Migrating DHCP servers

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Jan 10 10:03:47 UTC 2011

Peter Rathlev wrote:
>We're about to move several networks from being serviced by a Windows
>DHCP server to being served by ISC DHCP instead.
>What can we expect of trouble if we simply replace the "ip
>helper-address" configuration on the router? I tried poking around in
>the archives, and stumbled upon this:
>But it didn't make me significantly wiser.
>Two specific questions:
>1) Clients that already have a lease would keep asking (unicast) their
>current server. Will they do that for as long as the old server keeps
>acknowledging their lease, never returning to broadcast again to hear
>about the new server? If yes, do I have other options than either
>forcing all clients to reboot or disabling the old DHCP server?

Correct. As long as the old server keeps answering, then clients can 
keep an address leased from it. Unless you take specific steps, then 
nit's a bad idea to run multiple DHCP servers anyway - so you 
wouldn't want to keep the old one active after the migration.

>2) With "ping-check true;", and assuming all clients will answer to ping
>requests, should I have any serious concerns about migrating without
>keeping lease state?

Not really.

There are a lot of variables, and one that can cause problems is 
dynamic DNS updates. If you were to just "switch off" an old server, 
then you could find the new server unable to update the DNS as 
records will already exist but they won't be "tagged" (with the 
corresponding TXT record) as owned by the new DNS server.

With shortish leases and a fairly stable set of clients, you should 
be able to just turn off one server and turn on the new one. As their 
leases expire, clients will broadcast a request to use their current 
address and the new server will acknowledge that - thus clients will 
keep their current addresses.

Clients which come onto the network without a current address will 
however get a new lease - and that could clash with an address 
already in use. Ping check would detect that (subject to the "clients 
respond to ping" you've already mentioned) - but will create an 
abandoned lease.
Abandoned leases are only use in a "last resort" when all other 
options have failed to provide a free lease - so you would probably 
want to go through your leases and remove these once the migration 
was complete.

There is a trick you can use to your advantage. Bear in mind this is 
only an incidental effect of the way internal tables are hashed, but 
the ISC server current allocates unused addresses in a top-down (high 
to low) order. So if your new pool extends to higher addresses than 
teh old server had available, clients asking for a new address (ie 
not requesting a specific address) will get one from the top of the 
range where it doesn't overlap with the old one. This will 
significantly reduce the likelyhood of a collision.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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