[Dnsmasq-discuss] dhcp-option and vendor class

Simon Kelley simon at thekelleys.org.uk
Mon Dec 29 13:20:38 GMT 2008

Ben wrote:
> Simon Kelley wrote:
>> Ben wrote:
>>> This works:
>>>  dhcp-vendorclass=apc,APC
>>>  dhcp-option=apc,43,01:04:31:41:50:43:02:01:01
>>> but this doesn't:
>>>  dhcp-option=vendor:APC,43,01:04:31:41:50:43:02:01:01
>>> What am I missing here?  Shouldn't the "vendor:APC" syntax work?
>> It does work: it does the encapsulation for you, so you need
>> dhcp-option=vendor:APC,01,31:41:50:43
>> dhcp-option=vendor:APC,02,01:01
>> depending on what those options are, you may what to use a more 
>> natural representation than hex bytes.
>> Your second example is taking the already-encapsulated options, 
>> treating them as a single option and encapsulating them again, which 
>> confuses things somewhat.
>> Cheers,
>> Simon.
> Simon,
> Thanks for the reply but i am afraid i still don't get it.
> I pulled that string straight from the APC manual without knowing what 
> it means. http://www.apcmedia.com/salestools/ASTE-6Z5QEW_R0_EN.pdf page 
> 61/62.
> How would i go about representing it in a more natural way?
> Ben

Apolgies, I assumed too much in my first reply.

dhcp options are represented like this:

<option number> <length of data> <bytes of data>

packed together, with an 0xff option number to mark the end. The option 
numbers are defined by IETF. Encapulated options is the IETF option 
number 43, and the data part contains a complete set of options "nested" 
inside it, but of course with the options numbers privately defined by 
the vendor.

So your


will generate the byte-string (43 is 2b hex)


and the encapsulated options in the data part are

01 (length 4) 31:41:50:43
02 (length 1) 01

Looking at the documentation, the APC is expecting the string "1APC" as 
encapsulated option 1 and a flag which enables use of BOOTP or DHCP as 
encapsulated option 2

so we can represnt these as


The "b" after the one tells dnsmasq that the integer "1" is to be 
represented as a single byte. (It knows this stuff for IETF options, but 
not for privately defined ones)

dnsmasq will send these options only if the client send a vendor class 
which matches "APC" and will gather them together withing an 
encapsulating option 43 - just what you want.



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