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Trying to explain 2 DHCP host on same network.Help


Having setup LANs for many years and understand why you dont allow 2 DHCP host on the same LAN, I recently ran across 2 solutions where I need to appeal to you guys/girls for an explanation.

I first discovered this in Knoppix, 2 years ago and recently found a similar model in Puppy. In Knoppix it is, incorrect in my opinion, referred to as Terminal Server and in Puppy it is called Netboot...but they do the same thing for their LANs. They will provide everything needed so that another PC can use its BIOS to direct the PC so that if a "PXE" host is listening on the LAN, it will get its boot image and boot from the LAN instead of local storage devices. This ability works from a LiveCD running either Knoppix or Puppy.

Scenario: I have a router with LAN PCs all of which get their IP creditials from the router's DHCP service. One of the PC is Knoppix for example (or, it could be Puppy. It doesnt matter because the discussion is the same).

Question: Now for my request for explanation. The PXE host (Knoppix/Puppy) starts it service to allow the LAN PC to connect to boot. That service includes a DHCP service being started on the PXE host. When the LAN PC starts, it broadcasts for DHCP credentials.

How does that LAN PC know to use the PXE host's DHCP to boot, then use the router's DHCP once its OS starts the console desktop? Can anyone explain this?

Additional Info: My Puppy logs show that the DHCP address range that he is using is the a different range from my router's range. Both DHCP host are operation on the same LAN with the same subnet. Still, how does the LAN PC know which to use??? Is a PXE request coming from a LAN PC somehow different from a "normal" DHCP request for service? Is this why during PXE operations to get the LAN PC to boot, the LAN PC has an IP in the PXE host's range, then, when the LAN PC boots, how does it know to use the IP from the router's DHCP service? What constitutes/causes this magic?



  • saqman2060
    Do you have the option of writing scripts that tells the LAN PCs what DHCP host to use to obtain the boot image from and what DHCP host to use for network routing?
  • ben
    ben Posts: 134
    In your linux installation there're different components (software) involved in your network booting.

    PXE: on the client side (your desktop PCs)
    DHCP Server: on your host side, it gives you IP addresses
    TFTP Server: this is what you've not mentioned, the TFTP server is responsable for providing a boot image to your clients through the Trivial File Transfer Protocol (TFTP)

    DHCP itself provides only a valid ip address, then you've a kernel image loaded through the TFTP Server (still in your linux host), they work as a couple and they're responsable for network booting
    When your client pc receives the ip address and the boot image it's ready to go and get more information from your Linux Server (mounting NFS drives or whatever it needs)
    Your client tries to locate PXE boot servers with DHCP services available and tries to locate a tftp server with a valid boot image for its platform (x86 in your case I guess), that's why you've a bootable machine with a valid kernel, maybe your router does not provide TFTP server information (not configured, not available at all, ...) so your client parse every dhcp response to find the valid one, please remember: it's still a bad thing to have two dhcp services available in the same piece of network !
    But that's the reason why you've your machines happy with a linux kernel loaded, take a look at: http://en.wikipedia.org/wiki/Preboot_Execution_Environment , it's not the holy bible but it's a nice place to start; then study deeply how PXE works and you'll discover it's a really interesting service due to it's simplicity (not on the server configuration I mean !!!)

    Hope it helps


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