[Kea-users] kea and ha modes

Philippe Maechler pmaechler-ml at glattnet.ch
Fri Oct 5 07:02:26 UTC 2018


hello kea users

 

I'm once again looking for a way to make our dhcp setup more stable.
Currently we use the isc dhcpd without failover. For each active dhcp
server, there is an inactive one (cold standby?), which gets the
configuration and lease file every few minutes. If something breaks, we have
to manually take the broken server offline and activate the second one. This
setup worked more or less stable for more than 10 years.

 

But still this setup has it's disadvantages that I'd like to get rid of. Of
we have a maintenance window and have to take the router, where the active
server is connected to down, we don't have a running dhcp setup. New clients
won't be able to get a lease and existing clients fail to renew their lease.


 

>From the kea-user-guid
(https://kea.isc.org/docs/kea-guide.html#high-availability-library)

 

The Kea HA hook library supports two configurations also known as HA modes:
load balancing and hot standby.

 

In the load balancing mode.

When one of the servers allocates a lease for a client, it notifies the
partner server over the control channel (RESTful API), so as the partner can
save the lease information in its own database. If the communication with
the partner is unsuccessful, the DHCP query is dropped and the response is
not returned to the DHCP client. If the lease update is successful, the
response is returned to the DHCP client by the server which has allocated
the lease.

 

 

In the hot standby configuration.

The secodary server receives lease updates from the primary over the control
channel.

When the secondary server detects the failure of the primary, it starts
responding to all DHCP queries.

 

My question is:

In the load balancing mode, when one server dies, both servers won't hand
out leases, because the notification over the control channel fails, right?.
If so, where is the HA part in here?

 

In a hot standby configuration, what happens when the lease update fails?
Will the "primary" server hand out the lease or not?

 

 

Prior to 1.4 there were several messages on this list about a setup with
multiple (two) kea instances accessing both the same database (cluster). Is
such a setup supported/recommended or not? I guess not, because the initial
text on the above url states, that "supports two configurations also known
as HA modes"

 

Best regards

 

Philippe

 

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