Clarity on DHCP

JohnnyBiggles

From what I understand, the steps for DHCP to assign a host its approriate info are as follows:

1. Client broadcasts DHCP Discover message
2. All DHCP servers receive the message and
3. All available DHCP servers make an offer with Unicast to that client but only one is going to be used by the host
4. The client then sends a broadcast back to the server with a DHCP request
5. Then the server returns with an ACK message and the necessary assignment info.

My question is, once the client gets info from a DHCP server, why would it need to broadcast again back to the server? Shouldn't it be something like the following:
Broadcast to server > Unicast from server > Unicast from host > Unicast from server with info

(with the initial broadcast being the only one since it's looking for a server, but each after would be uni since it found one to establish communication with)?

Am I off with this logic? Why does the host broadcast twice? I hope I'm being clear...

Todd Burrell

Here is a good link that explains the DORA process in good detail. The REQUEST sent back to the DHCP server is a broadcast because you are not sure at that time WHICH DHCP server gave you the address. In actuality I believe that the DOR portion of the DHCP address request process can all three be broadcasts.

The TCP/IP Guide - DHCP Lease Allocation Process

JohnnyBiggles

That's a good graphic to explain it. Thanks for the response. I didn't realize the host sends out requests to all the servers that sent offers. I thought it somehow chooses the best from the pool of servers who sent offers, then sends a unicast to only one server once one is selected.

JohnnyBiggles

Also, what would be a (THE) deciding factor in the DHCP server selection? Is it the PC or Server that ultimately makes that decision? Why would one be chosen over the other? (Let's assume there are 2 available DHCP servers the same distance away, connected by the same type of cabling, # of routers, # of switches, etc.)

*EDIT: If anyone has a response, that's cool but I re-read the link past the graphic and it kind of explained it.

Todd Burrell

My understanding was that the client always takes the first OFFER it receives, so there would be no way to accurately predict which of the 2 DHCP servers would be the one to get its offer back first. My guess would be whichever one is less busy. But BOTH DHCP servers would send offers.

instant000

This is a good article, that kind of fits this topic of discussion:

DHCP Consumption Attack and Mitigation Techniques [Cisco Catalyst 6500 Series Switches] - Cisco Systems

I think the coolest part about it is that they show you step-by-step how it can be a problem for you, and then how you can remedy it.