[Kea-users] Load-Balancing Network issue between Relay and Kea

[Simon]

Francis Dupont <fdupont at isc.org> wrote:

> I leave details to our HA expert but it seems your setup requires an active
> load-balancer:
> - the path between clients and the first server is broken so this server
>   does not receive queries or clients do not receive responses
> - the path between the two servers work so for the second server the
>   first server is ok
> - the path between clients and the second server works so the second
>   server believes queries from first server clients are served by the
>   first server so it does not serve them
> - the second server has no way to detect the problem as it does not follow
>   responses
> 
> I suggest to use an active load-balancer i.e. a box between clients and
> servers which splits and monitors exchanges: not only it should solve the
> problem but it will avoid extra traffic. With other words you are outside
> what the Kea load-balancing can support...

I see an alternative, making assumption about what Kea can do.

Scenario like this are why the ISC DHCP server did not do automatic state change to partner-down - there are too many variables in terms of what the two partners see traffic wise and what they can assume about their partner. So by default, it is up to the admin to determine if a partner is down and set the other server to partner-down state accordingly.

So it would seem that an alternative to a load balancer is to script detection of the problem and handle it according to the automation level desired by the admin - at one extreme, simply alert it so manual intervention can be done; at the other extreme, automatically put a server into partner-down state.



Kevin P. Fleming <lists.kea-users at kevin.km6g.us> wrote:

> If 'max-unacked-clients' isn't sufficient to address this, then this leaves a fairly large opening in the Kea high-availability story, as any network disruption which causes a server to no longer receive discovery packets from clients, but otherwise receives all expected network traffic, won't be noticed except by the clients! This concerns me, as (like other users here) my Kea servers receive all client traffic via DHCP relays, and misconfiguration of the relay such that it only relays to one server and not both will result in half of my clients not getting DHCP service at all.

Surely, if you misconfigure a relay agent in that way, around half your clients will initially be unable to renew their leases, but eventually will get serviced by the available server once their active lease has expired ? That would mean the clients would drop their network config momentarily before setting up a new one - meaning that active connections would drop, but new ones would connect just fine once the new settings are in place.

Simon