OpenLDAP Everywhere Reloaded, Part I

One last note on the DHCP failover protocol: if you have more devices on one subnet than 50% of the overall amount available in the pool range, seriously consider re-engineering your network before implementing DHCP failover.

The protocol inherently relies on having an excess of free addresses to allocate, even after the pool range is cut in half by split 128;.

The maximum amount of available IP addresses for DHCP in a C Class subnet most of the time is 253 (255 addresses, minus 1 address for broadcast, minus 1 address for the router).

If you have more than 126 devices within one failover subnet, either split it into more subnets (for example, one subnet for each floor of the building), or use a larger subnet than C Class. Configure the subnet declaration in /etc/dhcpd.conf to increase its pool range accordingly. It will save you problems later on.

Now that the DHCP servers are configured with failover pools, the final thing to do is configure the and to forward DHCP client broadcasts through the LAN/WAN to and

This is done on with IP Helper addresses. On, it's done with ISC's DHCP Relay Agent.

Assume is a Cisco Catalyst Multi-layer Switch. Configure IP Helper addresses by entering privileged mode (run the enable command). Using the ip helper-address command, apply the two DHCP server IP addresses to the router interface that has the address. On the Catalyst 3750G in my lab, this is interface "vlan20". The commands are applied like so:

router03#show running-config
Building configuration...

  --- output suppressed ---

interface Vlan20
 description linuxjournal_vlan
 ip address

  --- output suppressed ---

router03#configure terminal
router03(config)#interface vlan 20
router03(config-if)#ip helper-address
router03(config-if)#ip helper-address
router03#copy running-config startup-config
Destination filename [startup-config]?
Building configuration...
0 bytes copied in 8.715 secs (0 bytes/sec)
router03#show running-config interface vlan 20
Building configuration...

Current configuration : 154 bytes
interface Vlan20
 description linuxjournal_vlan
 ip address
 ip helper-address
 ip helper-address


On, you need to install the isc-debian-relay package. Once it's installed, it will ask for the "multiple server names be provided as a space-separated list". Enter "", or if this host isn't configured to resolve from our DNS server pair, "". It will ask on which interface to listen. If you have no preference, leave it blank and press Enter. It will ask you to specify additional options, but you simply can press Enter.

If you make a mistake, you can reconfigure by running the command dpkg-reconfigure isc-dhcp-relay or modify the SERVERS variable in /etc/default/isc-dhcp-relay.

Your DHCP clients now should be able to contact either DHCP server.

In Part II of this series, I'll explain how to configure OpenLDAP on the two Linux servers and start to populate the directory with data.


Example configuration files for this article:

Debian GNU/Linux:

Download Debian

Manual Page for ntp.conf(5):

Manual Page for named.conf(5):

Manual Page for dhcpd.conf(5):

Manual Page for dhcp-options(5):

ISC dhcp-users Mailing List:

Cisco IOS 12.3 T Command Reference for Idle through IP local-proxy-arp (includes ip helper-address):


Stewart Walters is a Solutions Architect with more than 15 years' experience in the Information Technology industry. Amongst other industry certifications, he is a Senior Level Linux Professional (LPIC-3).


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