zero free leases

Simon Hobson dhcp1 at
Fri Jun 29 06:45:52 UTC 2007

Koichi Mori wrote:

>Is it in specification of ISC DHCP server?
>Dose it work well DHCP?
>failover MUST 2 servers be working?

Yes, for failover you need two working servers.

>I thought failover is duplicate system of DHCP.
>Usually duplicate system is still working when other side be down.
>#same as both systems are working.

The problem with DHCP is that it's dynamic. It's trivial to duplicate 
something static, but for something dynamic you have TWO systems 
which are both updating a shared database - if you think about that 
then I think you'll quickly realise that it's far from trivial !

>This specification means "If you use failover, ISC DHCP doesn't work
>well when a server to be down" dosen't it?
>I hope(need) failover work same as both server working when other
>side be down.

AIUI (I don't use failover myself), when one system dies, the other 
cannot take over immediately because it cannot be certain that the 
other system is not still offering leases to clients* and so cannot 
automatically take over full responsibility.

What has to happen is for the administrator to tell the surviving 
server that it's partner really is down and then it will behave as 
though it was the sole server and there never was any failover - when 
the partner is restored then things will be synchronised and 
redundancy returns.

I can't recall exactly what happens during "communications 
interrupted". I think the surviving server will continue to service 
it's own clients** normally, but cannot issue new leases from those 
'held' by the down partner.

While failover won't give you "fit and forget" redundancy, it does 
give you a situation where you can specifically tell a working server 
that it's partner is down - and then you have a fully working DHCP 
while you sort out the dead service/server.

* Sounds crazy, but I've seen routing issues such that A can see B 
and B can see C but A can't see C. In this case, A&C could be the 
servers and B a client - so the client can contact both servers but 
the servers can't communicate themselves. Another possibility is if 
the servers are set up to communicate between themselves on a 
separate management network/subnet which goes down.

** Effectively, each client is bound to one server and will normally 
renew from that server (which will tell it's partner about the new 
lease to update the other copy of the lease database.

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