Dueling DHCP Servers

Dueling DHCP Servers

(Or, when servers fight it's only the clients which suffer.)

(This item is under Linux for convenience only; it probably applies to any and all DHCP servers.)

I noticed on my DHCP server that I was getting a lot of incidents where windows clients would repeatedly and aggressively request IP addresses (ie one cycle every ten seconds). The client would eventually give up and require a static IP address be configured on it.

What turned out to be happening is that we had a second DHCP server on the network handing out a completely different network scope. An ethereal grab on a client having trouble looked like this:

client: can I have an IP address please?
server1: have IP1
server2: have IP2
client: can I use IP1?
server1: yes
server2: no
… ten seconds go by…
client: can I have an IP address please?
… several hours of this go by…
client: arrgh, fuck it.  I quit.

The client was selecting the IP address at random, either IP1 or IP2, but honoring the DHCPNAK sent by the competing server.

The solution, of course, was to locate the competing DHCP server, and turn it off with a hammer.