Index: tools/tools/netmap/pkt-gen.c
===================================================================
--- tools/tools/netmap/pkt-gen.c	(revision 262618)
+++ tools/tools/netmap/pkt-gen.c	(working copy)
@@ -546,6 +546,7 @@
 {
 	uint32_t a;
 	uint16_t p;
+	struct ether_header *eh = &pkt->eh;
 	struct ip *ip = &pkt->ip;
 	struct udphdr *udp = &pkt->udp;
 
@@ -580,6 +581,25 @@
 	ip->ip_dst.s_addr = htonl(g->dst_ip.start);
     } while (0);
     // update checksum
+	/* Missing code here ? the comment "update checksum" should be followed
+	 * by updating IP & UDP checksum. I will try to fix by simply copy/past
+	 * checksum code from initialize_packet function */
+
+	ip->ip_sum = 0;
+	ip->ip_sum = wrapsum(checksum(ip, sizeof(*ip), 0));
+
+	/* UDP checksum */
+	uint16_t paylen = g->pkt_size - sizeof(*eh) - sizeof(struct ip);
+	udp->uh_sum = 0;
+	udp->uh_sum = wrapsum(checksum(udp, sizeof(*udp),
+                    checksum(pkt->body,
+                        paylen - sizeof(*udp),
+                        checksum(&ip->ip_src, 2 * sizeof(ip->ip_src),
+                            IPPROTO_UDP + (u_int32_t)ntohs(udp->uh_ulen)
+                        )
+                    )
+                ));
+
 }
 
 /*