[Dnsmasq-discuss] Do "proxy-DHCP" without PXE

[Simon Kelley]

On 29/04/2025 06:58, Steven Maddox wrote:
> Ah and I just realised the final sentence I used in my original post 
> said 'what' and not 'why' ... it meant to say...
> 
> " But I have a feeling I'm going to be told that "proxy-DHCP" is, and 
> can only be, a PXE specific thing?  if so, **why** is that? "

Exactly so. It's client behaviour that stops it, not server. The DHCP 
protocol allows multiple _servers_ on a network, and the client 
broadcasts, collects answers from potentially more than one, chooses 
which one it likes best, and then transacts with that single server. 
That's what the spec says, and that's what clients do.  Only PXE clients 
have the ability to talk to both a DHCP server and a PXE server.

The only way to implement this that I can see is to put the clients you 
want to mess with in a separate subnet with a DHCP relay back to the 
main the DHCP server and make a relay that can add arbitrary options. 
But that's kind of pointless, since if you've moved the phones (or 
whatever) to a new subnet you may as well configure a DHCP server there.

Cheers,

Simon.


> 
> Steven Maddox
> Lantizia
> 
> On 29/04/2025 06:55, Steven Maddox wrote:
>> Hi Simon,
>>
>> I didn't know about that, I'll take a look at it.
>>
>> But I have a feeling that it's not what I'm after, as the scenario I'm 
>> talking about doesn't involve any PXE clients.  See the example 
>> scenario I gave before where I referred to a Polycom phone.
>>
>> Steven Maddox
>> Lantizia
>>
>> On 28/04/2025 19:52, Simon Kelley wrote:
>>> The latest release, 2.91, includes a new option --dhcp-option-pxe, 
>>> which may be of use here.
>>>
>>> Cheers,
>>>
>>> Simon.
>>>
>>>
>>> On 28/04/2025 13:32, Steven Maddox wrote:
>>>> Hello,
>>>>
>>>> Sorry if what I'm about to ask is impossible, but I just really 
>>>> needed to know one way or another.
>>>>
>>>> I find dnsmasq such a handy utility for all kinds of situations (so 
>>>> huge thanks for creating it!)... one such situation was back when I 
>>>> was working with ThinStation and I'd do something like this to 
>>>> quickly test how a new build would boot on some PXE enabled machines...
>>>>
>>>> dnsmasq -d -i enp2s0 -F192.168.1.0,proxy --enable-tftp --tftp- 
>>>> root="$(pwd)/build/boot-images/pxe" -p0 --log-dhcp --pxe- 
>>>> service=x86PC,'Network Boot',boot/lpxelinux/lpxelinux
>>>>
>>>> Ultimately it's the "proxy-DHCP" that I'm focussing on here.
>>>>
>>>> Would it be possible to use this technique to add extra arbitrary 
>>>> DHCP options to a reply instead?  without touching the config of the 
>>>> authoritative "other" DHCP server? e.g...
>>>>
>>>> dnsmasq -d -i enp2s0 -F192.168.1.0,proxy --enable-tftp --tftp- 
>>>> root="$(pwd)" -p0 --log-dhcp -O 160,tftp://192.168.1.2
>>>>
>>>> This would be really handy in a scenario where you need to tell a 
>>>> bunch of devices (e.g. Polycom phones) a new provisioning URL (in 
>>>> that example option 160) on a temporary basis whilst you reboot/ 
>>>> factory-reset them. Especially when you're stuck using whatever 
>>>> existing DHCP server is on the network with no ability (or great 
>>>> difficulty) to reconfigure it.
>>>>
>>>> But I have a feeling I'm going to be told that "proxy-DHCP" is, and 
>>>> can only be, a PXE specific thing?  if so, what is that?
>>>>
>>>> Thanks
>>>>
>>>
>>>
>>> _______________________________________________
>>> Dnsmasq-discuss mailing list
>>> Dnsmasq-discuss at lists.thekelleys.org.uk
>>> https://lists.thekelleys.org.uk/cgi-bin/mailman/listinfo/dnsmasq-discuss
> 
> _______________________________________________
> Dnsmasq-discuss mailing list
> Dnsmasq-discuss at lists.thekelleys.org.uk
> https://lists.thekelleys.org.uk/cgi-bin/mailman/listinfo/dnsmasq-discuss