Re: How to configure NetworkManager to use DUID-LL type with dhclient?

[Dan Williams <dcbw redhat com>]

On Mon, 2016-05-09 at 14:23 -0300, Jorge L. Corrêa wrote:
> Hi, I've been looking for information about how to configure network
> manager to pass parameters to dhclient. The goal is use dhclient with
> "-D LL" (use DUID-LL as default type of DUID generation). I can see
> that NetworkManager is calling dhclient in a way that DUID-LLT is
> being used. 
>
> For IPv6 networks we need to ensure that a host has, at minimum, one
> known IPv6 address. So, after the first DUID generation, we need to
> register this value in a system and ensure that this value doesn't
> change anymore. If we use DUID-LLT we cannot do that. So, we need
> that NetworkManager calls dhclient using DUID-LL (dhclient -D LL
> ...). 

If there is no preconfigured DUID, then NetworkManager will generate a
DUID-UUID from the machine-id file following RFC 6355.

https://tools.ietf.org/html/rfc6355

NM uses DUID-UUID because DUID-LL/LLT are not guaranteed to remain
constant across boots, since network device addresses can change and
many devices don't bother to store a MAC address in NVRAM but generate
a random one each boot (especially for embedded devices).

Once generated, this DUID-UUID gets stored into the leasefile for the
connection.  Since it's generated from the machine-id (which by
definition shouldn't change for the life of the machine/install) it
will be the same regardless of whether you swap out NICs or the
timestamp, which follows the DHCPv6 standards which say the DUID should
not change when network hardware changes (RFC 3315, section 9).

If you really want a DUID-LL, you can override it by specifying a DUID
in the connection specific leasefile (usually /var/lib/NetworkManager)
or in more standard leasefile locations for dhclient, like
/etc/dhclient6.leases or /var/lib/dhcp/dhclient6.leases.  NM will check
those locations first if no connection specific leasefile from
/var/lib/NetworkManager is found, and then copy that DUID to the
connection specific leasefile when it gets created.

Does that help?

Dan