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Is this possible?

On my LAN, I would like to setup the network properties such that the DHCP server's address is static. However, I want that server to hand out the IP Addresses and DNS addresses dynamically.

The reason is that some devices on the LAN will try to behave like a DHCP server.

For example, we use software to push images to computers on the LAN (our computer software configurations are centrally managed). When that imaging distribution software happens to be running, the machines being imaged will get confused as to which device is the DHCP -- the real one or the machine that is sending them the image.

So, to remove the confusion, I would like to setup my Windows 7 images such that the DHCP server address is statically assigned. And then that server would assign the IP Addresses and the DNS addresses dynamically.

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This cannot be done. DHCP is inherently a broadcast protocol. A DHCP OFFER response contains the DHCP server's address so that the device can send renewal requests directly without broadcasting again. A DHCP reply can even contain a different DHCP server address to send renewal requests to. But a DHCP server address cannot be set statically on the device because if it were ever in a situation where it had no valid DHCP lease, it has no choice but to broadcase for one.

If the software is using PXE to boot your clients, then that's a special type of DHCP request. There should be an option within that software to configure an actual DHCP server's address (it may be called ProxyDHCP or something similar). Or you may have to disable the DHCP response from the software and configure the real DHCP server with an option specifying the PXE server's address.

As a last resort, you'd want to configure an exclusion range on your DHCP server and then configure the software to only offer address within that range.

No matter how you slice it though, there has got to be a way to accomodate a real DHCP server within that software. If not, then it's poorly designed software.

I'd have to know more about the software to give you any further details.

  • I was afraid this was the answer though I am not surprised. The "real" dhcp server is not under my control and, whatever it is, it doesn't do a really good job either. The ip addr assignment appears to be a function of the office's router/gateway to the ISP. However, this device/software doesn't work (well) consistently. You are correct that the deploy software is PXE boot. I think I will just use static IP/dhcp. Can static dhcp/ip addr be done but retain dynamic DNS assignments? – mkstreet Jun 12 '14 at 14:46
  • Can't do that either :-( You can have a DHCP-assigned address with static DNS servers, but you can't have a static IP with DHCP assigned DNS servers. If you define a static IP you've told the computer that you do not wish to use DHCP, so it would never contact a DHCP server to ask for one. I'd still like to help you though, because I'm confident this can be made to work for you. What's the name of the software? – Wes Sayeed Jun 12 '14 at 15:33
  • a proxyDHCP is not an option within PXE software to configure an actual DHCP server's address – Pat Jun 13 '14 at 12:59
  • I think I got what I wanted now. On Win 7, I went into network control and properties. I set it as static IP. This allowed me to fill in the DNS server's IP address. Then I clicked the radio button to make it dynamic IP. So the IP Addr/Mask/Gateway-DHCP are dynamically assigned, but the DNS server IPs are filled in. For now, I will work with the assumption that the IP Addr can be assigned dynamically as-is, and the other situation I saw was a kind of aberration since it doesn't seem reproducible so far. Probably that was caused by some other factor which can't be known right now. – mkstreet Jun 14 '14 at 16:27
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If the unwanted DHCP is on a regular computer you can try blocking it with a firewall rule. Maybe this answer can help.

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you need only one DHCP Server; the one that you already have; keep it untouched assigning addresses to all the stations (including the PXE booting clients) and set the PXE server DHCP component as proxyDHCP Server.

This way when a PXE clients boots up it'll get its IP address from the main DHCP server while it will get its PXE additional parameters from the proxyDHCP Server included with your PXE server. This schema is completely "collision free" from a DHCP point of view

There are Automated PXE Server Solution Accelerator today (including proxyDHCP) that you can quickly integrate into an existing LAN environment, specially handy in cases like yours when the DHCP configuration cannot be altered.

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