Warning about packet streams and fixed dst port validation (#738)

src/probe_modules/packet.h

@@ -155,7 +155,11 @@ static inline int check_dst_port(uint16_t port, int num_ports,
 	int32_t min = validation[1] % num_ports;
 	int32_t max = (validation[1] + zconf.packet_streams - 1) % num_ports;
 
-	return (((max - min) % num_ports) >= ((to_validate - min) % num_ports));
+	if (min <= max) {
+		return (to_validate <= max && to_validate >= min);
+	} else {
+		return (to_validate <= max ^ to_validate >= min);
+	}
 }
 
 static inline uint16_t get_src_port(int num_ports, int probe_num,

src/zmap.c

@@ -553,6 +553,7 @@ int main(int argc, char *argv[])
 	SET_IF_GIVEN(zconf.max_sendto_failures, max_sendto_failures);
 	SET_IF_GIVEN(zconf.min_hitrate, min_hitrate);
 
+
 	if (zconf.num_retries < 0) {
 		log_fatal("zmap", "Invalid retry count");
 	}
@@ -646,6 +647,19 @@ int main(int argc, char *argv[])
 				zconf.source_port_first = port;
 				zconf.source_port_last = port;
 			}
+			int num_source_ports = (zconf.source_port_last - zconf.source_port_first) + 1;
+			if (zconf.packet_streams > num_source_ports) {
+				log_fatal("zmap", "The number of probes sent to each target ip/port (%i) "
+						"must be smaller than the size of the source port range (%u-%u, size: %i). "
+						"Otherwise, some generated probe packets will be identical.", zconf.packet_streams,
+						zconf.source_port_first, zconf.source_port_last,
+						(zconf.source_port_last - zconf.source_port_first) + 1);
+			} else if (((float)zconf.packet_streams / (float)num_source_ports) < 0.1) {
+				log_warn("zmap", "ZMap is configured to use a relatively small number"
+						" of source ports (fewer than 10x the number of probe packets per target ip/port),"
+						" which limits the entropy that ZMap has available for "
+						" validating responses. We recommend that you use a larger port range.");
+			}
 		}
 		if (!args.target_ports_given) {
 			log_fatal("zmap",