[kea-dev] ISC DHCP 4o6

[Francis Dupont]

In order to help the design of DHCP 4o6 support in Kea, here are some
notes about the DHCP 4o6 support in ISC DHCP.

First ISC DHCP uses a monolithic configuration file so the first question
is about handling both DHCPv4 and DHCPv6 in the same server process
with one configuration file and one lease file for both.

The answer is it can work but it is nearly impossible to use. There is
a negative proof of this in the #36639 ticket / rt36639 branch.

So I went to a 2 server design: one for DHCPv6 and one for DHCPv4,
all DHCP 4o6 messages are given to the companion server using a pair
of consecutive bound sockets (make easier to detect when the companion
is not available). A minimal information is prepended to help the
subnet selection, itself a mixed of DHCPv6 and DHCPv4 usual checks.

For similar reasons I found the 2 process design easier for the client:
the DHCPv6 client handles standard DHCPv6 messages and communicates
with a DHCPv4 client (usually a child launched through a script)
for DHCP 4o6.

Of course there is nothing for relays as DHCP 4o6 is transparent
for DHCPv6 relays...

The whole stuff has been under review since September 2014 and is
available in ticket #35711 / branch rt35711b (note the 'b').

Regards

Francis Dupont <fdupont at isc.org>