[Dnsmasq-discuss] DHCPv6 and MAC

[Simon Kelley]

On 07/02/13 21:27, Gene Czarcinski wrote:
> On 02/07/2013 04:22 PM, Gene Czarcinski wrote:
>> I was googling around and found this:
>>>
>>> Looks like I got it working. I must admit that I was going off of
>>> information back when IPv6 and DHCPv6 first came out.
>>>
>>> So it was my impression that in my DHCP server configuration I had to
>>> use the old style of:
>>> host-identifier option dhcp6.client-id
>>> 00:01:00:01:17:0B:B9:14:00:1D:60:B7:5F:D4;
>>> fixed-address6 FD35:4776:6804:1::1;
>>>
>>> Turns out there is now compatibility with the IPv4 style of setting
>>> fixed ip addresses where a clientid / duid isn't required and I was
>>> able to set it up as:
>>> hardware ethernet 00:0C:29:37:12:85;
>>> fixed-address6 FD35:4776:6804:2:1::1;
>>>
>>> I remember trying the previous a long time ago when hardware ethernet
>>> wasn't allowed to be used with IPv6 but it appears that has now changed.
>>>
>>> All is well now and things are finally back on track.
>>>
>> at: https://bbs.archlinux.org/viewtopic.php?id=139567
>>
>> The quoted text is comment #5
>>
>> The writer claims to be using dhclient and dhcpd.
>>
>> The problems described are the same ones which trouble me. Has
>> something changed with the DHCPv6 standard or did the folks at isc.org
>> just want something that worked?
>>
>>
> I should have waited because I found what might be a slightly different
> solution to the problem here:
> http://blog.geoff.co.uk/2011/07/08/dhcpv6-surprises/
>>
>> *Surprise #2: You can no longer simply use the MAC address of an
>> interface to assign a fixed IP address via DHCP.* Unlike DHCPv4, in
>> which the MAC address of the client interface is included in a DHCP
>> request, DHCPv6 uses a DHCP Unique Identifier
>> <http://tools.ietf.org/html/rfc3315#section-9>, or DUID, to uniquely
>> identify the client. By default, the DUID is synthesized from the MAC
>> address of an arbitrary interface on the host plus the system time at
>> the moment the DUID is generated. The same DUID is then used
>> regardless of which interface a DHCPv6 message originates from.
>>
>> The DUID normally persists across reboots, but if it’s deleted on the
>> client, e.g., when a new operating system is installed, the DUID is
>> automatically regenerated with a new time, and the server will no
>> longer recognize it. RFC 3315 <http://tools.ietf.org/html/rfc3315>
>> prohibits using a portion of the DUID as an identifier (it must be
>> treated as opaque), so DHCPv6 servers can’t be told to “just look at
>> the MAC address part” of the DUID.
>>
>> Fortunately for those wishing to use old-style MAC addresses, there’s
>> an alternative DUID type that let you do this. The DUID described
>> above is a “Type 1” DUID, and is referred to as a “Link-Layer plus
>> Time DUID”, or DUID-LLT. Many older DHCPv6 clients implement only Type
>> 1 DUIDs. You can configure ISC DHCPv6 client to use link-layer DUIDs
>> <http://tools.ietf.org/html/rfc3315#section-9.4> (DUID-LLs), which is
>> simply the MAC address prepended with two identifiers (the sequence
>> “00:03:00:01”). Moreover, these can be configured on a per-interface
>> basis in the ISC DHCP client.^3
>> <http://blog.geoff.co.uk/2011/07/08/dhcpv6-surprises/#footnote-3>
>> Problem solved.
>>
> 
> Now I just need to figure out how to get dhclient to use DUID-LL rather
> than DUID-LLT.
> 

Sadly, it ain't that easy. a DUID-LL is _generated_ using _a_ mac
address, but if the machine has more than one interface, the client can
use any of them, and use the same DUID for all the interfaces. Then if
the MAC address is changed for any reason, the client will likely
continue to use the existing DUID, not make a new one.


Essentially, the MAC address is used just to ensure that the DUID is
unique, not to make ide ntifying interfaces/machines easy. That wasn't
considered necessary by the definers if IPv6.


Cheers,

Simon.