Recovering from DHCP failure

Question

Hi
I am on Windows 2008. Just wondering what kind of problems I can get into if my DHCP server fails. We are backing up the DHCP DB once a day and the lease period for some odd reason has been set to 1 day.
For the sake of discussion I want you to assume that I backed up my DB yesterday at 6 PM and my DHCP crashed today at 4 PM.
I can restore from my backup. But the entries in the backup could be stale.
There could be some IP Address that was allocated by the DHCP server today at 2 PM but is see as available in yesterday's backup. Post restoration, the server could try to lease out that IP Address to some other system. (I assume that the system that got
the address at 2 PM would not contact the DHCP server again until 2 AM in the morning (50% of its least period)
There could be some IP Address that was assigned to some system at 3 PM (just before the crash) or not assigned at all but the backup taken yesterday could show the IP Address against a different system with active lease. In this case, if I try to use the
DHCP log to identify a system on the network then I am likely to reach the incorrect system.
Is there some kind of reconciliation option where the DHCP could force all the clients to renew leases to get to some consistent state of the network?
Thanks
-GW

Answer 1

Hi,
Windows Server 2008 DHCP has both server-side and client-side conflict detection to prevent duplicate IP addresses on your network.
Client conflict detection
Client computers running Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP, Windows 2000, Windows NT 4.0, Windows Millennium Edition, and Windows 98 automatically check to
determine if an IP address is already in use before using it.
After the DHCP client receives a lease from the DHCP server, the client sends an Address Resolution Protocol (ARP) request to the address that it has been assigned. If a reply to the ARP request
is received, the client has detected a conflict and sends a DHCPDecline message to the DHCP server. The DHCP server attaches a BAD_ADDRESS value to the IP address in the scope for the length of
the lease. The client then begins the lease process again, and is offered the next available address in the scope.
Server conflict detection
If your network includes older DHCP clients that do not perform conflict detection themselves, you can enable conflict detection on the DHCP server. By default, the Windows Server 2008 DHCP Server
service does not perform any conflict detection.
To detect conflicts, the DHCP server pings (sends an ICMP Echo message to) an IP address before offering that address to clients in a new lease. The DHCP server only pings addresses that have not
been successfully and previously leased. If a client requests a lease on an IP address that it already had or is requesting a renewal, the DHCP server does not ping the IP address.
If conflict detection is enabled, an administrator-defined number of pings are sent. The server waits 1 second for a reply. Because the time required for a client to obtain a lease is equal to the
number of pings used, choose this value carefully because it directly impacts the overall performance of the server. In general, one ping is sufficient.
If a response to the ping is received, a conflict is registered and that address is not offered to clients requesting a lease from the server. The DHCP server then attaches a BAD_ADDRESS value to
that IP address in the scope. The DHCP server then tries to lease the next available address. If the duplicate address is removed from the network, the BAD_ADDRESS value attached to the IP address
can be deleted from the scope’s list of active leases, and then the address returns to the pool. Addresses are marked as BAD_ADDRESS for the length of the lease for which the scope is configured.
If the BAD_ADDRESS entry is not manually removed, it will automatically be removed after a period of time equal to the lease time for the scope.
Note: In general, using server conflict detection only as a troubleshooting aid when you suspect that duplicate IP addresses are in use on your network. Each additional conflict detection attempt
adds to the time required to negotiate leases for DHCP clients.
Hope this helps.
Steven Lee
TechNet Community Support