moving subnet to another server

Gregory Sloop gregs at
Fri Jan 9 19:54:50 UTC 2015

To keep track of the already leased IP's and prevent duplicates, either
1) [As suggested] setup the leases in different ranges on the new vs old server.
2) Move the dhcp.leases file [I've done this myself, IRL] along with the config to the new server. [You'd shut down the old server, copy the leases file to the new and start it.]

Someone here should know more about #2, but if the versions don't vary too greatly this should be fairly safe.
[The docs seem to indicate that a v3 dhcp.leases file should also be compatible with a v4 dhcp.leases binary/release. They suggest that you check the syntax of both the config and the .leases files before you actually start the new dhcpd.]

[As I understand it, you're simply moving the DHCP server to new hardware, using all the same configs etc. - hopefully I've not misunderstood anything.]

How busy is busy? 10 leases an hour, 1000, 10K, 100K?
DHCPd isn't very resource intensive. Disk I/O is usually the first thing that constrains the server.
If disk IO isn't a problem, DHCPd can crank out a boat-load of leases very quickly.
DDNS is a bit more load, but still shouldn't be huge.

Finally, in a heavy load situation, where reliability is critical, it might be worth considering using fail-over peers with 4.2.X or newer.
[All that said, I'm no super-guru for DHCPd - so take my contributions with your own review of the docs and list etc.]



Thanks Patrick.   I don't know how to handle that. Not sure how smart dhcp is to detect such a condition or how long it would take for things to sort itself out.
The default-lease-time in the subnet declaration is 600.   Is there a preferred way to handle this? 

On Fri, Jan 9, 2015 at 10:59 AM, Patrick Trapp <ptrapp at> wrote:
How are you planning to prevent the new server from handing out addresses already assigned by the old server?

From: dhcp-users-bounces at [dhcp-users-bounces at] on behalf of Dana Huggard [dana.huggard at]
Sent: Friday, January 09, 2015 12:33 PM
To: dhcp-users at
Subject: moving subnet to another server


I'm planning on moving a subnet over to another server and am not sure what kind of devil hides in the details. 

The subnet is 10.x.x.0/18 and it is very busy. Its a subnet devoted to a test harness of servers that go up and down a lot putting a big load on dhcpd and named (using ddns).

Along with the dhcpd change the new dhcp server will also serve as a dns server for a new domain. 

To implement the move I have a new dhcp server I plan to remove the subnet declaration from dhcpd.conf off the old server, add it to the new server, then restart dhcpd on the old server, and start dhcpd on the new server.  

What I am wondering about is what to expect for the behaviour of the dhcp clients. How disruptive might this be?

Thank you.

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Dana Huggard
System Administrator
778.724.0761 ext 117
dana.huggard at

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