I have a Mac Mini running OS X Server in Mountain Lion that I use for imaging via DeployStudio. A couple of days ago, I tried to boot a MacbookPro using Netboot by going to Start-up Disks, and selecting the Netboot image. When the machine restarted, it just sat at the grey screen for about a minute, then it started flashing the globe icon, as it was trying to find the Mini. After about a minute of that, it would give up and then boot back to the OS. I tried this with a NetInstall image, another NetBoot image, another machine, and so on. All of them had the same behavior.
After going through some logs and looking at the documentation, it turned out that the machines really couldn't find the Mini to boot from. Basically, if the DHCP service wasn't turned on, and configured for the same subnet that the Mini was on, then nothing could find the Mini to boot from it. This was bad, really bad, as l work for a large achadimc instatution where they run their own DHCP services for all of the subnets.
The issue turnes out to be that as of 2.2 of OS X Xerver, that the bootpd service doesn't launch by itself anymore. The DHCP service must be running for bootpd to launch. Netboot needs bootpd for the clients to find the host. The solution that I came up with was to modify the /etc/bootpd.plist file.
There are, as of when I am writting this, two versions of Netboot. Netboot 1, or old Netboot, allows the bootpd service to run without DHCP, and Netboot 2 that requires DHCP to be on for bootdp to launch. I went into the /etc/bootpd.plist file and added the following lines to the bottom, just before the closing </dict> tag:
This turns on the old netboot so bootpd could run on its own. You'll have to restart the machine running OS X Server for the change to take affect.
There a couple of downsides to this method. One is if you turn on Internet Sharing in the Sharing system pane, or if you happen to turn on or change the DHCP settings in any way, your changes will be wipped out.
I hope this helps somebody out that that had the same issues that I had.