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The problem with OpenSSH delayed compression is that the spec has a race condition. Compression is enabled when the server sends USERAUTH_SUCCESS. In the server->client direction, that's fine: the USERAUTH_SUCCESS packet is not itself compressed, and the next packet in the same direction is. But in the client->server direction, this specification relies on there being a moment of half-duplex in the connection: the client can't send any outgoing packet _after_ whatever userauth packet the USERAUTH_SUCCESS was a response to, and _before_ finding out whether the response is USERAUTH_SUCCESS or something else. If it emitted, say, an SSH_MSG_IGNORE or initiated a rekey (perhaps due to a timeout), then that might cross in the network with USERAUTH_SUCCESS and the server wouldn't be able to know whether to treat it as compressed. My previous solution was to note the presence of delayed compression options in the server KEXINIT, but not to negotiate them in the initial key exchange. Instead, we conduct the userauth exchange with compression="none", and then once userauth has concluded, we trigger an immediate rekey in which we do accept delayed compression methods - because of course by that time they're no different from the non- delayed versions. And that means compression is enabled by the bidirectional NEWKEYS exchange, which lacks that race condition. I think OpenSSH itself gets away with this because its layer structure is structure so as to never send any such asynchronous transport-layer message in the middle of userauth. Ours is not. But my cunning plan is that now that my BPP abstraction includes a queue of packets to be sent and a callback that processes that queue on to the output raw data bufchain, it's possible to make that callback terminate early, to leave any dangerous transport-layer messages unsent while we wait for a userauth response. Specifically: if we've negotiated a delayed compression method and not yet seen USERAUTH_SUCCESS, then ssh2_bpp_handle_output will emit all packets from its queue up to and including the last one in the userauth type-code range, and keep back any further ones. The idea is that _if_ that last userauth message was one that might provoke USERAUTH_SUCCESS, we don't want to send any difficult things after it; if it's not (e.g. it's in the middle of some ongoing userauth process like k-i or GSS) then the userauth layer will know that, and will emit some further userauth packet on its own initiative which will clue us in that it's OK to release everything up to and including that one. (So in particular it wasn't even necessary to forbid _all_ transport- layer packets during userauth. I could have done that by reordering the output queue - packets in that queue haven't been assigned their sequence numbers yet, so that would have been safe - but it's more elegant not to have to.) One particular case we do have to be careful about is not trying to initiate a _rekey_ during userauth, if delayed compression is in the offing. That's because when we start rekeying, ssh2transport stops sending any higher-layer packets at all, to discourage servers from trying to ignore the KEXINIT and press on regardless - you don't get your higher-layer replies until you actually respond to the lower-layer interrupt. But in this case, if ssh2transport sent a KEXINIT, which ssh2bpp kept back in the queue to avoid a delayed compression race and would only send if another userauth packet followed it, which ssh2transport would never pass on to ssh2bpp's output queue, there'd be a complete protocol deadlock. So instead I defer any attempt to start a rekey until after userauth finishes (using the existing system for starting a deferred rekey at that moment, which was previously used for the _old_ delayed-compression strategy, and still has to be here anyway for GSSAPI purposes).
This is the README for the source archive of PuTTY, a free Windows
and Unix Telnet and SSH client.
If you want to rebuild PuTTY from source, we provide a variety of
Makefiles and equivalents. (If you have fetched the source from
Git, you'll have to generate the Makefiles yourself -- see
below.)
There are various compile-time directives that you can use to
disable or modify certain features; it may be necessary to do this
in some environments. They are documented in `Recipe', and in
comments in many of the generated Makefiles.
For building on Windows:
- windows/Makefile.vc is for command-line builds on MS Visual C++
systems. Change into the `windows' subdirectory and type `nmake
-f Makefile.vc' to build all the PuTTY binaries.
As of 2017, we successfully compile PuTTY with both Visual Studio
7 (2003) and Visual Studio 14 (2015), so our guess is that it will
probably build with versions in between those as well.
(The binaries from Visual Studio 14 are only compatible with
Windows XP and up. Binaries from Visual Studio 7 ought to work
with anything from Windows 95 onward.)
- Inside the windows/MSVC subdirectory are MS Visual Studio project
files for doing GUI-based builds of the various PuTTY utilities.
These have been tested on Visual Studio 7 and 10.
You should be able to build each PuTTY utility by loading the
corresponding .dsp file in Visual Studio. For example,
MSVC/putty/putty.dsp builds PuTTY itself, MSVC/plink/plink.dsp
builds Plink, and so on.
- windows/Makefile.mgw is for MinGW / Cygwin installations. Type
`make -f Makefile.mgw' while in the `windows' subdirectory to
build all the PuTTY binaries.
MinGW and friends can lag behind other toolchains in their support
for the Windows API. Compile-time levers are provided to exclude
some features; the defaults are set appropriately for the
'mingw-w64' cross-compiler provided with Ubuntu 14.04. If you are
using an older toolchain, you may need to exclude more features;
alternatively, you may find that upgrading to a recent version of
the 'w32api' package helps.
- windows/Makefile.lcc is for lcc-win32. Type `make -f
Makefile.lcc' while in the `windows' subdirectory. (You will
probably need to specify COMPAT=-DNO_MULTIMON.)
- Inside the windows/DEVCPP subdirectory are Dev-C++ project
files for doing GUI-based builds of the various PuTTY utilities.
The PuTTY team actively use Makefile.vc (with VC7/10) and Makefile.mgw
(with mingw32), so we'll probably notice problems with those
toolchains fairly quickly. Please report any problems with the other
toolchains mentioned above.
For building on Unix:
- unix/configure is for Unix and GTK. If you don't have GTK, you
should still be able to build the command-line utilities (PSCP,
PSFTP, Plink, PuTTYgen) using this script. To use it, change into
the `unix' subdirectory, run `./configure' and then `make'. Or you
can do the same in the top-level directory (we provide a little
wrapper that invokes configure one level down), which is more like
a normal Unix source archive but doesn't do so well at keeping the
per-platform stuff in each platform's subdirectory; it's up to you.
- unix/Makefile.gtk and unix/Makefile.ux are for non-autoconfigured
builds. These makefiles expect you to change into the `unix'
subdirectory, then run `make -f Makefile.gtk' or `make -f
Makefile.ux' respectively. Makefile.gtk builds all the programs but
relies on Gtk, whereas Makefile.ux builds only the command-line
utilities and has no Gtk dependence.
- For the graphical utilities, any of Gtk+-1.2, Gtk+-2.0, and Gtk+-3.0
should be supported. If you have more than one installed, you can
manually specify which one you want by giving the option
'--with-gtk=N' to the configure script where N is 1, 2, or 3.
(The default is the newest available, of course.) In the absence
of any Gtk version, the configure script will automatically
construct a Makefile which builds only the command-line utilities;
you can manually create this condition by giving configure the
option '--without-gtk'.
- pterm would like to be setuid or setgid, as appropriate, to permit
it to write records of user logins to /var/run/utmp and
/var/log/wtmp. (Of course it will not use this privilege for
anything else, and in particular it will drop all privileges before
starting up complex subsystems like GTK.) By default the makefile
will not attempt to add privileges to the pterm executable at 'make
install' time, but you can ask it to do so by running configure
with the option '--enable-setuid=USER' or '--enable-setgid=GROUP'.
- The Unix Makefiles have an `install' target. Note that by default
it tries to install `man' pages; if you have fetched the source via
Git then you will need to have built these using Halibut
first - see below.
- It's also possible to build the Windows version of PuTTY to run
on Unix by using Winelib. To do this, change to the `windows'
directory and run `make -f Makefile.mgw CC=winegcc RC=wrc'.
All of the Makefiles are generated automatically from the file
`Recipe' by the Perl script `mkfiles.pl' (except for the Unix one,
which is generated by the `configure' script; mkfiles.pl only
generates the input to automake). Additions and corrections to Recipe,
mkfiles.pl and/or configure.ac are much more useful than additions and
corrections to the actual Makefiles, Makefile.am or Makefile.in.
The Unix `configure' script and its various requirements are generated
by the shell script `mkauto.sh', which requires GNU Autoconf, GNU
Automake, and Gtk; if you've got the source from Git rather
than using one of our source snapshots, you'll need to run this
yourself. The input file to Automake is generated by mkfiles.pl along
with all the rest of the makefiles, so you will need to run mkfiles.pl
and then mkauto.sh.
Documentation (in various formats including Windows Help and Unix
`man' pages) is built from the Halibut (`.but') files in the `doc'
subdirectory using `doc/Makefile'. If you aren't using one of our
source snapshots, you'll need to do this yourself. Halibut can be
found at <https://www.chiark.greenend.org.uk/~sgtatham/halibut/>.
The PuTTY home web site is
https://www.chiark.greenend.org.uk/~sgtatham/putty/
If you want to send bug reports or feature requests, please read the
Feedback section of the web site before doing so. Sending one-line
reports saying `it doesn't work' will waste your time as much as
ours.
See the file LICENCE for the licence conditions.