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We have a Windows 2008 R2 Server with a DHCP server that manages a public IP net and a internal IP net. We use the latter for machine computers that are not supposed to show in the network and have restricted internet access.

All clients are supposed to get IPs from the public IP range. However, for a few weeks now, clients often get an IP from the private IP range (where they don't have network or internet access because they are not defined that way). The workaround that I tell my users is to try ipconfig /release and ipconfig /renew until they got the correct IP. I also tried ipconfig /registerdns on the clients, but they keep getting the wrong IP.

As far as I know, nothing changed on the server. It is possible that this behaviour started with a round of Windows updates.

How do I get client PCs to alway grab the correct IP? Is it a DHCP or a client problem?

Disclaimer: I do administrate the server, but I basically inherited the duty and I am a complete newbie. I can manage users and register new clients, but that's about it.

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  • Are they getting the leases from the correct server? This is pretty much what happens if someone connects a second DHCP server (e.g. the wrong end of a SOHO router) to the same network.
    – user1686
    Nov 13, 2019 at 9:50
  • @grawity Yes. The private subnet should only be leased to static IPs that are connected to the MAC address of the clients in question. It did work like that in the past.
    – Ian
    Nov 13, 2019 at 9:52
  • Does the wrong lease show up on the Win2008 "DHCP server" configuration? Does it have the same client ID, etc.?
    – user1686
    Nov 13, 2019 at 9:54
  • @grawity I need to check that the next time the problem occurs. Will report back.
    – Ian
    Nov 13, 2019 at 9:54
  • I think it would be useful to check the same from the client side, i.e. get a Wireshark packet capture of the actual DHCP leases/renews happening.
    – user1686
    Nov 13, 2019 at 9:58

1 Answer 1

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I'm guessing that one of two things is happening:

  1. Your DHCP server has two different NICs, one for Public and one for private, and is not filtering the DHCP requests based on the NIC it is receiving them from. In this case, you'll need to configure the DHCP server to restrict traffic for the particular network.

  2. Your network has multiple 'DHCP helpers' configured and is confusing the DHCP server and/or clients. DHCP traffic works by broadcast and as such cannot work normally when the DHCP server is on a different subnet from the client. To resolve this issue, the networking equipment can implement a "DHCP helper" which forwards the DHCP request broadcast from one network to the IP of the DHCP server (converts a broadcast to a unicast so it is routable). Make sure that you don't have multiple conflicting DHCP helpers on a single network or DHCP helpers on a network that should not have them.

Another workaround would be to restrict who can receive DHCP leases on the private network. 802.1x authentication or MAC filtering would prevent 'unauthorized' systems from receiving a DHCP lease on that scope.

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