From 7f68901d28ccbbc4717737170803364076288814 Mon Sep 17 00:00:00 2001
From: =?utf8?q?Markus=20K=C3=B6tter?= <koetter@luis.uni-hannover.de>
Date: Mon, 29 Oct 2018 11:29:22 +0100
Subject: [PATCH] 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
---
 packet/construct_unix.c | 49 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/packet/construct_unix.c b/packet/construct_unix.c
index 43e6e51..d31cba0 100644
--- 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;
-- 
2.47.3