[Kea-users] Impact of UDP mode on HA Hot-Standby configuration

[Marcin Siodelski]

Waldek,

High Availability hook has no dependency on the type of the socket used. 
However, typically, UDP and raw sockets are used in two different cases. 
The raw sockets are necessary when the clients are in the same network 
segment as the DHCP server. In this case, the server receives the 
broadcast traffic from the clients. The UDP sockets are typically used 
for the relayed traffic. In this case, the relay agent must be 
configured to send the traffic to both DHCP servers behind it, so all 
the messages are received by both servers. In case of an outage of one 
of the servers, the relay still sends the traffic to both but only the 
one online receives it and takes over when necessary.

Can you tell me if your "UDP test" involves relayed traffic, and whether 
or not the relay is configured to forward the traffic to both HA partners?

Marcin Siodelski

Principal Software Engineer

ISC

On 11/3/26 3:57 PM, Waldek Maleska wrote:
> Hello,
>
> This is my first message to this mailing list so first let me thank 
> you for making Kea available to everyone and for all the excellent 
> features.
>
> I'm in the process of preparing migration from ISC-DHCP to Kea and 
> trying to improve our situation by deploying it in HA mode (hot 
> standby) so I'm testing the behaviour of Kea (and its HA) in our 
> environment. So far all the testing was done using a raw socket where 
> HA worked great - standby node listened to the DHCP to/from the active 
> node and upon losing connectivity of the control link and noticing 
> unacked client requests - standby node got promoted to active and took 
> over.
>
> Now I'm trying to repeat the same in UDP mode and I'm not sure if 
> there is an impact here on HA parameters or not. My switchover test 
> wasn't successful, the standby node never switched to the active role. 
> Documentation doesn't mention any dependency between 
> max-unacked-clients (I currently have it at 2) and dhcp-socket-type 
> but I wonder: if Kea isn't listening to the broadcast traffic, is it 
> still able to count the unacked requests? and if it is, how does it do 
> that? Should max-unacked-clients be always 0 in UDP socket mode or is 
> that somehow not a problem?
> If someone could shed some life on this, I'd appreciate it a lot.
> Thank you,
> Waldek Maleska
>