[Kea-users] Need Help Overriding Standard Option in Kea DHCPv4

[Doug Freed]

On 6/24/25 14:21, Dipesh Kayastha wrote:
> Hi Kea Dev Team,
> 
> I’m migrating from ISC DHCP to Kea and encountered an issue when 
> attempting to define a custom option using code 121. In ISC DHCP, I was 
> able to define option 121 for custom use as follows:
> 
> option custom-option-121 code 121 = array of integer 8;
> 
> In Kea, when I try to replicate that using:
> 
> "option-def": [
>    {
>      "name": "custom-option-121",
>      "code": 121,
>      "type": "uint8",
> "array": true
> 
>    }
> ]
> 
> I get the following error:
> DHCP4_PARSER_FAIL failed to create or run parser for configuration 
> element option-def: unable to override definition of option '121' in 
> standard option space 'dhcp4'
> 
> 
> I understand from the Kea ARM documentation (https://kea.readthedocs.io/ 
> en/latest/arm/dhcp4-srv.html#id5 <https://kea.readthedocs.io/en/latest/ 
> arm/dhcp4-srv.html#id5>) that Option 121 is already part of the standard 
> DHCPv4 option definitions. However, in my use case, I need to send 
> custom data under option 121 because the client expects it.
> 
> Is there a supported way in Kea to:
> 
>  1.
> 
>     Override standard options like 121 with custom format/data?
> 
>  2.
> 
>     Send custom data using option 121 in a way compatible with clients
>     expecting it?
> 
> Any advice or suggested workarounds would be greatly appreciated.
> Note that this is not only for option 121, I am also using other option 
> numbers also as custom option.
> 
> Best regards,
> Dipesh
> 
> 

This is covered in this KB article:

https://kb.isc.org/docs/redefining-standard-options

If the values you need to send are arbitrary bytes (not just ASCII 
printable characters), you'll most likely need to use a hex string 
literal rather than a regular string literal.  See:

https://kea.readthedocs.io/en/stable/arm/classify.html#using-expressions-in-classification

-Doug