--- check_dhcp.c.orig	2007-12-14 21:04:26.000000000 +0100
+++ check_dhcp.c	2007-12-15 16:04:12.000000000 +0100
@@ -165,6 +165,7 @@
 	u_int32_t lease_time;            /* lease time in seconds */
 	u_int32_t renewal_time;          /* renewal time in seconds */
 	u_int32_t rebinding_time;        /* rebinding time in seconds */
+	u_int8_t desired;                /* is this offer desired (necessary in exclusive mode) */
 	struct dhcp_offer_struct *next;
         }dhcp_offer;
 
@@ -209,6 +210,7 @@
 #define ETHERNET_HARDWARE_ADDRESS_LENGTH     6     /* length of Ethernet hardware addresses */
 
 u_int8_t unicast = 0;        /* unicast mode: mimic a DHCP relay */
+u_int8_t exclusive = 0;        /* exclusive mode aka "rogue DHCP server detection" */
 struct in_addr my_ip;        /* our address (required for relay) */
 struct in_addr dhcp_ip;      /* server to query (if in unicast mode) */
 unsigned char client_hardware_address[MAX_DHCP_CHADDR_LENGTH]="";
@@ -919,6 +921,7 @@
 	new_offer->lease_time=dhcp_lease_time;
 	new_offer->renewal_time=dhcp_renewal_time;
 	new_offer->rebinding_time=dhcp_rebinding_time;
+	new_offer->desired=FALSE; /* exclusive mode: we'll check that in get_results */
 
 
 	if(verbose){
@@ -964,7 +967,7 @@
 
 /* gets state and plugin output to return */
 int get_results(void){
-	dhcp_offer *temp_offer;
+	dhcp_offer *temp_offer, *undesired_offer=NULL;
 	requested_server *temp_server;
 	int result;
 	u_int32_t max_lease_time=0;
@@ -999,11 +1002,18 @@
 					if(temp_server->answered == FALSE){
 						requested_responses++;
 						temp_server->answered=TRUE;
+						temp_offer->desired=TRUE;
 						}
 				        }
 		                }
 		        }
 
+		/* exclusive mode: check for undesired offers */
+		for(temp_offer=dhcp_offer_list;temp_offer!=NULL;temp_offer=temp_offer->next)
+			if (temp_offer->desired == FALSE) {
+				undesired_offer=temp_offer; /* Checks only for the first undesired offer */
+				break; /* no further checks needed */
+			}
 	        }
 
 	/* else check and see if we got our requested address from any server */
@@ -1030,6 +1040,9 @@
 		result=STATE_WARNING;
 	else if(request_specific_address==TRUE && received_requested_address==FALSE)
 		result=STATE_WARNING;
+	
+	if(exclusive && undesired_offer)
+                result=STATE_CRITICAL;
 
 	if(result==0)               /* garrett honeycutt 2005 */
 		printf("OK: ");
@@ -1046,6 +1059,12 @@
 		return result;
 	        }
 
+	if(exclusive && undesired_offer){
+                printf(_("Rogue DHCP Server detected! Server %s"),inet_ntoa(undesired_offer->server_address));
+                printf(_(" offered %s \n"),inet_ntoa(undesired_offer->offered_address));
+                return result;
+		}
+
 	printf(_("Received %d DHCPOFFER(s)"),valid_responses);
 
 	if(requested_servers>0)
@@ -1100,6 +1119,7 @@
 		{"interface",      required_argument,0,'i'},
 		{"mac",            required_argument,0,'m'},
 		{"unicast",        no_argument,      0,'u'},
+		{"exclusive",      no_argument,      0,'x'},
 		{"verbose",        no_argument,      0,'v'},
 		{"version",        no_argument,      0,'V'},
 		{"help",           no_argument,      0,'h'},
@@ -1107,7 +1127,7 @@
 	};
 
 	while(1){
-		c=getopt_long(argc,argv,"+hVvt:s:r:t:i:m:u",long_options,&option_index);
+		c=getopt_long(argc,argv,"+hVvxt:s:r:t:i:m:u",long_options,&option_index);
 
 		i++;
 
@@ -1170,6 +1190,10 @@
 			unicast=1;
 			break;
 
+                case 'x': /* exclusive testing aka "rogue DHCP server detection" */
+                        exclusive=1;
+                        break;
+		
 		case 'V': /* version */
 			print_revision(progname,revision);
 			exit(STATE_OK);
@@ -1415,6 +1439,8 @@
   printf ("    %s\n", _("MAC address to use in the DHCP request"));
   printf (" %s\n", "-u, --unicast");
   printf ("    %s\n", _("Unicast testing: mimic a DHCP relay, requires -s"));
+  printf (" %s\n", "-x, --exclusive");
+  printf ("    %s\n", _("Only requested DHCP server may response (rogue DHCP server detection), requires -s"));
 
 	return;
 	}
@@ -1424,7 +1450,7 @@
 print_usage(void){
 	
   printf (_("Usage:"));
-  printf (" %s [-v] [-u] [-s serverip] [-r requestedip] [-t timeout]\n",progname);
+  printf (" %s [-v] [-u] [-x] [-s serverip] [-r requestedip] [-t timeout]\n",progname);
   printf ("                  [-i interface] [-m mac]\n");
   
 	return;