how can DHCP offer be addressed other than broadcast ?

[fran1942]

Hello, I have attached a Wireshark capture of a DHCP offer packet being sent back to a host. I am confused by how thte DHCPOFFER is addressed to the IP the host will be alllocated.
I thought all DHCPOFFER packets were addressed to the broadcast address due to the host not having an IP address at that stage ?

thanks for any help.

[alf55]

The DHCP protocol is done via UDP.

Remember that DHCP sequence of events can be remembered as DORA (Discovery, Offer, Request, Accept). Communication is done at the UDP layer and it can not cross sub-networks. Communication to the DHCP server(s) is done on port 67 and all the responses are sent on port 68. (Yes, the DHCP specification does not limit a single DHCP server per sub-network; however, it is the normal case that there is only one.) Every packet that is sent during the process contains the MAC (Media Access Control) address which is unique to the NIC (Network Interface Card) which are assigned during manufacturing. The computer that is making the DHCP request drops any packet found on port 68 that does not have its MAC address.

Step 1 (Discovery) is from the computer (NIC) that is attempting the DHCP assigned IP address (via port 67) then it switches to listen on port 68 waiting on one or more Offers.

Step 2 (Offer) is from the DHCP server offering an IP address, network mask, gateway IP address, DNS server IP address, lease time, etc via port 68. The computer gets the Offer(s) and when it decides to request to use the IP address it must ask the DHCP server to use the offered address.

Step 3 (Request) is from the computer (NIC) that is attempting the DHCP assign IP address makes a "request" to use the IP address (if that address is still available for use, the server will send the "accept" packet back to the computer again on port 68.

Step 4 (Accept) message is read by the computer, it assigns the IP address and updates other network related information in the packet including the lease time.

Some use the term "Acknowledge" for the last step of the DORA.