1 \versionid $Id: faq.but,v 1.51 2003/11/19 19:09:07 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/}{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.
32 Public key authentication (both RSA and DSA) in SSH v2 is new in
35 \S{faq-ssh2-keyfmt}{Question} Does PuTTY support reading OpenSSH or
36 \cw{ssh.com} SSHv2 private key files?
38 PuTTY doesn't support this natively, but as of 0.53
39 PuTTYgen can convert both OpenSSH and \cw{ssh.com} private key
40 files into PuTTY's format.
42 \S{faq-ssh1}{Question} Does PuTTY support SSH v1?
44 Yes. SSH 1 support has always been available in PuTTY.
46 \S{faq-localecho}{Question} Does PuTTY support local echo?
48 Yes. Version 0.52 has proper support for local echo.
50 In version 0.51 and before, local echo could not be separated from
51 local line editing (where you type a line of text locally, and it is
52 not sent to the server until you press Return, so you have the
53 chance to edit it and correct mistakes \e{before} the server sees
54 it). New in version 0.52, local echo and local line editing are
55 separate options, and by default PuTTY will try to determine
56 automatically whether to enable them or not, based on which protocol
57 you have selected and also based on hints from the server. If you
58 have a problem with PuTTY's default choice, you can force each
59 option to be enabled or disabled as you choose. The controls are in
60 the Terminal panel, in the section marked \q{Line discipline
63 \S{faq-disksettings}{Question} Does PuTTY support storing its
64 settings in a disk file?
66 Not at present, although \k{config-file} in the documentation gives
67 a method of achieving the same effect.
69 \S{faq-fullscreen}{Question} Does PuTTY support full-screen mode,
72 Yes; this is a new feature in version 0.52.
74 \S{faq-password-remember}{Question} Does PuTTY have the ability to
75 remember my password so I don't have to type it every time?
79 Remembering your password is a bad plan for obvious security
80 reasons: anyone who gains access to your machine while you're away
81 from your desk can find out the remembered password, and use it,
82 abuse it or change it.
84 In addition, it's not even \e{possible} for PuTTY to automatically
85 send your password in a Telnet session, because Telnet doesn't give
86 the client software any indication of which part of the login
87 process is the password prompt. PuTTY would have to guess, by
88 looking for words like \q{password} in the session data; and if your
89 login program is written in something other than English, this won't
92 In SSH, remembering your password would be possible in theory, but
93 there doesn't seem to be much point since SSH supports public key
94 authentication, which is more flexible and more secure. See
95 \k{pubkey} in the documentation for a full discussion of public key
98 \S{faq-hostkeys}{Question} Is there an option to turn off the
99 annoying host key prompts?
101 No, there isn't. And there won't be. Even if you write it yourself
102 and send us the patch, we won't accept it.
104 Those annoying host key prompts are the \e{whole point} of SSH.
105 Without them, all the cryptographic technology SSH uses to secure
106 your session is doing nothing more than making an attacker's job
107 slightly harder; instead of sitting between you and the server with
108 a packet sniffer, the attacker must actually subvert a router and
109 start modifying the packets going back and forth. But that's not all
110 that much harder than just sniffing; and without host key checking,
111 it will go completely undetected by client or server.
113 Host key checking is your guarantee that the encryption you put on
114 your data at the client end is the \e{same} encryption taken off the
115 data at the server end; it's your guarantee that it hasn't been
116 removed and replaced somewhere on the way. Host key checking makes
117 the attacker's job \e{astronomically} hard, compared to packet
118 sniffing, and even compared to subverting a router. Instead of
119 applying a little intelligence and keeping an eye on Bugtraq, the
120 attacker must now perform a brute-force attack against at least one
121 military-strength cipher. That insignificant host key prompt really
122 does make \e{that} much difference.
124 If you're having a specific problem with host key checking - perhaps
125 you want an automated batch job to make use of PSCP or Plink, and
126 the interactive host key prompt is hanging the batch process - then
127 the right way to fix it is to add the correct host key to the
128 Registry in advance. That way, you retain the \e{important} feature
129 of host key checking: the right key will be accepted and the wrong
130 ones will not. Adding an option to turn host key checking off
131 completely is the wrong solution and we will not do it.
133 \S{faq-server}{Question} Will you write an SSH server for the PuTTY
134 suite, to go with the client?
136 No. The only reason we might want to would be if we could easily
137 re-use existing code and significantly cut down the effort. We don't
138 believe this is the case; there just isn't enough common ground
139 between an SSH client and server to make it worthwhile.
141 If someone else wants to use bits of PuTTY in the process of writing
142 a Windows SSH server, they'd be perfectly welcome to of course, but
143 I really can't see it being a lot less effort for us to do that than
144 it would be for us to write a server from the ground up. We don't
145 have time, and we don't have motivation. The code is available if
146 anyone else wants to try it.
148 \S{faq-pscp-ascii}{Question} Can PSCP or PSFTP transfer files in
153 Until recently, this was a limitation of the file transfer protocols:
154 the SCP and SFTP protocols had no notion of transferring a file in
155 anything other than binary mode. (This is still true of SCP.)
157 The current draft protocol spec of SFTP proposes a means of
158 implementing ASCII transfer. At some point PSCP/PSFTP may implement
161 \H{faq-ports} Ports to other operating systems
163 The eventual goal is for PuTTY to be a multi-platform program, able
164 to run on at least Windows, Mac OS and Unix.
166 Porting will become easier once PuTTY has a generalised porting
167 layer, drawing a clear line between platform-dependent and
168 platform-independent code. The general intention was for this
169 porting layer to evolve naturally as part of the process of doing
170 the first port; a Unix port is now under way and the plan seems to
173 \S{faq-ports-general}{Question} What ports of PuTTY exist?
175 Currently, release versions of PuTTY only run on full Win32 systems.
176 This includes Windows 95, 98, and ME, and it includes Windows NT,
177 Windows 2000 and Windows XP. In the development code, partial ports
178 to Unix (see \k{faq-unix}) and the Mac OS (see \k{faq-mac-port}).
181 Currently PuTTY does \e{not} run on Windows CE (see \k{faq-wince}),
182 and it does not quite run on the Win32s environment under Windows
183 3.1 (see \k{faq-win31}).
185 We do not have release-quality ports for any other systems at the
186 present time. If anyone told you we had a Mac port, or an iPaq port,
187 or any other port of PuTTY, they were mistaken. We don't.
189 \S{faq-unix}{Question} Will there be a port to Unix?
191 It's currently being worked on. If you look at the nightly source
192 snapshots, you should find a \c{unix} subdirectory, which should
193 build you Unix ports of Plink, PuTTY itself, PSCP, PSFTP, and
194 also \c{pterm} - an \cw{xterm}-type program which supports the
195 same terminal emulation as PuTTY. We do not yet have Unix ports of
198 \S{faq-wince}{Question} Will there be a port to Windows CE or PocketPC?
200 It's currently being worked on, but it's only in its early stages yet,
201 and certainly isn't yet useful. PuTTY on portable devices would
202 clearly be a useful thing, so in the long term I hope it can be
203 brought up to release quality.
205 \S{faq-win31}{Question} Is there a port to Windows 3.1?
207 PuTTY is a 32-bit application from the ground up, so it won't run on
208 Windows 3.1 as a native 16-bit program; and it would be \e{very}
209 hard to port it to do so, because of Windows 3.1's vile memory
210 allocation mechanisms.
212 However, it is possible in theory to compile the existing PuTTY
213 source in such a way that it will run under Win32s (an extension to
214 Windows 3.1 to let you run 32-bit programs). In order to do this
215 you'll need the right kind of C compiler - modern versions of Visual
216 C at least have stopped being backwards compatible to Win32s. Also,
217 the last time we tried this it didn't work very well.
219 If you're interested in running PuTTY under Windows 3.1, help and
220 testing in this area would be very welcome!
222 \S{faq-mac-port}{Question} Will there be a port to the Mac?
224 There is a port to the Mac OS in progress. It's just about usable, but
225 has an awful lot of gaps and rough edges that will need cleaning up
228 \S{faq-epoc}{Question} Will there be a port to EPOC?
230 I hope so, but given that ports aren't really progressing very fast
231 even on systems the developers \e{do} already know how to program
232 for, it might be a long time before any of us get round to learning
233 a new system and doing the port for that.
235 However, some of the work has been done by other people, and a beta
236 port of PuTTY for the Nokia 9200 Communicator series is available
237 from \W{http://www.s2.org/putty/}\cw{http://www.s2.org/putty/}
239 \H{faq-embedding} Embedding PuTTY in other programs
241 \S{faq-dll}{Question} Is the SSH or Telnet code available as a DLL?
243 No, it isn't. It would take a reasonable amount of rewriting for
244 this to be possible, and since the PuTTY project itself doesn't
245 believe in DLLs (they make installation more error-prone) none of us
246 has taken the time to do it.
248 Most of the code cleanup work would be a good thing to happen in
249 general, so if anyone feels like helping, we wouldn't say no.
251 \S{faq-vb}{Question} Is the SSH or Telnet code available as a Visual
254 No, it isn't. None of the PuTTY team uses Visual Basic, and none of
255 us has any particular need to make SSH connections from a Visual
256 Basic application. In addition, all the preliminary work to turn it
257 into a DLL would be necessary first; and furthermore, we don't even
258 know how to write VB components.
260 If someone offers to do some of this work for us, we might consider
261 it, but unless that happens I can't see VB integration being
262 anywhere other than the very bottom of our priority list.
264 \S{faq-ipc}{Question} How can I use PuTTY to make an SSH connection
265 from within another program?
267 Probably your best bet is to use Plink, the command-line connection
268 tool. If you can start Plink as a second Windows process, and
269 arrange for your primary process to be able to send data to the
270 Plink process, and receive data from it, through pipes, then you
271 should be able to make SSH connections from your program.
273 This is what CVS for Windows does, for example.
275 \H{faq-details} Details of PuTTY's operation
277 \S{faq-term}{Question} What terminal type does PuTTY use?
279 For most purposes, PuTTY can be considered to be an \cw{xterm}
282 PuTTY also supports some terminal control sequences not supported by
283 the real \cw{xterm}: notably the Linux console sequences that
284 reconfigure the colour palette, and the title bar control sequences
285 used by \cw{DECterm} (which are different from the \cw{xterm} ones;
286 PuTTY supports both).
288 By default, PuTTY announces its terminal type to the server as
289 \c{xterm}. If you have a problem with this, you can reconfigure it
290 to say something else; \c{vt220} might help if you have trouble.
292 \S{faq-settings}{Question} Where does PuTTY store its data?
294 PuTTY stores most of its data (saved sessions, SSH host keys) in the
295 Registry. The precise location is
297 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
299 and within that area, saved sessions are stored under \c{Sessions}
300 while host keys are stored under \c{SshHostKeys}.
302 PuTTY also requires a random number seed file, to improve the
303 unpredictability of randomly chosen data needed as part of the SSH
304 cryptography. This is stored by default in your Windows home
305 directory (\c{%HOMEDRIVE%\\%HOMEPATH%}), or in the actual Windows
306 directory (such as \c{C:\\WINDOWS}) if the home directory doesn't
307 exist, for example if you're using Win95. If you want to change the
308 location of the random number seed file, you can put your chosen
309 pathname in the Registry, at
311 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
313 \H{faq-howto} HOWTO questions
315 \S{faq-startmax}{Question} How can I make PuTTY start up maximised?
317 Create a Windows shortcut to start PuTTY from, and set it as \q{Run
320 \S{faq-startsess}{Question} How can I create a Windows shortcut to
321 start a particular saved session directly?
323 To run a PuTTY session saved under the name \q{\cw{mysession}},
324 create a Windows shortcut that invokes PuTTY with a command line
327 \c \path\name\to\putty.exe -load mysession
329 (Note: prior to 0.53, the syntax was \c{@session}. This is now
330 deprecated and may be removed at some point.)
332 \S{faq-startssh}{Question} How can I start an SSH session straight
333 from the command line?
335 Use the command line \c{putty -ssh host.name}. Alternatively, create
336 a saved session that specifies the SSH protocol, and start the saved
337 session as shown in \k{faq-startsess}.
339 \S{faq-cutpaste}{Question} How do I copy and paste between PuTTY and
340 other Windows applications?
342 Copy and paste works similarly to the X Window System. You use the
343 left mouse button to select text in the PuTTY window. The act of
344 selection \e{automatically} copies the text to the clipboard: there
345 is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
346 pressing Ctrl-C will send a Ctrl-C character to the other end of
347 your connection (just like it does the rest of the time), which may
348 have unpleasant effects. The \e{only} thing you need to do, to copy
349 text to the clipboard, is to select it.
351 To paste the clipboard contents into a PuTTY window, by default you
352 click the right mouse button. If you have a three-button mouse and
353 are used to X applications, you can configure pasting to be done by
354 the middle button instead, but this is not the default because most
355 Windows users don't have a middle button at all.
357 You can also paste by pressing Shift-Ins.
359 \S{faq-tunnels}{Question} How do I use X forwarding and port
360 forwarding? I can't find the Tunnels panel.
362 This is a new feature in version 0.52. You should upgrade.
364 \S{faq-options}{Question} How do I use all PuTTY's features (public
365 keys, proxying, cipher selection, etc.) in PSCP, PSFTP and Plink?
367 Most major features (e.g., public keys, port forwarding) are available
368 through command line options. See the documentation.
370 Not all features are accessible from the command line yet, although
371 we'd like to fix this. In the meantime, you can use most of
372 PuTTY's features if you create a PuTTY saved session, and then use
373 the name of the saved session on the command line in place of a
374 hostname. This works for PSCP, PSFTP and Plink (but don't expect
375 port forwarding in the file transfer applications!).
377 \S{faq-pscp}{Question} How do I use PSCP.EXE? When I double-click it
378 gives me a command prompt window which then closes instantly.
380 PSCP is a command-line application, not a GUI application. If you
381 run it without arguments, it will simply print a help message and
384 To use PSCP properly, run it from a Command Prompt window. See
385 \k{pscp} in the documentation for more details.
387 \S{faq-pscp-spaces}{Question} How do I use PSCP to copy a file whose
390 If PSCP is using the traditional SCP protocol, this is confusing. If
391 you're specifying a file at the local end, you just use one set of
392 quotes as you would normally do:
394 \c pscp "local filename with spaces" user@host:
395 \c pscp user@host:myfile "local filename with spaces"
397 But if the filename you're specifying is on the \e{remote} side, you
398 have to use backslashes and two sets of quotes:
400 \c pscp user@host:"\"remote filename with spaces\"" local_filename
401 \c pscp local_filename user@host:"\"remote filename with spaces\""
403 Worse still, in a remote-to-local copy you have to specify the local
404 file name explicitly, otherwise PSCP will complain that they don't
405 match (unless you specified the \c{-unsafe} option). The following
406 command will give an error message:
408 \c c:\>pscp user@host:"\"oo er\"" .
409 \c warning: remote host tried to write to a file called 'oo er'
410 \c when we requested a file called '"oo er"'.
412 Instead, you need to specify the local file name in full:
414 \c c:\>pscp user@host:"\"oo er\"" "oo er"
416 If PSCP is using the newer SFTP protocol, none of this is a problem,
417 and all filenames with spaces in are specified using a single pair
418 of quotes in the obvious way:
420 \c pscp "local file" user@host:
421 \c pscp user@host:"remote file" .
423 \H{faq-trouble} Troubleshooting
425 \S{faq-incorrect-mac}{Question} Why do I see \q{Incorrect MAC
428 One possible cause of this that used to be common is a bug in old
429 SSH 2 servers distributed by \cw{ssh.com}. (This is not the only
430 possible cause; see \k{errors-crc} in the documentation.)
431 Version 2.3.0 and below of their SSH 2 server
432 constructs Message Authentication Codes in the wrong way, and
433 expects the client to construct them in the same wrong way. PuTTY
434 constructs the MACs correctly by default, and hence these old
435 servers will fail to work with it.
437 If you are using PuTTY version 0.52 or better, this should work
438 automatically: PuTTY should detect the buggy servers from their
439 version number announcement, and automatically start to construct
440 its MACs in the same incorrect manner as they do, so it will be able
443 If you are using PuTTY version 0.51 or below, you can enable the
444 workaround by going to the SSH panel and ticking the box labelled
445 \q{Imitate SSH 2 MAC bug}. It's possible that you might have to do
446 this with 0.52 as well, if a buggy server exists that PuTTY doesn't
449 In this context MAC stands for Message Authentication Code. It's a
450 cryptographic term, and it has nothing at all to do with Ethernet
451 MAC (Media Access Control) addresses.
453 \S{faq-pscp-protocol}{Question} Why do I see \q{Fatal: Protocol
454 error: Expected control record} in PSCP?
456 This happens because PSCP was expecting to see data from the server
457 that was part of the PSCP protocol exchange, and instead it saw data
458 that it couldn't make any sense of at all.
460 This almost always happens because the startup scripts in your
461 account on the server machine are generating output. This is
462 impossible for PSCP, or any other SCP client, to work around. You
463 should never use startup files (\c{.bashrc}, \c{.cshrc} and so on)
464 which generate output in non-interactive sessions.
466 This is not actually a PuTTY problem. If PSCP fails in this way,
467 then all other SCP clients are likely to fail in exactly the same
468 way. The problem is at the server end.
470 \S{faq-colours}{Question} I clicked on a colour in the Colours
471 panel, and the colour didn't change in my terminal.
473 That isn't how you're supposed to use the Colours panel.
475 During the course of a session, PuTTY potentially uses \e{all} the
476 colours listed in the Colours panel. It's not a question of using
477 only one of them and you choosing which one; PuTTY will use them
478 \e{all}. The purpose of the Colours panel is to let you adjust the
479 appearance of all the colours. So to change the colour of the
480 cursor, for example, you would select \q{Cursor Colour}, press the
481 \q{Modify} button, and select a new colour from the dialog box that
482 appeared. Similarly, if you want your session to appear in green,
483 you should select \q{Default Foreground} and press \q{Modify}.
484 Clicking on \q{ANSI Green} won't turn your session green; it will
485 only allow you to adjust the \e{shade} of green used when PuTTY is
486 instructed by the server to display green text.
488 \S{faq-winsock2}{Question} Plink on Windows 95 says it can't find
491 Plink requires the extended Windows network library, WinSock version
492 2. This is installed as standard on Windows 98 and above, and on
493 Windows NT, and even on later versions of Windows 95; but early
494 Win95 installations don't have it.
496 In order to use Plink on these systems, you will need to download
498 \W{http://www.microsoft.com/windows95/downloads/contents/wuadmintools/s_wunetworkingtools/w95sockets2/}{WinSock 2 upgrade}:
500 \c http://www.microsoft.com/windows95/downloads/contents/wuadmintools/
501 \c s_wunetworkingtools/w95sockets2/
503 \S{faq-rekey}{Question} My PuTTY sessions close after an hour and
504 tell me \q{Server failed host key check}.
506 This is a bug in all versions of PuTTY up to and including 0.51. SSH
507 v2 servers from \cw{ssh.com} will require the key exchange to be
508 repeated one hour after the start of the connection, and PuTTY will
511 Upgrade to version 0.52 or better and the problem should go away.
513 \S{faq-outofmem}{Question} After trying to establish an SSH 2
514 connection, PuTTY says \q{Out of memory} and dies.
516 If this happens just while the connection is starting up, this often
517 indicates that for some reason the client and server have failed to
518 establish a session encryption key. Somehow, they have performed
519 calculations that should have given each of them the same key, but
520 have ended up with different keys; so data encrypted by one and
521 decrypted by the other looks like random garbage.
523 This causes an \q{out of memory} error because the first encrypted
524 data PuTTY expects to see is the length of an SSH message. Normally
525 this will be something well under 100 bytes. If the decryption has
526 failed, PuTTY will see a completely random length in the region of
527 two \e{gigabytes}, and will try to allocate enough memory to store
528 this non-existent message. This will immediately lead to it thinking
529 it doesn't have enough memory, and panicking.
531 If this happens to you, it is quite likely to still be a PuTTY bug
532 and you should report it (although it might be a bug in your SSH
533 server instead); but it doesn't necessarily mean you've actually run
536 \S{faq-outofmem2}{Question} When attempting a file transfer, either
537 PSCP or PSFTP says \q{Out of memory} and dies.
539 This is almost always caused by your login scripts on the server
540 generating output. PSCP or PSFTP will receive that output when they
541 were expecting to see the start of a file transfer protocol, and
542 they will attempt to interpret the output as file-transfer protocol.
543 This will usually lead to an \q{out of memory} error for much the
544 same reasons as given in \k{faq-outofmem}.
546 This is a setup problem in your account on your server, \e{not} a
547 PSCP/PSFTP bug. Your login scripts should \e{never} generate output
548 during non-interactive sessions; secure file transfer is not the
549 only form of remote access that will break if they do.
551 On Unix, a simple fix is to ensure that all the parts of your login
552 script that might generate output are in \c{.profile} (if you use a
553 Bourne shell derivative) or \c{.login} (if you use a C shell).
554 Putting them in more general files such as \c{.bashrc} or \c{.cshrc}
555 is liable to lead to problems.
557 \S{faq-psftp-slow}{Question} PSFTP transfers files much slower than PSCP.
559 We believe this is because the SFTP and SSH2 protocols are less
560 efficient at bulk data transfer than SCP and SSH1, because every
561 block of data transferred requires an acknowledgment from the far
562 end. It would in theory be possible to queue several blocks of data
563 to get round this speed problem, but as yet we haven't done the
564 coding. If you really want this fixed, feel free to offer to help.
566 \S{faq-bce}{Question} When I run full-colour applications, I see
567 areas of black space where colour ought to be.
569 You almost certainly need to enable the \q{Use background colour to
570 erase screen} setting in the Terminal panel. Note that if you do
571 this in mid-session, it won't take effect until you reset the
572 terminal (see \k{faq-resetterm}).
574 \S{faq-resetterm}{Question} When I change some terminal settings,
577 Some of the terminal options (notably Auto Wrap and
578 background-colour screen erase) actually represent the \e{default}
579 setting, rather than the currently active setting. The server can
580 send sequences that modify these options in mid-session, but when
581 the terminal is reset (by server action, or by you choosing \q{Reset
582 Terminal} from the System menu) the defaults are restored.
584 If you want to change one of these options in the middle of a
585 session, you will find that the change does not immediately take
586 effect. It will only take effect once you reset the terminal.
588 \S{faq-altgr}{Question} I can't type characters that require the
591 In PuTTY version 0.51, the AltGr key was broken. Upgrade to version
594 \S{faq-idleout}{Question} My PuTTY sessions unexpectedly close after
595 they are idle for a while.
597 Some types of firewall, and almost any router doing Network Address
598 Translation (NAT, also known as IP masquerading), will forget about
599 a connection through them if the connection does nothing for too
600 long. This will cause the connection to be rudely cut off when
603 You can try to combat this by telling PuTTY to send \e{keepalives}:
604 packets of data which have no effect on the actual session, but
605 which reassure the router or firewall that the network connection is
606 still active and worth remembering about.
608 Keepalives don't solve everything, unfortunately; although they
609 cause greater robustness against this sort of router, they can also
610 cause a \e{loss} of robustness against network dropouts. See
611 \k{config-keepalive} in the documentation for more discussion of
614 \S{faq-timeout}{Question} PuTTY's network connections time out too
615 quickly when network connectivity is temporarily lost.
617 This is a Windows problem, not a PuTTY problem. The timeout value
618 can't be set on per application or per session basis. To increase
619 the TCP timeout globally, you need to tinker with the Registry.
621 On Windows 95, 98 or ME, the registry key you need to change is
623 \c HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
624 \c MSTCP\MaxDataRetries
626 (it must be of type DWORD in Win95, or String in Win98/ME).
628 On Windows NT or 2000, the registry key is
630 \c HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
631 \c Parameters\TcpMaxDataRetransmissions
633 and it must be of type DWORD.
635 Set the key's value to something like 10. This will cause Windows to
636 try harder to keep connections alive instead of abandoning them.
638 \S{faq-puttyputty}{Question} When I \cw{cat} a binary file, I get
639 `PuTTYPuTTYPuTTY' on my command line.
643 This is designed behaviour; when PuTTY receives the character
644 Control-E from the remote server, it interprets it as a request to
645 identify itself, and so it sends back the string \q{\cw{PuTTY}} as
646 if that string had been entered at the keyboard. Control-E should
647 only be sent by programs that are prepared to deal with the
648 response. Writing a binary file to your terminal is likely to output
649 many Control-E characters, and cause this behaviour. Don't do it.
652 To mitigate the effects, you could configure the answerback string
653 to be empty (see \k{config-answerback}); but writing binary files to
654 your terminal is likely to cause various other unpleasant behaviour,
655 so this is only a small remedy.
657 \S{faq-wintitle}{Question} When I \cw{cat} a binary file, my window
658 title changes to a nonsense string.
662 It is designed behaviour that PuTTY should have the ability to
663 adjust the window title on instructions from the server. Normally
664 the control sequence that does this should only be sent
665 deliberately, by programs that know what they are doing and intend
666 to put meaningful text in the window title. Writing a binary file to
667 your terminal runs the risk of sending the same control sequence by
668 accident, and cause unexpected changes in the window title. Don't do
671 \S{faq-password-fails}{Question} My keyboard stops working once
672 PuTTY displays the password prompt.
674 No, it doesn't. PuTTY just doesn't display the password you type, so
675 that someone looking at your screen can't see what it is.
677 Unlike the Windows login prompts, PuTTY doesn't display the password
678 as a row of asterisks either. This is so that someone looking at
679 your screen can't even tell how \e{long} your password is, which
680 might be valuable information.
682 \S{faq-keyboard}{Question} One or more function keys don't do what I
683 expected in a server-side application.
685 If you've already tried all the relevant options in the PuTTY
686 Keyboard panel, you may need to mail the PuTTY maintainers and ask.
688 It is \e{not} usually helpful just to tell us which application,
689 which server operating system, and which key isn't working; in order
690 to replicate the problem we would need to have a copy of every
691 operating system, and every application, that anyone has ever
694 PuTTY responds to function key presses by sending a sequence of
695 control characters to the server. If a function key isn't doing what
696 you expect, it's likely that the character sequence your application
697 is expecting to receive is not the same as the one PuTTY is sending.
698 Therefore what we really need to know is \e{what} sequence the
699 application is expecting.
701 The simplest way to investigate this is to find some other terminal
702 environment, in which that function key \e{does} work; and then
703 investigate what sequence the function key is sending in that
704 situation. One reasonably easy way to do this on a Unix system is to
705 type the command \c{cat}, and then press the function key. This is
706 likely to produce output of the form \c{^[[11~}. You can also do
707 this in PuTTY, to find out what sequence the function key is
708 producing in that. Then you can mail the PuTTY maintainers and tell
709 us \q{I wanted the F1 key to send \c{^[[11~}, but instead it's
710 sending \c{^[OP}, can this be done?}, or something similar.
712 You should still read the
713 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/feedback.html}{Feedback
714 page} on the PuTTY website (also provided as \k{feedback} in the
715 manual), and follow the guidelines contained in that.
717 \S{faq-openssh-bad-openssl}{Question} Since my SSH server was upgraded
718 to OpenSSH 3.1p1/3.4p1, I can no longer connect with PuTTY.
720 There is a known problem when OpenSSH has been built against an
721 incorrect version of OpenSSL; the quick workaround is to configure
722 PuTTY to use SSH protocol 2 and the Blowfish cipher.
724 For more details and OpenSSH patches, see
725 \W{http://bugzilla.mindrot.org/show_bug.cgi?id=138}{bug 138} in the
728 This is not a PuTTY-specific problem; if you try to connect with
729 another client you'll likely have similar problems. (Although PuTTY's
730 default cipher differs from many other clients.)
732 \e{OpenSSH 3.1p1:} configurations known to be broken (and symptoms):
734 \b SSH 2 with AES cipher (PuTTY says "Assertion failed! Expression:
735 (len & 15) == 0" in sshaes.c, or "Out of memory", or crashes)
737 \b SSH 2 with 3DES (PuTTY says "Incorrect MAC received on packet")
739 \b SSH 1 with Blowfish (PuTTY says "Incorrect CRC received on
744 \e{OpenSSH 3.4p1:} as of 3.4p1, only the problem with SSH 1 and
745 Blowfish remains. Rebuild your server, apply the patch linked to from
746 bug 138 above, or use another cipher (e.g., 3DES) instead.
748 \e{Other versions:} we occasionally get reports of the same symptom
749 and workarounds with older versions of OpenSSH, although it's not
750 clear the underlying cause is the same.
752 \S{faq-ssh2key-ssh1conn}{Question} Why do I see "Couldn't load private
753 key from ..."? Why can PuTTYgen load my key but not PuTTY?
755 It's likely that you've generated an SSH protocol 2 key with PuTTYgen,
756 but you're trying to use it in an SSH 1 connection. SSH1 and SSH2 keys
757 have different formats, and (at least in 0.52) PuTTY's reporting of a
758 key in the wrong format isn't optimal.
760 To connect using SSH 2 to a server that supports both versions, you
761 need to change the configuration from the default (see \k{faq-ssh2}).
763 \S{faq-rh8-utf8}{Question} When I'm connected to a Red Hat Linux 8.0
764 system, some characters don't display properly.
766 A common complaint is that hyphens in man pages show up as a-acute.
768 With release 8.0, Red Hat appear to have made UTF-8 the default
769 character set. There appears to be no way for terminal emulators such
770 as PuTTY to know this (as far as we know, the appropriate escape
771 sequence to switch into UTF-8 mode isn't sent).
773 A fix is to configure sessions to RH8 systems to use UTF-8
774 translation - see \k{config-charset} in the documentation. (Note that
775 if you use \q{Change Settings}, changes may not take place immediately
776 - see \k{faq-resetterm}.)
778 If you really want to change the character set used by the server, the
779 right place is \c{/etc/sysconfig/i18n}, but this shouldn't be
782 \H{faq-secure} Security questions
784 \S{faq-publicpc}{Question} Is it safe for me to download PuTTY and
785 use it on a public PC?
787 It depends on whether you trust that PC. If you don't trust the
788 public PC, don't use PuTTY on it, and don't use any other software
789 you plan to type passwords into either. It might be watching your
790 keystrokes, or it might tamper with the PuTTY binary you download.
791 There is \e{no} program safe enough that you can run it on an
792 actively malicious PC and get away with typing passwords into it.
794 If you do trust the PC, then it's probably OK to use PuTTY on it
795 (but if you don't trust the network, then the PuTTY download might
796 be tampered with, so it would be better to carry PuTTY with you on a
799 \S{faq-cleanup}{Question} What does PuTTY leave on a system? How can
802 PuTTY will leave some Registry entries, and a random seed file, on
803 the PC (see \k{faq-settings}). If you are using PuTTY on a public
804 PC, or somebody else's PC, you might want to clean these up when you
805 leave. You can do that automatically, by running the command
808 \S{faq-dsa}{Question} How come PuTTY now supports DSA, when the
809 website used to say how insecure it was?
811 DSA has a major weakness \e{if badly implemented}: it relies on a
812 random number generator to far too great an extent. If the random
813 number generator produces a number an attacker can predict, the DSA
814 private key is exposed - meaning that the attacker can log in as you
815 on all systems that accept that key.
817 The PuTTY policy changed because the developers were informed of
818 ways to implement DSA which do not suffer nearly as badly from this
819 weakness, and indeed which don't need to rely on random numbers at
820 all. For this reason we now believe PuTTY's DSA implementation is
821 probably OK. However, if you have the choice, we still recommend you
824 \S{faq-virtuallock}{Question} Couldn't Pageant use
825 \cw{VirtualLock()} to stop private keys being written to disk?
827 Unfortunately not. The \cw{VirtualLock()} function in the Windows
828 API doesn't do a proper job: it may prevent small pieces of a
829 process's memory from being paged to disk while the process is
830 running, but it doesn't stop the process's memory as a whole from
831 being swapped completely out to disk when the process is long-term
832 inactive. And Pageant spends most of its time inactive.
834 \H{faq-admin} Administrative questions
836 \S{faq-domain}{Question} Would you like me to register you a nicer
839 No, thank you. Even if you can find one (most of them seem to have
840 been registered already, by people who didn't ask whether we
841 actually wanted it before they applied), we're happy with the PuTTY
842 web site being exactly where it is. It's not hard to find (just type
843 \q{putty} into \W{http://www.google.com/}{google.com} and we're the
844 first link returned), and we don't believe the administrative hassle
845 of moving the site would be worth the benefit.
847 In addition, if we \e{did} want a custom domain name, we would want
848 to run it ourselves, so we knew for certain that it would continue
849 to point where we wanted it, and wouldn't suddenly change or do
850 strange things. Having it registered for us by a third party who we
851 don't even know is not the best way to achieve this.
853 \S{faq-webhosting}{Question} Would you like free web hosting for the
856 We already have some, thanks.
858 \S{faq-sourceforge}{Question} Why don't you move PuTTY to
861 Partly, because we don't want to move the web site location (see
864 Also, security reasons. PuTTY is a security product, and as such it
865 is particularly important to guard the code and the web site against
866 unauthorised modifications which might introduce subtle security
867 flaws. Therefore, we prefer that the CVS repository, web site and
868 FTP site remain where they are, under the direct control of system
869 administrators we know and trust personally, rather than being run
870 by a large organisation full of people we've never met and which is
871 known to have had breakins in the past.
873 No offence to SourceForge; I think they do a wonderful job. But
874 they're not ideal for everyone, and in particular they're not ideal
877 \S{faq-mailinglist1}{Question} Why can't I subscribe to the
878 putty-bugs mailing list?
880 Because you're not a member of the PuTTY core development team. The
881 putty-bugs mailing list is not a general newsgroup-like discussion
882 forum; it's a contact address for the core developers, and an
883 \e{internal} mailing list for us to discuss things among ourselves.
884 If we opened it up for everybody to subscribe to, it would turn into
885 something more like a newsgroup and we would be completely
886 overwhelmed by the volume of traffic. It's hard enough to keep up
887 with the list as it is.
889 \S{faq-mailinglist2}{Question} If putty-bugs isn't a
890 general-subscription mailing list, what is?
892 There isn't one, that we know of.
894 If someone else wants to set up a mailing list or other forum for
895 PuTTY users to help each other with common problems, that would be
896 fine with us, though the PuTTY team would almost certainly not have the
897 time to read it. It's probably better to use the established
898 newsgroup \cw{comp.security.ssh} for this purpose.
900 \S{faq-donations}{Question} How can I donate to PuTTY development?
902 Please, \e{please} don't feel you have to. PuTTY is completely free
903 software, and not shareware. We think it's very important that
904 \e{everybody} who wants to use PuTTY should be able to, whether they
905 have any money or not; so the last thing we would want is for a
906 PuTTY user to feel guilty because they haven't paid us any money. If
907 you want to keep your money, please do keep it. We wouldn't dream of
910 Having said all that, if you still really \e{want} to give us money,
911 we won't argue :-) The easiest way for us to accept donations is if
912 you go to \W{http://www.e-gold.com}\cw{www.e-gold.com}, and deposit
913 your donation in account number 174769. Then send us e-mail to let
914 us know you've done so (otherwise we might not notice for months!).
915 Alternatively, if e-gold isn't convenient for you, you can donate to
916 \cw{<anakin@pobox.com>} using PayPal
917 (\W{http://www.paypal.com/}\cw{www.paypal.com}).
919 Small donations (tens of dollars or tens of euros) will probably be
920 spent on beer or curry, which helps motivate our volunteer team to
921 continue doing this for the world. Larger donations will be spent on
922 something that actually helps development, if we can find anything
923 (perhaps new hardware, or a copy of Windows XP), but if we can't
924 find anything then we'll just distribute the money among the
925 developers. If you want to be sure your donation is going towards
926 something worthwhile, ask us first. If you don't like these terms,
927 feel perfectly free not to donate. We don't mind.
929 \H{faq-misc} Miscellaneous questions
931 \S{faq-openssh}{Question} Is PuTTY a port of OpenSSH, or based on
934 No, it isn't. PuTTY is almost completely composed of code written
935 from scratch for PuTTY. The only code we share with OpenSSH is the
936 detector for SSH1 CRC compensation attacks, written by CORE SDI S.A.
938 \S{faq-sillyputty}{Question} Where can I buy silly putty?
940 You're looking at the wrong web site; the only PuTTY we know about
941 here is the name of a computer program.
943 If you want the kind of putty you can buy as an executive toy, the
944 PuTTY team can personally recommend Thinking Putty, which you can
945 buy from Crazy Aaron's Putty World, at
946 \W{http://www.puttyworld.com}\cw{www.puttyworld.com}.
948 \S{faq-meaning}{Question} What does \q{PuTTY} mean?
950 It's the name of a popular SSH and Telnet client. Any other meaning
951 is in the eye of the beholder. It's been rumoured that \q{PuTTY}
952 is the antonym of \q{\cw{getty}}, or that it's the stuff that makes your
953 Windows useful, or that it's a kind of plutonium Teletype. We
954 couldn't possibly comment on such allegations.
956 \S{faq-pronounce}{Question} How do I pronounce \q{PuTTY}?
958 Exactly like the English word \q{putty}, which we pronounce
959 /\u02C8{'}p\u028C{V}t\u026A{I}/.