DHCP Discovery from a non-0.0.0.0 IP

[BenNin]

Help! I'm trying to get a device to obtain an IP using DHCP. I have programmed it to send out a DHCP Discovery packet however it receives offers from one server (a Netgear router), but not another (a different model Netgear router). I captured the packets using Ethereal, modified them then injected them back onto the network using 'Network Packet Generator' to discover what was happening.

The device automatically configs a 0.0.0.1 IP on startup (I am not able to change this). I have discovered that simply changing the captured packets source IP (and the associated checksums) to 0.0.0.0 allows both routers to answer with an offer.

My knowledge in this subject is basic and am wondering if there is any way of getting the second router to respond to a device with a non-0.0.0.0 IP. Which router is perforimg correctly?

Ben.

 

[WhoKilledKenny]

Ben,
I'm going to take a stab at this. Your discover is comming from a uni-cast (0.0.0.0) because the client is on the same subnet as the DHCP server. For a DCHP server to recieve a multi-cast braodcast (255.255.255.255) you need to enable an ip forwarder (ip helper address) on your router. The ip helper will route multi-cast dhcp dicovers accross the router. This allows clients, not on the same subnet as the DHCP server, the ability to communicate with DHCP. Most routers are AKA collision domain boundaries which do not allow broadcasts to traverse from one network or subnet to the another.