1 \versionid $Id: faq.but,v 1.45 2003/03/04 14:14:17 ben Exp $
5 This FAQ is published on the PuTTY web site, and also provided as an
6 appendix in the manual.
8 \H{faq-support} Features supported in PuTTY
10 In general, if you want to know if PuTTY supports a particular
11 feature, you should look for it on the
12 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/}{PuTTY web site}.
16 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html}{changes
17 page}, and see if you can find the feature on there. If a feature is
18 listed there, it's been implemented. If it's listed as a change made
19 \e{since} the latest version, it should be available in the
20 development snapshots, in which case testing will be very welcome.
23 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/}{Wishlist
24 page}, and see if you can find the feature there. If it's on there,
25 and not in the \q{Recently fixed} section, it probably \e{hasn't} been
28 \S{faq-ssh2}{Question} Does PuTTY support SSH v2?
30 Yes. SSH v2 support has been available in PuTTY since version 0.50.
31 However, currently the \e{default} SSH protocol is v1; to select SSH
32 v2 if your server supports both, go to the SSH panel and change the
33 \e{Preferred SSH protocol version} option. (The factory default will
34 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/ssh2-default.html}{change to v2}
35 in the next full release.)
37 Public key authentication (both RSA and DSA) in SSH v2 is new in
40 \S{faq-ssh2-keyfmt}{Question} Does PuTTY support reading OpenSSH or
41 \cw{ssh.com} SSHv2 private key files?
43 PuTTY doesn't support this natively, but as of 0.53
44 PuTTYgen can convert both OpenSSH and \cw{ssh.com} private key
45 files into PuTTY's format.
47 \S{faq-ssh1}{Question} Does PuTTY support SSH v1?
49 Yes. SSH 1 support has always been available in PuTTY.
51 \S{faq-localecho}{Question} Does PuTTY support local echo?
53 Yes. Version 0.52 has proper support for local echo.
55 In version 0.51 and before, local echo could not be separated from
56 local line editing (where you type a line of text locally, and it is
57 not sent to the server until you press Return, so you have the
58 chance to edit it and correct mistakes \e{before} the server sees
59 it). New in version 0.52, local echo and local line editing are
60 separate options, and by default PuTTY will try to determine
61 automatically whether to enable them or not, based on which protocol
62 you have selected and also based on hints from the server. If you
63 have a problem with PuTTY's default choice, you can force each
64 option to be enabled or disabled as you choose. The controls are in
65 the Terminal panel, in the section marked \q{Line discipline
68 \S{faq-disksettings}{Question} Does PuTTY support storing its
69 settings in a disk file?
71 Not at present, although \k{config-file} in the documentation gives
72 a method of achieving the same effect.
74 \S{faq-fullscreen}{Question} Does PuTTY support full-screen mode,
77 Yes; this is a new feature in version 0.52.
79 \S{faq-password-remember}{Question} Does PuTTY have the ability to
80 remember my password so I don't have to type it every time?
84 Remembering your password is a bad plan for obvious security
85 reasons: anyone who gains access to your machine while you're away
86 from your desk can find out the remembered password, and use it,
87 abuse it or change it.
89 In addition, it's not even \e{possible} for PuTTY to automatically
90 send your password in a Telnet session, because Telnet doesn't give
91 the client software any indication of which part of the login
92 process is the password prompt. PuTTY would have to guess, by
93 looking for words like \q{password} in the session data; and if your
94 login program is written in something other than English, this won't
97 In SSH, remembering your password would be possible in theory, but
98 there doesn't seem to be much point since SSH supports public key
99 authentication, which is more flexible and more secure. See
100 \k{pubkey} in the documentation for a full discussion of public key
103 \S{faq-hostkeys}{Question} Is there an option to turn off the
104 annoying host key prompts?
106 No, there isn't. And there won't be. Even if you write it yourself
107 and send us the patch, we won't accept it.
109 Those annoying host key prompts are the \e{whole point} of SSH.
110 Without them, all the cryptographic technology SSH uses to secure
111 your session is doing nothing more than making an attacker's job
112 slightly harder; instead of sitting between you and the server with
113 a packet sniffer, the attacker must actually subvert a router and
114 start modifying the packets going back and forth. But that's not all
115 that much harder than just sniffing; and without host key checking,
116 it will go completely undetected by client or server.
118 Host key checking is your guarantee that the encryption you put on
119 your data at the client end is the \e{same} encryption taken off the
120 data at the server end; it's your guarantee that it hasn't been
121 removed and replaced somewhere on the way. Host key checking makes
122 the attacker's job \e{astronomically} hard, compared to packet
123 sniffing, and even compared to subverting a router. Instead of
124 applying a little intelligence and keeping an eye on Bugtraq, the
125 attacker must now perform a brute-force attack against at least one
126 military-strength cipher. That insignificant host key prompt really
127 does make \e{that} much difference.
129 If you're having a specific problem with host key checking - perhaps
130 you want an automated batch job to make use of PSCP or Plink, and
131 the interactive host key prompt is hanging the batch process - then
132 the right way to fix it is to add the correct host key to the
133 Registry in advance. That way, you retain the \e{important} feature
134 of host key checking: the right key will be accepted and the wrong
135 ones will not. Adding an option to turn host key checking off
136 completely is the wrong solution and we will not do it.
138 \S{faq-server}{Question} Will you write an SSH server for the PuTTY
139 suite, to go with the client?
141 No. The only reason we might want to would be if we could easily
142 re-use existing code and significantly cut down the effort. We don't
143 believe this is the case; there just isn't enough common ground
144 between an SSH client and server to make it worthwhile.
146 If someone else wants to use bits of PuTTY in the process of writing
147 a Windows SSH server, they'd be perfectly welcome to of course, but
148 I really can't see it being a lot less effort for us to do that than
149 it would be for us to write a server from the ground up. We don't
150 have time, and we don't have motivation. The code is available if
151 anyone else wants to try it.
153 \S{faq-pscp-ascii}{Question} Can PSCP or PSFTP transfer files in
156 Unfortunately not. This is a limitation of the file transfer
157 protocols: the SCP and SFTP protocols have no notion of transferring
158 a file in anything other than binary mode.
160 SFTP is designed to be extensible, so it's possible that an
161 extension might be proposed at some later date that implements ASCII
162 transfer. But the PuTTY team can't do anything about it until that
165 \H{faq-ports} Ports to other operating systems
167 The eventual goal is for PuTTY to be a multi-platform program, able
168 to run on at least Windows, Mac OS and Unix.
170 Porting will become easier once PuTTY has a generalised porting
171 layer, drawing a clear line between platform-dependent and
172 platform-independent code. The general intention was for this
173 porting layer to evolve naturally as part of the process of doing
174 the first port; a Unix port is now under way and the plan seems to
177 \S{faq-ports-general}{Question} What ports of PuTTY exist?
179 Currently, release versions of PuTTY only run on full Win32 systems.
180 This includes Windows 95, 98, and ME, and it includes Windows NT,
181 Windows 2000 and Windows XP. In the development code, partial ports
182 to Unix (see \k{faq-unix}) and the Mac OS (see \k{faq-mac-port}).
185 Currently PuTTY does \e{not} run on Windows CE (see \k{faq-wince}),
186 and it does not quite run on the Win32s environment under Windows
187 3.1 (see \k{faq-win31}).
189 We do not have release-quality ports for any other systems at the
190 present time. If anyone told you we had a Mac port, or an iPaq port,
191 or any other port of PuTTY, they were mistaken. We don't.
193 \S{faq-unix}{Question} Will there be a port to Unix?
195 It's currently being worked on. If you look at the nightly source
196 snapshots, you should find a \c{unix} subdirectory, which should
197 build you a Unix port of Plink, and also \c{pterm} - an
198 \cw{xterm}-type program which supports the same terminal emulation
201 It isn't yet clear whether we will bother combining the terminal
202 emulator and network back end into the same process, to provide a
203 Unix port of the full GUI form of PuTTY. It wouldn't be as useful a
204 thing on Unix as it would be on Windows; its major value would
205 probably be as a pathfinding effort for other ports. If anyone
206 really wants it, we'd be interested to know why :-)
208 \S{faq-wince}{Question} Will there be a port to Windows CE or PocketPC?
210 Probably not in the particularly near future. Despite sharing large
211 parts of the Windows API, in practice WinCE doesn't appear to be
212 significantly easier to port to than a totally different operating
215 However, PuTTY on portable devices would clearly be a useful thing,
216 so in the long term I hope there will be a WinCE port.
218 \S{faq-win31}{Question} Is there a port to Windows 3.1?
220 PuTTY is a 32-bit application from the ground up, so it won't run on
221 Windows 3.1 as a native 16-bit program; and it would be \e{very}
222 hard to port it to do so, because of Windows 3.1's vile memory
223 allocation mechanisms.
225 However, it is possible in theory to compile the existing PuTTY
226 source in such a way that it will run under Win32s (an extension to
227 Windows 3.1 to let you run 32-bit programs). In order to do this
228 you'll need the right kind of C compiler - modern versions of Visual
229 C at least have stopped being backwards compatible to Win32s. Also,
230 the last time we tried this it didn't work very well.
232 If you're interested in running PuTTY under Windows 3.1, help and
233 testing in this area would be very welcome!
235 \S{faq-mac-port}{Question} Will there be a port to the Mac?
237 Eventually. The terminal emulation code has been ported, as has the
238 saved-settings infrastructure, but networking and a configuration GUI
239 still need to be done before the port will be of any use.
241 \S{faq-epoc}{Question} Will there be a port to EPOC?
243 I hope so, but given that ports aren't really progressing very fast
244 even on systems the developers \e{do} already know how to program
245 for, it might be a long time before any of us get round to learning
246 a new system and doing the port for that.
248 \H{faq-embedding} Embedding PuTTY in other programs
250 \S{faq-dll}{Question} Is the SSH or Telnet code available as a DLL?
252 No, it isn't. It would take a reasonable amount of rewriting for
253 this to be possible, and since the PuTTY project itself doesn't
254 believe in DLLs (they make installation more error-prone) none of us
255 has taken the time to do it.
257 Most of the code cleanup work would be a good thing to happen in
258 general, so if anyone feels like helping, we wouldn't say no.
260 \S{faq-vb}{Question} Is the SSH or Telnet code available as a Visual
263 No, it isn't. None of the PuTTY team uses Visual Basic, and none of
264 us has any particular need to make SSH connections from a Visual
265 Basic application. In addition, all the preliminary work to turn it
266 into a DLL would be necessary first; and furthermore, we don't even
267 know how to write VB components.
269 If someone offers to do some of this work for us, we might consider
270 it, but unless that happens I can't see VB integration being
271 anywhere other than the very bottom of our priority list.
273 \S{faq-ipc}{Question} How can I use PuTTY to make an SSH connection
274 from within another program?
276 Probably your best bet is to use Plink, the command-line connection
277 tool. If you can start Plink as a second Windows process, and
278 arrange for your primary process to be able to send data to the
279 Plink process, and receive data from it, through pipes, then you
280 should be able to make SSH connections from your program.
282 This is what CVS for Windows does, for example.
284 \H{faq-details} Details of PuTTY's operation
286 \S{faq-term}{Question} What terminal type does PuTTY use?
288 For most purposes, PuTTY can be considered to be an \cw{xterm}
291 PuTTY also supports some terminal control sequences not supported by
292 the real \cw{xterm}: notably the Linux console sequences that
293 reconfigure the colour palette, and the title bar control sequences
294 used by \cw{DECterm} (which are different from the \cw{xterm} ones;
295 PuTTY supports both).
297 By default, PuTTY announces its terminal type to the server as
298 \c{xterm}. If you have a problem with this, you can reconfigure it
299 to say something else; \c{vt220} might help if you have trouble.
301 \S{faq-settings}{Question} Where does PuTTY store its data?
303 PuTTY stores most of its data (saved sessions, SSH host keys) in the
304 Registry. The precise location is
306 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
308 and within that area, saved sessions are stored under \c{Sessions}
309 while host keys are stored under \c{SshHostKeys}.
311 PuTTY also requires a random number seed file, to improve the
312 unpredictability of randomly chosen data needed as part of the SSH
313 cryptography. This is stored by default in your Windows home
314 directory (\c{%HOMEDRIVE%\\%HOMEPATH%}), or in the actual Windows
315 directory (such as \c{C:\\WINDOWS}) if the home directory doesn't
316 exist, for example if you're using Win95. If you want to change the
317 location of the random number seed file, you can put your chosen
318 pathname in the Registry, at
320 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
322 \H{faq-howto} HOWTO questions
324 \S{faq-startmax}{Question} How can I make PuTTY start up maximised?
326 Create a Windows shortcut to start PuTTY from, and set it as \q{Run
329 \S{faq-startsess}{Question} How can I create a Windows shortcut to
330 start a particular saved session directly?
332 To run a PuTTY session saved under the name \q{\cw{mysession}},
333 create a Windows shortcut that invokes PuTTY with a command line
336 \c \path\name\to\putty.exe -load mysession
338 (Note: prior to 0.53, the syntax was \c{@session}. This is now
339 deprecated and may be removed at some point.)
341 \S{faq-startssh}{Question} How can I start an SSH session straight
342 from the command line?
344 Use the command line \c{putty -ssh host.name}. Alternatively, create
345 a saved session that specifies the SSH protocol, and start the saved
346 session as shown in \k{faq-startsess}.
348 \S{faq-cutpaste}{Question} How do I copy and paste between PuTTY and
349 other Windows applications?
351 Copy and paste works similarly to the X Window System. You use the
352 left mouse button to select text in the PuTTY window. The act of
353 selection \e{automatically} copies the text to the clipboard: there
354 is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
355 pressing Ctrl-C will send a Ctrl-C character to the other end of
356 your connection (just like it does the rest of the time), which may
357 have unpleasant effects. The \e{only} thing you need to do, to copy
358 text to the clipboard, is to select it.
360 To paste the clipboard contents into a PuTTY window, by default you
361 click the right mouse button. If you have a three-button mouse and
362 are used to X applications, you can configure pasting to be done by
363 the middle button instead, but this is not the default because most
364 Windows users don't have a middle button at all.
366 You can also paste by pressing Shift-Ins.
368 \S{faq-tunnels}{Question} How do I use X forwarding and port
369 forwarding? I can't find the Tunnels panel.
371 This is a new feature in version 0.52. You should upgrade.
373 \S{faq-options}{Question} How do I use all PuTTY's features (public
374 keys, proxying, cipher selection, etc.) in PSCP, PSFTP and Plink?
376 Most major features (e.g., public keys, port forwarding) are available
377 through command line options. See the documentation.
379 Not all features are accessible from the command line yet, although
380 we'd like to fix this. In the meantime, you can use most of
381 PuTTY's features if you create a PuTTY saved session, and then use
382 the name of the saved session on the command line in place of a
383 hostname. This works for PSCP, PSFTP and Plink (but don't expect
384 port forwarding in the file transfer applications!).
386 \S{faq-pscp}{Question} How do I use PSCP.EXE? When I double-click it
387 gives me a command prompt window which then closes instantly.
389 PSCP is a command-line application, not a GUI application. If you
390 run it without arguments, it will simply print a help message and
393 To use PSCP properly, run it from a Command Prompt window. See
394 \k{pscp} in the documentation for more details.
396 \S{faq-pscp-spaces}{Question} How do I use PSCP to copy a file whose
399 If PSCP is using the traditional SCP protocol, this is confusing. If
400 you're specifying a file at the local end, you just use one set of
401 quotes as you would normally do:
403 \c pscp "local filename with spaces" user@host:
404 \c pscp user@host:myfile "local filename with spaces"
406 But if the filename you're specifying is on the \e{remote} side, you
407 have to use backslashes and two sets of quotes:
409 \c pscp user@host:"\"remote filename with spaces\"" local_filename
410 \c pscp local_filename user@host:"\"remote filename with spaces\""
412 Worse still, in a remote-to-local copy you have to specify the local
413 file name explicitly, otherwise PSCP will complain that they don't
414 match (unless you specified the \c{-unsafe} option). The following
415 command will give an error message:
417 \c c:\>pscp user@host:"\"oo er\"" .
418 \c warning: remote host tried to write to a file called 'oo er'
419 \c when we requested a file called '"oo er"'.
421 Instead, you need to specify the local file name in full:
423 \c c:\>pscp user@host:"\"oo er\"" "oo er"
425 If PSCP is using the newer SFTP protocol, none of this is a problem,
426 and all filenames with spaces in are specified using a single pair
427 of quotes in the obvious way:
429 \c pscp "local file" user@host:
430 \c pscp user@host:"remote file" .
432 \H{faq-trouble} Troubleshooting
434 \S{faq-incorrect-mac}{Question} Why do I see \q{Incorrect MAC
437 This is due to a bug in old SSH 2 servers distributed by
438 \cw{ssh.com}. Version 2.3.0 and below of their SSH 2 server
439 constructs Message Authentication Codes in the wrong way, and
440 expects the client to construct them in the same wrong way. PuTTY
441 constructs the MACs correctly by default, and hence these old
442 servers will fail to work with it.
444 If you are using PuTTY version 0.52 or better, this should work
445 automatically: PuTTY should detect the buggy servers from their
446 version number announcement, and automatically start to construct
447 its MACs in the same incorrect manner as they do, so it will be able
450 If you are using PuTTY version 0.51 or below, you can enable the
451 workaround by going to the SSH panel and ticking the box labelled
452 \q{Imitate SSH 2 MAC bug}. It's possible that you might have to do
453 this with 0.52 as well, if a buggy server exists that PuTTY doesn't
456 In this context MAC stands for Message Authentication Code. It's a
457 cryptographic term, and it has nothing at all to do with Ethernet
458 MAC (Media Access Control) addresses.
460 \S{faq-pscp-protocol}{Question} Why do I see \q{Fatal: Protocol
461 error: Expected control record} in PSCP?
463 This happens because PSCP was expecting to see data from the server
464 that was part of the PSCP protocol exchange, and instead it saw data
465 that it couldn't make any sense of at all.
467 This almost always happens because the startup scripts in your
468 account on the server machine are generating output. This is
469 impossible for PSCP, or any other SCP client, to work around. You
470 should never use startup files (\c{.bashrc}, \c{.cshrc} and so on)
471 which generate output in non-interactive sessions.
473 This is not actually a PuTTY problem. If PSCP fails in this way,
474 then all other SCP clients are likely to fail in exactly the same
475 way. The problem is at the server end.
477 \S{faq-colours}{Question} I clicked on a colour in the Colours
478 panel, and the colour didn't change in my terminal.
480 That isn't how you're supposed to use the Colours panel.
482 During the course of a session, PuTTY potentially uses \e{all} the
483 colours listed in the Colours panel. It's not a question of using
484 only one of them and you choosing which one; PuTTY will use them
485 \e{all}. The purpose of the Colours panel is to let you adjust the
486 appearance of all the colours. So to change the colour of the
487 cursor, for example, you would select \q{Cursor Colour}, press the
488 \q{Modify} button, and select a new colour from the dialog box that
489 appeared. Similarly, if you want your session to appear in green,
490 you should select \q{Default Foreground} and press \q{Modify}.
491 Clicking on \q{ANSI Green} won't turn your session green; it will
492 only allow you to adjust the \e{shade} of green used when PuTTY is
493 instructed by the server to display green text.
495 \S{faq-winsock2}{Question} Plink on Windows 95 says it can't find
498 Plink requires the extended Windows network library, WinSock version
499 2. This is installed as standard on Windows 98 and above, and on
500 Windows NT, and even on later versions of Windows 95; but early
501 Win95 installations don't have it.
503 In order to use Plink on these systems, you will need to download
505 \W{http://www.microsoft.com/windows95/downloads/contents/wuadmintools/s_wunetworkingtools/w95sockets2/}{WinSock 2 upgrade}:
507 \c http://www.microsoft.com/windows95/downloads/contents/wuadmintools/
508 \c s_wunetworkingtools/w95sockets2/
510 \S{faq-rekey}{Question} My PuTTY sessions close after an hour and
511 tell me \q{Server failed host key check}.
513 This is a bug in all versions of PuTTY up to and including 0.51. SSH
514 v2 servers from \cw{ssh.com} will require the key exchange to be
515 repeated one hour after the start of the connection, and PuTTY will
518 Upgrade to version 0.52 or better and the problem should go away.
520 \S{faq-outofmem}{Question} After trying to establish an SSH 2
521 connection, PuTTY says \q{Out of memory} and dies.
523 If this happens just while the connection is starting up, this often
524 indicates that for some reason the client and server have failed to
525 establish a session encryption key. Somehow, they have performed
526 calculations that should have given each of them the same key, but
527 have ended up with different keys; so data encrypted by one and
528 decrypted by the other looks like random garbage.
530 This causes an \q{out of memory} error because the first encrypted
531 data PuTTY expects to see is the length of an SSH message. Normally
532 this will be something well under 100 bytes. If the decryption has
533 failed, PuTTY will see a completely random length in the region of
534 two \e{gigabytes}, and will try to allocate enough memory to store
535 this non-existent message. This will immediately lead to it thinking
536 it doesn't have enough memory, and panicking.
538 If this happens to you, it is quite likely to still be a PuTTY bug
539 and you should report it (although it might be a bug in your SSH
540 server instead); but it doesn't necessarily mean you've actually run
543 \S{faq-outofmem2}{Question} When attempting a file transfer, either
544 PSCP or PSFTP says \q{Out of memory} and dies.
546 This is almost always caused by your login scripts on the server
547 generating output. PSCP or PSFTP will receive that output when they
548 were expecting to see the start of a file transfer protocol, and
549 they will attempt to interpret the output as file-transfer protocol.
550 This will usually lead to an \q{out of memory} error for much the
551 same reasons as given in \k{faq-outofmem}.
553 This is a setup problem in your account on your server, \e{not} a
554 PSCP/PSFTP bug. Your login scripts should \e{never} generate output
555 during non-interactive sessions; secure file transfer is not the
556 only form of remote access that will break if they do.
558 On Unix, a simple fix is to ensure that all the parts of your login
559 script that might generate output are in \c{.profile} (if you use a
560 Bourne shell derivative) or \c{.login} (if you use a C shell).
561 Putting them in more general files such as \c{.bashrc} or \c{.cshrc}
562 is liable to lead to problems.
564 \S{faq-psftp-slow}{Question} PSFTP transfers files much slower than PSCP.
566 We believe this is because the SFTP and SSH2 protocols are less
567 efficient at bulk data transfer than SCP and SSH1, because every
568 block of data transferred requires an acknowledgment from the far
569 end. It would in theory be possible to queue several blocks of data
570 to get round this speed problem, but as yet we haven't done the
571 coding. If you really want this fixed, feel free to offer to help.
573 \S{faq-bce}{Question} When I run full-colour applications, I see
574 areas of black space where colour ought to be.
576 You almost certainly need to enable the \q{Use background colour to
577 erase screen} setting in the Terminal panel. Note that if you do
578 this in mid-session, it won't take effect until you reset the
579 terminal (see \k{faq-resetterm}).
581 \S{faq-resetterm}{Question} When I change some terminal settings,
584 Some of the terminal options (notably Auto Wrap and
585 background-colour screen erase) actually represent the \e{default}
586 setting, rather than the currently active setting. The server can
587 send sequences that modify these options in mid-session, but when
588 the terminal is reset (by server action, or by you choosing \q{Reset
589 Terminal} from the System menu) the defaults are restored.
591 If you want to change one of these options in the middle of a
592 session, you will find that the change does not immediately take
593 effect. It will only take effect once you reset the terminal.
595 \S{faq-altgr}{Question} I can't type characters that require the
598 In PuTTY version 0.51, the AltGr key was broken. Upgrade to version
601 \S{faq-idleout}{Question} My PuTTY sessions unexpectedly close after
602 they are idle for a while.
604 Some types of firewall, and almost any router doing Network Address
605 Translation (NAT, also known as IP masquerading), will forget about
606 a connection through them if the connection does nothing for too
607 long. This will cause the connection to be rudely cut off when
610 You can try to combat this by telling PuTTY to send \e{keepalives}:
611 packets of data which have no effect on the actual session, but
612 which reassure the router or firewall that the network connection is
613 still active and worth remembering about.
615 Keepalives don't solve everything, unfortunately; although they
616 cause greater robustness against this sort of router, they can also
617 cause a \e{loss} of robustness against network dropouts. See
618 \k{config-keepalive} in the documentation for more discussion of
621 \S{faq-timeout}{Question} PuTTY's network connections time out too
622 quickly when network connectivity is temporarily lost.
624 This is a Windows problem, not a PuTTY problem. The timeout value
625 can't be set on per application or per session basis. To increase
626 the TCP timeout globally, you need to tinker with the Registry.
628 On Windows 95, 98 or ME, the registry key you need to change is
630 \c HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
631 \c MSTCP\MaxDataRetries
633 (it must be of type DWORD in Win95, or String in Win98/ME).
635 On Windows NT or 2000, the registry key is
637 \c HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
638 \c Parameters\TcpMaxDataRetransmissions
640 and it must be of type DWORD.
642 Set the key's value to something like 10. This will cause Windows to
643 try harder to keep connections alive instead of abandoning them.
645 \S{faq-puttyputty}{Question} When I \cw{cat} a binary file, I get
646 `PuTTYPuTTYPuTTY' on my command line.
650 This is designed behaviour; when PuTTY receives the character
651 Control-E from the remote server, it interprets it as a request to
652 identify itself, and so it sends back the string \q{\cw{PuTTY}} as
653 if that string had been entered at the keyboard. Control-E should
654 only be sent by programs that are prepared to deal with the
655 response. Writing a binary file to your terminal is likely to output
656 many Control-E characters, and cause this behaviour. Don't do it.
659 To mitigate the effects, you could configure the answerback string
660 to be empty (see \k{config-answerback}); but writing binary files to
661 your terminal is likely to cause various other unpleasant behaviour,
662 so this is only a small remedy.
664 \S{faq-wintitle}{Question} When I \cw{cat} a binary file, my window
665 title changes to a nonsense string.
669 It is designed behaviour that PuTTY should have the ability to
670 adjust the window title on instructions from the server. Normally
671 the control sequence that does this should only be sent
672 deliberately, by programs that know what they are doing and intend
673 to put meaningful text in the window title. Writing a binary file to
674 your terminal runs the risk of sending the same control sequence by
675 accident, and cause unexpected changes in the window title. Don't do
678 \S{faq-password-fails}{Question} My keyboard stops working once
679 PuTTY displays the password prompt.
681 No, it doesn't. PuTTY just doesn't display the password you type, so
682 that someone looking at your screen can't see what it is.
684 Unlike the Windows login prompts, PuTTY doesn't display the password
685 as a row of asterisks either. This is so that someone looking at
686 your screen can't even tell how \e{long} your password is, which
687 might be valuable information.
689 \S{faq-keyboard}{Question} One or more function keys don't do what I
690 expected in a server-side application.
692 If you've already tried all the relevant options in the PuTTY
693 Keyboard panel, you may need to mail the PuTTY maintainers and ask.
695 It is \e{not} usually helpful just to tell us which application,
696 which server operating system, and which key isn't working; in order
697 to replicate the problem we would need to have a copy of every
698 operating system, and every application, that anyone has ever
701 PuTTY responds to function key presses by sending a sequence of
702 control characters to the server. If a function key isn't doing what
703 you expect, it's likely that the character sequence your application
704 is expecting to receive is not the same as the one PuTTY is sending.
705 Therefore what we really need to know is \e{what} sequence the
706 application is expecting.
708 The simplest way to investigate this is to find some other terminal
709 environment, in which that function key \e{does} work; and then
710 investigate what sequence the function key is sending in that
711 situation. One reasonably easy way to do this on a Unix system is to
712 type the command \c{cat}, and then press the function key. This is
713 likely to produce output of the form \c{^[[11~}. You can also do
714 this in PuTTY, to find out what sequence the function key is
715 producing in that. Then you can mail the PuTTY maintainers and tell
716 us \q{I wanted the F1 key to send \c{^[[11~}, but instead it's
717 sending \c{^[OP}, can this be done?}, or something similar.
719 You should still read the
720 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/feedback.html}{Feedback
721 page} on the PuTTY website (also provided as \k{feedback} in the
722 manual), and follow the guidelines contained in that.
724 \S{faq-openssh-bad-openssl}{Question} Since my SSH server was upgraded
725 to OpenSSH 3.1p1/3.4p1, I can no longer connect with PuTTY.
727 There is a known problem when OpenSSH has been built against an
728 incorrect version of OpenSSL; the quick workaround is to configure
729 PuTTY to use SSH protocol 2 and the Blowfish cipher.
731 For more details and OpenSSH patches, see
732 \W{http://bugzilla.mindrot.org/show_bug.cgi?id=138}{bug 138} in the
735 This is not a PuTTY-specific problem; if you try to connect with
736 another client you'll likely have similar problems. (Although PuTTY's
737 default cipher differs from many other clients.)
739 \e{OpenSSH 3.1p1:} configurations known to be broken (and symptoms):
741 \b SSH 2 with AES cipher (PuTTY says "Assertion failed! Expression:
742 (len & 15) == 0" in sshaes.c, or "Out of memory", or crashes)
744 \b SSH 2 with 3DES (PuTTY says "Incorrect MAC received on packet")
746 \b SSH 1 with Blowfish (PuTTY says "Incorrect CRC received on
751 \e{OpenSSH 3.4p1:} as of 3.4p1, only the problem with SSH 1 and
752 Blowfish remains. Rebuild your server, apply the patch linked to from
753 bug 138 above, or use another cipher (e.g., 3DES) instead.
755 \e{Other versions:} we occasionally get reports of the same symptom
756 and workarounds with older versions of OpenSSH, although it's not
757 clear the underlying cause is the same.
759 \S{faq-ssh2key-ssh1conn}{Question} Why do I see "Couldn't load private
760 key from ..."? Why can PuTTYgen load my key but not PuTTY?
762 It's likely that you've generated an SSH protocol 2 key with PuTTYgen,
763 but you're trying to use it in an SSH 1 connection. SSH1 and SSH2 keys
764 have different formats, and (at least in 0.52) PuTTY's reporting of a
765 key in the wrong format isn't optimal.
767 To connect using SSH 2 to a server that supports both versions, you
768 need to change the configuration from the default (see \k{faq-ssh2}).
770 \S{faq-rh8-utf8}{Question} When I'm connected to a Red Hat Linux 8.0
771 system, some characters don't display properly.
773 A common complaint is that hyphens in man pages show up as a-acute.
775 With release 8.0, Red Hat appear to have made UTF-8 the default
776 character set. There appears to be no way for terminal emulators such
777 as PuTTY to know this (as far as we know, the appropriate escape
778 sequence to switch into UTF-8 mode isn't sent).
780 A fix is to configure sessions to RH8 systems to use UTF-8
781 translation - see \k{config-charset} in the documentation. (Note that
782 if you use \q{Change Settings}, changes may not take place immediately
783 - see \k{faq-resetterm}.)
785 If you really want to change the character set used by the server, the
786 right place is \c{/etc/sysconfig/i18n}, but this shouldn't be
789 \H{faq-secure} Security questions
791 \S{faq-publicpc}{Question} Is it safe for me to download PuTTY and
792 use it on a public PC?
794 It depends on whether you trust that PC. If you don't trust the
795 public PC, don't use PuTTY on it, and don't use any other software
796 you plan to type passwords into either. It might be watching your
797 keystrokes, or it might tamper with the PuTTY binary you download.
798 There is \e{no} program safe enough that you can run it on an
799 actively malicious PC and get away with typing passwords into it.
801 If you do trust the PC, then it's probably OK to use PuTTY on it
802 (but if you don't trust the network, then the PuTTY download might
803 be tampered with, so it would be better to carry PuTTY with you on a
806 \S{faq-cleanup}{Question} What does PuTTY leave on a system? How can
809 PuTTY will leave some Registry entries, and a random seed file, on
810 the PC (see \k{faq-settings}). If you are using PuTTY on a public
811 PC, or somebody else's PC, you might want to clean these up when you
812 leave. You can do that automatically, by running the command
815 \S{faq-dsa}{Question} How come PuTTY now supports DSA, when the
816 website used to say how insecure it was?
818 DSA has a major weakness \e{if badly implemented}: it relies on a
819 random number generator to far too great an extent. If the random
820 number generator produces a number an attacker can predict, the DSA
821 private key is exposed - meaning that the attacker can log in as you
822 on all systems that accept that key.
824 The PuTTY policy changed because the developers were informed of
825 ways to implement DSA which do not suffer nearly as badly from this
826 weakness, and indeed which don't need to rely on random numbers at
827 all. For this reason we now believe PuTTY's DSA implementation is
828 probably OK. However, if you have the choice, we still recommend you
831 \S{faq-virtuallock}{Question} Couldn't Pageant use
832 \cw{VirtualLock()} to stop private keys being written to disk?
834 Unfortunately not. The \cw{VirtualLock()} function in the Windows
835 API doesn't do a proper job: it may prevent small pieces of a
836 process's memory from being paged to disk while the process is
837 running, but it doesn't stop the process's memory as a whole from
838 being swapped completely out to disk when the process is long-term
839 inactive. And Pageant spends most of its time inactive.
841 \H{faq-admin} Administrative questions
843 \S{faq-domain}{Question} Would you like me to register you a nicer
846 No, thank you. Even if you can find one (most of them seem to have
847 been registered already, by people who didn't ask whether we
848 actually wanted it before they applied), we're happy with the PuTTY
849 web site being exactly where it is. It's not hard to find (just type
850 \q{putty} into \W{http://www.google.com/}{google.com} and we're the
851 first link returned), and we don't believe the administrative hassle
852 of moving the site would be worth the benefit.
854 In addition, if we \e{did} want a custom domain name, we would want
855 to run it ourselves, so we knew for certain that it would continue
856 to point where we wanted it, and wouldn't suddenly change or do
857 strange things. Having it registered for us by a third party who we
858 don't even know is not the best way to achieve this.
860 \S{faq-webhosting}{Question} Would you like free web hosting for the
863 We already have some, thanks.
865 \S{faq-sourceforge}{Question} Why don't you move PuTTY to
868 Partly, because we don't want to move the web site location (see
871 Also, security reasons. PuTTY is a security product, and as such it
872 is particularly important to guard the code and the web site against
873 unauthorised modifications which might introduce subtle security
874 flaws. Therefore, we prefer that the CVS repository, web site and
875 FTP site remain where they are, under the direct control of system
876 administrators we know and trust personally, rather than being run
877 by a large organisation full of people we've never met and which is
878 known to have had breakins in the past.
880 No offence to SourceForge; I think they do a wonderful job. But
881 they're not ideal for everyone, and in particular they're not ideal
884 \S{faq-mailinglist1}{Question} Why can't I subscribe to the
885 putty-bugs mailing list?
887 Because you're not a member of the PuTTY core development team. The
888 putty-bugs mailing list is not a general newsgroup-like discussion
889 forum; it's a contact address for the core developers, and an
890 \e{internal} mailing list for us to discuss things among ourselves.
891 If we opened it up for everybody to subscribe to, it would turn into
892 something more like a newsgroup and we would be completely
893 overwhelmed by the volume of traffic. It's hard enough to keep up
894 with the list as it is.
896 \S{faq-mailinglist2}{Question} If putty-bugs isn't a
897 general-subscription mailing list, what is?
899 There isn't one, that we know of.
901 If someone else wants to set up a mailing list or other forum for
902 PuTTY users to help each other with common problems, that would be
903 fine with us, though the PuTTY team would almost certainly not have the
904 time to read it. It's probably better to use the established
905 newsgroup \cw{comp.security.ssh} for this purpose.
907 \S{faq-donations}{Question} How can I donate to PuTTY development?
909 Please, \e{please} don't feel you have to. PuTTY is completely free
910 software, and not shareware. We think it's very important that
911 \e{everybody} who wants to use PuTTY should be able to, whether they
912 have any money or not; so the last thing we would want is for a
913 PuTTY user to feel guilty because they haven't paid us any money. If
914 you want to keep your money, please do keep it. We wouldn't dream of
917 Having said all that, if you still really \e{want} to give us money,
918 we won't argue :-) The easiest way for us to accept donations is if
919 you go to \W{http://www.e-gold.com}\cw{www.e-gold.com}, and deposit
920 your donation in account number 174769. Then send us e-mail to let
921 us know you've done so (otherwise we might not notice for months!).
922 Alternatively, if e-gold isn't convenient for you, you can donate to
923 \cw{<anakin@pobox.com>} using PayPal
924 (\W{http://www.paypal.com/}\cw{www.paypal.com}).
926 Small donations (tens of dollars or tens of euros) will probably be
927 spent on beer or curry, which helps motivate our volunteer team to
928 continue doing this for the world. Larger donations will be spent on
929 something that actually helps development, if we can find anything
930 (perhaps new hardware, or a copy of Windows XP), but if we can't
931 find anything then we'll just distribute the money among the
932 developers. If you want to be sure your donation is going towards
933 something worthwhile, ask us first. If you don't like these terms,
934 feel perfectly free not to donate. We don't mind.
936 \H{faq-misc} Miscellaneous questions
938 \S{faq-openssh}{Question} Is PuTTY a port of OpenSSH, or based on
941 No, it isn't. PuTTY is almost completely composed of code written
942 from scratch for PuTTY. The only code we share with OpenSSH is the
943 detector for SSH1 CRC compensation attacks, written by CORE SDI S.A.
945 \S{faq-sillyputty}{Question} Where can I buy silly putty?
947 You're looking at the wrong web site; the only PuTTY we know about
948 here is the name of a computer program.
950 If you want the kind of putty you can buy as an executive toy, the
951 PuTTY team can personally recommend Thinking Putty, which you can
952 buy from Crazy Aaron's Putty World, at
953 \W{http://www.puttyworld.com}\cw{www.puttyworld.com}.
955 \S{faq-meaning}{Question} What does \q{PuTTY} mean?
957 It's the name of a popular SSH and Telnet client. Any other meaning
958 is in the eye of the beholder. It's been rumoured that \q{PuTTY}
959 is the antonym of \q{\cw{getty}}, or that it's the stuff that makes your
960 Windows useful, or that it's a kind of plutonium Teletype. We
961 couldn't possibly comment on such allegations.
963 \S{faq-pronounce}{Question} How do I pronounce \q{PuTTY}?
965 Exactly like the English word \q{putty}, which we pronounce
966 /\u02C8{'}p\u028C{V}t\u026A{I}/.