[Kea-users] subnet-mask not sent in DHCPINFORM flow

[Darren Ankney]

Hi Adonis,

If I understand correctly, you are performing a stateless
configuration (https://kea.readthedocs.io/en/stable/arm/dhcp4-srv.html#stateless-configuration-of-dhcpv4-clients)?
 Does it make sense to send a subnet mask since you must have already
configured such IP information on the client?  RFC 2131 section 3.4
(https://www.rfc-editor.org/info/rfc2131/#section-3.4) says:

> Servers receiving a
> DHCPINFORM message construct a DHCPACK message with any local
> configuration parameters appropriate for the client without:
> allocating a new address, checking for an existing binding, filling
> in 'yiaddr' or including lease time parameters.  The servers SHOULD
> unicast the DHCPACK reply to the address given in the 'ciaddr' field
> of the DHCPINFORM message.

which would seem to imply that Kea would not be able to find such
address related information as subnet mask (since it did not go
through the process of allocating the address).  Does the MacOS not
let you manually configure the subnet mask when you are setting an
address on the interface?

If you don't mind, a traffic capture file (pcap) that could be opened
in Wireshark would be very informative here. You can obtain such a
capture using tcpdump on the kea-dhcp4 server: "tcpdump -i <interface>
-w <file.pcap> port 67".  Run that command, cause MacOS to send a
DHCPINFORM, then stop the command.  This file can be opened in
Wireshark for packet inspection and filtering.  I would be interested
to see what MacOS requested and what kea-dhcp4 responded with.

Thank you,
Darren Ankney

On Mon, Nov 9, 2026 at 12:00 PM Adonis <smooth.map9660 at isadon.com> wrote:
>
> Hi all,
>
> So on macOS 27.2, when I configure DHCP with a manual address: 192.168.1.50, kea-dhcp4 (3.2.0) matches the client to one of its subnets in the configuration but sends no subnet-mask (option 1). This causes macOS to set the subnet mask to 255.255.255.255 (instead of 255.255.255.0) and break network connectivity. When using DHCP or DHCP with manual addressing in macOS the subnet-mask cannot be changed, it is meant to use whatever the server sends.
>
> In the initial DHCP request (DHCPINFORM option 53) by macOS (src ip 0.0.0.0 to dst ip 255.255.255.255) I can see the ciaddr is correctly set to 192.168.1.50 in the request and the request asks for a subnet mask (DHCP option 55 param 1). Yet in the response from kea no subnet-mask is sent while other options like domain-name, domain-search, domain-name-servers configured for the subnet for 192.168.1.50 are sent. The subnet declaration in kea-dhcp4.conf has "192.168.1.0/24" set and is being matched properly, but it seems the auto subnet mask calculation is not run and the respective subnet-mask field is not sent in response. Given the subnet-mask is explicitly stated to be auto-calculated and managed, this behavior seems like a bug to me.
>
> --
> Adonis
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