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Given a Windows 2012 Server DHCP server

When a add an IP Reservation for an IP address providing the client's MAC address

And this client is powered off

And a different Windows client with a different MAC address requests this IP address

Then the new client will get the IP address despite MAC mismatch

And the reservation will be renamed to the new clients FQDN.

How is that then a reservation!?

H.-G. Buttentee
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  • DHCP clients should not be dictating to the DHCP server which IP address they want. Like you said below, you don't even see a DHCPDISCOVER. If you are trying to prove a hypothetical, that's fine, but the process is: 1) client asks for an available IP 2) Server suggests an IP 3) client accepts. In which case the server will never suggest a reserved IP address. Please update your question and specify how you have forced your computers to "steal" an IP. – Appleoddity Oct 16 '17 at 19:54

3 Answers3

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When an IP addressed is reserved by MAC address the server will not offer it to some other client, whether it specifically requests it or not.

Do you use multiple scopes? These must not overlap.

Multiple DHCP servers? Are they synchronized?

Zac67
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  • One server, one scope. Bewilderingly, Wireshark shows that sometimes there's neither DHCPDISCOVER nor DHCPOFFER, only DHCPREQUEST and DHCPACK. – H.-G. Buttentee Oct 16 '17 at 17:48
  • At 50% of lease duration a Windows client will attempt to renew it's lease without going through the full DORA process. See figure 4.3 in this article sections. https://technet.microsoft.com/en-us/library/cc958935.aspx also check the renewing and rebinding sections for further useful info – martin81 Oct 18 '17 at 07:24
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Then the new client will get the IP address despite MAC mismatch

DHCP does not deny a IP address to a client; longer lease times or manual configuration ognore reservations. A reservation only kicks in when the client is reuqesting a address.

And the reservation will be renamed to the new clients FQDN.

Yes, thats the default behaviour. For any IP address - the 'name' field is dynamically filled by the client.

bjoster
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And a different Windows client with a different MAC address requests this IP address

You are 100% sure that other computer is not in static mode or does not have that IP as a secondary IP ? I ask as you don't even see the DHCP message in wireshark.

yagmoth555
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