Fix #104: IGMP header checksum missing from mrdisc frames

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/inet.c

@@ -36,6 +36,37 @@
 
 #define MC_ALL_SNOOPERS "224.0.0.106"
 
+/* Checksum routine for Internet Protocol family headers */
+static unsigned short in_cksum(unsigned short *addr, int len)
+{
+ unsigned short *w = addr;
+ unsigned short answer = 0;
+ int nleft = len;
+ int sum = 0;
+
+ /*
+ * Our algorithm is simple, using a 32 bit accumulator (sum), we add
+ * sequential 16 bit words to it, and at the end, fold back all the
+ * carry bits from the top 16 bits into the lower 16 bits.
+ */
+ while (nleft > 1) {
+ sum += *w++;
+ nleft -= 2;
+ }
+
+ /* mop up an odd byte, if necessary */
+ if (nleft == 1) {
+ *(unsigned char *)(&answer) = *(unsigned char *)w;
+ sum += answer;
+ }
+
+ /* add back carry outs from top 16 bits to low 16 bits */
+ sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
+ sum += (sum >> 16); /* add carry */
+ answer = ~sum; /* truncate to 16 bits */
+
+ return answer;
+}
 
 int inet_open(char *ifname)
 {
@@ -121,7 +152,7 @@ int inet_send(int sd, uint8_t type, uint8_t interval)
  memset(&igmp, 0, sizeof(igmp));
  igmp.igmp_type = type;
  igmp.igmp_code = interval;
- igmp.igmp_cksum = 0;
+ igmp.igmp_cksum = in_cksum(&igmp, sizeof(igmp));
 
  compose_addr((struct sockaddr_in *)&dest, MC_ALL_SNOOPERS);