1 \versionid $Id: faq.but,v 1.47 2003/05/22 08:43:50 simon 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
151 Unfortunately not. This is a limitation of the file transfer
152 protocols: the SCP and SFTP protocols have no notion of transferring
153 a file in anything other than binary mode.
155 SFTP is designed to be extensible, so it's possible that an
156 extension might be proposed at some later date that implements ASCII
157 transfer. But the PuTTY team can't do anything about it until that
160 \H{faq-ports} Ports to other operating systems
162 The eventual goal is for PuTTY to be a multi-platform program, able
163 to run on at least Windows, Mac OS and Unix.
165 Porting will become easier once PuTTY has a generalised porting
166 layer, drawing a clear line between platform-dependent and
167 platform-independent code. The general intention was for this
168 porting layer to evolve naturally as part of the process of doing
169 the first port; a Unix port is now under way and the plan seems to
172 \S{faq-ports-general}{Question} What ports of PuTTY exist?
174 Currently, release versions of PuTTY only run on full Win32 systems.
175 This includes Windows 95, 98, and ME, and it includes Windows NT,
176 Windows 2000 and Windows XP. In the development code, partial ports
177 to Unix (see \k{faq-unix}) and the Mac OS (see \k{faq-mac-port}).
180 Currently PuTTY does \e{not} run on Windows CE (see \k{faq-wince}),
181 and it does not quite run on the Win32s environment under Windows
182 3.1 (see \k{faq-win31}).
184 We do not have release-quality ports for any other systems at the
185 present time. If anyone told you we had a Mac port, or an iPaq port,
186 or any other port of PuTTY, they were mistaken. We don't.
188 \S{faq-unix}{Question} Will there be a port to Unix?
190 It's currently being worked on. If you look at the nightly source
191 snapshots, you should find a \c{unix} subdirectory, which should
192 build you Unix ports of Plink, PuTTY itself, and also \c{pterm} - an
193 \cw{xterm}-type program which supports the same terminal emulation
194 as PuTTY. We do not yet have Unix ports of PSCP, PSFTP, Pageant or
197 \S{faq-wince}{Question} Will there be a port to Windows CE or PocketPC?
199 It's currently being worked on, but it's only in its early stages yet,
200 and certainly isn't yet useful. PuTTY on portable devices would
201 clearly be a useful thing, so in the long term I hope it can be
202 brought up to release quality.
204 \S{faq-win31}{Question} Is there a port to Windows 3.1?
206 PuTTY is a 32-bit application from the ground up, so it won't run on
207 Windows 3.1 as a native 16-bit program; and it would be \e{very}
208 hard to port it to do so, because of Windows 3.1's vile memory
209 allocation mechanisms.
211 However, it is possible in theory to compile the existing PuTTY
212 source in such a way that it will run under Win32s (an extension to
213 Windows 3.1 to let you run 32-bit programs). In order to do this
214 you'll need the right kind of C compiler - modern versions of Visual
215 C at least have stopped being backwards compatible to Win32s. Also,
216 the last time we tried this it didn't work very well.
218 If you're interested in running PuTTY under Windows 3.1, help and
219 testing in this area would be very welcome!
221 \S{faq-mac-port}{Question} Will there be a port to the Mac?
223 There is a port to the Mac OS in progress. It's just about usable, but
224 has an awful lot of gaps and rough edges that will need cleaning up
227 \S{faq-epoc}{Question} Will there be a port to EPOC?
229 I hope so, but given that ports aren't really progressing very fast
230 even on systems the developers \e{do} already know how to program
231 for, it might be a long time before any of us get round to learning
232 a new system and doing the port for that.
234 However, some of the work has been done by other people, and a beta
235 port of PuTTY for the Nokia 9200 Communicator series is available
236 from \W{http://www.s2.org/putty/}\cw{http://www.s2.org/putty/}
238 \H{faq-embedding} Embedding PuTTY in other programs
240 \S{faq-dll}{Question} Is the SSH or Telnet code available as a DLL?
242 No, it isn't. It would take a reasonable amount of rewriting for
243 this to be possible, and since the PuTTY project itself doesn't
244 believe in DLLs (they make installation more error-prone) none of us
245 has taken the time to do it.
247 Most of the code cleanup work would be a good thing to happen in
248 general, so if anyone feels like helping, we wouldn't say no.
250 \S{faq-vb}{Question} Is the SSH or Telnet code available as a Visual
253 No, it isn't. None of the PuTTY team uses Visual Basic, and none of
254 us has any particular need to make SSH connections from a Visual
255 Basic application. In addition, all the preliminary work to turn it
256 into a DLL would be necessary first; and furthermore, we don't even
257 know how to write VB components.
259 If someone offers to do some of this work for us, we might consider
260 it, but unless that happens I can't see VB integration being
261 anywhere other than the very bottom of our priority list.
263 \S{faq-ipc}{Question} How can I use PuTTY to make an SSH connection
264 from within another program?
266 Probably your best bet is to use Plink, the command-line connection
267 tool. If you can start Plink as a second Windows process, and
268 arrange for your primary process to be able to send data to the
269 Plink process, and receive data from it, through pipes, then you
270 should be able to make SSH connections from your program.
272 This is what CVS for Windows does, for example.
274 \H{faq-details} Details of PuTTY's operation
276 \S{faq-term}{Question} What terminal type does PuTTY use?
278 For most purposes, PuTTY can be considered to be an \cw{xterm}
281 PuTTY also supports some terminal control sequences not supported by
282 the real \cw{xterm}: notably the Linux console sequences that
283 reconfigure the colour palette, and the title bar control sequences
284 used by \cw{DECterm} (which are different from the \cw{xterm} ones;
285 PuTTY supports both).
287 By default, PuTTY announces its terminal type to the server as
288 \c{xterm}. If you have a problem with this, you can reconfigure it
289 to say something else; \c{vt220} might help if you have trouble.
291 \S{faq-settings}{Question} Where does PuTTY store its data?
293 PuTTY stores most of its data (saved sessions, SSH host keys) in the
294 Registry. The precise location is
296 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY
298 and within that area, saved sessions are stored under \c{Sessions}
299 while host keys are stored under \c{SshHostKeys}.
301 PuTTY also requires a random number seed file, to improve the
302 unpredictability of randomly chosen data needed as part of the SSH
303 cryptography. This is stored by default in your Windows home
304 directory (\c{%HOMEDRIVE%\\%HOMEPATH%}), or in the actual Windows
305 directory (such as \c{C:\\WINDOWS}) if the home directory doesn't
306 exist, for example if you're using Win95. If you want to change the
307 location of the random number seed file, you can put your chosen
308 pathname in the Registry, at
310 \c HKEY_CURRENT_USER\Software\SimonTatham\PuTTY\RandSeedFile
312 \H{faq-howto} HOWTO questions
314 \S{faq-startmax}{Question} How can I make PuTTY start up maximised?
316 Create a Windows shortcut to start PuTTY from, and set it as \q{Run
319 \S{faq-startsess}{Question} How can I create a Windows shortcut to
320 start a particular saved session directly?
322 To run a PuTTY session saved under the name \q{\cw{mysession}},
323 create a Windows shortcut that invokes PuTTY with a command line
326 \c \path\name\to\putty.exe -load mysession
328 (Note: prior to 0.53, the syntax was \c{@session}. This is now
329 deprecated and may be removed at some point.)
331 \S{faq-startssh}{Question} How can I start an SSH session straight
332 from the command line?
334 Use the command line \c{putty -ssh host.name}. Alternatively, create
335 a saved session that specifies the SSH protocol, and start the saved
336 session as shown in \k{faq-startsess}.
338 \S{faq-cutpaste}{Question} How do I copy and paste between PuTTY and
339 other Windows applications?
341 Copy and paste works similarly to the X Window System. You use the
342 left mouse button to select text in the PuTTY window. The act of
343 selection \e{automatically} copies the text to the clipboard: there
344 is no need to press Ctrl-Ins or Ctrl-C or anything else. In fact,
345 pressing Ctrl-C will send a Ctrl-C character to the other end of
346 your connection (just like it does the rest of the time), which may
347 have unpleasant effects. The \e{only} thing you need to do, to copy
348 text to the clipboard, is to select it.
350 To paste the clipboard contents into a PuTTY window, by default you
351 click the right mouse button. If you have a three-button mouse and
352 are used to X applications, you can configure pasting to be done by
353 the middle button instead, but this is not the default because most
354 Windows users don't have a middle button at all.
356 You can also paste by pressing Shift-Ins.
358 \S{faq-tunnels}{Question} How do I use X forwarding and port
359 forwarding? I can't find the Tunnels panel.
361 This is a new feature in version 0.52. You should upgrade.
363 \S{faq-options}{Question} How do I use all PuTTY's features (public
364 keys, proxying, cipher selection, etc.) in PSCP, PSFTP and Plink?
366 Most major features (e.g., public keys, port forwarding) are available
367 through command line options. See the documentation.
369 Not all features are accessible from the command line yet, although
370 we'd like to fix this. In the meantime, you can use most of
371 PuTTY's features if you create a PuTTY saved session, and then use
372 the name of the saved session on the command line in place of a
373 hostname. This works for PSCP, PSFTP and Plink (but don't expect
374 port forwarding in the file transfer applications!).
376 \S{faq-pscp}{Question} How do I use PSCP.EXE? When I double-click it
377 gives me a command prompt window which then closes instantly.
379 PSCP is a command-line application, not a GUI application. If you
380 run it without arguments, it will simply print a help message and
383 To use PSCP properly, run it from a Command Prompt window. See
384 \k{pscp} in the documentation for more details.
386 \S{faq-pscp-spaces}{Question} How do I use PSCP to copy a file whose
389 If PSCP is using the traditional SCP protocol, this is confusing. If
390 you're specifying a file at the local end, you just use one set of
391 quotes as you would normally do:
393 \c pscp "local filename with spaces" user@host:
394 \c pscp user@host:myfile "local filename with spaces"
396 But if the filename you're specifying is on the \e{remote} side, you
397 have to use backslashes and two sets of quotes:
399 \c pscp user@host:"\"remote filename with spaces\"" local_filename
400 \c pscp local_filename user@host:"\"remote filename with spaces\""
402 Worse still, in a remote-to-local copy you have to specify the local
403 file name explicitly, otherwise PSCP will complain that they don't
404 match (unless you specified the \c{-unsafe} option). The following
405 command will give an error message:
407 \c c:\>pscp user@host:"\"oo er\"" .
408 \c warning: remote host tried to write to a file called 'oo er'
409 \c when we requested a file called '"oo er"'.
411 Instead, you need to specify the local file name in full:
413 \c c:\>pscp user@host:"\"oo er\"" "oo er"
415 If PSCP is using the newer SFTP protocol, none of this is a problem,
416 and all filenames with spaces in are specified using a single pair
417 of quotes in the obvious way:
419 \c pscp "local file" user@host:
420 \c pscp user@host:"remote file" .
422 \H{faq-trouble} Troubleshooting
424 \S{faq-incorrect-mac}{Question} Why do I see \q{Incorrect MAC
427 This is due to a bug in old SSH 2 servers distributed by
428 \cw{ssh.com}. Version 2.3.0 and below of their SSH 2 server
429 constructs Message Authentication Codes in the wrong way, and
430 expects the client to construct them in the same wrong way. PuTTY
431 constructs the MACs correctly by default, and hence these old
432 servers will fail to work with it.
434 If you are using PuTTY version 0.52 or better, this should work
435 automatically: PuTTY should detect the buggy servers from their
436 version number announcement, and automatically start to construct
437 its MACs in the same incorrect manner as they do, so it will be able
440 If you are using PuTTY version 0.51 or below, you can enable the
441 workaround by going to the SSH panel and ticking the box labelled
442 \q{Imitate SSH 2 MAC bug}. It's possible that you might have to do
443 this with 0.52 as well, if a buggy server exists that PuTTY doesn't
446 In this context MAC stands for Message Authentication Code. It's a
447 cryptographic term, and it has nothing at all to do with Ethernet
448 MAC (Media Access Control) addresses.
450 \S{faq-pscp-protocol}{Question} Why do I see \q{Fatal: Protocol
451 error: Expected control record} in PSCP?
453 This happens because PSCP was expecting to see data from the server
454 that was part of the PSCP protocol exchange, and instead it saw data
455 that it couldn't make any sense of at all.
457 This almost always happens because the startup scripts in your
458 account on the server machine are generating output. This is
459 impossible for PSCP, or any other SCP client, to work around. You
460 should never use startup files (\c{.bashrc}, \c{.cshrc} and so on)
461 which generate output in non-interactive sessions.
463 This is not actually a PuTTY problem. If PSCP fails in this way,
464 then all other SCP clients are likely to fail in exactly the same
465 way. The problem is at the server end.
467 \S{faq-colours}{Question} I clicked on a colour in the Colours
468 panel, and the colour didn't change in my terminal.
470 That isn't how you're supposed to use the Colours panel.
472 During the course of a session, PuTTY potentially uses \e{all} the
473 colours listed in the Colours panel. It's not a question of using
474 only one of them and you choosing which one; PuTTY will use them
475 \e{all}. The purpose of the Colours panel is to let you adjust the
476 appearance of all the colours. So to change the colour of the
477 cursor, for example, you would select \q{Cursor Colour}, press the
478 \q{Modify} button, and select a new colour from the dialog box that
479 appeared. Similarly, if you want your session to appear in green,
480 you should select \q{Default Foreground} and press \q{Modify}.
481 Clicking on \q{ANSI Green} won't turn your session green; it will
482 only allow you to adjust the \e{shade} of green used when PuTTY is
483 instructed by the server to display green text.
485 \S{faq-winsock2}{Question} Plink on Windows 95 says it can't find
488 Plink requires the extended Windows network library, WinSock version
489 2. This is installed as standard on Windows 98 and above, and on
490 Windows NT, and even on later versions of Windows 95; but early
491 Win95 installations don't have it.
493 In order to use Plink on these systems, you will need to download
495 \W{http://www.microsoft.com/windows95/downloads/contents/wuadmintools/s_wunetworkingtools/w95sockets2/}{WinSock 2 upgrade}:
497 \c http://www.microsoft.com/windows95/downloads/contents/wuadmintools/
498 \c s_wunetworkingtools/w95sockets2/
500 \S{faq-rekey}{Question} My PuTTY sessions close after an hour and
501 tell me \q{Server failed host key check}.
503 This is a bug in all versions of PuTTY up to and including 0.51. SSH
504 v2 servers from \cw{ssh.com} will require the key exchange to be
505 repeated one hour after the start of the connection, and PuTTY will
508 Upgrade to version 0.52 or better and the problem should go away.
510 \S{faq-outofmem}{Question} After trying to establish an SSH 2
511 connection, PuTTY says \q{Out of memory} and dies.
513 If this happens just while the connection is starting up, this often
514 indicates that for some reason the client and server have failed to
515 establish a session encryption key. Somehow, they have performed
516 calculations that should have given each of them the same key, but
517 have ended up with different keys; so data encrypted by one and
518 decrypted by the other looks like random garbage.
520 This causes an \q{out of memory} error because the first encrypted
521 data PuTTY expects to see is the length of an SSH message. Normally
522 this will be something well under 100 bytes. If the decryption has
523 failed, PuTTY will see a completely random length in the region of
524 two \e{gigabytes}, and will try to allocate enough memory to store
525 this non-existent message. This will immediately lead to it thinking
526 it doesn't have enough memory, and panicking.
528 If this happens to you, it is quite likely to still be a PuTTY bug
529 and you should report it (although it might be a bug in your SSH
530 server instead); but it doesn't necessarily mean you've actually run
533 \S{faq-outofmem2}{Question} When attempting a file transfer, either
534 PSCP or PSFTP says \q{Out of memory} and dies.
536 This is almost always caused by your login scripts on the server
537 generating output. PSCP or PSFTP will receive that output when they
538 were expecting to see the start of a file transfer protocol, and
539 they will attempt to interpret the output as file-transfer protocol.
540 This will usually lead to an \q{out of memory} error for much the
541 same reasons as given in \k{faq-outofmem}.
543 This is a setup problem in your account on your server, \e{not} a
544 PSCP/PSFTP bug. Your login scripts should \e{never} generate output
545 during non-interactive sessions; secure file transfer is not the
546 only form of remote access that will break if they do.
548 On Unix, a simple fix is to ensure that all the parts of your login
549 script that might generate output are in \c{.profile} (if you use a
550 Bourne shell derivative) or \c{.login} (if you use a C shell).
551 Putting them in more general files such as \c{.bashrc} or \c{.cshrc}
552 is liable to lead to problems.
554 \S{faq-psftp-slow}{Question} PSFTP transfers files much slower than PSCP.
556 We believe this is because the SFTP and SSH2 protocols are less
557 efficient at bulk data transfer than SCP and SSH1, because every
558 block of data transferred requires an acknowledgment from the far
559 end. It would in theory be possible to queue several blocks of data
560 to get round this speed problem, but as yet we haven't done the
561 coding. If you really want this fixed, feel free to offer to help.
563 \S{faq-bce}{Question} When I run full-colour applications, I see
564 areas of black space where colour ought to be.
566 You almost certainly need to enable the \q{Use background colour to
567 erase screen} setting in the Terminal panel. Note that if you do
568 this in mid-session, it won't take effect until you reset the
569 terminal (see \k{faq-resetterm}).
571 \S{faq-resetterm}{Question} When I change some terminal settings,
574 Some of the terminal options (notably Auto Wrap and
575 background-colour screen erase) actually represent the \e{default}
576 setting, rather than the currently active setting. The server can
577 send sequences that modify these options in mid-session, but when
578 the terminal is reset (by server action, or by you choosing \q{Reset
579 Terminal} from the System menu) the defaults are restored.
581 If you want to change one of these options in the middle of a
582 session, you will find that the change does not immediately take
583 effect. It will only take effect once you reset the terminal.
585 \S{faq-altgr}{Question} I can't type characters that require the
588 In PuTTY version 0.51, the AltGr key was broken. Upgrade to version
591 \S{faq-idleout}{Question} My PuTTY sessions unexpectedly close after
592 they are idle for a while.
594 Some types of firewall, and almost any router doing Network Address
595 Translation (NAT, also known as IP masquerading), will forget about
596 a connection through them if the connection does nothing for too
597 long. This will cause the connection to be rudely cut off when
600 You can try to combat this by telling PuTTY to send \e{keepalives}:
601 packets of data which have no effect on the actual session, but
602 which reassure the router or firewall that the network connection is
603 still active and worth remembering about.
605 Keepalives don't solve everything, unfortunately; although they
606 cause greater robustness against this sort of router, they can also
607 cause a \e{loss} of robustness against network dropouts. See
608 \k{config-keepalive} in the documentation for more discussion of
611 \S{faq-timeout}{Question} PuTTY's network connections time out too
612 quickly when network connectivity is temporarily lost.
614 This is a Windows problem, not a PuTTY problem. The timeout value
615 can't be set on per application or per session basis. To increase
616 the TCP timeout globally, you need to tinker with the Registry.
618 On Windows 95, 98 or ME, the registry key you need to change is
620 \c HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\
621 \c MSTCP\MaxDataRetries
623 (it must be of type DWORD in Win95, or String in Win98/ME).
625 On Windows NT or 2000, the registry key is
627 \c HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
628 \c Parameters\TcpMaxDataRetransmissions
630 and it must be of type DWORD.
632 Set the key's value to something like 10. This will cause Windows to
633 try harder to keep connections alive instead of abandoning them.
635 \S{faq-puttyputty}{Question} When I \cw{cat} a binary file, I get
636 `PuTTYPuTTYPuTTY' on my command line.
640 This is designed behaviour; when PuTTY receives the character
641 Control-E from the remote server, it interprets it as a request to
642 identify itself, and so it sends back the string \q{\cw{PuTTY}} as
643 if that string had been entered at the keyboard. Control-E should
644 only be sent by programs that are prepared to deal with the
645 response. Writing a binary file to your terminal is likely to output
646 many Control-E characters, and cause this behaviour. Don't do it.
649 To mitigate the effects, you could configure the answerback string
650 to be empty (see \k{config-answerback}); but writing binary files to
651 your terminal is likely to cause various other unpleasant behaviour,
652 so this is only a small remedy.
654 \S{faq-wintitle}{Question} When I \cw{cat} a binary file, my window
655 title changes to a nonsense string.
659 It is designed behaviour that PuTTY should have the ability to
660 adjust the window title on instructions from the server. Normally
661 the control sequence that does this should only be sent
662 deliberately, by programs that know what they are doing and intend
663 to put meaningful text in the window title. Writing a binary file to
664 your terminal runs the risk of sending the same control sequence by
665 accident, and cause unexpected changes in the window title. Don't do
668 \S{faq-password-fails}{Question} My keyboard stops working once
669 PuTTY displays the password prompt.
671 No, it doesn't. PuTTY just doesn't display the password you type, so
672 that someone looking at your screen can't see what it is.
674 Unlike the Windows login prompts, PuTTY doesn't display the password
675 as a row of asterisks either. This is so that someone looking at
676 your screen can't even tell how \e{long} your password is, which
677 might be valuable information.
679 \S{faq-keyboard}{Question} One or more function keys don't do what I
680 expected in a server-side application.
682 If you've already tried all the relevant options in the PuTTY
683 Keyboard panel, you may need to mail the PuTTY maintainers and ask.
685 It is \e{not} usually helpful just to tell us which application,
686 which server operating system, and which key isn't working; in order
687 to replicate the problem we would need to have a copy of every
688 operating system, and every application, that anyone has ever
691 PuTTY responds to function key presses by sending a sequence of
692 control characters to the server. If a function key isn't doing what
693 you expect, it's likely that the character sequence your application
694 is expecting to receive is not the same as the one PuTTY is sending.
695 Therefore what we really need to know is \e{what} sequence the
696 application is expecting.
698 The simplest way to investigate this is to find some other terminal
699 environment, in which that function key \e{does} work; and then
700 investigate what sequence the function key is sending in that
701 situation. One reasonably easy way to do this on a Unix system is to
702 type the command \c{cat}, and then press the function key. This is
703 likely to produce output of the form \c{^[[11~}. You can also do
704 this in PuTTY, to find out what sequence the function key is
705 producing in that. Then you can mail the PuTTY maintainers and tell
706 us \q{I wanted the F1 key to send \c{^[[11~}, but instead it's
707 sending \c{^[OP}, can this be done?}, or something similar.
709 You should still read the
710 \W{http://www.chiark.greenend.org.uk/~sgtatham/putty/feedback.html}{Feedback
711 page} on the PuTTY website (also provided as \k{feedback} in the
712 manual), and follow the guidelines contained in that.
714 \S{faq-openssh-bad-openssl}{Question} Since my SSH server was upgraded
715 to OpenSSH 3.1p1/3.4p1, I can no longer connect with PuTTY.
717 There is a known problem when OpenSSH has been built against an
718 incorrect version of OpenSSL; the quick workaround is to configure
719 PuTTY to use SSH protocol 2 and the Blowfish cipher.
721 For more details and OpenSSH patches, see
722 \W{http://bugzilla.mindrot.org/show_bug.cgi?id=138}{bug 138} in the
725 This is not a PuTTY-specific problem; if you try to connect with
726 another client you'll likely have similar problems. (Although PuTTY's
727 default cipher differs from many other clients.)
729 \e{OpenSSH 3.1p1:} configurations known to be broken (and symptoms):
731 \b SSH 2 with AES cipher (PuTTY says "Assertion failed! Expression:
732 (len & 15) == 0" in sshaes.c, or "Out of memory", or crashes)
734 \b SSH 2 with 3DES (PuTTY says "Incorrect MAC received on packet")
736 \b SSH 1 with Blowfish (PuTTY says "Incorrect CRC received on
741 \e{OpenSSH 3.4p1:} as of 3.4p1, only the problem with SSH 1 and
742 Blowfish remains. Rebuild your server, apply the patch linked to from
743 bug 138 above, or use another cipher (e.g., 3DES) instead.
745 \e{Other versions:} we occasionally get reports of the same symptom
746 and workarounds with older versions of OpenSSH, although it's not
747 clear the underlying cause is the same.
749 \S{faq-ssh2key-ssh1conn}{Question} Why do I see "Couldn't load private
750 key from ..."? Why can PuTTYgen load my key but not PuTTY?
752 It's likely that you've generated an SSH protocol 2 key with PuTTYgen,
753 but you're trying to use it in an SSH 1 connection. SSH1 and SSH2 keys
754 have different formats, and (at least in 0.52) PuTTY's reporting of a
755 key in the wrong format isn't optimal.
757 To connect using SSH 2 to a server that supports both versions, you
758 need to change the configuration from the default (see \k{faq-ssh2}).
760 \S{faq-rh8-utf8}{Question} When I'm connected to a Red Hat Linux 8.0
761 system, some characters don't display properly.
763 A common complaint is that hyphens in man pages show up as a-acute.
765 With release 8.0, Red Hat appear to have made UTF-8 the default
766 character set. There appears to be no way for terminal emulators such
767 as PuTTY to know this (as far as we know, the appropriate escape
768 sequence to switch into UTF-8 mode isn't sent).
770 A fix is to configure sessions to RH8 systems to use UTF-8
771 translation - see \k{config-charset} in the documentation. (Note that
772 if you use \q{Change Settings}, changes may not take place immediately
773 - see \k{faq-resetterm}.)
775 If you really want to change the character set used by the server, the
776 right place is \c{/etc/sysconfig/i18n}, but this shouldn't be
779 \H{faq-secure} Security questions
781 \S{faq-publicpc}{Question} Is it safe for me to download PuTTY and
782 use it on a public PC?
784 It depends on whether you trust that PC. If you don't trust the
785 public PC, don't use PuTTY on it, and don't use any other software
786 you plan to type passwords into either. It might be watching your
787 keystrokes, or it might tamper with the PuTTY binary you download.
788 There is \e{no} program safe enough that you can run it on an
789 actively malicious PC and get away with typing passwords into it.
791 If you do trust the PC, then it's probably OK to use PuTTY on it
792 (but if you don't trust the network, then the PuTTY download might
793 be tampered with, so it would be better to carry PuTTY with you on a
796 \S{faq-cleanup}{Question} What does PuTTY leave on a system? How can
799 PuTTY will leave some Registry entries, and a random seed file, on
800 the PC (see \k{faq-settings}). If you are using PuTTY on a public
801 PC, or somebody else's PC, you might want to clean these up when you
802 leave. You can do that automatically, by running the command
805 \S{faq-dsa}{Question} How come PuTTY now supports DSA, when the
806 website used to say how insecure it was?
808 DSA has a major weakness \e{if badly implemented}: it relies on a
809 random number generator to far too great an extent. If the random
810 number generator produces a number an attacker can predict, the DSA
811 private key is exposed - meaning that the attacker can log in as you
812 on all systems that accept that key.
814 The PuTTY policy changed because the developers were informed of
815 ways to implement DSA which do not suffer nearly as badly from this
816 weakness, and indeed which don't need to rely on random numbers at
817 all. For this reason we now believe PuTTY's DSA implementation is
818 probably OK. However, if you have the choice, we still recommend you
821 \S{faq-virtuallock}{Question} Couldn't Pageant use
822 \cw{VirtualLock()} to stop private keys being written to disk?
824 Unfortunately not. The \cw{VirtualLock()} function in the Windows
825 API doesn't do a proper job: it may prevent small pieces of a
826 process's memory from being paged to disk while the process is
827 running, but it doesn't stop the process's memory as a whole from
828 being swapped completely out to disk when the process is long-term
829 inactive. And Pageant spends most of its time inactive.
831 \H{faq-admin} Administrative questions
833 \S{faq-domain}{Question} Would you like me to register you a nicer
836 No, thank you. Even if you can find one (most of them seem to have
837 been registered already, by people who didn't ask whether we
838 actually wanted it before they applied), we're happy with the PuTTY
839 web site being exactly where it is. It's not hard to find (just type
840 \q{putty} into \W{http://www.google.com/}{google.com} and we're the
841 first link returned), and we don't believe the administrative hassle
842 of moving the site would be worth the benefit.
844 In addition, if we \e{did} want a custom domain name, we would want
845 to run it ourselves, so we knew for certain that it would continue
846 to point where we wanted it, and wouldn't suddenly change or do
847 strange things. Having it registered for us by a third party who we
848 don't even know is not the best way to achieve this.
850 \S{faq-webhosting}{Question} Would you like free web hosting for the
853 We already have some, thanks.
855 \S{faq-sourceforge}{Question} Why don't you move PuTTY to
858 Partly, because we don't want to move the web site location (see
861 Also, security reasons. PuTTY is a security product, and as such it
862 is particularly important to guard the code and the web site against
863 unauthorised modifications which might introduce subtle security
864 flaws. Therefore, we prefer that the CVS repository, web site and
865 FTP site remain where they are, under the direct control of system
866 administrators we know and trust personally, rather than being run
867 by a large organisation full of people we've never met and which is
868 known to have had breakins in the past.
870 No offence to SourceForge; I think they do a wonderful job. But
871 they're not ideal for everyone, and in particular they're not ideal
874 \S{faq-mailinglist1}{Question} Why can't I subscribe to the
875 putty-bugs mailing list?
877 Because you're not a member of the PuTTY core development team. The
878 putty-bugs mailing list is not a general newsgroup-like discussion
879 forum; it's a contact address for the core developers, and an
880 \e{internal} mailing list for us to discuss things among ourselves.
881 If we opened it up for everybody to subscribe to, it would turn into
882 something more like a newsgroup and we would be completely
883 overwhelmed by the volume of traffic. It's hard enough to keep up
884 with the list as it is.
886 \S{faq-mailinglist2}{Question} If putty-bugs isn't a
887 general-subscription mailing list, what is?
889 There isn't one, that we know of.
891 If someone else wants to set up a mailing list or other forum for
892 PuTTY users to help each other with common problems, that would be
893 fine with us, though the PuTTY team would almost certainly not have the
894 time to read it. It's probably better to use the established
895 newsgroup \cw{comp.security.ssh} for this purpose.
897 \S{faq-donations}{Question} How can I donate to PuTTY development?
899 Please, \e{please} don't feel you have to. PuTTY is completely free
900 software, and not shareware. We think it's very important that
901 \e{everybody} who wants to use PuTTY should be able to, whether they
902 have any money or not; so the last thing we would want is for a
903 PuTTY user to feel guilty because they haven't paid us any money. If
904 you want to keep your money, please do keep it. We wouldn't dream of
907 Having said all that, if you still really \e{want} to give us money,
908 we won't argue :-) The easiest way for us to accept donations is if
909 you go to \W{http://www.e-gold.com}\cw{www.e-gold.com}, and deposit
910 your donation in account number 174769. Then send us e-mail to let
911 us know you've done so (otherwise we might not notice for months!).
912 Alternatively, if e-gold isn't convenient for you, you can donate to
913 \cw{<anakin@pobox.com>} using PayPal
914 (\W{http://www.paypal.com/}\cw{www.paypal.com}).
916 Small donations (tens of dollars or tens of euros) will probably be
917 spent on beer or curry, which helps motivate our volunteer team to
918 continue doing this for the world. Larger donations will be spent on
919 something that actually helps development, if we can find anything
920 (perhaps new hardware, or a copy of Windows XP), but if we can't
921 find anything then we'll just distribute the money among the
922 developers. If you want to be sure your donation is going towards
923 something worthwhile, ask us first. If you don't like these terms,
924 feel perfectly free not to donate. We don't mind.
926 \H{faq-misc} Miscellaneous questions
928 \S{faq-openssh}{Question} Is PuTTY a port of OpenSSH, or based on
931 No, it isn't. PuTTY is almost completely composed of code written
932 from scratch for PuTTY. The only code we share with OpenSSH is the
933 detector for SSH1 CRC compensation attacks, written by CORE SDI S.A.
935 \S{faq-sillyputty}{Question} Where can I buy silly putty?
937 You're looking at the wrong web site; the only PuTTY we know about
938 here is the name of a computer program.
940 If you want the kind of putty you can buy as an executive toy, the
941 PuTTY team can personally recommend Thinking Putty, which you can
942 buy from Crazy Aaron's Putty World, at
943 \W{http://www.puttyworld.com}\cw{www.puttyworld.com}.
945 \S{faq-meaning}{Question} What does \q{PuTTY} mean?
947 It's the name of a popular SSH and Telnet client. Any other meaning
948 is in the eye of the beholder. It's been rumoured that \q{PuTTY}
949 is the antonym of \q{\cw{getty}}, or that it's the stuff that makes your
950 Windows useful, or that it's a kind of plutonium Teletype. We
951 couldn't possibly comment on such allegations.
953 \S{faq-pronounce}{Question} How do I pronounce \q{PuTTY}?
955 Exactly like the English word \q{putty}, which we pronounce
956 /\u02C8{'}p\u028C{V}t\u026A{I}/.