Remote DHCP

[John A. Sullivan III]

On Mon, 2007-12-03 at 18:02 +0100, Jose Manuel García wrote: 
> Hi,
> 
>  
> 
> I’m writting from Spain and my english is not very good, thought, I’ll
> try to do my best.
> 
>  
> 
> I have a Nagios 2.9 under Ubuntu 6.10. Everything works fine but I
> need to monitor a Remote Windows DHCP Server and I don’t know how.
> Evidently check_dhcp is not useful in remote hosts.
> 
>  
> 
> Any ideas ?
<snip>

It is possible although a bit tricky.  I have a somewhat old
installation so I had to download a later DHCP plugin than shipped with
my Nagios.  The dhcp plugin version I am using is 1.13.  Yours may
already be current enough.

Run check_dhcp --help to see the options.  Here is a snippet from my
command file:

# 'check_dhcp-remote' command definition - Only works where host is
configured to return a DHCP OFFER to the monitoring station network (can
be dummy configuration)
# ARG1 is the interface the monitoring system will use to send the DHCP
DISCOVER, e.g., tun0
define command{
        command_name    check_dhcp-remote
        command_line    $USER1$/check_dhcp-uni -s $HOSTADDRESS$ -u
$HOSTADDRESS$ -i $ARG1$
}


Besides getting the right version and command line, there are two other
issues as I remember.

1) The DHCP server will need to be configured to respond to requests
from the monitoring network.  In other words, if you are monitoring from
a remote network, there is a good chance the DHCP server is not
configured to assign addresses for the monitoring network.  Thus, it
will not respond to DHCP requests from the monitoring station.  It is
sufficient to just have a stub or dummy configuration.

2) Be aware that the actual packet will not be a DHCP packet but a
closely related DHCP-Relay packet.  I forget the port numbers exactly.
I believe DHCP is destination port 67 and source port 68 whereas
DHCP-Relay is both destination and source port 67.  We had an unusual
problem where the DHCP service on a small UTM device was failing.
Unfortunately the DHCP monitoring did not pick up the failures because,
although the UTM was not responding to DHCP packets, it was responding
to DHCP-Relay packets :(

Hope this helps - John
-- 
John A. Sullivan III
Open Source Development Corporation
+1 207-985-7880
jsullivan at opensourcedevel.com

If you would like to participate in the development of an open source
enterprise class network security management system, please visit
http://iscs.sourceforge.net


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