ipv6 udp checksums like ipv4 but with ipv6 pseudoheader

this fixes a problem where both source and destination port are supplied on the commandline,
this lead to packets not showing up any more due to checksum being different.
this was mentioned first on traviscross#351

packet/construct_unix.c

@@ -306,18 +306,25 @@ int construct_udp6_packet(
     set_udp_ports(udp, probe, param);
     udp->length = htons(udp_size);
 
-    if (net_state->platform.ip6_socket_raw) {
-        /*
-           Instruct the kernel to put the pseudoheader checksum into the
-           UDP header, this is only needed when using RAW socket.
-         */
-        int chksum_offset = (char *) &udp->checksum - (char *) udp;
-        if (setsockopt(udp_socket, IPPROTO_IPV6,
-                       IPV6_CHECKSUM, &chksum_offset, sizeof(int))) {
-            return -1;
-        }
-    }
+    struct IP6PseudoHeader udph = {
+        .zero = {0,0,0},
+        .protocol = 17,
+        .len = udp->length
+    };
+    memcpy(udph.saddr, sockaddr_addr_offset(&probe->local_addr), 16);
+    memcpy(udph.daddr, sockaddr_addr_offset(&probe->remote_addr), 16);
 
+    /* get position to write checksum */
+    uint16_t *checksum_off = &udp->checksum;
+
+    if (udp->checksum != 0)
+    { /* checksum is sequence number - correct the payload to match the checksum
+         checksum_off is udp payload */
+        checksum_off = (uint16_t *)&packet_buffer[sizeof(struct UDPHeader)];
+    }
+    *checksum_off = htons(udp4_checksum(&udph, udp,
+                                        sizeof(struct IP6PseudoHeader),
+                                        udp_size, udp->checksum != 0));
     return 0;
 }