dissecting DHCP - Q2

[Simon Hobson]

Defryn, Guy wrote:

>Also, I had a tcpdump process running on both servers.
>On a shutdown/start tcpdump shows me the following on both servers 
>(timestamp omitted and hostname differs for each server)
>
>IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, request from 
><MAC Address> (oui unknown), length: 309
>IP tur-net1.domain.com.bootps > tur-xp1.domain.com.bootpc: 
>BOOTP/DHCP, Reply, length: 315

Yes.
The client will still have an unexpired lease (unless you leave it 
turned off long enough for it to expire). When it brings up the 
interface, all it needs to do is ask (via a DHCP Request message) 
"Can I still use this address, and can I extend the lease on it". In 
this case, the answer is yes, and the server responds.

If the client has moved network, then the DHCP server on the new 
network would probably respond "No, that address isn't valid for this 
network" - by sending back a DHCP Nak message. The server would then 
discard the old lease and start from scratch to get a new one.

There is a third case - no DHCP server responds. In that case, many 
modern clients will then check the MAC address of the device they 
last had as the router. If it's the same MAC at the same address, 
then it's a good bet that you're on the same network - and so they 
will continue using an unexpired lease until it expires. This makes 
the network less brittle during a temporary outage of the DHCP 
service.


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Simon Hobson

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