[Dnsmasq-discuss] Boot file name not given (DHCP Option: 67 Bootfile name - is set but not used in PXE clients)

[Christoph Loesch]

Hi,

Am 08.03.2021 um 04:24 schrieb Oliver Freyermuth:
> Hi,
>
> Am 08.03.21 um 00:42 schrieb Christoph Loesch:
>> Hi,
>>
>> with dnsmasq (version 2.80 on current OpenWRT 19.07.7) I noticed following (in my opinion wrong) behaviour:
>>
>> when using the setting "dhcp-boot=/pxelinux.0,servername,192.168.1.123" the DHCP Offer/ACK packets sent do not contain the "Boot file name" in the header but "Option: (67) Bootfile name" is set.
>
> it's not "wrong" in my opinion, however, there are still many legacy / broken clients out there which don't conform to the DHCP standard completely,

Thanks for the pointer about legacy / broken clients!


> and only check the classic BOOTP headers. A noteworthy quite recent example is Grub2, which only learnt this in 2019:
>  https://github.com/rhboot/grub2/commit/93289dc67c7e213b21df0eb09afea5e3b00ad7df#diff-93cd75cf8712d66c15ae5885f7cac5dae3531aaef211a17ef9885eb443ecdb0b
>
>> Some DHCP clients, who read Option 67 can work with that, but some clients like PXE do not read that option therefor are failing for first tftp file rrq.
>>
>> As I could not find a git commit regarding a fix for this, I wanted to ask if someone can confirm (and/or possibly fix) this on recent dnsmasq version?
>
> I also have such legacy clients in use. The option "dhcp-no-override" forces dnsmasq to use the classic BOOTP headers.

Thanks, I tried that but it still didn't work out, I compared DHCP packets between a working and my non-working environment but besides TFTP server not beeing on same host as the DHCP server, the packets were same.


> Note that in general, using the dedicated options for servername and filename is preferrable, since it allows to reuse the space in the headers for options,
> but if you have legacy clients (as we also have) you have to set "dhcp-no-override".
>
> Does this also solve your issue?

Unluckily not, but your hint about broken clients got me thinking, I should test to run a TFTP server also on OpenWRT instead on another host, which then worked as expected.

So I assume my legacy PXE 2.00 clients are indeed broken, because they are not able to use a TFTP server on another host other than on the DHCP server it seems.

Thanks again and kind regards,
Christoph

>
> Cheers,
>     Oliver
>
>>
>> Kind regards,
>> Christoph
>>
>>
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