dhcp failover behalve

Simon Hobson dhcp1 at thehobsons.co.uk
Sun Oct 16 09:21:42 UTC 2016

Julie Xu <J.Xu at westernsydney.edu.au> wrote:

> When dhcp failover setup, for a dynamic range on a subnet, when first dhcp server offline, I will expect all machine in the subnet will get second dhcp server for the service to give a new ip address, or to confirm the use of existed ip address which the machine is using.

Yes, but this is not completely automatic.

> And I believe, most pairs in my control is working this way. But, one pair is not working as others. If the  first dhcp server offline, than the machine who was get ip address from the server, has problem to get ip address from second dhcp server.

You'd need to post your configs and any logs that show what's going on at the time.

> Could anyone advise what is  the correct action the dhcp server should do? and is there  any special configuration required to make the service continue even one of the server is offline?

If one server dies, the other goes into "communications Interrupted" state. This is because there are failure modes which can mean the two servers can't communicate with each other, but they can both still service clients. It would be "rather bad" if the two servers then started doing their own thing and causing IP conflicts. Knowing whether this is a problem is site specific - so there's no generic AND SAFE way to deal with it from the developers PoV.

So when one server is down, you need to manually put the other into "partner down" state - and then it can continue servicing all the clients. There is an option to have the servers assume partner down when communications are interrupted - but you have to set that yourself after considering your network topology and the risks involved.

Also, bear in mind that clients will initially attempt to contact the server that issued them their lease. Only if renewals from this server don't happen, and the lease is nearing it's end, will they revert to broadcasts - at which point the other server will see the requests and be able to service it.

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