Can we disable diffie-hellman-group-exchange-sha1 by default?

Jochen Bern Jochen.Bern at
Sat Feb 16 18:47:30 AEDT 2019

On 02/15/2019 06:07 PM, Christoph Anton Mitterer wrote:
> On Fri, 2019-02-15 at 15:57 +1100, Darren Tucker wrote:
>> That was the original intent (and it's mentioned in RFC4419) however
>> each moduli file we ship (70-80 instances of 6 sizes)  takes about 1
>> cpu-month to generate on a lowish-power x86-64 machine.  Most of it
>> is parallelizable, but even then it'd likely take a few hours to
>> generate one of each size.  I imagine that'd cause some complaints
>> about startup time.
> One way of handling this, at least if the moduli generation was nicely
> interruptable, is that distros ship a file, e.g. /etc/ssh/moduli.dist
> but by some mean (init script, systemd unit, maybe even sshd itself) a
> daemon that does calculation of new moduli values is started whenever
> /etc/ssh/moduli (no ".dist") isn't found.

I'm (manually) creating the shorter moduli anew for every VM I set up
(which may have but one core), but have the longer ones copied after
being created *once* for every "platform" (definition subject to
practicality). Running *that* much of ssh-keygen already takes longer
than people are willing to wait for the VM that's needed oh-so-urgently
for a customer-visible change. I can't say that I would be very happy
about an out-of-the-box "Hulk smash puny CPU!!" behavior.

Considering that the moduli file already varies with the distro, I'ld
say that the duty of recognizing it / that situation (and reacting to
it) is *not* on the OpenSSH maintainers, beyond maybe suggesting a
global mechanism to do so (*). Since /etc/ssh/moduli's syntax supports
comment lines, how about having the distro moduli files include one alike

# DO-MODULI-WARNING This is an unchanged $DISTRO moduli file. You *want*
to (create and) install one of your own (or delete this comment to hide
your disgrace and make this OpenSSH shut up).

and then have the distro's choice of escalating warnings /
countermeasures (**) triggered by a '^# DO-MODULI-WARNING' regexp?

(*) I first wanted to suggest recognizing distro's default moduli files
by checksum, but that would discourage having it updated / recomputed
frequently by the package maintainers (boatload of historic checksums to
check for), which is *not* what we want.

(**) Say, start with the equivalent of a hardcoded login banner,
escalate all the way up to running a *very* niced-down moduli generation
when nobody intervened for a year ...

Jochen Bern

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