[Dnsmasq-discuss] dhcp-host precedence request due multiple matches

Jesus M Diaz jesusm.diazperez at gmail.com
Mon Jun 21 21:15:52 UTC 2021


Yes, I agree that those are the general assumptions, but sometimes there
are exceptions or rare cases, and I simply asked, but apparently, what I
was trying to get is not possible. And that's ok.

I tried to set up client-id but still mac-address seems to have preference,
and I cannot force all physical devices to use client-id (I don't have full
control of all of them).

Thanks for the suggestions anyway.

What actually surprises me from this forum (I will admit that I am pretty
new in it), is that quite often people use more time judging the reasons
one may have to do things, or to try things, instead of simply answering
questions. I totally understand that to answer a question some context is
needed, and understanding why you want to do something can help to find a
solution or an alternative way to get the result, but this is the second
question I asked in here and the second time I was questioned about whether
what I wanted to do made sense or not, to the point of even arguing about
if I mentioned some words or others (as VM versus 'a virtual computer'). I
am finding people pretty interested on doing things in the only way to do
things, when in my opinion, networking in general and Internet specifically
have evolved by thinking out of the box and using the tools in ways they
were not initially designed for (a very simple example, initially the DNS
didn't include IPv6 -rfc 1034-,and DNS software was updated to get that
later on, and I cannot imagine anyone saying 'you want to use DNS for
something different, that's not the purpose of DNS').

Anyway, thanks everybody for all the comments, there is always a chance to
learn from them.

On Mon, 21 Jun 2021 at 18:35, Dominick C. Pastore <dominickpastore at dcpx.org>
wrote:

> There are much smarter people than me here, but if I might chime in:
>
> Two basic assumptions in networking are that no two separate devices will
> share the same MAC address, and a device's MAC address doesn't change after
> it's manufactured. Obviously there are practical exceptions (spoofing,
> privacy
> MAC addresses, etc.), but those are the assumptions we operate under (I
> think
> this is the real takeaway from the discussion about VMs). Meanwhile, no
> such
> assumptions can be made about hostnames.
>
> So, from Dnsmasq's point of view, given conflicting configuration, it makes
> sense to always prefer the MAC address over the hostname, since it's a
> stronger
> identifier.
>
> Accordingly, if the assumption about MAC address <=> machine doesn't hold
> true
> in your environment, it seems to me the best option is probably to avoid
> using
> MAC addresses as machine identifiers at all. Can you set DHCP client IDs?
> Or
> use hostnames for the host machines' regular OSes as well?
>
> Alternatively, can you set up the live USBs to use custom MAC addresses,
> so the assumption does hold true?
>
> Regards,
> Nick
>
> On Mon, Jun 21, 2021, at 12:09 PM, Jesus M Diaz wrote:
>
> I never talked of VM on purpose, because they are not VM (hence, no
> hypervisor or any other controller plane). Indeed I talked of live linux
> distributions running on USB that I only use for very specific tasks, and
> of course, I run them over physical machines, but these physical
> machines are usually running their own OS.
>
> But all this, imho, is not relevant at all, it was explained just to
> express that *sometimes* it might happen that one DHCP request matches more
> than one dhcp-host entry in the dnsmasq.con file. That is the only relevant
> situation, and my question was: is there any way to set a precedence among
> the potentially matching dhcp-hosts entries?
>
> I don't really understand why the philosophical discussion on what a
> computer is or is not.
>
> Thanks anyway everybody for the comments.
>
>
> On Mon, 21 Jun 2021 at 14:54, Alex Morris <
> dnsmasq-discuss at alexmorris.co.uk> wrote:
>
> On Mon, 21 Jun 2021, at 13:42, Jesus M Diaz wrote:
> >
> > so, if I have two virtual computers running over the same hardware,
> >
>
> ...then your hypervisor ought to provide unique MAC addresses for each
> guest NIC.
>
> >
> > what should be used as identifier for dnsmasq?
> >
>
> The unique MAC address allocated by the hypervisor to the guest NIC.
>
>
> --
> Alex
>
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