[Nagiosplug-devel] check_dhcp patch #2968305

Mike Holloway mikhollo at cisco.com
Thu Apr 15 18:06:24 CEST 2010

Howdy all, 

I recently submitted a patch for the check_dhcp plugin, and I wanted to provide some additional feedback to the committers to help them in their consideration of including this patch in the next release.  We've deployed this patched sensor to over 100 networks so far with widely varying DHCP server implementations, and have had no reported issues.  This patch corrects behavior we observed in a few DHCP server implementations where the DHCP servers exhausted their free IP pools because the current check_dhcp sensor doesn't broadcast a subsequent DHCPREQUEST to allow these servers to clear out their previous offered IPs.  Details of the patch below.


check_dhcp: send a REQUEST after receiving OFFERs - ID: 2968305

Initial Comment:
Patch against Plugin Version (-V output): 1.4.14
Plugin Name: check_dhcp
Example Plugin Commandline: check_dhcp -i egiga0
Tested on operating system: linux
Tested on architecture: arm/freebsd/linux
Tested with compiler: gcc

Patch adds the sending of a DHCPREQUEST after receiving OFFERs. This allow
s DHCP servers to release any IPs allocated by the OFFER back to their free
pools immediately, rather than waiting for the offer to timeout. Some DHC
P servers hold these offers for a long time, and will happily issue a new I
P to the same device each time because the previous OFFER wasn't followed b
y a subsequent broadcast/unicast REQUEST. This could lead to an exhaustion
of IP addresses from the DHCP server's free pool on some implementations.


Send a DHCP REQUEST after receiving OFFERs, marking our own address as SERVER IDENTIFIER in DHCP OPTIONS, tidy some tabs
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