Server Move...

Simon Hobson dhcp1 at
Sat Oct 14 22:07:25 UTC 2006

Keith Woodworth wrote:

>Having just moved the first subnet from one old server to the new one a
>question arose as I was watching the dhcpd log on the new server.
>All clients by default get a 1 week lease. I changed that to 30 mins prior
>to the changeover, let the lease time pass, changed the ip helper and
>commented out the subnet on the old server, restarted the old dhcp server
>and within about 30 seconds started to get traffic on the new server.
>Now it was stated that the client would fall back to a DISCOVER when the
>old dhcp server didnt reply.
>As the new server is starting out with an empty lease file, it will see a
>DISCOVER and pick a lease and hand it out. The problem I have just seen is
>the new server pinged an address, saw that its in use (that client had not
>renewed yet) and abandoned it and gave the DISOVERing client a different

One reason for copying the leases file from the old server, or 
alternatively, make the move when as many machines as possible are 
switched off.

It does depend on the client though. IIRC, Windows XP clients request 
the last address they had when they broadcast for a server and so 
would normally get the same address.*

>I'm not sure if this is going to be a problem? Having the new server
>abandon IP's like that right after a cutover? Would setting a shorter
>lease time help? Or cause more problems?

When the transition is complete, remove the abandoned leases from the 
leases file, that will free up the addresses. You can leave them 
there and they will be reclaimed in required, but it will increase 
address churn in the subnet.

Abandoned leases are the lowest priority when allocating new leases, 
so expired leases will be reallocated in preference - thus reducing 
the likelyhood of a client returning and getting it's old address.

>I do have much larger subnets than active clients so I'm hoping that will
>help too.

It will

* How did I find this out ? Well I was sorting out connectivity at a 
clients site (a campus business park). Their networking provider had 
setup a VLAN to allow reception at a new building to access their 
office network and I could not get their PC connected to the network 
- it just kept assigning itself a 'link local' address. I assumed 
there was a problem with the VLAN setup until I plugged in my Mac and 
got an address straight away.

It turned out that their DHCP server was not acting authoratatively 
and didn't respond with a NACK when the client requested it's link 
local address - so the client never stopped using it, and never got a 
real address. I had to manually assign it an address in the correct 
range then set it back to DHCP to make it work !

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