DH Group Exchange Fallback

[Damien Miller]

On Mon, 25 Sep 2017, Darren Tucker wrote:

> On 25 September 2017 at 08:01, Damien Miller <djm at mindrot.org> wrote:
> [...]
> > What behaviour would you like for this case? We can't send a group that is
> > larger than 2048, because the client has said that is the maxiumum that
> > will accept. Would you prefer the server disconnect at that point?
> 
> Maybe only offer group14 in dh-gex the fallback path if the group14
> methods are in KexAlgorithms?

Well, the problem with being restrictive in DH-GEX is that it can
then fail in common circumstances after it has been selected. KEX
algorithms don't usually do that - if you offer one then it will
usually complete (barring internal errors).

-d