dhcp server ignoring nagios check_dhcp requests

[Guillaume Rousse]

Hello list.

I'm facing a very curious problem here. We have a DHCP server working
perfectly, with the only exception than it seems to ignore all test dhcp
queries from our nagios server n°1, while honouring test queries from
our nagios server n°2. The problems is probably caused by the fact than
all of those hosts are actually xen-based virtual machines, with a quite
complex network setting (channel bonding + vlan + bridges), and that the
nagios server n°1 (the failing one) is actually hosted on the same xen
server, while the nagios server n°2 (the working one) is hosted on
another xen server. But nothing in the dchp server logs, nor in the
captured traffic, show any evidence of what actually differs between
honoured and ignored requests.

I'm attaching a network capture from dhcp server, showing two requests,
and one only answer. Here is a quick description of the situation:

xen-server1:
- 195.83.213.219 nagios server 1 (mac 00:16:3e:00:00:05)
- 195.83.213.206 dhcp server

xen-server2:
- 195.83.213.210 nagios server 2 (mac 00:16:3e:00:00:10)

The dhcp server has fixed addres configuration for both of them
host tipunch {
    hardware ethernet 00:16:3e:00:00:05;
    fixed-address 195.83.213.219;
}
host zombie {
    hardware ethernet 00:16:3e:00:00:10;
    fixed-address 195.83.213.210;
}
And normal dhcp operations (getting an IP adress at boot, or renewing
lease later) works perfectly for both hosts.

All of those hosts are paravirtualized x86_64 mandriva 2008.0 linux,
running 2.6.18 kernel on xen 3.1.3. The dhcp server version is 3.0.6.
The guilty dhcp client is check_dhcp plugin from nagios plugins 1.4.11,
whose source code is available at:
http://nagiosplug.svn.sourceforge.net/viewvc/nagiosplug/nagiosplug/tags/release-1.4.11/plugins-root/check_dhcp.c?view=log

Any hint welcome, as I'm a bit lost here.
-- 
Guillaume Rousse
Moyens Informatiques - INRIA Futurs
Tel: 01 69 35 69 62
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