the setting you want if you have no better ideas.

‘Window title’

PuTTY responds with the actual window title. This is dangerous for the reasons described above.

4.6.7 Disabling remote scrollback clearing

PuTTY has the ability to clear the terminal's scrollback buffer in response to a command from the server. If you find PuTTY is doing this unexpectedly or inconveniently, you can tell PuTTY not to respond to that server command.

4.6.8 Disabling destructive backspace

Normally, when PuTTY receives character 127 (^?) from the server, it will perform a ‘destructive backspace’: move the cursor one space left and delete the character under it. This can apparently cause problems in some applications, so PuTTY provides the ability to configure character 127 to perform a normal backspace (without deleting a character) instead.

4.6.9 Disabling remote character set configuration

PuTTY has the ability to change its character set configuration in response to commands from the server. Some programs send these commands unexpectedly or inconveniently. In particular, BitchX (an IRC client) seems to have a habit of reconfiguring the character set to something other than the user intended.

If you find that accented characters are not showing up the way you expect them to, particularly if you're running BitchX, you could try disabling the remote character set configuration commands.

4.6.10 Disabling Arabic text shaping

PuTTY supports shaping of Arabic text, which means that if your server sends text written in the basic Unicode Arabic alphabet then it will convert it to the correct display forms before printing it on the screen.

If you are using full-screen software which was not expecting this to happen (especially if you are not an Arabic speaker and you unexpectedly find yourself dealing with Arabic text files in applications which are not Arabic-aware), you might find that the display becomes corrupted. By ticking this box, you can disable Arabic text shaping so that PuTTY displays precisely the characters it is told to display.

You may also find you need to disable bidirectional text display; see section 4.6.11.

4.6.11 Disabling bidirectional text display

PuTTY supports bidirectional text display, which means that if your server sends text written in a language which is usually displayed from right to left (such as Arabic or Hebrew) then PuTTY will automatically flip it round so that it is displayed in the right direction on the screen.

If you are using full-screen software which was not expecting this to happen (especially if you are not an Arabic speaker and you unexpectedly find yourself dealing with Arabic text files in applications which are not Arabic-aware), you might find that the display becomes corrupted. By ticking this box, you can disable bidirectional text display, so that PuTTY displays text from left to right in all situations.

You may also find you need to disable Arabic text shaping; see section 4.6.10.

4.7 The Window panel

The Window configuration panel allows you to control aspects of the PuTTY window.

4.7.1 Setting the size of the PuTTY window

The ‘Columns’ and ‘Rows’ boxes let you set the PuTTY window to a precise size. Of course you can also drag the window to a new size while a session is running.

4.7.2 What to do when the window is resized

These options allow you to control what happens when the user tries to resize the PuTTY window using its window furniture.

There are four options here:

• ‘Change the number of rows and columns’: the font size will not change. (This is the default.)
• ‘Change the size of the font’: the number of rows and columns in the terminal will stay the same, and the font size will change.
• ‘Change font size when maximised’: when the window is resized, the number of rows and columns will change, except when the window is maximised (or restored), when the font size will change. (In this mode, holding down the Alt key while resizing will also cause the font size to change.)
• ‘Forbid resizing completely’: the terminal will refuse to be resized at all.

4.7.3 Controlling scrollback

These options let you configure the way PuTTY keeps text after it scrolls off the top of the screen (see section 3.1.2).

The ‘Lines of scrollback’ box lets you configure how many lines of text PuTTY keeps. The ‘Display scrollbar’ options allow you to hide the scrollbar (although you can still view the scrollback using the keyboard as described in section 3.1.2). You can separately configure whether the scrollbar is shown in full-screen mode and in normal modes.

If you are viewing part of the scrollback when the server sends more text to PuTTY, the screen will revert to showing the current terminal contents. You can disable this behaviour by turning off ‘Reset scrollback on display activity’. You can also make the screen revert when you press a key, by turning on ‘Reset scrollback on keypress’.

4.7.4 ‘Push erased text into scrollback’

When this option is enabled, the contents of the terminal screen will be pushed into the scrollback when a server-side application clears the screen, so that your scrollback will contain a better record of what was on your screen in the past.

If the application switches to the alternate screen (see section 4.6.4 for more about this), then the contents of the primary screen will be visible in the scrollback until the application switches back again.

This option is enabled by default.

4.8 The Appearance panel

The Appearance configuration panel allows you to control aspects of the appearance of PuTTY's window.

4.8.1 Controlling the appearance of the cursor

The ‘Cursor appearance’ option lets you configure the cursor to be a block, an underline, or a vertical line. A block cursor becomes an empty box when the window loses focus; an underline or a vertical line becomes dotted.

The ‘Cursor blinks’ option makes the cursor blink on and off. This works in any of the cursor modes.

4.8.2 Controlling the font used in the terminal window

This option allows you to choose what font, in what size, the PuTTY terminal window uses to display the text in the session.

By default, you will be offered a choice from all the fixed-width fonts installed on the system, since VT100-style terminal handling expects a fixed-width font. If you tick the box marked ‘Allow selection of variable-pitch fonts’, however, PuTTY will offer variable-width fonts as well: if you select one of these, the font will be coerced into fixed-size character cells, which will probably not look very good (but can work OK with some fonts).

4.8.3 ‘Hide mouse pointer when typing in window’

If you enable this option, the mouse pointer will disappear if the PuTTY window is selected and you press a key. This way, it will not obscure any of the text in the window while you work in your session. As soon as you move the mouse, the pointer will reappear.

This option is disabled by default, so the mouse pointer remains visible at all times.

4.8.4 Controlling the window border

PuTTY allows you to configure the appearance of the window border to some extent.

The checkbox marked ‘Sunken-edge border’ changes the appearance of the window border to something more like a DOS box: the inside edge of the border is highlighted as if it sank down to meet the surface inside the window. This makes the border a little bit thicker as well. It's hard to describe well. Try it and see if you like it.

You can also configure a completely blank gap between the text in the window and the border, using the ‘Gap between text and window edge’ control. By default this is set at one pixel. You can reduce it to zero, or increase it further.

4.9 The Behaviour panel

The Behaviour configuration panel allows you to control aspects of the behaviour of PuTTY's window.

4.9.1 Controlling the window title

The ‘Window title’ edit box allows you to set the title of the PuTTY window. By default the window title will contain the host name followed by ‘PuTTY’, for example . If you want a different window title, this is where to set it.

PuTTY allows the server to send control sequences which modify the title of the window in mid-session (unless this is disabled - see section 4.6.5); the title string set here is therefore only the initial window title.

As well as the window title, there is also an sequence to modify the title of the window's icon. This makes sense in a windowing system where the window becomes an icon when minimised, such as Windows 3.1 or most X Window System setups; but in the Windows 95-like user interface it isn't as applicable.

By default, PuTTY only uses the server-supplied window title, and ignores the icon title entirely. If for some reason you want to see both titles, check the box marked ‘Separate window and icon titles’. If you do this, PuTTY's window title and Taskbar caption will change into the server-supplied icon title if you minimise the PuTTY window, and change back to the server-supplied window title if you restore it. (If the server has not bothered to supply a window or icon title, none of this will happen.)

4.9.2 ‘Warn before closing window’

If you press the Close button in a PuTTY window that contains a running session, PuTTY will put up a warning window asking if you really meant to close the window. A window whose session has already terminated can always be closed without a warning.

If you want to be able to close a window quickly, you can disable the ‘Warn before closing window’ option.

4.9.3 ‘Window closes on ALT-F4’

By default, pressing ALT-F4 causes the window to close (or a warning box to appear; see section 4.9.2). If you disable the ‘Window closes on ALT-F4’ option, then pressing ALT-F4 will simply send a key sequence to the server.

4.9.4 ‘System menu appears on ALT-Space’

If this option is enabled, then pressing ALT-Space will bring up the PuTTY window's menu, like clicking on the top left corner. If it is disabled, then pressing ALT-Space will just send to the server.

Some accessibility programs for Windows may need this option enabling to be able to control PuTTY's window successfully. For instance, Dragon NaturallySpeaking requires it both to open the system menu via voice, and to close, minimise, maximise and restore the window.

4.9.5 ‘System menu appears on Alt alone’

If this option is enabled, then pressing and releasing ALT will bring up the PuTTY window's menu, like clicking on the top left corner. If it is disabled, then pressing and releasing ALT will have no effect.

4.9.6 ‘Ensure window is always on top’

If this option is enabled, the PuTTY window will stay on top of all other windows.

4.9.7 ‘Full screen on Alt-Enter’

If this option is enabled, then pressing Alt-Enter will cause the PuTTY window to become full-screen. Pressing Alt-Enter again will restore the previous window size.

The full-screen feature is also available from the System menu, even when it is configured not to be available on the Alt-Enter key. See section 3.1.3.7.

4.10 The Translation panel

The Translation configuration panel allows you to control the translation between the character set understood by the server and the character set understood by PuTTY.

4.10.1 Controlling character set translation

During an interactive session, PuTTY receives a stream of 8-bit bytes from the server, and in order to display them on the screen it needs to know what character set to interpret them in. Similarly, PuTTY needs to know how to translate your keystrokes into the encoding the server expects. Unfortunately, there is no satisfactory mechanism for PuTTY and the server to communicate this information, so it must usually be manually configured.

There are a lot of character sets to choose from. The ‘Remote character set’ option lets you select one.

By default PuTTY will use the UTF-8 encoding of Unicode, which can represent pretty much any character; data coming from the server is interpreted as UTF-8, and keystrokes are sent UTF-8 encoded. This is what most modern distributions of Linux will expect by default. However, if this is wrong for your server, you can select a different character set using this control.

A few other notable character sets are:

• The ISO-8859 series are all standard character sets that include various accented characters appropriate for different sets of languages.
• The Win125x series are defined by Microsoft, for similar purposes. In particular Win1252 is almost equivalent to ISO-8859-1, but contains a few extra characters such as matched quotes and the Euro symbol.
• If you want the old IBM PC character set with block graphics and line-drawing characters, you can select ‘CP437’.

If you need support for a numeric code page which is not listed in the drop-down list, such as code page 866, then you can try entering its name manually ( for example) in the list box. If the underlying version of Windows has the appropriate translation table installed, PuTTY will use it.

4.10.2 ‘Treat CJK ambiguous characters as wide’

There are some Unicode characters whose width is not well-defined. In most contexts, such characters should be treated as single-width for the purposes of wrapping and so on; however, in some CJK contexts, they are better treated as double-width for historical reasons, and some server-side applications may expect them to be displayed as such. Setting this option will cause PuTTY to take the double-width interpretation.

If you use legacy CJK applications, and you find your lines are wrapping in the wrong places, or you are having other display problems, you might want to play with this setting.

This option only has any effect in UTF-8 mode (see section 4.10.1).

4.10.3 ‘Caps Lock acts as Cyrillic switch’

This feature allows you to switch between a US/UK keyboard layout and a Cyrillic keyboard layout by using the Caps Lock key, if you need to type (for example) Russian and English side by side in the same document.

Currently this feature is not expected to work properly if your native keyboard layout is not US or UK.

4.10.4 Controlling display of line-drawing characters

VT100-series terminals allow the server to send control sequences that shift temporarily into a separate character set for drawing simple lines and boxes. However, there are a variety of ways in which PuTTY can attempt to find appropriate characters, and the right one to use depends on the locally configured font. In general you should probably try lots of options until you find one that your particular font supports.

• ‘Use Unicode line drawing code points’ tries to use the box characters that are present in Unicode. For good Unicode-supporting fonts this is probably the most reliable and functional option.
• ‘Poor man's line drawing’ assumes that the font cannot generate the line and box characters at all, so it will use the , and characters to draw approximations to boxes. You should use this option if none of the other options works.
• ‘Font has XWindows encoding’ is for use with fonts that have a special encoding, where the lowest 32 character positions (below the ASCII printable range) contain the line-drawing characters. This is unlikely to be the case with any standard Windows font; it will probably only apply to custom-built fonts or fonts that have been automatically converted from the X Window System.
• ‘Use font in both ANSI and OEM modes’ tries to use the same font in two different character sets, to obtain a wider range of characters. This doesn't always work; some fonts claim to be a different size depending on which character set you try to use.
• ‘Use font in OEM mode only’ is more reliable than that, but can miss out other characters from the main character set.

4.10.5 Controlling copy and paste of line drawing characters

By default, when you copy and paste a piece of the PuTTY screen that contains VT100 line and box drawing characters, PuTTY will paste them in the form they appear on the screen: either Unicode line drawing code points, or the ‘poor man's’ line-drawing characters , and . The checkbox ‘Copy and paste VT100 line drawing chars as lqqqk’ disables this feature, so line-drawing characters will be pasted as the ASCII characters that were printed to produce them. This will typically mean they come out mostly as and , with a scattering of at the corners. This might be useful if you were trying to recreate the same box layout in another program, for example.

Note that this option only applies to line-drawing characters which were printed by using the VT100 mechanism. Line-drawing characters that were received as Unicode code points will paste as Unicode always.

4.11 The Selection panel

The Selection panel allows you to control the way copy and paste work in the PuTTY window.

4.11.1 Pasting in Rich Text Format

If you enable ‘Paste to clipboard in RTF as well as plain text’, PuTTY will write formatting information to the clipboard as well as the actual text you copy. The effect of this is that if you paste into (say) a word processor, the text will appear in the word processor in the same font, colour, and style (e.g. bold, underline) PuTTY was using to display it.

This option can easily be inconvenient, so by default it is disabled.

4.11.2 Changing the actions of the mouse buttons

PuTTY's copy and paste mechanism is by default modelled on the Unix application. The X Window System uses a three-button mouse, and the convention is that the left button selects, the right button extends an existing selection, and the middle button pastes.

Windows often only has two mouse buttons, so in PuTTY's default configuration (‘Compromise’), the right button pastes, and the middle button (if you have one) extends a selection.

If you have a three-button mouse and you are already used to the arrangement, you can select it using the ‘Action of mouse buttons’ control.

Alternatively, with the ‘Windows’ option selected, the middle button extends, and the right button brings up a context menu (on which one of the options is ‘Paste’). (This context menu is always available by holding down Ctrl and right-clicking, regardless of the setting of this option.)

4.11.3 ‘Shift overrides application's use of mouse’

PuTTY allows the server to send control codes that let it take over the mouse and use it for purposes other than copy and paste. Applications which use this feature include the text-mode web browser , the Usenet newsreader version 4, and the file manager (Midnight Commander).

When running one of these applications, pressing the mouse buttons no longer performs copy and paste. If you do need to copy and paste, you can still do so if you hold down Shift while you do your mouse clicks.

However, it is possible in theory for applications to even detect and make use of Shift + mouse clicks. We don't know of any applications that do this, but in case someone ever writes one, unchecking the ‘Shift overrides application's use of mouse’ checkbox will cause Shift + mouse clicks to go to the server as well (so that mouse-driven copy and paste will be completely disabled).

If you want to prevent the application from taking over the mouse at all, you can do this using the Features control panel; see section 4.6.2.

4.11.4 Default selection mode

As described in section 3.1.1, PuTTY has two modes of selecting text to be copied to the clipboard. In the default mode (‘Normal’), dragging the mouse from point A to point B selects to the end of the line containing A, all the lines in between, and from the very beginning of the line containing B. In the other mode (‘Rectangular block’), dragging the mouse between two points defines a rectangle, and everything within that rectangle is copied.

Normally, you have to hold down Alt while dragging the mouse to select a rectangular block. Using the ‘Default selection mode’ control, you can set rectangular selection as the default, and then you have to hold down Alt to get the normal behaviour.

4.11.5 Configuring word-by-word selection

PuTTY will select a word at a time in the terminal window if you double-click to begin the drag. This panel allows you to control precisely what is considered to be a word.

Each character is given a class, which is a small number (typically 0, 1 or 2). PuTTY considers a single word to be any number of adjacent characters in the same class. So by modifying the assignment of characters to classes, you can modify the word-by-word selection behaviour.

In the default configuration, the character classes are:

• Class 0 contains white space and control characters.
• Class 1 contains most punctuation.
• Class 2 contains letters, numbers and a few pieces of punctuation (the double quote, minus sign, period, forward slash and underscore).

So, for example, if you assign the symbol into character class 2, you will be able to select an e-mail address with just a double click.

In order to adjust these assignments, you start by selecting a group of characters in the list box. Then enter a class number in the edit box below, and press the ‘Set’ button.

This mechanism currently only covers ASCII characters, because it isn't feasible to expand the list to cover the whole of Unicode.

Character class definitions can be modified by control sequences sent by the server. This configuration option controls the default state, which will be restored when you reset the terminal (see section 3.1.3.6). However, if you modify this option in mid-session using ‘Change Settings’, it will take effect immediately.

4.12 The Colours panel

The Colours panel allows you to control PuTTY's use of colour.

4.12.1 ‘Allow terminal to specify ANSI colours’

This option is enabled by default. If it is disabled, PuTTY will ignore any control sequences sent by the server to request coloured text.

If you have a particularly garish application, you might want to turn this option off and make PuTTY only use the default foreground and background colours.

4.12.2 ‘Allow terminal to use xterm 256-colour mode’

This option is enabled by default. If it is disabled, PuTTY will ignore any control sequences sent by the server which use the extended 256-colour mode supported by recent versions of .

If you have an application which is supposed to use 256-colour mode and it isn't working, you may find you need to tell your server that your terminal supports 256 colours. On Unix, you do this by ensuring that the setting of describes a 256-colour-capable terminal. You can check this using a command such as :

If you do not see ‘’ in the output, you may need to change your terminal setting. On modern Linux machines, you could try ‘’.

4.12.3 ‘Indicate bolded text by changing...’

When the server sends a control sequence indicating that some text should be displayed in bold, PuTTY can handle this in several ways. It can either change the font for a bold version, or use the same font in a brighter colour, or it can do both (brighten the colour and embolden the font). This control lets you choose which.

By default bold is indicated by colour, so non-bold text is displayed in light grey and bold text is displayed in bright white (and similarly in other colours). If you change the setting to ‘The font’ box, bold and non-bold text will be displayed in the same colour, and instead the font will change to indicate the difference. If you select ‘Both’, the font and the colour will both change.

Some applications rely on ‘bold black’ being distinguishable from a black background; if you choose ‘The font’, their text may become invisible.

4.12.4 ‘Attempt to use logical palettes’

Logical palettes are a mechanism by which a Windows application running on an 8-bit colour display can select precisely the colours it wants instead of going with the Windows standard defaults.

If you are not getting the colours you ask for on an 8-bit display, you can try enabling this option. However, be warned that it's never worked very well.

4.12.5 ‘Use system colours’

Enabling this option will cause PuTTY to ignore the configured colours for ‘Default Background/Foreground’ and ‘Cursor Colour/Text’ (see section 4.12.6), instead going with the system-wide defaults.

Note that non-bold and bold text will be the same colour if this option is enabled. You might want to change to indicating bold text by font changes (see section 4.12.3).

4.12.6 Adjusting the colours in the terminal window

The main colour control allows you to specify exactly what colours things should be displayed in. To modify one of the PuTTY colours, use the list box to select which colour you want to modify. The RGB values for that colour will appear on the right-hand side of the list box. Now, if you press the ‘Modify’ button, you will be presented with a colour selector, in which you can choose a new colour to go in place of the old one. (You may also edit the RGB values directly in the edit boxes, if you wish; each value is an integer from 0 to 255.)

PuTTY allows you to set the cursor colour, the default foreground and background, and the precise shades of all the ANSI configurable colours (black, red, green, yellow, blue, magenta, cyan, and white). You can also modify the precise shades used for the bold versions of these colours; these are used to display bold text if you have chosen to indicate that by colour (see section 4.12.3), and can also be used if the server asks specifically to use them. (Note that ‘Default Bold Background’ is not the background colour used for bold text; it is only used if the server specifically asks for a bold background.)

4.13 The Connection panel

The Connection panel allows you to configure options that apply to more than one type of connection.

4.13.1 Using keepalives to prevent disconnection

— OpenSSH authentication key utility

generates, manages and converts authentication keys for ssh(1). can create keys for use by SSH protocol version 2.

The type of key to be generated is specified with the option. If invoked without any arguments, will generate an RSA key.

is also used to generate groups for use in Diffie-Hellman group exchange (DH-GEX). See the MODULI GENERATION section for details.

Finally, can be used to generate and update Key Revocation Lists, and to test whether given keys have been revoked by one. See the KEY REVOCATION LISTS section for details.

Normally each user wishing to use SSH with public key authentication runs this once to create the authentication key in ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk or ~/.ssh/id_rsa. Additionally, the system administrator may use this to generate host keys, as seen in /etc/rc.

Normally this program generates the key and asks for a file in which to store the private key. The public key is stored in a file with the same name but “.pub” appended. The program also asks for a passphrase. The passphrase may be empty to indicate no passphrase (host keys must have an empty passphrase), or it may be a string of arbitrary length. A passphrase is similar to a password, except it can be a phrase with a series of words, punctuation, numbers, whitespace, or any string of characters you want. Good passphrases are 10-30 characters long, are not simple sentences or otherwise easily guessable (English prose has only 1-2 bits of entropy per character, and provides very bad passphrases), and contain a mix of upper and lowercase letters, numbers, and non-alphanumeric characters. The passphrase can be changed later by using the option.

There is no way to recover a lost passphrase. If the passphrase is lost or forgotten, a new key must be generated and the corresponding public key copied to other machines.

will by default write keys in an OpenSSH-specific format. This format is preferred as it offers better protection for keys at rest as well as allowing storage of key comments within the private key file itself. The key comment may be useful to help identify the key. The comment is initialized to “user@host” when the key is created, but can be changed using the option.

It is still possible for to write the previously-used PEM format private keys using the flag. This may be used when generating new keys, and existing new-format keys may be converted using this option in conjunction with the (change passphrase) flag.

After a key is generated, will ask where the keys should be placed to be activated.

The options are as follows:

For each of the key types (rsa, dsa, ecdsa and ed25519) for which host keys do not exist, generate the host keys with the default key file path, an empty passphrase, default bits for the key type, and default comment. If has also been specified, its argument is used as a prefix to the default path for the resulting host key files. This is used by /etc/rc to generate new host keys.

When saving a private key, this option specifies the number of KDF (key derivation function) rounds used. Higher numbers result in slower passphrase verification and increased resistance to brute-force password cracking (should the keys be stolen). The default is 16 rounds.

Show the bubblebabble digest of specified private or public key file.

Specifies the number of bits in the key to create. For RSA keys, the minimum size is 1024 bits and the default is 3072 bits. Generally, 3072 bits is considered sufficient. DSA keys must be exactly 1024 bits as specified by FIPS 186-2. For ECDSA keys, the flag determines the key length by selecting from one of three elliptic curve sizes: 256, 384 or 521 bits. Attempting to use bit lengths other than these three values for ECDSA keys will fail. ECDSA-SK, Ed25519 and Ed25519-SK keys have a fixed length and the flag will be ignored.

Provides a new comment.

Requests changing the comment in the private and public key files. The program will prompt for the file containing the private keys, for the passphrase if the key has one, and for the new comment.

Download the public keys provided by the PKCS#11 shared library . When used in combination with , this option indicates that a CA key resides in a PKCS#11 token (see the CERTIFICATES section for details).

Specifies the hash algorithm used when displaying key fingerprints. Valid options are: “md5” and “sha256”. The default is “sha256”.

This option will read a private or public OpenSSH key file and print to stdout a public key in one of the formats specified by the option. The default export format is “RFC4716”. This option allows exporting OpenSSH keys for use by other programs, including several commercial SSH implementations.

|

Search for the specified (with optional port number) in a known_hosts file, listing any occurrences found. This option is useful to find hashed host names or addresses and may also be used in conjunction with the option to print found keys in a hashed format.

Specifies the filename of the key file.

Use generic DNS format when printing fingerprint resource records using the command.

Hash a known_hosts file. This replaces all hostnames and addresses with hashed representations within the specified file; the original content is moved to a file with a .old suffix. These hashes may be used normally by and , but they do not reveal identifying information should the file's contents be disclosed. This option will not modify existing hashed hostnames and is therefore safe to use on files that mix hashed and non-hashed names.

When signing a key, create a host certificate instead of a user certificate. Please see the CERTIFICATES section for details.

Specify the key identity when signing a public key. Please see the CERTIFICATES section for details.

This option will read an unencrypted private (or public) key file in the format specified by the option and print an OpenSSH compatible private (or public) key to stdout. This option allows importing keys from other software, including several commercial SSH implementations. The default import format is “RFC4716”.

Download resident keys from a FIDO authenticator. Public and private key files will be written to the current directory for each downloaded key. If multiple FIDO authenticators are attached, keys will be downloaded from the first touched authenticator.

Generate a KRL file. In this mode, will generate a KRL file at the location specified via the flag that revokes every key or certificate presented on the command line. Keys/certificates to be revoked may be specified by public key file or using the format described in the KEY REVOCATION LISTS section.

Prints the contents of one or more certificates.

Show fingerprint of specified public key file. For RSA and DSA keys tries to find the matching public key file and prints its fingerprint. If combined with , a visual ASCII art representation of the key is supplied with the fingerprint.

Generate candidate Diffie-Hellman Group Exchange (DH-GEX) parameters for eventual use by the ‘diffie-hellman-group-exchange-*’ key exchange methods. The numbers generated by this operation must be further screened before use. See the MODULI GENERATION section for more information.

Screen candidate parameters for Diffie-Hellman Group Exchange. This will accept a list of candidate numbers and test that they are safe (Sophie Germain) primes with acceptable group generators. The results of this operation may be added to the /etc/moduli file. See the MODULI GENERATION section for more information.

Specify a key format for key generation, the (import), (export) conversion options, and the change passphrase operation. The latter may be used to convert between OpenSSH private key and PEM private key formats. The supported key formats are: “RFC4716” (RFC 4716/SSH2 public or private key), “PKCS8” (PKCS8 public or private key) or “PEM” (PEM public key). By default OpenSSH will write newly-generated private keys in its own format, but when converting public keys for export the default format is “RFC4716”. Setting a format of “PEM” when generating or updating a supported private key type will cause the key to be stored in the legacy PEM private key format.

Provides the new passphrase.

Specify one or more principals (user or host names) to be included in a certificate when signing a key. Multiple principals may be specified, separated by commas. Please see the CERTIFICATES section for details.

Specify a key/value option. These are specific to the operation that has been requested to perform.

When signing certificates, one of the options listed in the CERTIFICATES section may be specified here.

When performing moduli generation or screening, one of the options listed in the MODULI GENERATION section may be specified.

When generating a key that will be hosted on a FIDO authenticator, this flag may be used to specify key-specific options. Those supported at present are:

Override the default FIDO application/origin string of “ssh:”. This may be useful when generating host or domain-specific resident keys. The specified application string must begin with “ssh:”.

=

Specifies a path to a challenge string that will be passed to the FIDO token during key generation. The challenge string may be used as part of an out-of-band protocol for key enrollment (a random challenge is used by default).

Explicitly specify a fido(4) device to use, rather than letting the token middleware select one.

Indicate that the generated private key should not require touch events (user presence) when making signatures. Note that sshd(8) will refuse such signatures by default, unless overridden via an authorized_keys option.

Indicate that the key should be stored on the FIDO authenticator itself. Resident keys may be supported on FIDO2 tokens and typically require that a PIN be set on the token prior to generation. Resident keys may be loaded off the token using ssh-add(1).

A username to be associated with a resident key, overriding the empty default username. Specifying a username may be useful when generating multiple resident keys for the same application name.

Indicate that this private key should require user verification for each signature. Not all FIDO tokens support this option. Currently PIN authentication is the only supported verification method, but other methods may be supported in the future.

=

May be used at key generation time to record the attestation data returned from FIDO tokens during key generation. Please note that this information is potentially sensitive. By default, this information is discarded.

The option may be specified multiple times.

Provides the (old) passphrase.

Requests changing the passphrase of a private key file instead of creating a new private key. The program will prompt for the file containing the private key, for the old passphrase, and twice for the new passphrase.

Test whether keys have been revoked in a KRL. If the option is also specified then the contents of the KRL will be printed.

Silence .

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Removes all keys belonging to the specified (with optional port number) from a known_hosts file. This option is useful to delete hashed hosts (see the option above).

Print the SSHFP fingerprint resource record named for the specified public key file.

Certify (sign) a public key using the specified CA key. Please see the CERTIFICATES section for details.

When generating a KRL, specifies a path to a CA public key file used to revoke certificates directly by key ID or serial number. See the KEY REVOCATION LISTS section for details.

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Specifies the type of key to create. The possible values are “dsa”, “ecdsa”, “ecdsa-sk”, “ed25519”, “ed25519-sk”, or “rsa”.

This flag may also be used to specify the desired signature type when signing certificates using an RSA CA key. The available RSA signature variants are “ssh-rsa” (SHA1 signatures, not recommended), “rsa-sha2-256”, and “rsa-sha2-512” (the default).

When used in combination with , this option indicates that a CA key resides in a ssh-agent(1). See the CERTIFICATES section for more information.

Update a KRL. When specified with , keys listed via the command line are added to the existing KRL rather than a new KRL being created.

Specify a validity interval when signing a certificate. A validity interval may consist of a single time, indicating that the certificate is valid beginning now and expiring at that time, or may consist of two times separated by a colon to indicate an explicit time interval.

The start time may be specified as the string “always” to indicate the certificate has no specified start time, a date in YYYYMMDD format, a time in YYYYMMDDHHMM[SS] format, a relative time (to the current time) consisting of a minus sign followed by an interval in the format described in the TIME FORMATS section of sshd_config(5).

The end time may be specified as a YYYYMMDD date, a YYYYMMDDHHMM[SS] time, a relative time starting with a plus character or the string “forever” to indicate that the certificate has no expiry date.

For example: “+52w1d” (valid from now to 52 weeks and one day from now), “-4w:+4w” (valid from four weeks ago to four weeks from now), “20100101123000:20110101123000” (valid from 12:30 PM, January 1st, 2010 to 12:30 PM, January 1st, 2011), “-1d:20110101” (valid from yesterday to midnight, January 1st, 2011). “-1m:forever” (valid from one minute ago and never expiring).

Verbose mode. Causes to print debugging messages about its progress. This is helpful for debugging moduli generation. Multiple options increase the verbosity. The maximum is 3.

Specifies a path to a library that will be used when creating FIDO authenticator-hosted keys, overriding the default of using the internal USB HID support.

Find the principal(s) associated with the public key of a signature, provided using the flag in an authorized signers file provided using the flag. The format of the allowed signers file is documented in the ALLOWED SIGNERS section below. If one or more matching principals are found, they are returned on standard output.

Checks that a signature generated using has a valid structure. This does not validate if a signature comes from an authorized signer. When testing a signature, accepts a message on standard input and a signature namespace using . A file containing the corresponding signature must also be supplied using the flag. Successful testing of the signature is signalled by returning a zero exit status.

Cryptographically sign a file or some data using a SSH key. When signing, accepts zero or more files to sign on the command-line - if no files are specified then will sign data presented on standard input. Signatures are written to the path of the input file with “.sig” appended, or to standard output if the message to be signed was read from standard input.

The key used for signing is specified using the option and may refer to either a private key, or a public key with the private half available via ssh-agent(1). An additional signature namespace, used to prevent signature confusion across different domains of use (e.g. file signing vs email signing) must be provided via the flag. Namespaces are arbitrary strings, and may include: “file” for file signing, “email” for email signing. For custom uses, it is recommended to use names following a NAMESPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.

Request to verify a signature generated using as described above. When verifying a signature, accepts a message on standard input and a signature namespace using . A file containing the corresponding signature must also be supplied using the flag, along with the identity of the signer using and a list of allowed signers via the flag. The format of the allowed signers file is documented in the ALLOWED SIGNERS section below. A file containing revoked keys can be passed using the flag. The revocation file may be a KRL or a one-per-line list of public keys. Successful verification by an authorized signer is signalled by returning a zero exit status.

This option will read a private OpenSSH format file and print an OpenSSH public key to stdout.

Specifies a serial number to be embedded in the certificate to distinguish this certificate from others from the same CA. If the is prefixed with a ‘+’ character, then the serial number will be incremented for each certificate signed on a single command-line. The default serial number is zero.

When generating a KRL, the flag is used to specify a KRL version number.

may be used to generate groups for the Diffie-Hellman Group Exchange (DH-GEX) protocol. Generating these groups is a two-step process: first, candidate primes are generated using a fast, but memory intensive process. These candidate primes are then tested for suitability (a CPU-intensive process).

Generation of primes is performed using the option. The desired length of the primes may be specified by the option. For example:

By default, the search for primes begins at a random point in the desired length range. This may be overridden using the option, which specifies a different start point (in hex).

Once a set of candidates have been generated, they must be screened for suitability. This may be performed using the option. In this mode will read candidates from standard input (or a file specified using the option). For example:

By default, each candidate will be subjected to 100 primality tests. This may be overridden using the option. The DH generator value will be chosen automatically for the prime under consideration. If a specific generator is desired, it may be requested using the option. Valid generator values are 2, 3, and 5.

Screened DH groups may be installed in /etc/moduli. It is important that this file contains moduli of a range of bit lengths and that both ends of a connection share common moduli.

A number of options are available for moduli generation and screening via the flag:

=

Exit after screening the specified number of lines while performing DH candidate screening.

=

Start screening at the specified line number while performing DH candidate screening.

=

Write the last line processed to the specified file while performing DH candidate screening. This will be used to skip lines in the input file that have already been processed if the job is restarted.

=

Specify the amount of memory to use (in megabytes) when generating candidate moduli for DH-GEX.

=

Specify start point (in hex) when generating candidate moduli for DH-GEX.

=

Specify desired generator (in decimal) when testing candidate moduli for DH-GEX.

supports signing of keys to produce certificates that may be used for user or host authentication. Certificates consist of a public key, some identity information, zero or more principal (user or host) names and a set of options that are signed by a Certification Authority (CA) key. Clients or servers may then trust only the CA key and verify its signature on a certificate rather than trusting many user/host keys. Note that OpenSSH certificates are a different, and much simpler, format to the X.509 certificates used in ssl(8).

supports two types of certificates: user and host. User certificates authenticate users to servers, whereas host certificates authenticate server hosts to users. To generate a user certificate:

The resultant certificate will be placed in /path/to/user_key-cert.pub. A host certificate requires the option:

The host certificate will be output to /path/to/host_key-cert.pub.

It is possible to sign using a CA key stored in a PKCS#11 token by providing the token library using and identifying the CA key by providing its public half as an argument to :

Similarly, it is possible for the CA key to be hosted in a ssh-agent(1). This is indicated by the flag and, again, the CA key must be identified by its public half.

In all cases, is a "key identifier" that is logged by the server when the certificate is used for authentication.

Certificates may be limited to be valid for a set of principal (user/host) names. By default, generated certificates are valid for all users or hosts. To generate a certificate for a specified set of principals:

Additional limitations on the validity and use of user certificates may be specified through certificate options. A certificate option may disable features of the SSH session, may be valid only when presented from particular source addresses or may force the use of a specific command.

The options that are valid for user certificates are:

Clear all enabled permissions. This is useful for clearing the default set of permissions so permissions may be added individually.

:[=]

 

:[=]

Includes an arbitrary certificate critical option or extension. The specified should include a domain suffix, e.g. “name@example.com”. If is specified then it is included as the contents of the extension/option encoded as a string, otherwise the extension/option is created with no contents (usually indicating a flag). Extensions may be ignored by a client or server that does not recognise them, whereas unknown critical options will cause the certificate to be refused.

=

Forces the execution of instead of any shell or command specified by the user when the certificate is used for authentication.

Disable ssh-agent(1) forwarding (permitted by default).

Disable port forwarding (permitted by default).

Disable PTY allocation (permitted by default).

Disable execution of ~/.ssh/rc by sshd(8) (permitted by default).

Disable X11 forwarding (permitted by default).

Allows ssh-agent(1) forwarding.

Allows port forwarding.

Allows PTY allocation.

Allows execution of ~/.ssh/rc by sshd(8).

Allows X11 forwarding.

Do not require signatures made using this key include demonstration of user presence (e.g. by having the user touch the authenticator). This option only makes sense for the FIDO authenticator algorithms and .

=

Restrict the source addresses from which the certificate is considered valid. The is a comma-separated list of one or more address/netmask pairs in CIDR format.

Require signatures made using this key indicate that the user was first verified. This option only makes sense for the FIDO authenticator algorithms and . Currently PIN authentication is the only supported verification method, but other methods may be supported in the future.

At present, no standard options are valid for host keys.

Finally, certificates may be defined with a validity lifetime. The option allows specification of certificate start and end times. A certificate that is presented at a time outside this range will not be considered valid. By default, certificates are valid from UNIX Epoch to the distant future.

For certificates to be used for user or host authentication, the CA public key must be trusted by sshd(8) or ssh(1). Please refer to those manual pages for details.

is able to manage OpenSSH format Key Revocation Lists (KRLs). These binary files specify keys or certificates to be revoked using a compact format, taking as little as one bit per certificate if they are being revoked by serial number.

KRLs may be generated using the flag. This option reads one or more files from the command line and generates a new KRL. The files may either contain a KRL specification (see below) or public keys, listed one per line. Plain public keys are revoked by listing their hash or contents in the KRL and certificates revoked by serial number or key ID (if the serial is zero or not available).

Revoking keys using a KRL specification offers explicit control over the types of record used to revoke keys and may be used to directly revoke certificates by serial number or key ID without having the complete original certificate on hand. A KRL specification consists of lines containing one of the following directives followed by a colon and some directive-specific information.

: [-]

Revokes a certificate with the specified serial number. Serial numbers are 64-bit values, not including zero and may be expressed in decimal, hex or octal. If two serial numbers are specified separated by a hyphen, then the range of serial numbers including and between each is revoked. The CA key must have been specified on the command line using the option.

:

Revokes a certificate with the specified key ID string. The CA key must have been specified on the command line using the option.

:

Revokes the specified key. If a certificate is listed, then it is revoked as a plain public key.

:

Revokes the specified key by including its SHA1 hash in the KRL.

:

Revokes the specified key by including its SHA256 hash in the KRL. KRLs that revoke keys by SHA256 hash are not supported by OpenSSH versions prior to 7.9.

:

Revokes a key using a fingerprint hash, as obtained from a sshd(8) authentication log message or the flag. Only SHA256 fingerprints are supported here and resultant KRLs are not supported by OpenSSH versions prior to 7.9.

KRLs may be updated using the flag in addition to . When this option is specified, keys listed via the command line are merged into the KRL, adding to those already there.

It is also possible, given a KRL, to test whether it revokes a particular key (or keys). The flag will query an existing KRL, testing each key specified on the command line. If any key listed on the command line has been revoked (or an error encountered) then will exit with a non-zero exit status. A zero exit status will only be returned if no key was revoked.

When verifying signatures, uses a simple list of identities and keys to determine whether a signature comes from an authorized source. This "allowed signers" file uses a format patterned after the AUTHORIZED_KEYS FILE FORMAT described in sshd(8). Each line of the file contains the following space-separated fields: principals, options, keytype, base64-encoded key. Empty lines and lines starting with a ‘’ are ignored as comments.

The principals field is a pattern-list (See PATTERNS in ssh_config(5)) consisting of one or more comma-separated USER@DOMAIN identity patterns that are accepted for signing. When verifying, the identity presented via the option must match a principals pattern in order for the corresponding key to be considered acceptable for verification.

The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported (note that option keywords are case-insensitive):

Indicates that this key is accepted as a certificate authority (CA) and that certificates signed by this CA may be accepted for verification.

Specifies a pattern-list of namespaces that are accepted for this key. If this option is present, the signature namespace embedded in the signature object and presented on the verification command-line must match the specified list before the key will be considered acceptable.

When verifying signatures made by certificates, the expected principal name must match both the principals pattern in the allowed signers file and the principals embedded in the certificate itself.

An example allowed signers file:

Specifies a path to a library that will be used when loading any FIDO authenticator-hosted keys, overriding the default of using the built-in USB HID support.

~/.ssh/id_dsa

 

~/.ssh/id_ecdsa

 

~/.ssh/id_ecdsa_sk

 

~/.ssh/id_ed25519

 

~/.ssh/id_ed25519_sk

 

~/.ssh/id_rsa

Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA authentication identity of the user. This file should not be readable by anyone but the user. It is possible to specify a passphrase when generating the key; that passphrase will be used to encrypt the private part of this file using 128-bit AES. This file is not automatically accessed by but it is offered as the default file for the private key. ssh(1) will read this file when a login attempt is made.

~/.ssh/id_dsa.pub

 

~/.ssh/id_ecdsa.pub

 

~/.ssh/id_ecdsa_sk.pub

 

~/.ssh/id_ed25519.pub

 

~/.ssh/id_ed25519_sk.pub

 

~/.ssh/id_rsa.pub

Contains the DSA, ECDSA, authenticator-hosted ECDSA, Ed25519, authenticator-hosted Ed25519 or RSA public key for authentication. The contents of this file should be added to ~/.ssh/authorized_keys on all machines where the user wishes to log in using public key authentication. There is no need to keep the contents of this file secret.

/etc/moduli

Contains Diffie-Hellman groups used for DH-GEX. The file format is described in moduli(5).

ssh(1), ssh-add(1), ssh-agent(1), moduli(5), sshd(8)

The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.

OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0.

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