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We've occasionally had reports of SSH servers disconnecting as soon as they receive PuTTY's KEXINIT. I think all such reports have involved the kind of simple ROM-based SSH server software you find in small embedded devices. I've never been able to prove it, but I've always suspected that one possible cause of this is simply that PuTTY's KEXINIT is _too long_, either in number of algorithms listed or in total length (especially given all the ones that end in @very.long.domain.name suffixes). If I'm right about either of those being the cause, then it's just become even more likely to happen, because of all the extra Diffie-Hellman groups and GSSAPI algorithms we just threw into our already-long list in the previous few commits. A workaround I've had in mind for ages is to wait for the server's KEXINIT, and then filter our own down to just the algorithms the server also mentioned. Then our KEXINIT is no longer than that of the server, and hence, presumably fits in whatever buffer it has. So I've implemented that workaround, in anticipation of it being needed in the near future. (Well ... it's not _quite_ true that our KEXINIT is at most the same length as the server. In fact I had to leave in one KEXINIT item that won't match anything in the server's list, namely "ext-info-c" which gates access to SHA-2 based RSA. So if we turn out to support absolutely everything on all the server's lists, then our KEXINIT would be a few bytes longer than the server's, even with this workaround. But that would only cause trouble if the server's outgoing KEXINIT was skating very close to whatever buffer size it has for the incoming one, and I'm guessing that's not very likely.) ((Another possible cause of this kind of disconnection would be a server that simply objects to seeing any KEXINIT string it doesn't know how to speak. But _surely_ no such server would have survived initial testing against any full-featured client at all!))
This is the README for PuTTY, a free Windows and Unix Telnet and SSH
client.
PuTTY is built using CMake <https://cmake.org/>. To compile in the
simplest way (on any of Linux, Windows or Mac), run these commands in
the source directory:
cmake .
cmake --build .
Then, to install in the simplest way on Linux or Mac:
cmake --build . --target install
On Unix, 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.) The cmake install step
doesn't attempt to add these privileges, so if you want user login
recording to work, you should manually ch{own,grp} and chmod the
pterm binary yourself after installation. If you don't do this,
pterm will still work, but not update the user login databases.
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.