1 \versionid $Id: faq.but,v 1.36 2002/10/10 14:39:35 jacob 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.html}{Wishlist
24 page}, and see if you can find the feature there. If it's on there,
25 it probably \e{hasn't} been implemented.
27 \S{faq-ssh2}{Question} Does PuTTY support SSH v2?
29 Yes. SSH v2 support has been available in PuTTY since version 0.50.
30 However, currently the \e{default} SSH protocol is v1; to select SSH
31 v2 if your server supports both, go to the SSH panel and change the
32 \e{Preferred SSH protocol version} option.
34 Public key authentication (both RSA and DSA) in SSH v2 is new in
37 \S{faq-ssh2-keyfmt}{Question} Does PuTTY support reading OpenSSH or
38 \cw{ssh.com} SSHv2 private key files?
40 PuTTY doesn't support this natively, but as of 0.53
41 PuTTYgen can convert both OpenSSH and \cw{ssh.com} private key
42 files into PuTTY's format.
44 \S{faq-ssh1}{Question} Does PuTTY support SSH v1?
46 Yes. SSH 1 support has always been available in PuTTY.
48 \S{faq-localecho}{Question} Does PuTTY support local echo?
50 Yes. Version 0.52 has proper support for local echo.
52 In version 0.51 and before, local echo could not be separated from
53 local line editing (where you type a line of text locally, and it is
54 not sent to the server until you press Return, so you have the
55 chance to edit it and correct mistakes \e{before} the server sees
56 it). New in version 0.52, local echo and local line editing are
57 separate options, and by default PuTTY will try to determine
58 automatically whether to enable them or not, based on which protocol
59 you have selected and also based on hints from the server. If you
60 have a problem with PuTTY's default choice, you can force each
61 option to be enabled or disabled as you choose. The controls are in
62 the Terminal panel, in the section marked \q{Line discipline
65 \S{faq-disksettings}{Question} Does PuTTY support storing its
66 settings in a disk file?
68 Not at present, although \k{config-file} in the documentation gives
69 a method of achieving the same effect.
71 \S{faq-fullscreen}{Question} Does PuTTY support full-screen mode,
74 Yes; this is a new feature in version 0.52.
76 \S{faq-password-remember}{Question} Does PuTTY have the ability to
77 remember my password so I don't have to type it every time?
81 Remembering your password is a bad plan for obvious security
82 reasons: anyone who gains access to your machine while you're away
83 from your desk can find out the remembered password, and use it,
84 abuse it or change it.
86 In addition, it's not even \e{possible} for PuTTY to automatically
87 send your password in a Telnet session, because Telnet doesn't give
88 the client software any indication of which part of the login
89 process is the password prompt. PuTTY would have to guess, by
90 looking for words like \q{password} in the session data; and if your
91 login program is written in something other than English, this won't
94 In SSH, remembering your password would be possible in theory, but
95 there doesn't seem to be much point since SSH supports public key
96 authentication, which is more flexible and more secure. See
97 \k{pubkey} in the documentation for a full discussion of public key
100 \S{faq-hostkeys}{Question} Is there an option to turn off the
101 annoying host key prompts?
103 No, there isn't. And there won't be. Even if you write it yourself
104 and send us the patch, we won't accept it.
106 Those annoying host key prompts are the \e{whole point} of SSH.
107 Without them, all the cryptographic technology SSH uses to secure
108 your session is doing nothing more than making an attacker's job
109 slightly harder; instead of sitting between you and the server with
110 a packet sniffer, the attacker must actually subvert a router and
111 start modifying the packets going back and forth. But that's not all
112 that much harder than just sniffing; and without host key checking,
113 it will go completely undetected by client or server.
115 Host key checking is your guarantee that the encryption you put on
116 your data at the client end is the \e{same} encryption taken off the
117 data at the server end; it's your guarantee that it hasn't been
118 removed and replaced somewhere on the way. Host key checking makes
119 the attacker's job \e{astronomically} hard, compared to packet
120 sniffing, and even compared to subverting a router. Instead of
121 applying a little intelligence and keeping an eye on Bugtraq, the
122 attacker must now perform a brute-force attack against at least one
123 military-strength cipher. That insignificant host key prompt really
124 does make \e{that} much difference.
126 If you're having a specific problem with host key checking - perhaps
127 you want an automated batch job to make use of PSCP or Plink, and
128 the interactive host key prompt is hanging the batch process - then
129 the right way to fix it is to add the correct host key to the
130 Registry in advance. That way, you retain the \e{important} feature
131 of host key checking: the right key will be accepted and the wrong
132 ones will not. Adding an option to turn host key checking off
133 completely is the wrong solution and we will not do it.
135 \S{faq-server}{Question} Will you write an SSH server for the PuTTY
136 suite, to go with the client?
138 No. The only reason we might want to would be if we could easily
139 re-use existing code and significantly cut down the effort. We don't
140 believe this is the case; there just isn't enough common ground
141 between an SSH client and server to make it worthwhile.
143 If someone else wants to use bits of PuTTY in the process of writing
144 a Windows SSH server, they'd be perfectly welcome to of course, but
145 I really can't see it being a lot less effort for us to do that than
146 it would be for us to write a server from the ground up. We don't
147 have time, and we don't have motivation. The code is available if
148 anyone else wants to try it.
150 \S{faq-pscp-ascii}{Question} Can PSCP or PSFTP transfer files in
153 Unfortunately not. This is a limitation of the file transfer
154 protocols: the SCP and SFTP protocols have no notion of transferring
155 a file in anything other than binary mode.
157 SFTP is designed to be extensible, so it's possible that an
158 extension might be proposed at some later date that implements ASCII
159 transfer. But the PuTTY team can't do anything about it until that
162 \H{faq-ports} Ports to other operating systems
164 The eventual goal is for PuTTY to be a multi-platform program, able
165 to run on at least Windows, MacOS and Unix. Whether this will
166 actually ever happen I have no idea, but it is the plan. A Mac port
167 has been started, but is only half-finished and currently not moving
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 is for this porting
173 layer to evolve naturally as part of the process of doing the first
174 port. One particularly nasty part of this will be separating the
175 many configuration options into platform-dependent and
176 platform-independent ones; for example, the options controlling when
177 the Windows System menu appears will be pretty much meaningless
178 under X11 or perhaps other windowing systems, whereas Telnet Passive
179 Mode is universal and shouldn't need to be specified once for each
182 \S{faq-ports-general}{Question} What ports of PuTTY exist?
184 Currently, PuTTY only runs on full Win32 systems. This includes
185 Windows 95, 98, and ME, and it includes Windows NT, Windows 2000 and
188 It does \e{not} include Windows CE (see \k{faq-wince}), and it does
189 not quite include the Win32s environment under Windows 3.1 (see
192 We do not have ports for any other systems at the present time. If
193 anyone told you we had a Unix port, or an iPaq port, or any other
194 port of PuTTY, they were mistaken. We don't.
196 \S{faq-wince}{Question} Will there be a port to Windows CE or PocketPC?
198 Probably not in the particularly near future. Despite sharing large
199 parts of the Windows API, in practice WinCE doesn't appear to be
200 significantly easier to port to than a totally different operating
203 However, PuTTY on portable devices would clearly be a useful thing,
204 so in the long term I hope there will be a WinCE port.
206 \S{faq-win31}{Question} Is there a port to Windows 3.1?
208 PuTTY is a 32-bit application from the ground up, so it won't run on
209 Windows 3.1 as a native 16-bit program; and it would be \e{very}
210 hard to port it to do so, because of Windows 3.1's vile memory
211 allocation mechanisms.
213 However, it is possible in theory to compile the existing PuTTY
214 source in such a way that it will run under Win32s (an extension to
215 Windows 3.1 to let you run 32-bit programs). In order to do this
216 you'll need the right kind of C compiler - modern versions of Visual
217 C at least have stopped being backwards compatible to Win32s. Also,
218 the last time we tried this it didn't work very well.
220 If you're interested in running PuTTY under Windows 3.1, help and
221 testing in this area would be very welcome!
223 \S{faq-mac-port}{Question} Will there be a port to the Mac?
225 A Mac port was started once and is half-finished, but development
226 has been static for some time and the main PuTTY code has moved on,
227 so it's not clear how quickly development would resume even if
228 developer effort were available.
230 \S{faq-unix}{Question} Will there be a port to Unix?
232 I hope so, if only so that I can have an \cw{xterm}-like program
233 that supports exactly the same terminal emulation as PuTTY. If and
234 when we do do a Unix port, it will have a local-terminal back end so
235 it can be used like an \cw{xterm}, rather than only being usable as
238 \S{faq-epoc}{Question} Will there be a port to EPOC?
240 I hope so, but given that ports aren't really progressing very fast
241 even on systems the developers \e{do} already know how to program
242 for, it might be a long time before any of us get round to learning
243 a new system and doing the port for that.
245 \H{faq-embedding} Embedding PuTTY in other programs
247 \S{faq-dll}{Question} Is the SSH or Telnet code available as a DLL?
249 No, it isn't. It would take a reasonable amount of rewriting for
250 this to be possible, and since the PuTTY project itself doesn't
251 believe in DLLs (they make installation more error-prone) none of us
252 has taken the time to do it.
254 Most of the code cleanup work would be a good thing to happen in
255 general, so if anyone feels like helping, we wouldn't say no.
257 \S{faq-vb}{Question} Is the SSH or Telnet code available as a Visual
260 No, it isn't. None of the PuTTY team uses Visual Basic, and none of
261 us has any particular need to make SSH connections from a Visual
262 Basic application. In addition, all the preliminary work to turn it
263 into a DLL would be necessary first; and furthermore, we don't even
264 know how to write VB components.
266 If someone offers to do some of this work for us, we might consider
267 it, but unless that happens I can't see VB integration being
268 anywhere other than the very bottom of our priority list.
270 \S{faq-ipc}{Question} How can I use PuTTY to make an SSH connection
271 from within another program?
273 Probably your best bet is to use Plink, the command-line connection
274 tool. If you can start Plink as a second Windows process, and
275 arrange for your primary process to be able to send data to the
276 Plink process, and receive data from it, through pipes, then you
277 should be able to make SSH connections from your program.
279 This is what CVS for Windows does, for example.
281 \H{faq-details} Details of PuTTY's operation
283 \S{faq-term}{Question} What terminal type does PuTTY use?
285 For most purposes, PuTTY can be considered to be an \cw{xterm}
288 PuTTY also supports some terminal control sequences not supported by
289 the real \cw{xterm}: notably the Linux console sequences that
290 reconfigure the colour palette, and the title bar control sequences
291 used by \cw{DECterm} (which are different from the \cw{xterm} ones;
292 PuTTY supports both).
294 By default, PuTTY announces its terminal type to the server as
295 \c{xterm}. If you have a problem with this, you can reconfigure it
296 to say something else; \c{vt220} might help if you have trouble.
298 \S{faq-settings}{Question} Where does PuTTY store its data?
300 PuTTY stores most of its data (saved sessions, SSH host keys) in the
301 Registry. The precise location is
303 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
305 and within that area, saved sessions are stored under \c{Sessions}
306 while host keys are stored under \c{SshHostKeys}.
308 PuTTY also requires a random number seed file, to improve the
309 unpredictability of randomly chosen data needed as part of the SSH
310 cryptography. This is stored by default in your Windows home
311 directory (\c{%HOMEDRIVE%\\%HOMEPATH%}), or in the actual Windows
312 directory (such as \c{C:\\WINDOWS}) if the home directory doesn't
313 exist, for example if you're using Win95. If you want to change the
314 location of the random number seed file, you can put your chosen
315 pathname in the Registry, at
317 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
319 \H{faq-howto} HOWTO questions
321 \S{faq-startmax}{Question} How can I make PuTTY start up maximised?
323 Create a Windows shortcut to start PuTTY from, and set it as \q{Run
326 \S{faq-startsess}{Question} How can I create a Windows shortcut to
327 start a particular saved session directly?
329 To run a PuTTY session saved under the name \q{\cw{mysession}},
330 create a Windows shortcut that invokes PuTTY with a command line
333 \c \path\name\to\putty.exe -load mysession
335 (Note: prior to 0.53, the syntax was \c{@session}. This is now
336 deprecated and may be removed at some point.)
338 \S{faq-startssh}{Question} How can I start an SSH session straight
339 from the command line?
341 Use the command line \c{putty -ssh host.name}. Alternatively, create
342 a saved session that specifies the SSH protocol, and start the saved
343 session as shown in \k{faq-startsess}.
345 \S{faq-cutpaste}{Question} How do I copy and paste between PuTTY and
346 other Windows applications?
348 Copy and paste works similarly to the X Window System. You use the
349 left mouse button to select text in the PuTTY window. The act of
350 selection \e{automatically} copies the text to the clipboard: there
351 is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
352 pressing Ctrl-C will send a Ctrl-C character to the other end of
353 your connection (just like it does the rest of the time), which may
354 have unpleasant effects. The \e{only} thing you need to do, to copy
355 text to the clipboard, is to select it.
357 To paste the clipboard contents into a PuTTY window, by default you
358 click the right mouse button. If you have a three-button mouse and
359 are used to X applications, you can configure pasting to be done by
360 the middle button instead, but this is not the default because most
361 Windows users don't have a middle button at all.
363 You can also paste by pressing Shift-Ins.
365 \S{faq-tunnels}{Question} How do I use X forwarding and port
366 forwarding? I can't find the Tunnels panel.
368 This is a new feature in version 0.52. You should upgrade.
370 \S{faq-options}{Question} How do I use all PuTTY's features (public
371 keys, proxying, cipher selection, etc.) in PSCP, PSFTP and Plink?
373 Most major features (e.g., public keys, port forwarding) are available
374 through command line options. See the documentation.
376 Not all features are accessible from the command line yet, although
377 we'd like to fix this. In the meantime, you can use most of
378 PuTTY's features if you create a PuTTY saved session, and then use
379 the name of the saved session on the command line in place of a
380 hostname. This works for PSCP, PSFTP and Plink (but don't expect
381 port forwarding in the file transfer applications!).
383 \S{faq-pscp}{Question} How do I use PSCP.EXE? When I double-click it
384 gives me a command prompt window which then closes instantly.
386 PSCP is a command-line application, not a GUI application. If you
387 run it without arguments, it will simply print a help message and
390 To use PSCP properly, run it from a Command Prompt window. See
391 \k{pscp} in the documentation for more details.
393 \S{faq-pscp-spaces}{Question} How do I use PSCP to copy a file whose
396 If PSCP is using the traditional SCP protocol, this is confusing. If
397 you're specifying a file at the local end, you just use one set of
398 quotes as you would normally do:
400 \c pscp "local filename with spaces" user@host:
401 \c pscp user@host:myfile "local filename with spaces"
403 But if the filename you're specifying is on the \e{remote} side, you
404 have to use backslashes and two sets of quotes:
406 \c pscp user@host:"\"remote filename with spaces\"" local_filename
407 \c pscp local_filename user@host:"\"remote filename with spaces\""
409 Worse still, in a remote-to-local copy you have to specify the local
410 file name explicitly, otherwise PSCP will complain that they don't
411 match (unless you specified the \c{-unsafe} option). The following
412 command will give an error message:
414 \c c:\>pscp user@host:"\"oo er\"" .
415 \c warning: remote host tried to write to a file called 'oo er'
416 \c when we requested a file called '"oo er"'.
418 Instead, you need to specify the local file name in full:
420 \c c:\>pscp user@host:"\"oo er\"" "oo er"
422 If PSCP is using the newer SFTP protocol, none of this is a problem,
423 and all filenames with spaces in are specified using a single pair
424 of quotes in the obvious way:
426 \c pscp "local file" user@host:
427 \c pscp user@host:"remote file" .
429 \H{faq-trouble} Troubleshooting
431 \S{faq-incorrect-mac}{Question} Why do I see \q{Incorrect MAC
434 This is due to a bug in old SSH 2 servers distributed by
435 \cw{ssh.com}. Version 2.3.0 and below of their SSH 2 server
436 constructs Message Authentication Codes in the wrong way, and
437 expects the client to construct them in the same wrong way. PuTTY
438 constructs the MACs correctly by default, and hence these old
439 servers will fail to work with it.
441 If you are using PuTTY version 0.52 or better, this should work
442 automatically: PuTTY should detect the buggy servers from their
443 version number announcement, and automatically start to construct
444 its MACs in the same incorrect manner as they do, so it will be able
447 If you are using PuTTY version 0.51 or below, you can enable the
448 workaround by going to the SSH panel and ticking the box labelled
449 \q{Imitate SSH 2 MAC bug}. It's possible that you might have to do
450 this with 0.52 as well, if a buggy server exists that PuTTY doesn't
453 In this context MAC stands for Message Authentication Code. It's a
454 cryptographic term, and it has nothing at all to do with Ethernet
455 MAC (Media Access Control) addresses.
457 \S{faq-pscp-protocol}{Question} Why do I see \q{Fatal: Protocol
458 error: Expected control record} in PSCP?
460 This happens because PSCP was expecting to see data from the server
461 that was part of the PSCP protocol exchange, and instead it saw data
462 that it couldn't make any sense of at all.
464 This almost always happens because the startup scripts in your
465 account on the server machine are generating output. This is
466 impossible for PSCP, or any other SCP client, to work around. You
467 should never use startup files (\c{.bashrc}, \c{.cshrc} and so on)
468 which generate output in non-interactive sessions.
470 This is not actually a PuTTY problem. If PSCP fails in this way,
471 then all other SCP clients are likely to fail in exactly the same
472 way. The problem is at the server end.
474 \S{faq-colours}{Question} I clicked on a colour in the Colours
475 panel, and the colour didn't change in my terminal.
477 That isn't how you're supposed to use the Colours panel.
479 During the course of a session, PuTTY potentially uses \e{all} the
480 colours listed in the Colours panel. It's not a question of using
481 only one of them and you choosing which one; PuTTY will use them
482 \e{all}. The purpose of the Colours panel is to let you adjust the
483 appearance of all the colours. So to change the colour of the
484 cursor, for example, you would select \q{Cursor Colour}, press the
485 \q{Modify} button, and select a new colour from the dialog box that
486 appeared. Similarly, if you want your session to appear in green,
487 you should select \q{Default Foreground} and press \q{Modify}.
488 Clicking on \q{ANSI Green} won't turn your session green; it will
489 only allow you to adjust the \e{shade} of green used when PuTTY is
490 instructed by the server to display green text.
492 \S{faq-winsock2}{Question} Plink on Windows 95 says it can't find
495 Plink requires the extended Windows network library, WinSock version
496 2. This is installed as standard on Windows 98 and above, and on
497 Windows NT, and even on later versions of Windows 95; but early
498 Win95 installations don't have it.
500 In order to use Plink on these systems, you will need to download
502 \W{http://www.microsoft.com/windows95/downloads/contents/wuadmintools/s_wunetworkingtools/w95sockets2/}{WinSock 2 upgrade}:
504 \c http://www.microsoft.com/windows95/downloads/contents/wuadmintools/
505 \c s_wunetworkingtools/w95sockets2/
507 \S{faq-rekey}{Question} My PuTTY sessions close after an hour and
508 tell me \q{Server failed host key check}.
510 This is a bug in all versions of PuTTY up to and including 0.51. SSH
511 v2 servers from \cw{ssh.com} will require the key exchange to be
512 repeated one hour after the start of the connection, and PuTTY will
515 Upgrade to version 0.52 or better and the problem should go away.
517 \S{faq-outofmem}{Question} After trying to establish an SSH 2
518 connection, PuTTY says \q{Out of memory} and dies.
520 If this happens just while the connection is starting up, this often
521 indicates that for some reason the client and server have failed to
522 establish a session encryption key. Somehow, they have performed
523 calculations that should have given each of them the same key, but
524 have ended up with different keys; so data encrypted by one and
525 decrypted by the other looks like random garbage.
527 This causes an \q{out of memory} error because the first encrypted
528 data PuTTY expects to see is the length of an SSH message. Normally
529 this will be something well under 100 bytes. If the decryption has
530 failed, PuTTY will see a completely random length in the region of
531 two \e{gigabytes}, and will try to allocate enough memory to store
532 this non-existent message. This will immediately lead to it thinking
533 it doesn't have enough memory, and panicking.
535 If this happens to you, it is quite likely to still be a PuTTY bug
536 and you should report it (although it might be a bug in your SSH
537 server instead); but it doesn't necessarily mean you've actually run
540 \S{faq-outofmem2}{Question} When attempting a file transfer, either
541 PSCP or PSFTP says \q{Out of memory} and dies.
543 This is almost always caused by your login scripts on the server
544 generating output. PSCP or PSFTP will receive that output when they
545 were expecting to see the start of a file transfer protocol, and
546 they will attempt to interpret the output as file-transfer protocol.
547 This will usually lead to an \q{out of memory} error for much the
548 same reasons as given in \k{faq-outofmem}.
550 This is a setup problem in your account on your server, \e{not} a
551 PSCP/PSFTP bug. Your login scripts should \e{never} generate output
552 during non-interactive sessions; secure file transfer is not the
553 only form of remote access that will break if they do.
555 On Unix, a simple fix is to ensure that all the parts of your login
556 script that might generate output are in \c{.profile} (if you use a
557 Bourne shell derivative) or \c{.login} (if you use a C shell).
558 Putting them in more general files such as \c{.bashrc} or \c{.cshrc}
559 is liable to lead to problems.
561 \S{faq-psftp-slow} PSFTP transfers files much slower than PSCP.
563 We believe this is because the SFTP and SSH2 protocols are less
564 efficient at bulk data transfer than SCP and SSH1, because every
565 block of data transferred requires an acknowledgment from the far
566 end. It would in theory be possible to queue several blocks of data
567 to get round this speed problem, but as yet we haven't done the
568 coding. If you really want this fixed, feel free to offer to help.
570 \S{faq-bce}{Question} When I run full-colour applications, I see
571 areas of black space where colour ought to be.
573 You almost certainly need to enable the \q{Use background colour to
574 erase screen} setting in the Terminal panel. Note that if you do
575 this in mid-session, it won't take effect until you reset the
576 terminal (see \k{faq-resetterm}).
578 \S{faq-resetterm}{Question} When I change some terminal settings,
581 Some of the terminal options (notably Auto Wrap and
582 background-colour screen erase) actually represent the \e{default}
583 setting, rather than the currently active setting. The server can
584 send sequences that modify these options in mid-session, but when
585 the terminal is reset (by server action, or by you choosing \q{Reset
586 Terminal} from the System menu) the defaults are restored.
588 If you want to change one of these options in the middle of a
589 session, you will find that the change does not immediately take
590 effect. It will only take effect once you reset the terminal.
592 \S{faq-altgr}{Question} I can't type characters that require the
595 In PuTTY version 0.51, the AltGr key was broken. Upgrade to version
598 \S{faq-idleout}{Question} My PuTTY sessions unexpectedly close after
599 they are idle for a while.
601 Some types of firewall, and almost any router doing Network Address
602 Translation (NAT, also known as IP masquerading), will forget about
603 a connection through them if the connection does nothing for too
604 long. This will cause the connection to be rudely cut off when
607 You can try to combat this by telling PuTTY to send \e{keepalives}:
608 packets of data which have no effect on the actual session, but
609 which reassure the router or firewall that the network connection is
610 still active and worth remembering about.
612 Keepalives don't solve everything, unfortunately; although they
613 cause greater robustness against this sort of router, they can also
614 cause a \e{loss} of robustness against network dropouts. See
615 \k{config-keepalive} in the documentation for more discussion of
618 \S{faq-timeout}{Question} PuTTY's network connections time out too
619 quickly when network connectivity is temporarily lost.
621 This is a Windows problem, not a PuTTY problem. The timeout value
622 can't be set on per application or per session basis. To increase
623 the TCP timeout globally, you need to tinker with the Registry.
625 On Windows 95, 98 or ME, the registry key you need to change is
627 \c HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
628 \c MSTCP\MaxDataRetries
630 (it must be of type DWORD in Win95, or String in Win98/ME).
632 On Windows NT or 2000, the registry key is
634 \c HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
635 \c Parameters\TcpMaxDataRetransmissions
637 and it must be of type DWORD.
639 Set the key's value to something like 10. This will cause Windows to
640 try harder to keep connections alive instead of abandoning them.
642 \S{faq-puttyputty}{Question} When I \cw{cat} a binary file, I get
643 `PuTTYPuTTYPuTTY' on my command line.
647 This is designed behaviour; when PuTTY receives the character
648 Control-E from the remote server, it interprets it as a request to
649 identify itself, and so it sends back the string \q{\cw{PuTTY}} as
650 if that string had been entered at the keyboard. Control-E should
651 only be sent by programs that are prepared to deal with the
652 response. Writing a binary file to your terminal is likely to output
653 many Control-E characters, and cause this behaviour. Don't do it.
656 To mitigate the effects, you could configure the answerback string
657 to be empty (see \k{config-answerback}); but writing binary files to
658 your terminal is likely to cause various other unpleasant behaviour,
659 so this is only a small remedy.
661 \S{faq-wintitle}{Question} When I \cw{cat} a binary file, my window
662 title changes to a nonsense string.
666 It is designed behaviour that PuTTY should have the ability to
667 adjust the window title on instructions from the server. Normally
668 the control sequence that does this should only be sent
669 deliberately, by programs that know what they are doing and intend
670 to put meaningful text in the window title. Writing a binary file to
671 your terminal runs the risk of sending the same control sequence by
672 accident, and cause unexpected changes in the window title. Don't do
675 \S{faq-password-fails}{Question} My keyboard stops working once
676 PuTTY displays the password prompt.
678 No, it doesn't. PuTTY just doesn't display the password you type, so
679 that someone looking at your screen can't see what it is.
681 Unlike the Windows login prompts, PuTTY doesn't display the password
682 as a row of asterisks either. This is so that someone looking at
683 your screen can't even tell how \e{long} your password is, which
684 might be valuable information.
686 \S{faq-keyboard}{Question} One or more function keys don't do what I
687 expected in a server-side application.
689 If you've already tried all the relevant options in the PuTTY
690 Keyboard panel, you may need to mail the PuTTY maintainers and ask.
692 It is \e{not} usually helpful just to tell us which application,
693 which server operating system, and which key isn't working; in order
694 to replicate the problem we would need to have a copy of every
695 operating system, and every application, that anyone has ever
698 PuTTY responds to function key presses by sending a sequence of
699 control characters to the server. If a function key isn't doing what
700 you expect, it's likely that the character sequence your application
701 is expecting to receive is not the same as the one PuTTY is sending.
702 Therefore what we really need to know is \e{what} sequence the
703 application is expecting.
705 The simplest way to investigate this is to find some other terminal
706 environment, in which that function key \e{does} work; and then
707 investigate what sequence the function key is sending in that
708 situation. One reasonably easy way to do this on a Unix system is to
709 type the command \c{cat}, and then press the function key. This is
710 likely to produce output of the form \c{^[[11~}. You can also do
711 this in PuTTY, to find out what sequence the function key is
712 producing in that. Then you can mail the PuTTY maintainers and tell
713 us \q{I wanted the F1 key to send \c{^[[11~}, but instead it's
714 sending \c{^[OP}, can this be done?}, or something similar.
716 You should still read the
717 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/feedback.html}{Feedback
718 page} on the PuTTY website (also provided as \k{feedback} in the
719 manual), and follow the guidelines contained in that.
721 \S{faq-openssh-bad-openssl}{Question} Since my SSH server was upgraded
722 to OpenSSH 3.1p1/3.4p1, I can no longer connect with PuTTY.
724 There is a known problem when OpenSSH has been built against an
725 incorrect version of OpenSSL; the quick workaround is to configure
726 PuTTY to use SSH protocol 2 and the Blowfish cipher.
728 For more details and OpenSSH patches, see
729 \W{http://bugzilla.mindrot.org/show_bug.cgi?id=138}{bug 138} in the
732 This is not a PuTTY-specific problem; if you try to connect with
733 another client you'll likely have similar problems. (Although PuTTY's
734 default cipher differs from many other clients.)
736 \e{OpenSSH 3.1p1:} configurations known to be broken (and symptoms):
738 \b SSH 2 with AES cipher (PuTTY says "Assertion failed! Expression:
739 (len & 15) == 0" in sshaes.c, or "Out of memory", or crashes)
741 \b SSH 2 with 3DES (PuTTY says "Incorrect MAC received on packet")
743 \b SSH 1 with Blowfish (PuTTY says "Incorrect CRC received on
748 \e{OpenSSH 3.4p1:} as of 3.4p1, only the problem with SSH 1 and
749 Blowfish remains. Rebuild your server, apply the patch linked to from
750 bug 138 above, or use another cipher (e.g., 3DES) instead.
752 \e{Other versions:} we occasionally get reports of the same symptom
753 and workarounds with older versions of OpenSSH, although it's not
754 clear the underlying cause is the same.
756 \S{faq-ssh2key-ssh1conn}{Question} Why do I see "Couldn't load private
757 key from ..."? Why can PuTTYgen load my key but not PuTTY?
759 It's likely that you've generated an SSH protocol 2 key with PuTTYgen,
760 but you're trying to use it in an SSH 1 connection. SSH1 and SSH2 keys
761 have different formats, and (at least in 0.52) PuTTY's reporting of a
762 key in the wrong format isn't optimal.
764 To connect using SSH 2 to a server that supports both versions, you
765 need to change the configuration from the default (see \k{faq-ssh2}).
767 \H{faq-secure} Security questions
769 \S{faq-publicpc}{Question} Is it safe for me to download PuTTY and
770 use it on a public PC?
772 It depends on whether you trust that PC. If you don't trust the
773 public PC, don't use PuTTY on it, and don't use any other software
774 you plan to type passwords into either. It might be watching your
775 keystrokes, or it might tamper with the PuTTY binary you download.
776 There is \e{no} program safe enough that you can run it on an
777 actively malicious PC and get away with typing passwords into it.
779 If you do trust the PC, then it's probably OK to use PuTTY on it
780 (but if you don't trust the network, then the PuTTY download might
781 be tampered with, so it would be better to carry PuTTY with you on a
784 \S{faq-cleanup}{Question} What does PuTTY leave on a system? How can
787 PuTTY will leave some Registry entries, and a random seed file, on
788 the PC (see \k{faq-settings}). If you are using PuTTY on a public
789 PC, or somebody else's PC, you might want to clean these up when you
790 leave. You can do that automatically, by running the command
793 \S{faq-dsa}{Question} How come PuTTY now supports DSA, when the
794 website used to say how insecure it was?
796 DSA has a major weakness \e{if badly implemented}: it relies on a
797 random number generator to far too great an extent. If the random
798 number generator produces a number an attacker can predict, the DSA
799 private key is exposed - meaning that the attacker can log in as you
800 on all systems that accept that key.
802 The PuTTY policy changed because the developers were informed of
803 ways to implement DSA which do not suffer nearly as badly from this
804 weakness, and indeed which don't need to rely on random numbers at
805 all. For this reason we now believe PuTTY's DSA implementation is
806 probably OK. However, if you have the choice, we still recommend you
809 \S{faq-virtuallock}{Question} Couldn't Pageant use
810 \cw{VirtualLock()} to stop private keys being written to disk?
812 Unfortunately not. The \cw{VirtualLock()} function in the Windows
813 API doesn't do a proper job: it may prevent small pieces of a
814 process's memory from being paged to disk while the process is
815 running, but it doesn't stop the process's memory as a whole from
816 being swapped completely out to disk when the process is long-term
817 inactive. And Pageant spends most of its time inactive.
819 \H{faq-admin} Administrative questions
821 \S{faq-domain}{Question} Would you like me to register you a nicer
824 No, thank you. Even if you can find one (most of them seem to have
825 been registered already, by people who didn't ask whether we
826 actually wanted it before they applied), we're happy with the PuTTY
827 web site being exactly where it is. It's not hard to find (just type
828 \q{putty} into \W{http://www.google.com/}{google.com} and we're the
829 first link returned), and we don't believe the administrative hassle
830 of moving the site would be worth the benefit.
832 In addition, if we \e{did} want a custom domain name, we would want
833 to run it ourselves, so we knew for certain that it would continue
834 to point where we wanted it, and wouldn't suddenly change or do
835 strange things. Having it registered for us by a third party who we
836 don't even know is not the best way to achieve this.
838 \S{faq-webhosting}{Question} Would you like free web hosting for the
841 We already have some, thanks.
843 \S{faq-sourceforge}{Question} Why don't you move PuTTY to
846 Partly, because we don't want to move the web site location (see
849 Also, security reasons. PuTTY is a security product, and as such it
850 is particularly important to guard the code and the web site against
851 unauthorised modifications which might introduce subtle security
852 flaws. Therefore, we prefer that the CVS repository, web site and
853 FTP site remain where they are, under the direct control of system
854 administrators we know and trust personally, rather than being run
855 by a large organisation full of people we've never met and which is
856 known to have had breakins in the past.
858 No offence to SourceForge; I think they do a wonderful job. But
859 they're not ideal for everyone, and in particular they're not ideal
862 \S{faq-mailinglist1}{Question} Why can't I subscribe to the
863 putty-bugs mailing list?
865 Because you're not a member of the PuTTY core development team. The
866 putty-bugs mailing list is not a general newsgroup-like discussion
867 forum; it's a contact address for the core developers, and an
868 \e{internal} mailing list for us to discuss things among ourselves.
869 If we opened it up for everybody to subscribe to, it would turn into
870 something more like a newsgroup and we would be completely
871 overwhelmed by the volume of traffic. It's hard enough to keep up
872 with the list as it is.
874 \S{faq-mailinglist2}{Question} If putty-bugs isn't a
875 general-subscription mailing list, what is?
877 There isn't one, that we know of.
879 If someone else wants to set up a mailing list for PuTTY users to
880 help each other with common problems, that would be fine with us;
881 but the PuTTY team would almost certainly not have the time to read
882 it, so any questions the list couldn't answer would have to be
883 forwarded on to us by the questioner. In any case, it's probably
884 better to use the established newsgroup \cw{comp.security.ssh} for
887 \S{faq-donations}{Question} How can I donate to PuTTY development?
889 Please, \e{please} don't feel you have to. PuTTY is completely free
890 software, and not shareware. We think it's very important that
891 \e{everybody} who wants to use PuTTY should be able to, whether they
892 have any money or not; so the last thing we would want is for a
893 PuTTY user to feel guilty because they haven't paid us any money. If
894 you want to keep your money, please do keep it. We wouldn't dream of
897 Having said all that, if you still really \e{want} to give us money,
898 we won't argue :-) The easiest way for us to accept donations is if
899 you go to \W{http://www.e-gold.com}\cw{www.e-gold.com}, and deposit
900 your donation in account number 174769. Then send us e-mail to let
901 us know you've done so (otherwise we might not notice for months!).
902 Alternatively, if e-gold isn't convenient for you, you can donate to
903 \cw{<anakin@pobox.com>} using PayPal
904 (\W{http://www.paypal.com/}\cw{www.paypal.com}).
906 Small donations (tens of dollars or tens of euros) will probably be
907 spent on beer or curry, which helps motivate our volunteer team to
908 continue doing this for the world. Larger donations will be spent on
909 something that actually helps development, if we can find anything
910 (perhaps new hardware, or a copy of Windows XP), but if we can't
911 find anything then we'll just distribute the money among the
912 developers. If you want to be sure your donation is going towards
913 something worthwhile, ask us first. If you don't like these terms,
914 feel perfectly free not to donate. We don't mind.
916 \H{faq-misc} Miscellaneous questions
918 \S{faq-openssh}{Question} Is PuTTY a port of OpenSSH, or based on
921 No, it isn't. PuTTY is almost completely composed of code written
922 from scratch for PuTTY. The only code we share with OpenSSH is the
923 detector for SSH1 CRC compensation attacks, written by CORE SDI S.A.
925 \S{faq-sillyputty}{Question} Where can I buy silly putty?
927 You're looking at the wrong web site; the only PuTTY we know about
928 here is the name of a computer program.
930 If you want the kind of putty you can buy as an executive toy, the
931 PuTTY team can personally recommend Thinking Putty, which you can
932 buy from Crazy Aaron's Putty World, at
933 \W{http://www.puttyworld.com}\cw{www.puttyworld.com}.
935 \S{faq-pronounce}{Question} How do I pronounce PuTTY?
937 Exactly like the normal word \q{putty}. Just like the stuff you put
938 on window frames. (One of the reasons it's called PuTTY is because
939 it makes Windows usable. :-)