[Kea-users] Problems trying to implement RFC 8925 (v6-only-preferred)

[Brian Candler]

On 30/07/2023 14:28, Darren Ankney wrote:
> I have not tested this, but you could use the 'v6-only-preferred'
> setting in the subnet (see:
> https://kea.readthedocs.io/en/kea-2.4.0/arm/dhcp4-srv.html#ipv6-only-preferred-networks).

As the config I included in my original mail shows, I'm already using 
that option.  Yes it works.  But I still end up *offering* clients an 
IPv4 address from the pool, even those which request the 
v6-only-preferred option and will actually not request the offered 
address; and RFC 8925 says the server should set yiaddr 0.0.0.0 in this 
situation.

> If I'm understanding what you are saying ... DON'T allocate v4 to
> clients who DON'T send v6 only option?  Won't that result in a bunch
> of clients with no IP at all?

No, because those clients will still be able to get an IPv6 address (via 
either SLAAC or DHCPv6, depending on what I set up on the network)

> Kea does not appear to offer native support for RFC 2563.  I must not
> be understanding what you are saying.  You also want the clients that
> didn't receive any ip address (v4 or v6) to not auto-configure a
> 169.254 address?

Correct. Clients which don't understand RFC 8925 will still try to 
obtain an IPv4 address.  However, if they support RFC 2563 then I can 
detect this, and use this option to control whether or not they assign a 
link-local IPv4 address.  Again: if I'm not offering them a real IPv4 
address, then the RFC says the yiaddr should be 0.0.0.0.

Clients which don't implement either RFC 2563 or RFC 8925, I don't want 
to respond to.  If they choose to assign a link-local IPv4 address, 
that's up to them.

Regards,

Brian.