dhcpcd-discuss

Hi Joachim

On 08/01/2015 03:31, Joachim Achtzehnter wrote:
>>> This is an answer only if one wanted to disable autonomous
>>> auto-configuration globally as a policy for the subnet, but this wasn't
>>> my question. Sorry, I should have made this clear.
>>>
>>> The question is about client-side configuration. On a subnet where the
>>> router allows both autonomous and managed address configuration we want
>>> to configure a host such that it only configures the address assigned by
>>> DHCP, but not generate addresses autonomously based on the hardware
>>> address or randomized (privacy extension).
>>
>> It helps if I read messages like this if not concentrating on
>> Christmas :)
>>
>> OK, so basically if Auto is set AND Managed is set, you want an option to
>> ignore Auto.
> 
> Yes, exactly. The addrconf RFC says that "Creation of global and
> site-local addresses and configuration of other parameters as described
> in this section SHOULD be locally configurable. However, the processing
> described below MUST be enabled by default."
> 
> So the standards do not object to having an option for a local system to
> opt-out of stateless auto-configuration. On Linux systems, this is
> usually controlled by the autoconf kernel parameter, which my original
> patch tried to honour.
> 
>> Here is a patch which allows this, but before I commit it, a question.
>> If we set this new option and receive a RA with Managed set so DHCPv6 is
>> kicking off but a new RA appears without Managed set but with prefixes
>> and auto
>> flags set, should dhcpcd ignore these also or generate SLAAC addresses
>> for
>> them?
> 
> Yes, otherwise the option would be pointless. When this option is
> engaged no other global scope addresses should be configured, except the
> one negotiated with a DHCP server.
> 
> We have a similar use case when the local administrator wants to assign
> an address manually, and not have anything assigned automatically,
> except for the link-local address. Of course, in this case we simply
> don't call the DHCP client.

So many use cases and possible problems.
I played around with my patch in various scenarios it and wasn't exactly
ideal.

So I revisited your original patch (respect Linux ipv6/conf/autoconf
parameter) and changed it to a generic dhcpcd option as this kernel
tunable doesn't exist in any BSD I know of. This puts the decision
entirely in the hands of the user.
Comitted here:
http://roy.marples.name/projects/dhcpcd/ci/4dd3f7b8d9e0ae20755b1224eb0375a141bf21f6?sbs=0

But still, this doesn't match your original request:
> On a subnet where the
> router allows both autonomous and managed address configuration we
> want to configure a host such that it only configures the address
> assigned by DHCP, but not generate addresses autonomously based on
> the hardware address or randomized (privacy extension).

So this is actually quite tricky, for a variety of reasons. What if
there is no DHCP server? What if it stops responding and the address
expires? What is there is another router with a different subnet on the
same network that requires autoconf as there is no DHCP?

Admittedly the last one is unlikely, but still technically possible.
Now, it's only really tricky because of your statement:
> only configures the address assigned by DHCP

So let us revisit the rationale for your request, ie why you asked this
in the first place. I'm hopeful that you are asking for this to control
source address selection - as all things being equal Linux will use the
last IPv6 address added to the interface. So, lets make things unequal
with a new setting which will set the preferred lifetime of the autoconf
addresses to 0 when a DHCPv6 address exists on the interface. If, for
any reason, the DHCPv6 addresses expire or are removed then the
preferred lifetimes are restored to the autoconf addresses.

Is this what you really want - source address selection?

Roy