How to make a kernel starting dhcp before it mount root?

[Magicloud <magicloud.magiclouds@gmail.com>]

Dear all,
     I want to make a diskless debian using nfsroot. While I followed the nfsboot package and some articles on the internet, and I got a kernel that panic on mounting root, it seems that the ethernet card is up but has no dhcp ip.
     So how to resolve this? And does this has something about the dhcp server?

Thanks.