Very basic Bash script to check dhcp server through DHCPDISCOVER or DHCPINFORM querys.
Results are parsed and returned to nagios.
requires nmap and dhcp-discover.nse (nmap script)

parameters: 
check_dhcp_ext [DHCPDISCOVER] hostname
if dhcpdiscover is not specified, it defaults to dhcpinform

eg
(reply to a discover)
Status information : DHCP OK: DHCP Message Type: DHCPOFFER
Performance Data : Server Identifier: 10.0.0.1 - IP Offered: 10.0.0.31 - IP Address Lease Time: 2h00m00s - Subnet Mask: 255.255.255.0 - Router: 10.0.0.1 - Domain Name Server: 10.0.0.1 - Domain Name: homenetwork - NetBIOS Name Server: 10.0.0.1 - MAC Address: 00:CC:66:E6:99:CC (Sitecom Europe BV)

(reply to an inform)
Status Information:	DHCP OK: DHCP Message Type: DHCPACK
Performance Data:	Server Identifier: 10.0.0.1 - Router: 10.0.0.1 - Domain Name Server: 10.0.0.1 - Domain Name: homenetwork - NetBIOS Name Server: 10.0.0.1 - MAC Address: 00:0C:F6:E6:59:CC (Sitecom Europe BV)

Installation Instructions : 

Install the script in nagios plugin directory (eg. /usr/local/nagios/libexec
Check the script for binary locations (sudo and nmap)
make root/nagios owned 
chown root:nagios check_dhcp_ext
make it executable
sudo chmod ug+x check_dhcp_ext

let nagios run nmap as root passwordless : 
add nagios ALL=(root) NOPASSWD:/usr/bin/nmap to /etc/sudoers.d/010_nagios-nopasswd

add nagios commands :
define command {
    command_name    check_dhcp_inform
    command_line    $USER1$/check_dhcp_ext $HOSTADDRESS$
}

define command {
    command_name    check_dhcp_discover
    command_line    $USER1$/check_dhcp_ext DHCPDISCOVER $HOSTADDRESS$
}

add nagios services
define service {
    use                           local-service
    host_name                gateway
    service_description    DHCP-discover
    check_command        check_dhcp_discover
}

Enjoy the plugin, feedback always welcome! 
kalanikta@gmail.com