When you send a "publickey" USERAUTH_REQUEST containing a certified
RSA key, and you want to use a SHA-2 based RSA algorithm, modern
OpenSSH expects you to send the algorithm string as
rsa-sha2-NNN-cert-v01@openssh.com. But 7.7 and earlier didn't
recognise those names, and expected the algorithm string in the
userauth request packet to be ssh-rsa-cert-v01@... and would then
follow it with an rsa-sha2-NNN signature.
OpenSSH itself has a bug workaround for its own older versions. Follow
suit.
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!))
My bulk indentation check also turned up a lot of cases where a run-on
function call or if statement didn't have its later lines aligned
correctly relative to the open paren.
I think this is quite easy to do by getting things out of
sync (editing the first line of the function call and forgetting to
update the rest, perhaps even because you never _saw_ the rest during
a search-replace). But a few didn't quite fit into that pattern, in
particular an outright misleading case in unix/askpass.c where the
second line of a call was aligned neatly below the _wrong_ one of the
open parens on the opening line.
Restored as many alignments as I could easily find.
It's a thoroughly pedantic point, but I just spotted that the
comparison of wire data against theoretically platform-dependent char
escapes is a violation of \k{udp-portability}.
Ian Jackson recently tried to use the recipe in the psusan manpage for
talking to UML, and found that the connection was not successfully set
up, because at some point during startup, UML read the SSH greeting
(ok, the bare-ssh-connection greeting) from its input fd and threw it
away. So by the time psusan was run by the guest init process, the
greeting wasn't there to be read.
Ian's report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=991958
I was also able to reproduce this locally, which makes me wonder why I
_didn't_ notice it when I originally wrote that part of the psusan man
page. It worked for me before, honest! But now it doesn't.
Anyway. The ssh verstring module already has a mode switch to decide
whether we ought to send our greeting before or after waiting for the
other side's greeting (because that decision varies between client and
server, and between SSH-1 and SSH-2). So it's easy to implement an
override that forces it to 'wait for the server greeting first'.
I've added this as yet another bug workaround flag. But unlike all the
others, it can't be autodetected from the server's version string,
because, of course, we have to act on it _before_ seeing the server's
greeting and version string! So it's a manual-only flag.
However, I've mentioned it in the UML section of the psusan man page,
since that's the place where I _know_ people are likely to need to use
this flag.
This clears up another large pile of clutter at the top level, and in
the process, allows me to rename source files to things that don't all
have that annoying 'ssh' prefix at the top.
