ISC DHCP server always ACKs DHCPREQUESTs without checking the server identier

[Simon Hobson]

Jordan Gordeev wrote:

>I am using the ISC DHCP server version 3.0.5. Most of the DHCP clients
>on my network have host declarations with fixed adresses. I have noticed
>that the server  sends DHCPACK messages to clients requesting (via
>DHCPREQUEST) leases from other DHCP server. I have looked at the source
>code and have noticed that the server identier in DHCPREQUEST messages
>is not used. I'm wondering why the server behaves like that.

What is wrong with that ?

If the client broadcasts a request, then it's valid for any server to 
respond - either with DHCP-NACK if the requested address is not 
valid, or DHCP-ACK if it is valid. If the client is simply renewing a 
lease then it will unicast (not broadcast) a request to the server 
from which it got it's current lease any any other servers will not 
even see the request.

It's important for a server to respond to a broadcast request because 
it might be from a client that has moved networks - in which case the 
server identifier is likely to be different (and probably 
unreachable). If you server simply ignores the request then the 
client will have to wait to be sure it isn't going to get a reply 
before reverting to doing DHCP-DISCOVER broadcasts and this will 
delay network configuration.