What is the dhcpnack and when do you get one name 2 scenarios?

Answer 1

Recently I saw a lot of queries regarding when the Microsoft DHCP server issues a NAK to DHCP clients.

For simplification purposes, I am listing down the possible scenarios in which the server should NOT issue a NAK. This should give you a good understanding of DHCP NAK behavior.

When a DHCP server receives a DHCPRequest with a previously assigned address specified, it first checks to see if it came from the local segment by checking the GIADDR field. If it originated from the local segment, the DHCP server compares the requested address to the IP address and subnet mask belonging to the local interface that received the request.

DHCP server will issue a NAK to the client ONLY IF it is sure that the client, "on the local subnet", is asking for an address that doesn't exist on that subnet.

The server will send a NAK EXCEPT in the following scenarios:-

1. Requested address from possibly the same subnet but not in the address pool of the server:-

This can be the failover scenario in which 2 DHCP servers are serving the same subnet so that when one goes down, the other should not NAK to clients which got an IP from the first server.

2. Requested address on a different subnet:-

If the Address is from the same superscope to which the subnet belongs, DHCP server will ACK the REQUEST.

The DHCPNACK or Negative Acknowledgment is a packet that the server sends if the IP address is not available in stand of DHCPACK (in use on other client for example) or the address is no longer valid. In case of DHCPNACK the client must restart the lease process in order to get an IP address.

Answer 2

The DHCPNACK or Negative Acknowledgment is a packet that the server sends if the IP address is not available in stand of DHCPACK (in use on other client for example) or the address is no longer valid. In case of DHCPNACK the client must restart the lease process in order to get an IP address.