DHCP feedback and a puzzle

DHCP, our topic for the past few weeks, was a popular one, and your feedback has warmed the cockles of Gearhead's heart. Kevin Noll wrote, "In your discussion of [Dynamic Host Configuration Protocol] servers allocating duplicate IP addresses [specifically when there are multiple DHCP servers], you mention that you didn't know of any DHCP servers that handle this problem very well.

First, note that RFC 2131 allows the existence of multiple DHCP servers on the network, and specifies that the client is responsible to choose one, and only one, if more than one server replies to the Discover.

Cisco's Network Registrar [formerly American Internet] implements a failover configuration. This system allows a secondary CNR DHCP server to allocate addresses from a scope configured on the primary when the primary is not available. When the primary is brought back online, the secondary transfers information about those leases to the primary so that duplicates are not handed out and so that leases continue to be renewed properly. CNR simultaneously allows a 'load-balancing' configuration that works with this failover configuration.

Nortel Networks' NetID allows a similar configuration of primary and secondary DHCP servers. Check Point's Meta IP claims to have the same capability. But I do not have experience with this particular server, so I cannot speak to its capabilities.

I'm not sure you were specifically referring to the failover functionality as much as the inability to correctly deal with duplicate allocations, but in a well-thought-out DHCP network [with a good DHCP server . . . that is, not the one that comes with you-know-who's operating system] this problem is minimized while still providing fault-tolerant IP address assignment."

Paul Villarno commented, "If I understand the problem you outlined in your article, you should try Novell's DHCP server. It overcomes the problem you mentioned because the DHCP data is not tied to any particular server but rather to the directory (Novell Directory Services). As such, it doesn't matter if your primary DHCP server goes down, you just nominate another server to take over its function, it reads the data from the directory, and voila. Further reading: www.nwc.com/1117/1117ws1.html."

Finally, a tricky problem. John Becker writes, "Thanks for the series of articles on DHCP. The latest one struck a chord with me. I am volunteer administrator for a small (four NT Servers, about 115 clients) school network. We have had problems with DHCP conflicts where two or more clients try to grab the same IP address. We have three NT servers with SP4 and one server is the primary domain controller. All servers have static IP addresses, and we have set scopes for specific IP address range for DHCP use.

Initially, I cranked the lease period down to eight hours, on the theory that overnight all clients would release. No luck, so I cranked it back up to two, five, 10, 30 days, and slowly the problems have disappeared. Can you tell me why?"

A great question and Gearhead has wild theories about the problem. Let us throw this one open to Gearhead readers. The first person to send us a solution will win something cool. We have no idea what, but it will be cool.

Your attempt at fame and the acquisition of cool to gh@gibbs.com.