From Nagios Wiki

[edit] example entries in commands.cfg

#taken from CentOS 4
define command{
        command_name    check_reverse_smtp
        command_line    $USER1$/negate "/usr/lib/nagios/plugins/check_smtp -H $HOSTADDRESS$"
        }

#taken from CentOS 4
define command{
        command_name    check_reverse_dhcp
        command_line    $USER1$/negate "/usr/lib/nagios/plugins/check_dhcp -H $HOSTADDRESS$"
        }