chicken egg problem

[Simon Hobson]

richard lucassen <mailinglists at lucassen.org> wrote:

> According to "man dhcpd.conf" i can use *hostnames* as DNS:
> 
> option domain-name-servers ns1.example.com, ns2.example.com;
> 
> That's very nice, but can someone explain how the ns1.example.com and
> ns2.example.com can be resolved by a new client?

They can't, and they aren't. They are resolved by the *server* when it starts and the resulting IPs are given to the clients.

> What I initially was looking for is a way to randomly assign multiple
> DNS servers to clients. With a simple:
> 
> option domain-name-servers 10.0.0.1, 10.0.0.2;
> 
> the 10.0.0.1 gets all the queries from all clients. Of course I can
> assign the nameservers the other way round for some ranges:
> 
> option domain-name-servers 10.0.0.2, 10.0.0.1;

You'll find it's more subtle than that !
Assuming a preponderance of Windows clients, try assigning "10.0.0.1, 10.0.0.2" and then take 10.0.0.1 offline for a while. When you turn it back on, you'll probably find that few clients will use it again - until 10.0.0.2 goes offline.

I know of people who've empirically found that if they specify a local server before external resolvers, they can get a form of split DNS - where the Windows clients check with the local server first and then go to an external resolver if the first one doesn't have the answer. They then found that this breaks if the internal server ever goes offline - because then the clients put it to the end of the list "permanently" and it only gets promoted if others above it fail.

IIRC there's also reports of Windows machines not changing settings (I suspect DNS servers was included) when the values change in the offers. Only if they lose their connection (lease expired, moved network, old lease no longer valid and NACKed) will they get a new lease and the new settings.