[Dibbler] Content of duid
linux at thehobsons.co.uk
Tue Jun 26 18:05:17 CEST 2012
There was a 'fairly heated' discussion over on the ISC DHCP Users
List not too long ago about DUIDs. As I recall, opinion seemed to be
divided into those who believe that the use of MAC addresses is evil
and we should all be using DUIDs - and those who think that in the
real world the MAC address is no worse (and potentially better, less
volatile, commonly available before a system is connected and turned
on) than most DUIDs, and necessary for anyone trying to track
If you read the relevant RFCs, it is expressly forbidden to look into
the DUID even though it's format is defined. Eg, if the client is
using DUID-LL or DUID-LLT then it's forbidden to extract the MAC from
that. I believe that most clients don't use DUID-LL or DUID-LLT
For the OP, the RFC is very clear - you cannot under any
circumstances use the DUID (of any type) for anything but a straight
match. In practical terms, that means you'll need to power up and
connect the client, see what DUID it comes up with, and then you can
copy/paste that to match against. You can only match against the
whole thing (ie <something> == <something else>) and not look inside
it (ie substring<something>,... == <something else>).
Bear in mind that the RFCs specify that DUID-LLT and DUID-LL should
only be used by the client where there is no persistent storage (and
for LL, no clock either). For many devices, if the user were to reset
it or re-install the OS etc, then the DUID may well change. On many
devices, the user may well be able to easily change it - more easily
than changing the MAC.
Can't help with config of the server though.
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