DH Group Exchange Fallback

[Joseph S Testa II]

On 09/24/2017 06:01 PM, Damien Miller wrote:
> On Thu, 21 Sep 2017, Joseph S Testa II wrote:
> 
>> Hi,
>>
>>     I'm interested in requiring a minimum of 3072-bit DH moduli when using the
>> "diffie-hellman-group-exchange-sha256" kex, so I edited my /etc/ssh/moduli
>> file such that only 3071+ moduli are left.  However, when clients ask for a
>> max of 2048-bit moduli, they actually get one (!).  I poked around and found
>> that a fallback mechanism exists (dh.c:185), which returns back the fixed
>> group14 Group in that case (dh.c:441).
> 
> 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?

Yeah, I suppose a disconnect would be the option that most closely 
follows RFC4419.  Or the code could bend the rules, return the smallest 
modulus (3072-bit), and let the client decide if it wants to continue or 
not (or crash).

I don't have a strong opinion on this part, personally.

   - Joe