DHCP Copy and Configuration

Keywords: 	 DHCP SETUP Copy Configuration

DHCP Services:

The DHCP server provides nodes with and IP address leases and IP network information at boot time. Because the workstation negotiates a lease with the DHCP server that is renewed at defined intervals, central IP administration is possible. Also, machines can be easily move from segment to segment. As a result of lease renewal, all IP related information is updated as well.

NT's DHCP service is not redundant by design. Only one DHCP scope can be active at a time on a physical network (broadcast domain). When the DHCP server for a segment fails, IP address assignment stops and new IP address leases are not available for workstations. More importantly, the DHCP database contains a list of all active leases and reservations. If the DHCP database is lost (due to a hard disk crash, for example), re-implementing the service on a new machine can easily cause duplicate address assignment that result in network connectivity problems for many, and usually all the computers on a segment. Having several recent copies of the database is desirable. Microsoft provides an automatic backup feature that copies the DHCP lease database to a directory on the server. Also, in a TechNet article, Microsoft outlines a manual process to migrate a DHCP lease database from one server to another. This article describes how to set up a stand-by DHCP server if a copy of the DHCP lease database is available. Since this approach would have to rely on manual or scheduled file copy operation and that a skilled person have access to the procedure during a failure. An automatic approach was designed that will only require that a person start the DHCP service on a standby DHCP server.

DHCPCopy:

DHCPCopy will allow for easy recovery in the event of a DHCP server failure. Several (as many as five) DHCP servers can be configured to participate in a recovery strategy on a given network segment. Each server should be configured with the DHCP service and the service should be stopped and set to Manual Start-up on all but one server. This is obvious, since only one DHCP server should be active on a network segment at a time. This group of servers will service a single network segment, so at least two DCHP servers must be configured - one active DHCP server and one standby. The DHCPCopy service is installed on each DHCP server. On the active DHCP server, this service will push copies of the DHCP lease database and scope definition information to each of the standby DCHP servers. If the active DHCP server were to fail, the DHCP service on any of the standby DHCP servers can be started with minimal loss of lease information.

More specifically, the DHCPCopy service monitors the status of the DHCP service. If it is stopped, the DHCPCopy service does nothing. If it is active, the backup DHCP lease database is copied to the backup DHCP lease database of the standby DHCP servers and sets the RestoreFlag registry parameter is set to 1 (restore). Also, registry information pertaining to the active DHCP server's scope configuration is copies to the standby DHCP server's registry. If the standby server is also running DHCP, an error is logged and no push occurs. Should the need arise to start the DHCP service on one of the standby DHCP servers, the RestoreFlag forces the DHCP service to load the backup lease database, which is actually the backup database from the most previously active DHCP server. Since the DHCP service only periodically backs up the lease database, some most recent leases can be lost in this process. To reduce the number of lost leases, the DCHP backup interval should be judiciously set. The DHCPCopy service uses this interval to determine its execution intercal. The leases lost could be as many as the number of leases that are renewed in one DHCP backup interval. The DHCP backup interval defaults to 60 minutes, and should not be set lower than 5 minutes. 10 to 20 minutes are recommended, unless you have a very short lease time (less than 4 hours) and many clients (above 500). All DHCPCopy services can run under different security, but username and password information must be configured manually, in this case. Normally, DHCPCopy services should run on Servers and/or Domain Controllers in the same NT Domain.

Configuring and Installing DHCPCopy:

A configuration program (DCConfig.exe) is provided to configure and install the DCHPCopy service on all DHCP servers in one group at one time. It is intended that DCConfig should be run from an NT Workstation, Server or Domain Controller within the same domain as the DHCP servers. The account used should have Domain Administrator privileges. Also, all DHCP servers must have fixed IP addresses in the same logical network on the same network segment (the same broadcast space). DCConfig checks none of this. It is assumed the user has sufficient skills to deal with site or enterprise DHCP issues.

To configure DCHPCopy, you must already have:

Configured at least 2 NT 4.0 servers with the DHCP Service and applied the same service packs after DHCP service installation. This is very important since DHCP has changed significantly between release and SP3.
Given these servers static IP addresses in the same logical network and placed them on the same network segment.
Identified the number of DHCP clients and pooled IP addresses you have available - this suggests scope lease time and DHCP backup interval.
Logged on using an Domain Administrator account.

For more information, download the zip file with complete documentation:

dhcpcopy.zip

Article ID:   W14719

Filename:   DHCP Copy and Config.txt

File Created: 2019:08:14:09:34:18

Last Updated: 2019:08:14:09:34:18

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