3 This FAQ is published on the PuTTY web site, and also provided as an
4 appendix in the manual.
6 \H{faq-intro} Introduction
8 \S{faq-what}{Question} What is PuTTY?
10 PuTTY is a client program for the SSH, Telnet and Rlogin network
13 These protocols are all used to run a remote session on a computer,
14 over a network. PuTTY implements the client end of that session: the
15 end at which the session is displayed, rather than the end at which
18 In really simple terms: you run PuTTY on a Windows machine, and tell
19 it to connect to (for example) a Unix machine. PuTTY opens a window.
20 Then, anything you type into that window is sent straight to the
21 Unix machine, and everything the Unix machine sends back is
22 displayed in the window. So you can work on the Unix machine as if
23 you were sitting at its console, while actually sitting somewhere
26 \H{faq-support} Features supported in PuTTY
28 \I{supported features}In general, if you want to know if PuTTY supports
29 a particular feature, you should look for it on the
30 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/}{PuTTY web site}.
34 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html}{changes
35 page}, and see if you can find the feature on there. If a feature is
36 listed there, it's been implemented. If it's listed as a change made
37 \e{since} the latest version, it should be available in the
38 development snapshots, in which case testing will be very welcome.
41 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/}{Wishlist
42 page}, and see if you can find the feature there. If it's on there,
43 and not in the \q{Recently fixed} section, it probably \e{hasn't} been
46 \S{faq-ssh2}{Question} Does PuTTY support SSH-2?
48 Yes. SSH-2 support has been available in PuTTY since version 0.50.
50 Public key authentication (both RSA and DSA) in SSH-2 is new in
53 \S{faq-ssh2-keyfmt}{Question} Does PuTTY support reading OpenSSH or
54 \cw{ssh.com} SSH-2 private key files?
56 PuTTY doesn't support this natively (see
57 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/key-formats-natively.html}{the wishlist entry}
58 for reasons why not), but as of 0.53
59 PuTTYgen can convert both OpenSSH and \cw{ssh.com} private key
60 files into PuTTY's format.
62 \S{faq-ssh1}{Question} Does PuTTY support SSH-1?
64 Yes. SSH-1 support has always been available in PuTTY.
66 However, the SSH-1 protocol has many weaknesses and is no longer
67 considered secure; it should be avoided if at all possible.
69 \S{faq-localecho}{Question} Does PuTTY support \i{local echo}?
71 Yes. Version 0.52 has proper support for local echo.
73 In version 0.51 and before, local echo could not be separated from
74 local line editing (where you type a line of text locally, and it is
75 not sent to the server until you press Return, so you have the
76 chance to edit it and correct mistakes \e{before} the server sees
77 it). New in version 0.52, local echo and local line editing are
78 separate options, and by default PuTTY will try to determine
79 automatically whether to enable them or not, based on which protocol
80 you have selected and also based on hints from the server. If you
81 have a problem with PuTTY's default choice, you can force each
82 option to be enabled or disabled as you choose. The controls are in
83 the Terminal panel, in the section marked \q{Line discipline
86 \S{faq-savedsettings}{Question} Does PuTTY support storing settings,
87 so I don't have to change them every time?
89 Yes, all of PuTTY's settings can be saved in named session profiles.
90 You can also change the default settings that are used for new sessions.
91 See \k{config-saving} in the documentation for how to do this.
93 \S{faq-disksettings}{Question} Does PuTTY support storing its
94 settings in a disk file?
96 Not at present, although \k{config-file} in the documentation gives
97 a method of achieving the same effect.
99 \S{faq-fullscreen}{Question} Does PuTTY support full-screen mode,
102 Yes; this is a new feature in version 0.52.
104 \S{faq-password-remember}{Question} Does PuTTY have the ability to
105 \i{remember my password} so I don't have to type it every time?
109 Remembering your password is a bad plan for obvious security
110 reasons: anyone who gains access to your machine while you're away
111 from your desk can find out the remembered password, and use it,
112 abuse it or change it.
114 In addition, it's not even \e{possible} for PuTTY to automatically
115 send your password in a Telnet session, because Telnet doesn't give
116 the client software any indication of which part of the login
117 process is the password prompt. PuTTY would have to guess, by
118 looking for words like \q{password} in the session data; and if your
119 login program is written in something other than English, this won't
122 In SSH, remembering your password would be possible in theory, but
123 there doesn't seem to be much point since SSH supports public key
124 authentication, which is more flexible and more secure. See
125 \k{pubkey} in the documentation for a full discussion of public key
128 \S{faq-hostkeys}{Question} Is there an option to turn off the
129 \I{verifying the host key}annoying host key prompts?
131 No, there isn't. And there won't be. Even if you write it yourself
132 and send us the patch, we won't accept it.
134 Those annoying host key prompts are the \e{whole point} of SSH.
135 Without them, all the cryptographic technology SSH uses to secure
136 your session is doing nothing more than making an attacker's job
137 slightly harder; instead of sitting between you and the server with
138 a packet sniffer, the attacker must actually subvert a router and
139 start modifying the packets going back and forth. But that's not all
140 that much harder than just sniffing; and without host key checking,
141 it will go completely undetected by client or server.
143 Host key checking is your guarantee that the encryption you put on
144 your data at the client end is the \e{same} encryption taken off the
145 data at the server end; it's your guarantee that it hasn't been
146 removed and replaced somewhere on the way. Host key checking makes
147 the attacker's job \e{astronomically} hard, compared to packet
148 sniffing, and even compared to subverting a router. Instead of
149 applying a little intelligence and keeping an eye on Bugtraq, the
150 attacker must now perform a brute-force attack against at least one
151 military-strength cipher. That insignificant host key prompt really
152 does make \e{that} much difference.
154 If you're having a specific problem with host key checking - perhaps
155 you want an automated batch job to make use of PSCP or Plink, and the
156 interactive host key prompt is hanging the batch process - then the
157 right way to fix it is to add the correct host key to the Registry in
158 advance, or if the Registry is not available, to use the \cw{-hostkey}
159 command-line option. That way, you retain the \e{important} feature of
160 host key checking: the right key will be accepted and the wrong ones
161 will not. Adding an option to turn host key checking off completely is
162 the wrong solution and we will not do it.
164 If you have host keys available in the common \i\c{known_hosts} format,
165 we have a script called
166 \W{http://tartarus.org/~simon-git/gitweb/?p=putty.git;a=blob;f=contrib/kh2reg.py;hb=HEAD}\c{kh2reg.py}
167 to convert them to a Windows .REG file, which can be installed ahead of
168 time by double-clicking or using \c{REGEDIT}.
170 \S{faq-server}{Question} Will you write an SSH server for the PuTTY
171 suite, to go with the client?
173 No. The only reason we might want to would be if we could easily
174 re-use existing code and significantly cut down the effort. We don't
175 believe this is the case; there just isn't enough common ground
176 between an SSH client and server to make it worthwhile.
178 If someone else wants to use bits of PuTTY in the process of writing
179 a Windows SSH server, they'd be perfectly welcome to of course, but
180 I really can't see it being a lot less effort for us to do that than
181 it would be for us to write a server from the ground up. We don't
182 have time, and we don't have motivation. The code is available if
183 anyone else wants to try it.
185 \S{faq-pscp-ascii}{Question} Can PSCP or PSFTP transfer files in
190 Until recently, this was a limitation of the file transfer protocols:
191 the SCP and SFTP protocols had no notion of transferring a file in
192 anything other than binary mode. (This is still true of SCP.)
194 The current draft protocol spec of SFTP proposes a means of
195 implementing ASCII transfer. At some point PSCP/PSFTP may implement
198 \H{faq-ports} Ports to other operating systems
200 The eventual goal is for PuTTY to be a multi-platform program, able
201 to run on at least Windows, Mac OS and Unix.
203 Porting will become easier once PuTTY has a generalised porting
204 layer, drawing a clear line between platform-dependent and
205 platform-independent code. The general intention was for this
206 porting layer to evolve naturally as part of the process of doing
207 the first port; a Unix port has now been released and the plan
208 seems to be working so far.
210 \S{faq-ports-general}{Question} What ports of PuTTY exist?
212 Currently, release versions of PuTTY tools only run on full Win32
213 systems and Unix. \q{\i{Win32}} includes versions of Windows from
214 Windows 95 onwards (as opposed to the 16-bit Windows 3.1; see
215 \k{faq-win31}), up to and including Windows 7; and we know of no
216 reason why PuTTY should not continue to work on future versions
219 The Windows executables we provide are for the 32-bit \q{\i{x86}}
220 processor architecture, but they should work fine on 64-bit
221 processors that are backward-compatible with that architecture.
222 (We used to also provide executables for Windows for the Alpha
223 processor, but stopped after 0.58 due to lack of interest.)
225 In the development code, partial ports to the Mac OSes exist (see
228 Currently PuTTY does \e{not} run on Windows CE (see \k{faq-wince}).
230 We do not have release-quality ports for any other systems at the
231 present time. If anyone told you we had an EPOC port, or an iPaq port,
232 or any other port of PuTTY, they were mistaken. We don't.
234 There are some third-party ports to various platforms, mentioned
236 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/links.html}{Links page of our website}.
238 \S{faq-unix}{Question} \I{Unix version}Is there a port to Unix?
240 As of 0.54, there are Unix ports of most of the traditional PuTTY
241 tools, and also one entirely new application.
243 If you look at the source release, you should find a \c{unix}
244 subdirectory. There are a couple of ways of building it,
245 including the usual \c{configure}/\c{make}; see the file \c{README}
246 in the source distribution. This should build you Unix
247 ports of Plink, PuTTY itself, PuTTYgen, PSCP, PSFTP, and also
248 \i\c{pterm} - an \cw{xterm}-type program which supports the same
249 terminal emulation as PuTTY. We do not yet have a Unix port of
252 If you don't have \i{Gtk}, you should still be able to build the
255 Note that Unix PuTTY has mostly only been tested on Linux so far;
256 portability problems such as BSD-style ptys or different header file
257 requirements are expected.
259 \S{faq-unix-why}{Question} What's the point of the Unix port? Unix
262 All sorts of little things. \c{pterm} is directly useful to anyone
263 who prefers PuTTY's terminal emulation to \c{xterm}'s, which at
264 least some people do. Unix Plink has apparently found a niche among
265 people who find the complexity of OpenSSL makes OpenSSH hard to
266 install (and who don't mind Plink not having as many features). Some
267 users want to generate a large number of SSH keys on Unix and then
268 copy them all into PuTTY, and the Unix PuTTYgen should allow them to
269 automate that conversion process.
271 There were development advantages as well; porting PuTTY to Unix was
272 a valuable path-finding effort for other future ports, and also
273 allowed us to use the excellent Linux tool
274 \W{http://valgrind.kde.org/}{Valgrind} to help with debugging, which
275 has already improved PuTTY's stability on \e{all} platforms.
277 However, if you're a Unix user and you can see no reason to switch
278 from OpenSSH to PuTTY/Plink, then you're probably right. We don't
279 expect our Unix port to be the right thing for everybody.
281 \S{faq-wince}{Question} Will there be a port to Windows CE or PocketPC?
283 We have done some work on such a port, but it only reached an early
284 stage, and certainly not a useful one. It's no longer being actively
287 However, there's a third-party port at
288 \W{http://www.pocketputty.net/}\c{http://www.pocketputty.net/}.
290 \S{faq-win31}{Question} Is there a port to \i{Windows 3.1}?
292 PuTTY is a 32-bit application from the ground up, so it won't run on
293 Windows 3.1 as a native 16-bit program; and it would be \e{very}
294 hard to port it to do so, because of Windows 3.1's vile memory
295 allocation mechanisms.
297 However, it is possible in theory to compile the existing PuTTY
298 source in such a way that it will run under \i{Win32s} (an extension to
299 Windows 3.1 to let you run 32-bit programs). In order to do this
300 you'll need the right kind of C compiler - modern versions of Visual
301 C at least have stopped being backwards compatible to Win32s. Also,
302 the last time we tried this it didn't work very well.
304 If you're interested in running PuTTY under Windows 3.1, help and
305 testing in this area would be very welcome!
307 \S{faq-mac-port}{Question} Will there be a port to the \I{Mac OS}Mac?
311 We attempted one around 2005, written as a native Cocoa application,
312 but it turned out to be very slow to redraw its window for some reason
313 we never got to the bottom of.
315 In 2015, after porting the GTK front end to work with GTK 3, we began
316 another attempt based on making small changes to the GTK code and
317 building it against the OS X Quartz version of GTK 3. This doesn't
318 seem to have the window redrawing problem any more, so it's already
319 got further than the last effort, but it is still substantially
322 If any OS X and/or GTK programming experts are keen to have a finished
323 version of this, we urge them to help out with some of the remaining
326 \S{faq-epoc}{Question} Will there be a port to EPOC?
328 I hope so, but given that ports aren't really progressing very fast
329 even on systems the developers \e{do} already know how to program
330 for, it might be a long time before any of us get round to learning
331 a new system and doing the port for that.
333 However, some of the work has been done by other people; see the
334 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/links.html}{Links page of our website}
335 for various third-party ports.
337 \S{faq-iphone}{Question} Will there be a port to the iPhone?
339 We have no plans to write such a port ourselves; none of us has an
340 iPhone, and developing and publishing applications for it looks
341 awkward and expensive. Such a port would probably depend upon the
342 stalled Mac OS X port (see \k{faq-mac-port}).
344 However, there is a third-party SSH client for the iPhone and
345 iPod\_Touch called \W{http://www.instantcocoa.com/products/pTerm/}{pTerm},
346 which is apparently based on PuTTY. (This is nothing to do with our
347 similarly-named \c{pterm}, which is a standalone terminal emulator for
348 Unix systems; see \k{faq-unix}.)
350 \H{faq-embedding} Embedding PuTTY in other programs
352 \S{faq-dll}{Question} Is the SSH or Telnet code available as a DLL?
354 No, it isn't. It would take a reasonable amount of rewriting for
355 this to be possible, and since the PuTTY project itself doesn't
356 believe in DLLs (they make installation more error-prone) none of us
357 has taken the time to do it.
359 Most of the code cleanup work would be a good thing to happen in
360 general, so if anyone feels like helping, we wouldn't say no.
363 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/dll-frontend.html}{the wishlist entry}.
365 \S{faq-vb}{Question} Is the SSH or Telnet code available as a Visual
368 No, it isn't. None of the PuTTY team uses Visual Basic, and none of
369 us has any particular need to make SSH connections from a Visual
370 Basic application. In addition, all the preliminary work to turn it
371 into a DLL would be necessary first; and furthermore, we don't even
372 know how to write VB components.
374 If someone offers to do some of this work for us, we might consider
375 it, but unless that happens I can't see VB integration being
376 anywhere other than the very bottom of our priority list.
378 \S{faq-ipc}{Question} How can I use PuTTY to make an SSH connection
379 from within another program?
381 Probably your best bet is to use Plink, the command-line connection
382 tool. If you can start Plink as a second Windows process, and
383 arrange for your primary process to be able to send data to the
384 Plink process, and receive data from it, through pipes, then you
385 should be able to make SSH connections from your program.
387 This is what CVS for Windows does, for example.
389 \H{faq-details} Details of PuTTY's operation
391 \S{faq-term}{Question} What \i{terminal type} does PuTTY use?
393 For most purposes, PuTTY can be considered to be an \cw{xterm}
396 PuTTY also supports some terminal \i{control sequences} not supported by
397 the real \cw{xterm}: notably the Linux console sequences that
398 reconfigure the colour palette, and the title bar control sequences
399 used by \i\cw{DECterm} (which are different from the \cw{xterm} ones;
400 PuTTY supports both).
402 By default, PuTTY announces its terminal type to the server as
403 \c{xterm}. If you have a problem with this, you can reconfigure it
404 to say something else; \c{vt220} might help if you have trouble.
406 \S{faq-settings}{Question} Where does PuTTY store its data?
408 On Windows, PuTTY stores most of its data (saved sessions, SSH host
409 keys) in the \i{Registry}. The precise location is
411 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
413 and within that area, saved sessions are stored under \c{Sessions}
414 while host keys are stored under \c{SshHostKeys}.
416 PuTTY also requires a random number seed file, to improve the
417 unpredictability of randomly chosen data needed as part of the SSH
418 cryptography. This is stored by default in a file called \i\c{PUTTY.RND};
419 this is stored by default in the \q{Application Data} directory,
420 or failing that, one of a number of fallback locations. If you
421 want to change the location of the random number seed file, you can
422 put your chosen pathname in the Registry, at
424 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
426 You can ask PuTTY to delete all this data; see \k{faq-cleanup}.
428 On Unix, PuTTY stores all of this data in a directory \cw{~/.putty}.
430 \H{faq-howto} HOWTO questions
432 \S{faq-login}{Question} What login name / password should I use?
434 This is not a question you should be asking \e{us}.
436 PuTTY is a communications tool, for making connections to other
437 computers. We maintain the tool; we \e{don't} administer any computers
438 that you're likely to be able to use, in the same way that the people
439 who make web browsers aren't responsible for most of the content you can
440 view in them. \#{FIXME: less technical analogy?} We cannot help with
441 questions of this sort.
443 If you know the name of the computer you want to connect to, but don't
444 know what login name or password to use, you should talk to whoever
445 administers that computer. If you don't know who that is, see the next
446 question for some possible ways to find out.
448 \# FIXME: some people ask us to provide them with a login name
449 apparently as random members of the public rather than in the
450 belief that we run a server belonging to an organisation they already
451 have some relationship with. Not sure what to say to such people.
453 \S{faq-commands}{Question} \I{commands on the server}What commands
454 can I type into my PuTTY terminal window?
456 Again, this is not a question you should be asking \e{us}. You need
457 to read the manuals, or ask the administrator, of \e{the computer
458 you have connected to}.
460 PuTTY does not process the commands you type into it. It's only a
461 communications tool. It makes a connection to another computer; it
462 passes the commands you type to that other computer; and it passes
463 the other computer's responses back to you. Therefore, the precise
464 range of commands you can use will not depend on PuTTY, but on what
465 kind of computer you have connected to and what software is running
466 on it. The PuTTY team cannot help you with that.
468 (Think of PuTTY as being a bit like a telephone. If you phone
469 somebody up and you don't know what language to speak to make them
470 understand you, it isn't \e{the telephone company}'s job to find
471 that out for you. We just provide the means for you to get in touch;
472 making yourself understood is somebody else's problem.)
474 If you are unsure of where to start looking for the administrator of
475 your server, a good place to start might be to remember how you
476 found out the host name in the PuTTY configuration. If you were
477 given that host name by e-mail, for example, you could try asking
478 the person who sent you that e-mail. If your company's IT department
479 provided you with ready-made PuTTY saved sessions, then that IT
480 department can probably also tell you something about what commands
481 you can type during those sessions. But the PuTTY maintainer team
482 does not administer any server you are likely to be connecting to,
483 and cannot help you with questions of this type.
485 \S{faq-startmax}{Question} How can I make PuTTY start up \i{maximise}d?
487 Create a Windows shortcut to start PuTTY from, and set it as \q{Run
490 \S{faq-startsess}{Question} How can I create a \i{Windows shortcut} to
491 start a particular saved session directly?
493 To run a PuTTY session saved under the name \q{\cw{mysession}},
494 create a Windows shortcut that invokes PuTTY with a command line
497 \c \path\name\to\putty.exe -load "mysession"
499 (Note: prior to 0.53, the syntax was \c{@session}. This is now
500 deprecated and may be removed at some point.)
502 \S{faq-startssh}{Question} How can I start an SSH session straight
503 from the command line?
505 Use the command line \c{putty -ssh host.name}. Alternatively, create
506 a saved session that specifies the SSH protocol, and start the saved
507 session as shown in \k{faq-startsess}.
509 \S{faq-cutpaste}{Question} How do I \i{copy and paste} between PuTTY and
510 other Windows applications?
512 Copy and paste works similarly to the X Window System. You use the
513 left mouse button to select text in the PuTTY window. The act of
514 selection \e{automatically} copies the text to the clipboard: there
515 is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
516 pressing Ctrl-C will send a Ctrl-C character to the other end of
517 your connection (just like it does the rest of the time), which may
518 have unpleasant effects. The \e{only} thing you need to do, to copy
519 text to the clipboard, is to select it.
521 To paste the clipboard contents into a PuTTY window, by default you
522 click the right mouse button. If you have a three-button mouse and
523 are used to X applications, you can configure pasting to be done by
524 the middle button instead, but this is not the default because most
525 Windows users don't have a middle button at all.
527 You can also paste by pressing Shift-Ins.
529 \S{faq-options}{Question} How do I use all PuTTY's features (public
530 keys, proxying, cipher selection, etc.) in PSCP, PSFTP and Plink?
532 Most major features (e.g., public keys, port forwarding) are available
533 through command line options. See the documentation.
535 Not all features are accessible from the command line yet, although
536 we'd like to fix this. In the meantime, you can use most of
537 PuTTY's features if you create a PuTTY saved session, and then use
538 the name of the saved session on the command line in place of a
539 hostname. This works for PSCP, PSFTP and Plink (but don't expect
540 port forwarding in the file transfer applications!).
542 \S{faq-pscp}{Question} How do I use PSCP.EXE? When I double-click it
543 gives me a command prompt window which then closes instantly.
545 PSCP is a command-line application, not a GUI application. If you
546 run it without arguments, it will simply print a help message and
549 To use PSCP properly, run it from a Command Prompt window. See
550 \k{pscp} in the documentation for more details.
552 \S{faq-pscp-spaces}{Question} \I{spaces in filenames}How do I use
553 PSCP to copy a file whose name has spaces in?
555 If PSCP is using the traditional SCP protocol, this is confusing. If
556 you're specifying a file at the local end, you just use one set of
557 quotes as you would normally do:
559 \c pscp "local filename with spaces" user@host:
560 \c pscp user@host:myfile "local filename with spaces"
562 But if the filename you're specifying is on the \e{remote} side, you
563 have to use backslashes and two sets of quotes:
565 \c pscp user@host:"\"remote filename with spaces\"" local_filename
566 \c pscp local_filename user@host:"\"remote filename with spaces\""
568 Worse still, in a remote-to-local copy you have to specify the local
569 file name explicitly, otherwise PSCP will complain that they don't
570 match (unless you specified the \c{-unsafe} option). The following
571 command will give an error message:
573 \c c:\>pscp user@host:"\"oo er\"" .
574 \c warning: remote host tried to write to a file called 'oo er'
575 \c when we requested a file called '"oo er"'.
577 Instead, you need to specify the local file name in full:
579 \c c:\>pscp user@host:"\"oo er\"" "oo er"
581 If PSCP is using the newer SFTP protocol, none of this is a problem,
582 and all filenames with spaces in are specified using a single pair
583 of quotes in the obvious way:
585 \c pscp "local file" user@host:
586 \c pscp user@host:"remote file" .
588 \H{faq-trouble} Troubleshooting
590 \S{faq-incorrect-mac}{Question} Why do I see \q{Incorrect MAC
593 One possible cause of this that used to be common is a bug in old
594 SSH-2 servers distributed by \cw{ssh.com}. (This is not the only
595 possible cause; see \k{errors-crc} in the documentation.)
596 Version 2.3.0 and below of their SSH-2 server
597 constructs Message Authentication Codes in the wrong way, and
598 expects the client to construct them in the same wrong way. PuTTY
599 constructs the MACs correctly by default, and hence these old
600 servers will fail to work with it.
602 If you are using PuTTY version 0.52 or better, this should work
603 automatically: PuTTY should detect the buggy servers from their
604 version number announcement, and automatically start to construct
605 its MACs in the same incorrect manner as they do, so it will be able
608 If you are using PuTTY version 0.51 or below, you can enable the
609 workaround by going to the SSH panel and ticking the box labelled
610 \q{Imitate SSH2 MAC bug}. It's possible that you might have to do
611 this with 0.52 as well, if a buggy server exists that PuTTY doesn't
614 In this context MAC stands for \ii{Message Authentication Code}. It's a
615 cryptographic term, and it has nothing at all to do with Ethernet
616 MAC (Media Access Control) addresses.
618 \S{faq-pscp-protocol}{Question} Why do I see \q{Fatal: Protocol
619 error: Expected control record} in PSCP?
621 This happens because PSCP was expecting to see data from the server
622 that was part of the PSCP protocol exchange, and instead it saw data
623 that it couldn't make any sense of at all.
625 This almost always happens because the \i{startup scripts} in your
626 account on the server machine are generating output. This is
627 impossible for PSCP, or any other SCP client, to work around. You
628 should never use startup files (\c{.bashrc}, \c{.cshrc} and so on)
629 which generate output in non-interactive sessions.
631 This is not actually a PuTTY problem. If PSCP fails in this way,
632 then all other SCP clients are likely to fail in exactly the same
633 way. The problem is at the server end.
635 \S{faq-colours}{Question} I clicked on a colour in the \ii{Colours}
636 panel, and the colour didn't change in my terminal.
638 That isn't how you're supposed to use the Colours panel.
640 During the course of a session, PuTTY potentially uses \e{all} the
641 colours listed in the Colours panel. It's not a question of using
642 only one of them and you choosing which one; PuTTY will use them
643 \e{all}. The purpose of the Colours panel is to let you adjust the
644 appearance of all the colours. So to change the colour of the
645 cursor, for example, you would select \q{Cursor Colour}, press the
646 \q{Modify} button, and select a new colour from the dialog box that
647 appeared. Similarly, if you want your session to appear in green,
648 you should select \q{Default Foreground} and press \q{Modify}.
649 Clicking on \q{ANSI Green} won't turn your session green; it will
650 only allow you to adjust the \e{shade} of green used when PuTTY is
651 instructed by the server to display green text.
653 \S{faq-winsock2}{Question} Plink on \i{Windows 95} says it can't find
656 Plink requires the extended Windows network library, WinSock version
657 2. This is installed as standard on Windows 98 and above, and on
658 Windows NT, and even on later versions of Windows 95; but early
659 Win95 installations don't have it.
661 In order to use Plink on these systems, you will need to download
663 \W{http://www.microsoft.com/windows95/downloads/contents/wuadmintools/s_wunetworkingtools/w95sockets2/}{WinSock 2 upgrade}:
665 \c http://www.microsoft.com/windows95/downloads/contents/
666 \c wuadmintools/s_wunetworkingtools/w95sockets2/
668 \S{faq-outofmem}{Question} After trying to establish an SSH-2
669 connection, PuTTY says \q{\ii{Out of memory}} and dies.
671 If this happens just while the connection is starting up, this often
672 indicates that for some reason the client and server have failed to
673 establish a session encryption key. Somehow, they have performed
674 calculations that should have given each of them the same key, but
675 have ended up with different keys; so data encrypted by one and
676 decrypted by the other looks like random garbage.
678 This causes an \q{out of memory} error because the first encrypted
679 data PuTTY expects to see is the length of an SSH message. Normally
680 this will be something well under 100 bytes. If the decryption has
681 failed, PuTTY will see a completely random length in the region of
682 two \e{gigabytes}, and will try to allocate enough memory to store
683 this non-existent message. This will immediately lead to it thinking
684 it doesn't have enough memory, and panicking.
686 If this happens to you, it is quite likely to still be a PuTTY bug
687 and you should report it (although it might be a bug in your SSH
688 server instead); but it doesn't necessarily mean you've actually run
691 \S{faq-outofmem2}{Question} When attempting a file transfer, either
692 PSCP or PSFTP says \q{\ii{Out of memory}} and dies.
694 This is almost always caused by your \i{login scripts} on the server
695 generating output. PSCP or PSFTP will receive that output when they
696 were expecting to see the start of a file transfer protocol, and
697 they will attempt to interpret the output as file-transfer protocol.
698 This will usually lead to an \q{out of memory} error for much the
699 same reasons as given in \k{faq-outofmem}.
701 This is a setup problem in your account on your server, \e{not} a
702 PSCP/PSFTP bug. Your login scripts should \e{never} generate output
703 during non-interactive sessions; secure file transfer is not the
704 only form of remote access that will break if they do.
706 On Unix, a simple fix is to ensure that all the parts of your login
707 script that might generate output are in \c{.profile} (if you use a
708 Bourne shell derivative) or \c{.login} (if you use a C shell).
709 Putting them in more general files such as \c{.bashrc} or \c{.cshrc}
710 is liable to lead to problems.
712 \S{faq-psftp-slow}{Question} PSFTP transfers files much slower than PSCP.
714 The throughput of PSFTP 0.54 should be much better than 0.53b and
715 prior; we've added code to the SFTP backend to queue several blocks
716 of data rather than waiting for an acknowledgement for each. (The
717 SCP backend did not suffer from this performance issue because SCP
718 is a much simpler protocol.)
720 \S{faq-bce}{Question} When I run full-colour applications, I see
721 areas of black space where colour ought to be, or vice versa.
723 You almost certainly need to change the \q{Use \i{background colour} to
724 erase screen} setting in the Terminal panel. If there is too much
725 black space (the commoner situation), you should enable it, while if
726 there is too much colour, you should disable it. (See \k{config-erase}.)
728 In old versions of PuTTY, this was disabled by default, and would not
729 take effect until you reset the terminal (see \k{faq-resetterm}).
730 Since 0.54, it is enabled by default, and changes take effect
733 \S{faq-resetterm}{Question} When I change some terminal settings,
736 Some of the terminal options (notably \ii{Auto Wrap} and
737 background-colour screen erase) actually represent the \e{default}
738 setting, rather than the currently active setting. The server can
739 send sequences that modify these options in mid-session, but when
740 the terminal is reset (by server action, or by you choosing \q{Reset
741 Terminal} from the System menu) the defaults are restored.
743 In versions 0.53b and prior, if you change one of these options in
744 the middle of a session, you will find that the change does not
745 immediately take effect. It will only take effect once you reset
748 In version 0.54, the behaviour has changed - changes to these
749 settings take effect immediately.
751 \S{faq-idleout}{Question} My PuTTY sessions unexpectedly close after
752 they are \I{idle connections}idle for a while.
754 Some types of \i{firewall}, and almost any router doing Network Address
755 Translation (\i{NAT}, also known as IP masquerading), will forget about
756 a connection through them if the connection does nothing for too
757 long. This will cause the connection to be rudely cut off when
760 You can try to combat this by telling PuTTY to send \e{keepalives}:
761 packets of data which have no effect on the actual session, but
762 which reassure the router or firewall that the network connection is
763 still active and worth remembering about.
765 Keepalives don't solve everything, unfortunately; although they
766 cause greater robustness against this sort of router, they can also
767 cause a \e{loss} of robustness against network dropouts. See
768 \k{config-keepalive} in the documentation for more discussion of
771 \S{faq-timeout}{Question} PuTTY's network connections time out too
772 quickly when \I{breaks in connectivity}network connectivity is
775 This is a Windows problem, not a PuTTY problem. The timeout value
776 can't be set on per application or per session basis. To increase
777 the TCP timeout globally, you need to tinker with the Registry.
779 On Windows 95, 98 or ME, the registry key you need to create or
782 \c HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
783 \c MSTCP\MaxDataRetries
785 (it must be of type DWORD in Win95, or String in Win98/ME).
786 (See MS Knowledge Base article
787 \W{http://support.microsoft.com/default.aspx?scid=kb;en-us;158474}{158474}
788 for more information.)
790 On Windows NT, 2000, or XP, the registry key to create or change is
792 \c HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
793 \c Parameters\TcpMaxDataRetransmissions
795 and it must be of type DWORD.
796 (See MS Knowledge Base articles
797 \W{http://support.microsoft.com/default.aspx?scid=kb;en-us;120642}{120642}
799 \W{http://support.microsoft.com/default.aspx?scid=kb;en-us;314053}{314053}
800 for more information.)
802 Set the key's value to something like 10. This will cause Windows to
803 try harder to keep connections alive instead of abandoning them.
805 \S{faq-puttyputty}{Question} When I \cw{cat} a binary file, I get
806 \q{PuTTYPuTTYPuTTY} on my command line.
810 This is designed behaviour; when PuTTY receives the character
811 Control-E from the remote server, it interprets it as a request to
812 identify itself, and so it sends back the string \q{\cw{PuTTY}} as
813 if that string had been entered at the keyboard. Control-E should
814 only be sent by programs that are prepared to deal with the
815 response. Writing a binary file to your terminal is likely to output
816 many Control-E characters, and cause this behaviour. Don't do it.
819 To mitigate the effects, you could configure the answerback string
820 to be empty (see \k{config-answerback}); but writing binary files to
821 your terminal is likely to cause various other unpleasant behaviour,
822 so this is only a small remedy.
824 \S{faq-wintitle}{Question} When I \cw{cat} a binary file, my \i{window
825 title} changes to a nonsense string.
829 It is designed behaviour that PuTTY should have the ability to
830 adjust the window title on instructions from the server. Normally
831 the control sequence that does this should only be sent
832 deliberately, by programs that know what they are doing and intend
833 to put meaningful text in the window title. Writing a binary file to
834 your terminal runs the risk of sending the same control sequence by
835 accident, and cause unexpected changes in the window title. Don't do
838 \S{faq-password-fails}{Question} My \i{keyboard} stops working once
839 PuTTY displays the \i{password prompt}.
841 No, it doesn't. PuTTY just doesn't display the password you type, so
842 that someone looking at your screen can't see what it is.
844 Unlike the Windows login prompts, PuTTY doesn't display the password
845 as a row of asterisks either. This is so that someone looking at
846 your screen can't even tell how \e{long} your password is, which
847 might be valuable information.
849 \S{faq-keyboard}{Question} One or more \I{keyboard}\i{function keys}
850 don't do what I expected in a server-side application.
852 If you've already tried all the relevant options in the PuTTY
853 Keyboard panel, you may need to mail the PuTTY maintainers and ask.
855 It is \e{not} usually helpful just to tell us which application,
856 which server operating system, and which key isn't working; in order
857 to replicate the problem we would need to have a copy of every
858 operating system, and every application, that anyone has ever
861 PuTTY responds to function key presses by sending a sequence of
862 control characters to the server. If a function key isn't doing what
863 you expect, it's likely that the character sequence your application
864 is expecting to receive is not the same as the one PuTTY is sending.
865 Therefore what we really need to know is \e{what} sequence the
866 application is expecting.
868 The simplest way to investigate this is to find some other terminal
869 environment, in which that function key \e{does} work; and then
870 investigate what sequence the function key is sending in that
871 situation. One reasonably easy way to do this on a \i{Unix} system is to
872 type the command \i\c{cat}, and then press the function key. This is
873 likely to produce output of the form \c{^[[11~}. You can also do
874 this in PuTTY, to find out what sequence the function key is
875 producing in that. Then you can mail the PuTTY maintainers and tell
876 us \q{I wanted the F1 key to send \c{^[[11~}, but instead it's
877 sending \c{^[OP}, can this be done?}, or something similar.
879 You should still read the
880 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/feedback.html}{Feedback
881 page} on the PuTTY website (also provided as \k{feedback} in the
882 manual), and follow the guidelines contained in that.
884 \S{faq-openssh-bad-openssl}{Question} Since my SSH server was upgraded
885 to \i{OpenSSH} 3.1p1/3.4p1, I can no longer connect with PuTTY.
887 There is a known problem when OpenSSH has been built against an
888 incorrect version of OpenSSL; the quick workaround is to configure
889 PuTTY to use SSH protocol 2 and the Blowfish cipher.
891 For more details and OpenSSH patches, see
892 \W{http://bugzilla.mindrot.org/show_bug.cgi?id=138}{bug 138} in the
895 This is not a PuTTY-specific problem; if you try to connect with
896 another client you'll likely have similar problems. (Although PuTTY's
897 default cipher differs from many other clients.)
899 \e{OpenSSH 3.1p1:} configurations known to be broken (and symptoms):
901 \b SSH-2 with AES cipher (PuTTY says \q{Assertion failed! Expression:
902 (len & 15) == 0} in \cw{sshaes.c}, or \q{Out of memory}, or crashes)
904 \b SSH-2 with 3DES (PuTTY says \q{Incorrect MAC received on packet})
906 \b SSH-1 with Blowfish (PuTTY says \q{Incorrect CRC received on
911 \e{OpenSSH 3.4p1:} as of 3.4p1, only the problem with SSH-1 and
912 Blowfish remains. Rebuild your server, apply the patch linked to from
913 bug 138 above, or use another cipher (e.g., 3DES) instead.
915 \e{Other versions:} we occasionally get reports of the same symptom
916 and workarounds with older versions of OpenSSH, although it's not
917 clear the underlying cause is the same.
919 \S{faq-ssh2key-ssh1conn}{Question} Why do I see \q{Couldn't load
920 private key from ...}? Why can PuTTYgen load my key but not PuTTY?
922 It's likely that you've generated an SSH protocol 2 key with PuTTYgen,
923 but you're trying to use it in an SSH-1 connection. SSH-1 and SSH-2 keys
924 have different formats, and (at least in 0.52) PuTTY's reporting of a
925 key in the wrong format isn't optimal.
927 To connect using SSH-2 to a server that supports both versions, you
928 need to change the configuration from the default (see \k{faq-ssh2}).
930 \S{faq-rh8-utf8}{Question} When I'm connected to a \i{Red Hat Linux} 8.0
931 system, some characters don't display properly.
933 A common complaint is that hyphens in man pages show up as a-acute.
935 With release 8.0, Red Hat appear to have made \i{UTF-8} the default
936 character set. There appears to be no way for terminal emulators such
937 as PuTTY to know this (as far as we know, the appropriate escape
938 sequence to switch into UTF-8 mode isn't sent).
940 A fix is to configure sessions to RH8 systems to use UTF-8
941 translation - see \k{config-charset} in the documentation. (Note that
942 if you use \q{Change Settings}, changes may not take place immediately
943 - see \k{faq-resetterm}.)
945 If you really want to change the character set used by the server, the
946 right place is \c{/etc/sysconfig/i18n}, but this shouldn't be
949 \S{faq-screen}{Question} Since I upgraded to PuTTY 0.54, the
950 scrollback has stopped working when I run \c{screen}.
952 PuTTY's terminal emulator has always had the policy that when the
953 \q{\i{alternate screen}} is in use, nothing is added to the scrollback.
954 This is because the usual sorts of programs which use the alternate
955 screen are things like text editors, which tend to scroll back and
956 forth in the same document a lot; so (a) they would fill up the
957 scrollback with a large amount of unhelpfully disordered text, and
958 (b) they contain their \e{own} method for the user to scroll back to
959 the bit they were interested in. We have generally found this policy
960 to do the Right Thing in almost all situations.
962 Unfortunately, \c{screen} is one exception: it uses the alternate
963 screen, but it's still usually helpful to have PuTTY's scrollback
964 continue working. The simplest solution is to go to the Features
965 control panel and tick \q{Disable switching to alternate terminal
966 screen}. (See \k{config-features-altscreen} for more details.)
967 Alternatively, you can tell \c{screen} itself not to use the
968 alternate screen: the
969 \W{http://www4.informatik.uni-erlangen.de/~jnweiger/screen-faq.html}{\c{screen}
970 FAQ} suggests adding the line \cq{termcapinfo xterm ti@:te@} to your
973 The reason why this only started to be a problem in 0.54 is because
974 \c{screen} typically uses an unusual control sequence to switch to
975 the alternate screen, and previous versions of PuTTY did not support
978 \S{faq-alternate-localhost}{Question} Since I upgraded \i{Windows XP}
979 to Service Pack 2, I can't use addresses like \cw{127.0.0.2}.
981 Some people who ask PuTTY to listen on \i{localhost} addresses other
982 than \cw{127.0.0.1} to forward services such as \i{SMB} and \i{Windows
983 Terminal Services} have found that doing so no longer works since
984 they upgraded to WinXP SP2.
986 This is apparently an issue with SP2 that is acknowledged by Microsoft
987 in MS Knowledge Base article
988 \W{http://support.microsoft.com/default.aspx?scid=kb;en-us;884020}{884020}.
989 The article links to a fix you can download.
991 (\e{However}, we've been told that SP2 \e{also} fixes the bug that
992 means you need to use non-\cw{127.0.0.1} addresses to forward
993 Terminal Services in the first place.)
995 \S{faq-missing-slash}{Question} PSFTP commands seem to be missing a
996 directory separator (slash).
998 Some people have reported the following incorrect behaviour with
1003 \c Remote directory is /dir1/dir2
1004 \c psftp> get filename.ext
1006 \c /dir1/dir2filename.ext: no such file or directory
1008 This is not a bug in PSFTP. There is a known bug in some versions of
1009 portable \i{OpenSSH}
1010 (\W{http://bugzilla.mindrot.org/show_bug.cgi?id=697}{bug 697}) that
1011 causes these symptoms; it appears to have been introduced around
1012 3.7.x. It manifests only on certain platforms (AIX is what has been
1015 There is a patch for OpenSSH attached to that bug; it's also fixed in
1016 recent versions of portable OpenSSH (from around 3.8).
1018 \S{faq-connaborted}{Question} Do you want to hear about \q{Software
1019 caused connection abort}?
1021 In the documentation for PuTTY 0.53 and 0.53b, we mentioned that we'd
1022 like to hear about any occurrences of this error. Since the release
1023 of PuTTY 0.54, however, we've been convinced that this error doesn't
1024 indicate that PuTTY's doing anything wrong, and we don't need to hear
1025 about further occurrences. See \k{errors-connaborted} for our current
1026 documentation of this error.
1028 \S{faq-rekey}{Question} My SSH-2 session \I{locking up, SSH-2
1029 sessions}locks up for a few seconds every so often.
1031 Recent versions of PuTTY automatically initiate \i{repeat key
1032 exchange} once per hour, to improve session security. If your client
1033 or server machine is slow, you may experience this as a delay of
1034 anything up to thirty seconds or so.
1036 These \I{delays, in SSH-2 sessions}delays are inconvenient, but they
1037 are there for your protection. If they really cause you a problem,
1038 you can choose to turn off periodic rekeying using the \q{Kex}
1039 configuration panel (see \k{config-ssh-kex}), but be aware that you
1040 will be sacrificing security for this. (Falling back to SSH-1 would
1041 also remove the delays, but would lose a \e{lot} more security
1042 still. We do not recommend it.)
1044 \S{faq-xpwontrun}{Question} PuTTY fails to start up. Windows claims that
1045 \q{the application configuration is incorrect}.
1047 This is caused by a bug in certain versions of \i{Windows XP} which
1048 is triggered by PuTTY 0.58. This was fixed in 0.59. The
1049 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/xp-wont-run}{\q{xp-wont-run}}
1050 entry in PuTTY's wishlist has more details.
1052 \S{faq-system32}{Question} When I put PuTTY in
1053 \cw{C:\\WINDOWS\\\i{SYSTEM32}} on my \i{64-bit Windows} system,
1054 \i{\q{Duplicate Session}} doesn't work.
1056 The short answer is not to put the PuTTY executables in that location.
1058 On 64-bit systems, \cw{C:\\WINDOWS\\SYSTEM32} is intended to contain
1059 only 64-bit binaries; Windows' 32-bit binaries live in
1060 \cw{C:\\WINDOWS\\SYSWOW64}. When a 32-bit program such as PuTTY runs
1061 on a 64-bit system, it cannot by default see the \q{real}
1062 \cw{C:\\WINDOWS\\SYSTEM32} at all, because the
1063 \W{http://msdn.microsoft.com/en-us/library/aa384187(v=vs.85).aspx}{File
1064 System Redirector} arranges that the running program sees the
1065 appropriate kind of binaries in \cw{SYSTEM32}. Thus, operations in
1066 the PuTTY suite that involve it accessing its own executables, such as
1067 \i{\q{New Session}} and \q{Duplicate Session}, will not work.
1069 \H{faq-secure} Security questions
1071 \S{faq-publicpc}{Question} Is it safe for me to download PuTTY and
1072 use it on a public PC?
1074 It depends on whether you trust that PC. If you don't trust the
1075 public PC, don't use PuTTY on it, and don't use any other software
1076 you plan to type passwords into either. It might be watching your
1077 keystrokes, or it might tamper with the PuTTY binary you download.
1078 There is \e{no} program safe enough that you can run it on an
1079 actively malicious PC and get away with typing passwords into it.
1081 If you do trust the PC, then it's probably OK to use PuTTY on it
1082 (but if you don't trust the network, then the PuTTY download might
1083 be tampered with, so it would be better to carry PuTTY with you on a
1086 \S{faq-cleanup}{Question} What does PuTTY leave on a system? How can
1087 I \i{clean up} after it?
1089 PuTTY will leave some Registry entries, and a random seed file, on
1090 the PC (see \k{faq-settings}). If you are using PuTTY on a public
1091 PC, or somebody else's PC, you might want to clean these up when you
1092 leave. You can do that automatically, by running the command
1093 \c{putty -cleanup}. (Note that this only removes settings for
1094 the currently logged-in user on \i{multi-user systems}.)
1096 If PuTTY was installed from the installer package, it will also
1097 appear in \q{Add/Remove Programs}. Older versions of the uninstaller
1098 do not remove the above-mentioned registry entries and file.
1100 \S{faq-dsa}{Question} How come PuTTY now supports \i{DSA}, when the
1101 website used to say how insecure it was?
1103 DSA has a major weakness \e{if badly implemented}: it relies on a
1104 random number generator to far too great an extent. If the random
1105 number generator produces a number an attacker can predict, the DSA
1106 private key is exposed - meaning that the attacker can log in as you
1107 on all systems that accept that key.
1109 The PuTTY policy changed because the developers were informed of
1110 ways to implement DSA which do not suffer nearly as badly from this
1111 weakness, and indeed which don't need to rely on random numbers at
1112 all. For this reason we now believe PuTTY's DSA implementation is
1113 probably OK. However, if you have the choice, we still recommend you
1116 \S{faq-virtuallock}{Question} Couldn't Pageant use
1117 \cw{VirtualLock()} to stop private keys being written to disk?
1119 Unfortunately not. The \cw{VirtualLock()} function in the Windows
1120 API doesn't do a proper job: it may prevent small pieces of a
1121 process's memory from being paged to disk while the process is
1122 running, but it doesn't stop the process's memory as a whole from
1123 being swapped completely out to disk when the process is long-term
1124 inactive. And Pageant spends most of its time inactive.
1126 \H{faq-admin} Administrative questions
1128 \S{faq-domain}{Question} Would you like me to register you a nicer
1131 No, thank you. Even if you can find one (most of them seem to have
1132 been registered already, by people who didn't ask whether we
1133 actually wanted it before they applied), we're happy with the PuTTY
1134 web site being exactly where it is. It's not hard to find (just type
1135 \q{putty} into \W{http://www.google.com/}{google.com} and we're the
1136 first link returned), and we don't believe the administrative hassle
1137 of moving the site would be worth the benefit.
1139 In addition, if we \e{did} want a custom domain name, we would want
1140 to run it ourselves, so we knew for certain that it would continue
1141 to point where we wanted it, and wouldn't suddenly change or do
1142 strange things. Having it registered for us by a third party who we
1143 don't even know is not the best way to achieve this.
1145 \S{faq-webhosting}{Question} Would you like free web hosting for the
1148 We already have some, thanks.
1150 \S{faq-link}{Question} Would you link to my web site from the PuTTY
1153 Only if the content of your web page is of definite direct interest
1154 to PuTTY users. If your content is unrelated, or only tangentially
1155 related, to PuTTY, then the link would simply be advertising for
1158 One very nice effect of the Google ranking mechanism is that by and
1159 large, the most popular web sites get the highest rankings. This
1160 means that when an ordinary person does a search, the top item in
1161 the search is very likely to be a high-quality site or the site they
1162 actually wanted, rather than the site which paid the most money for
1165 The PuTTY web site is held in high esteem by Google, for precisely
1166 this reason: lots of people have linked to it simply because they
1167 like PuTTY, without us ever having to ask anyone to link to us. We
1168 feel that it would be an abuse of this esteem to use it to boost the
1169 ranking of random advertisers' web sites. If you want your web site
1170 to have a high Google ranking, we'd prefer that you achieve this the
1171 way we did - by being good enough at what you do that people will
1172 link to you simply because they like you.
1174 In particular, we aren't interested in trading links for money (see
1175 above), and we \e{certainly} aren't interested in trading links for
1176 other links (since we have no advertising on our web site, our
1177 Google ranking is not even directly worth anything to us). If we
1178 don't want to link to you for free, then we probably won't want to
1181 If you have software based on PuTTY, or specifically designed to
1182 interoperate with PuTTY, or in some other way of genuine interest to
1183 PuTTY users, then we will probably be happy to add a link to you on
1184 our Links page. And if you're running a particularly valuable mirror
1185 of the PuTTY web site, we might be interested in linking to you from
1188 \S{faq-sourceforge}{Question} Why don't you move PuTTY to
1191 Partly, because we don't want to move the web site location (see
1194 Also, security reasons. PuTTY is a security product, and as such it
1195 is particularly important to guard the code and the web site against
1196 unauthorised modifications which might introduce subtle security
1197 flaws. Therefore, we prefer that the Git repository, web site and
1198 FTP site remain where they are, under the direct control of system
1199 administrators we know and trust personally, rather than being run
1200 by a large organisation full of people we've never met and which is
1201 known to have had breakins in the past.
1203 No offence to SourceForge; I think they do a wonderful job. But
1204 they're not ideal for everyone, and in particular they're not ideal
1207 \S{faq-mailinglist1}{Question} Why can't I subscribe to the
1208 putty-bugs mailing list?
1210 Because you're not a member of the PuTTY core development team. The
1211 putty-bugs mailing list is not a general newsgroup-like discussion
1212 forum; it's a contact address for the core developers, and an
1213 \e{internal} mailing list for us to discuss things among ourselves.
1214 If we opened it up for everybody to subscribe to, it would turn into
1215 something more like a newsgroup and we would be completely
1216 overwhelmed by the volume of traffic. It's hard enough to keep up
1217 with the list as it is.
1219 \S{faq-mailinglist2}{Question} If putty-bugs isn't a
1220 general-subscription mailing list, what is?
1222 There isn't one, that we know of.
1224 If someone else wants to set up a mailing list or other forum for
1225 PuTTY users to help each other with common problems, that would be
1226 fine with us, though the PuTTY team would almost certainly not have the
1227 time to read it. It's probably better to use one of the established
1228 newsgroups for this purpose (see \k{feedback-other-fora}).
1230 \S{faq-donations}{Question} How can I donate to PuTTY development?
1232 Please, \e{please} don't feel you have to. PuTTY is completely free
1233 software, and not shareware. We think it's very important that
1234 \e{everybody} who wants to use PuTTY should be able to, whether they
1235 have any money or not; so the last thing we would want is for a
1236 PuTTY user to feel guilty because they haven't paid us any money. If
1237 you want to keep your money, please do keep it. We wouldn't dream of
1240 Having said all that, if you still really \e{want} to give us money,
1241 we won't argue :-) The easiest way for us to accept donations is if
1242 you send money to \cw{<anakin@pobox.com>} using PayPal
1243 (\W{http://www.paypal.com/}\cw{www.paypal.com}). If you don't like
1244 PayPal, talk to us; we can probably arrange some alternative means.
1246 Small donations (tens of dollars or tens of euros) will probably be
1247 spent on beer or curry, which helps motivate our volunteer team to
1248 continue doing this for the world. Larger donations will be spent on
1249 something that actually helps development, if we can find anything
1250 (perhaps new hardware, or a copy of Windows XP), but if we can't
1251 find anything then we'll just distribute the money among the
1252 developers. If you want to be sure your donation is going towards
1253 something worthwhile, ask us first. If you don't like these terms,
1254 feel perfectly free not to donate. We don't mind.
1256 \S{faq-permission}{Question} Can I have permission to put PuTTY on a
1257 cover disk / distribute it with other software / etc?
1259 Yes. For most things, you need not bother asking us explicitly for
1260 permission; our licence already grants you permission.
1262 See \k{feedback-permission} for more details.
1264 \S{faq-indemnity}{Question} Can you sign an agreement indemnifying
1265 us against security problems in PuTTY?
1269 A vendor of physical security products (e.g. locks) might plausibly
1270 be willing to accept financial liability for a product that failed
1271 to perform as advertised and resulted in damage (e.g. valuables
1272 being stolen). The reason they can afford to do this is because they
1273 sell a \e{lot} of units, and only a small proportion of them will
1274 fail; so they can meet their financial liability out of the income
1275 from all the rest of their sales, and still have enough left over to
1276 make a profit. Financial liability is intrinsically linked to
1277 selling your product for money.
1279 There are two reasons why PuTTY is not analogous to a physical lock
1280 in this context. One is that software products don't exhibit random
1281 variation: \e{if} PuTTY has a security hole (which does happen,
1282 although we do our utmost to prevent it and to respond quickly when
1283 it does), every copy of PuTTY will have the same hole, so it's
1284 likely to affect all the users at the same time. So even if our
1285 users were all paying us to use PuTTY, we wouldn't be able to
1286 \e{simultaneously} pay every affected user compensation in excess of
1287 the amount they had paid us in the first place. It just wouldn't
1290 The second, much more important, reason is that PuTTY users
1291 \e{don't} pay us. The PuTTY team does not have an income; it's a
1292 volunteer effort composed of people spending their spare time to try
1293 to write useful software. We aren't even a company or any kind of
1294 legally recognised organisation. We're just a bunch of people who
1295 happen to do some stuff in our spare time.
1297 Therefore, to ask us to assume financial liability is to ask us to
1298 assume a risk of having to pay it out of our own \e{personal}
1299 pockets: out of the same budget from which we buy food and clothes
1300 and pay our rent. That's more than we're willing to give. We're
1301 already giving a lot of our spare \e{time} to developing software
1302 for free; if we had to pay our own \e{money} to do it as well, we'd
1303 start to wonder why we were bothering.
1305 Free software fundamentally does not work on the basis of financial
1306 guarantees. Your guarantee of the software functioning correctly is
1307 simply that you have the source code and can check it before you use
1308 it. If you want to be sure there aren't any security holes, do a
1309 security audit of the PuTTY code, or hire a security engineer if you
1310 don't have the necessary skills yourself: instead of trying to
1311 ensure you can get compensation in the event of a disaster, try to
1312 ensure there isn't a disaster in the first place.
1314 If you \e{really} want financial security, see if you can find a
1315 security engineer who will take financial responsibility for the
1316 correctness of their review. (This might be less likely to suffer
1317 from the everything-failing-at-once problem mentioned above, because
1318 such an engineer would probably be reviewing a lot of \e{different}
1319 products which would tend to fail independently.) Failing that, see
1320 if you can persuade an insurance company to insure you against
1321 security incidents, and if the insurer demands it as a condition
1322 then get our code reviewed by a security engineer they're happy
1325 \S{faq-permission-form}{Question} Can you sign this form granting us
1326 permission to use/distribute PuTTY?
1328 If your form contains any clause along the lines of \q{the
1329 undersigned represents and warrants}, we're not going to sign it.
1330 This is particularly true if it asks us to warrant that PuTTY is
1331 secure; see \k{faq-indemnity} for more discussion of this. But it
1332 doesn't really matter what we're supposed to be warranting: even if
1333 it's something we already believe is true, such as that we don't
1334 infringe any third-party copyright, we will not sign a document
1335 accepting any legal or financial liability. This is simply because
1336 the PuTTY development project has no income out of which to satisfy
1337 that liability, or pay legal costs, should it become necessary. We
1338 cannot afford to be sued. We are assuring you that \e{we have done
1339 our best}; if that isn't good enough for you, tough.
1341 The existing PuTTY licence document already gives you permission to
1342 use or distribute PuTTY in pretty much any way which does not
1343 involve pretending you wrote it or suing us if it goes wrong. We
1344 think that really ought to be enough for anybody.
1346 See also \k{faq-permission-general} for another reason why we don't
1347 want to do this sort of thing.
1349 \S{faq-permission-future}{Question} Can you write us a formal notice
1350 of permission to use PuTTY?
1352 We could, in principle, but it isn't clear what use it would be. If
1353 you think there's a serious chance of one of the PuTTY copyright
1354 holders suing you (which we don't!), you would presumably want a
1355 signed notice from \e{all} of them; and we couldn't provide that
1356 even if we wanted to, because many of the copyright holders are
1357 people who contributed some code in the past and with whom we
1358 subsequently lost contact. Therefore the best we would be able to do
1359 \e{even in theory} would be to have the core development team sign
1360 the document, which wouldn't guarantee you that some other copyright
1361 holder might not sue.
1363 See also \k{faq-permission-general} for another reason why we don't
1364 want to do this sort of thing.
1366 \S{faq-permission-general}{Question} Can you sign \e{anything} for
1369 Not unless there's an incredibly good reason.
1371 We are generally unwilling to set a precedent that involves us
1372 having to enter into individual agreements with PuTTY users. We
1373 estimate that we have literally \e{millions} of users, and we
1374 absolutely would not have time to go round signing specific
1375 agreements with every one of them. So if you want us to sign
1376 something specific for you, you might usefully stop to consider
1377 whether there's anything special that distinguishes you from 999,999
1378 other users, and therefore any reason we should be willing to sign
1379 something for you without it setting such a precedent.
1381 If your company policy requires you to have an individual agreement
1382 with the supplier of any software you use, then your company policy
1383 is simply not well suited to using popular free software, and we
1384 urge you to consider this as a flaw in your policy.
1386 \S{faq-permission-assurance}{Question} If you won't sign anything,
1387 can you give us some sort of assurance that you won't make PuTTY
1388 closed-source in future?
1392 If what you want is an assurance that some \e{current version} of
1393 PuTTY which you've already downloaded will remain free, then you
1394 already have that assurance: it's called the PuTTY Licence. It
1395 grants you permission to use, distribute and copy the software to
1396 which it applies; once we've granted that permission (which we
1397 have), we can't just revoke it.
1399 On the other hand, if you want an assurance that \e{future} versions
1400 of PuTTY won't be closed-source, that's more difficult. We could in
1401 principle sign a document stating that we would never release a
1402 closed-source PuTTY, but that wouldn't assure you that we \e{would}
1403 keep releasing \e{open}-source PuTTYs: we would still have the
1404 option of ceasing to develop PuTTY at all, which would surely be
1405 even worse for you than making it closed-source! (And we almost
1406 certainly wouldn't \e{want} to sign a document guaranteeing that we
1407 would actually continue to do development work on PuTTY; we
1408 certainly wouldn't sign it for free. Documents like that are called
1409 contracts of employment, and are generally not signed except in
1410 return for a sizeable salary.)
1412 If we \e{were} to stop developing PuTTY, or to decide to make all
1413 future releases closed-source, then you would still be free to copy
1414 the last open release in accordance with the current licence, and in
1415 particular you could start your own fork of the project from that
1416 release. If this happened, I confidently predict that \e{somebody}
1417 would do that, and that some kind of a free PuTTY would continue to
1418 be developed. There's already precedent for that sort of thing
1419 happening in free software. We can't guarantee that somebody
1420 \e{other than you} would do it, of course; you might have to do it
1421 yourself. But we can assure you that there would be nothing
1422 \e{preventing} anyone from continuing free development if we
1425 (Finally, we can also confidently predict that if we made PuTTY
1426 closed-source and someone made an open-source fork, most people
1427 would switch to the latter. Therefore, it would be pretty stupid of
1430 \S{faq-export-cert}{Question} Can you provide us with export control
1431 information / FIPS certification for PuTTY?
1433 Some people have asked us for an Export Control Classification Number
1434 (ECCN) for PuTTY. We don't know whether we have one, and as a team of
1435 free software developers based in the UK we don't have the time,
1436 money, or effort to deal with US bureaucracy to investigate any
1437 further. We believe that PuTTY falls under 5D002 on the US Commerce
1438 Control List, but that shouldn't be taken as definitive. If you need
1439 to know more you should seek professional legal advice. The same
1440 applies to any other country's legal requirements and restrictions.
1442 Similarly, some people have asked us for FIPS certification of the
1443 PuTTY tools. Unless someone else is prepared to do the necessary work
1444 and pay any costs, we can't provide this.
1446 \S{faq-vendor}{Question} As one of our existing software vendors, can
1447 you just fill in this questionnaire for us?
1449 We periodically receive requests like this, from organisations which
1450 have apparently sent out a form letter to everyone listed in their big
1451 spreadsheet of \q{software vendors} requiring them all to answer some
1452 long list of questions about supported OS versions, paid support
1453 arrangements, compliance with assorted local regulations we haven't
1454 heard of, contact phone numbers, and other such administrivia. Many of
1455 the questions are obviously meaningless when applied to PuTTY (we
1456 don't provide any paid support in the first place!), most of the rest
1457 could have been answered with only a very quick look at our website,
1458 and some we are actively unwilling to answer (we are private
1459 individuals, why would we want to give out our home phone numbers to
1460 large corporations?).
1462 We don't make a habit of responding in full to these questionnaires,
1463 because \e{we are not a software vendor}.
1465 A software \e{vendor} is a company to which you are paying lots of
1466 money in return for some software. They know who you are, and they
1467 know you're paying them money; so they have an incentive to fill in
1468 your forms and questionnaires, to research any local regulations you
1469 cite if they don't already know about them, and generally to provide
1470 every scrap of information you might possibly need in the most
1471 convenient manner for you, because they want to keep being paid.
1473 But we are a team of free software developers, and that means your
1474 relationship with us is nothing like that at all. If you once
1475 downloaded our software from our website, that's great and we hope you
1476 found it useful, but it doesn't mean we have the least idea who you
1477 are, or any incentive to do lots of unpaid work to support our
1478 \q{relationship} with you.
1480 It's not that we are unwilling to \e{provide information}. We put as
1481 much of it as we can on our website for your convenience, and if you
1482 actually need to know some fact about PuTTY which you haven't been
1483 able to find on the website (and which is not obviously inapplicable
1484 to free software in the first place) then please do ask us, and we'll
1485 try to answer as best we can. But we put up the website and this FAQ
1486 precisely so that we \e{don't} have to keep answering the same
1487 questions over and over again, so we aren't prepared to fill in
1488 completely generic form-letter questionnaires for people who haven't
1489 done their best to find the answers here first.
1491 If you work for an organisation which you think might be at risk of
1492 making this mistake, we urge you to reorganise your list of software
1493 suppliers so that it clearly distinguishes paid vendors who know about
1494 you from free software developers who don't have any idea who you are.
1495 Then, only send out these mass mailings to the former.
1497 \S{faq-checksums}{Question} The \c{sha1sums} / \c{sha256sums} / etc
1498 files on your download page don't match the binaries.
1500 People report this every so often, and usually the reason turns out to
1501 be that they've matched up the wrong checksums file with the wrong
1504 The PuTTY download page contains more than one version of the
1505 software. There's a \e{latest release} version; there are the
1506 \e{development snapshots}; and when we're in the run-up to making a
1507 release, there are also \e{pre-release} builds of the upcoming new
1508 version. Each one has its own collection of binaries, and its own
1509 collection of checksums files to go with them.
1511 So if you've downloaded the release version of the actual program, you
1512 need the release version of the checksums too, otherwise you will see
1513 a mismatch. Similarly, the development snapshot binaries go with the
1514 development snapshot checksums, and so on. (We've colour-coded the
1515 download page in an effort to reduce this confusion a bit.)
1517 If you have double-checked that, and you still think there's a real
1518 mismatch, then please send us a report carefully quoting everything
1521 \b the exact URL you got your binary from
1523 \b the checksum of the binary after you downloaded
1525 \b the exact URL you got your checksums file from
1527 \b the checksum that file says the binary should have.
1529 \H{faq-misc} Miscellaneous questions
1531 \S{faq-openssh}{Question} Is PuTTY a port of \i{OpenSSH}, or based on
1534 No, it isn't. PuTTY is almost completely composed of code written
1535 from scratch for PuTTY. The only code we share with OpenSSH is the
1536 detector for SSH-1 CRC compensation attacks, written by CORE SDI
1537 S.A; we share no code at all with OpenSSL.
1539 \S{faq-sillyputty}{Question} Where can I buy silly putty?
1541 You're looking at the wrong web site; the only PuTTY we know about
1542 here is the name of a computer program.
1544 If you want the kind of putty you can buy as an executive toy, the
1545 PuTTY team can personally recommend Thinking Putty, which you can
1546 buy from Crazy Aaron's Putty World, at
1547 \W{http://www.puttyworld.com}\cw{www.puttyworld.com}.
1549 \S{faq-meaning}{Question} What does \q{PuTTY} mean?
1551 It's the name of a popular SSH and Telnet client. Any other meaning
1552 is in the eye of the beholder. It's been rumoured that \q{PuTTY}
1553 is the antonym of \q{\cw{getty}}, or that it's the stuff that makes your
1554 Windows useful, or that it's a kind of plutonium Teletype. We
1555 couldn't possibly comment on such allegations.
1557 \S{faq-pronounce}{Question} How do I pronounce \q{PuTTY}?
1559 Exactly like the English word \q{putty}, which we pronounce
1560 /\u02C8{'}p\u028C{V}ti/.