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Hack 41 Ensure DHCP Server Availability
Making sure a DHCP server is always available
is critical if your network uses dynamic TCP/IP
addressing.
Microsoft DHCP server became much more
popular in Windows 2000 environments, where it became part of the
overall strategy for managing IP addressing, host namespace, and name
resolution (due to its close integration with
Microsoft's implementation of DNS). Because of its
significance, it is imperative to have a solid plan that allows you
to quickly recover from DHCP server failures.
Installing Redundant DHCP Servers
One approach to ensuring DHCP server availability is
to install multiple DHCP servers and divide the list of available IP
addresses on each subnet into multiple ranges, one per server. In the
simplest case of two DHCP servers, configure each with the scopes
that have matching start and end address. Next, for each one create
mutually exclusive exclusion lists. For example,
if your network is using class C nonsubnetted network
192.168.168.0/24, then, on both servers, you should create the scope
with the start IP address 192.168.0.1 and the end IP address
192.168.168.254. Your choice of exclusion lists depends on whether
you want both servers to share the load equally or whether one of
them will be a primary choice for your DHCP
clients. For example, to balance the load, you would configure the
range 192.168.168.1-192.168.168.127 on the first server and
192.168.168.128-192.16.168.254 on the second.
In order for this configuration to work, you have to ensure that
broadcasts from DHCP clients will reach both servers. Typically, this
is done either by installing DHCP relay agents on the servers that
reside on clients subnet or by configuring routers as BOOTP Relay
Agents.
Backing Up the DHCP Database
In addition to providing redundancy, you should also ensure
regular backups of the DHCP database. Fortunately, the backup takes
place automatically by default. Its behavior is determined by
Registry entries that reside in the following key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DHCPServer\Parameters
The Registry entries contain the following values:
BackupDatabasePath
Determines the location of the backup (set initially to
%SystemRoot%\System32\DHCP\Backup).
BackupInterval
Determines the frequency of the automatic backup, in minutes (the
default is 60).
RestoreFlag
Can be used to force the restore by using the existing backup (by
setting RestoreFlag to 1).
Typically, the operating system does this automatically if it detects
the DHCP database corruption.
Windows also automatically backs up the content of the Registry key
HKLM\SOFTWARE\Microsoft\DHCPServer\Configuration
to the DHCPCFG file, which resides in the
Backup folder.
Recovering the Database
Recovering the database involves
restoring both the database files and the Registry settings. You
should first stop the DHCP server and then copy the files and load
the Registry hive (using REGEDT32.EXE) to their
target location by overwriting the existing
HKLM\SOFTWARE\Microsoft\DHCPServer\Configuration
Registry key. After you have restored the database file, you should
change the default of 0 conflict-detection
attempts (from the Advanced tab of Server properties in the DHCP MMC
console) to a nonzero value (5 is the maximum).
Another option is to use the NETSH command-line
utility to back up and restore configuration of the DHCP server
database. NETSH's functionality
is provided through a number of helper DLLs, each dealing with a
particular type of Windows networking component.
NETSH allows you to dump the configuration of the
DHCP server (including all superscopes, scopes, exclusion ranges, and
reservations) into a text file that later can be used to restore.
Note, however, that NETSH does not back up
information about existing leases, which are stored in the DHCP
database.
To create the DHCP configuration dump file, execute the following
command, where IPAddressOrName is the IP
address or name of your DHCP server (note that this command can be
executed remotely):
NETSH DHCP SERVER IPAddressOrName DUMP > C:\DHCPCfg.txt
To restore the DHCP server configuration settings using the same
file, run this command:
NETSH EXEC C:\DHCPCfg.txt
Marcin Policht
