set udp checksum

in case the checksum field is used fpr the sequence number,
calculate the checksum and write to the udp payload field so the seq no
matches the checksum

diff --git a/packet/construct_unix.c b/packet/construct_unix.c
index 43e6e5136fc0dc0d936a6a274eee74dd7d8ff364..d31cba0d6ba98a01fce8a753f5e6bfb8d3b1c783 100644 (file)
--- a/packet/construct_unix.c
+++ b/packet/construct_unix.c
@@ -219,6 +219,25 @@ void set_udp_ports(
     }
 }
 
+/* Prepend pseudoheader to the udp datagram and calculate checksum */
+static
+int udp4_checksum(void *pheader, void *udata, int psize, int dsize,
+                  int alt_checksum)
+{
+    unsigned int totalsize = psize + dsize;
+    unsigned char csumpacket[totalsize];
+
+    memcpy(csumpacket, pheader, psize); /* pseudo header */
+    memcpy(csumpacket+psize, udata, dsize);   /* udp header & payload */
+
+    if (alt_checksum && dsize >= sizeof(struct UDPHeader) + 2) {
+        csumpacket[psize + sizeof(struct UDPHeader)] = 0;
+        csumpacket[psize + sizeof(struct UDPHeader) + 1] = 0;
+    }
+
+    return compute_checksum(csumpacket, totalsize);
+}
+
 /*
     Construct a header for UDP probes, using the port number associated
     with the probe.
@@ -229,6 +248,8 @@ void construct_udp4_header(
     int sequence,
     char *packet_buffer,
     int packet_size,
+    const struct sockaddr_in *src_addr,
+    const struct sockaddr_in *dst_addr,
     const struct probe_param_t *param)
 {
     struct UDPHeader *udp;
@@ -246,6 +267,29 @@ void construct_udp4_header(
 
     set_udp_ports(udp, sequence, param);
     udp->length = htons(udp_size);
+
+    /* calculate udp checksum */
+    struct UDPPseudoHeader udph = {
+        .saddr = src_addr->sin_addr.s_addr,
+        .daddr = dst_addr->sin_addr.s_addr,
+        .zero = 0,
+        .protocol = 17,
+        .len = udp->length
+    };
+
+    /* 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[packet_size -
+                                                  udp_size +
+                                                  sizeof(struct UDPHeader)];
+    }
+    *checksum_off = htons(udp4_checksum(&udph, udp,
+                                        sizeof(struct UDPPseudoHeader),
+                                        udp_size, udp->checksum != 0));
 }
 
 /*  Construct a header for UDPv6 probes  */
@@ -533,7 +577,10 @@ int construct_ip4_packet(
                                    packet_size, param);
         } else if (param->protocol == IPPROTO_UDP) {
             construct_udp4_header(net_state, sequence, packet_buffer,
-                                  packet_size, param);
+                                  packet_size,
+                                  (struct sockaddr_in *)src_sockaddr,
+                                  (struct sockaddr_in *)dest_sockaddr,
+                                  param);
         } else {
             errno = EINVAL;
             return -1;