Authoritative use.

Simon Hobson dhcp1 at
Thu Oct 12 23:03:36 UTC 2006

Keith Woodworth wrote:
>Is the use of Authoritative a required command to use in a subnet
>subnet netmask {
>         authoritative;
>         option domain-name-servers,;
>         option broadcast-address;
>         option routers;
>         max-lease-time 604800;
>         default-lease-time 604800;
>         range;

Not necessarily in the subnet declaration (it is usually a global 
option), but it is required somewhere in scope for the server to act 
correctly (NACK invalid requests).

>I'm cutting over and moving two old dhcp servers into one. I have about 8
>networks on one and 4 on another.
>On the above network I have set the lease time the old server hands out to
>clients to 30 mins. I'm going to change the helper address on the switch
>to the new server. Should I just comment out the subnet and restart dhcpd
>on the old server after that so the old server does not even answer for
>that subnet any longer?

The clients will attempt to contact the old server directly by 
unicast to renew their leases - so you will need to tell it to ignore 
them. Simply removing (or commenting out) the subnet declaration is 
the easiest way to do it.

>I was going to do this for all the networks. One at a time lower the lease
>time on the subnet, change the dhcp relay (ip-helper) address on the
>approprate vlan and just comment out the subnet on the old server and let
>the new one take over. Will this work ok?

Yes, but be aware that your cannot shorten existing leases. So for 
example, if you've been giving out 4 week leases, you need to start 
your work at least 4 weeks in advance - I would do it in stages by 
cutting down to 2 weeks at 4 weeks to go, then down to 1 week at 2 
weeks to go, and so on until you arrive at cutover time with no 
client having a long lease. If you just went to 30 minute leases with 
4 weeks to go, then you expose yourself to increased server load, and 
more risk from a server failure, for several weeks.

If you don't stat well in advance, you can end up with clients that 
still have weeks of lease left when you want to change over - if they 
are never rebooted, put to sleep, or disconnected from the network 
then they will not attempt to renew when you want them to.

>In the DHCP hand book it says to copy over all the config files, including
>the lease file too I'm guessing, but that wont work here as I have two
>lease files from two different servers, running two different versions of
>DHCP, each serving multpile networks. Acutally one DHCP server is running
>Netreg too, as will the new one.

Your method will work, but so too will the copy the files option - 
with some changes. You can copy the subnet definitions a subnet at a 
time, which is what you are effectively doing anyway.

You can also copy the leases files - you only need to copy the one 
for the subnet you are moving at any point. What you need to do is to 
append the lease file to the existing lease file, then when you start 
the new server again it will discard any leases for subnets it 
doesn't yet know about. The old server will also discard leases for 
the subnet you have commented out from it's config, so it will be 
safe to append it's (now smaller) lease file to the new servers file 
when you move the next subnet.

Don't forget that you must stop a server before doing anything with 
the leases file.

On the whole, with a migration like this it's as easy to just let the 
clients take on a new lease with the new server. Many clients will 
ask for their previous address anyway, so little will change.


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