How can we send data to ipv4 and ipv6 google interface using non connected socket udp in C?

Question

I'm trying to send some random datagram to host www.google.fr to both ipv4 and ipv6 interfaces, this work fine for ipv4 but not for ipv6. I wanted to do it with a single not-connected socket via UDP protocol.

I did something using getaddrinfo, and sendto function but when I call sendto on ipv6 interfaces, it fails and print "network is unreachable".

int main() {
  int s = socket(AF_INET6, SOCK_DGRAM, 0);
  char test[4];

  struct addrinfo hints = {0};
  hints.ai_family   = AF_INET6;
  hints.ai_socktype = SOCK_DGRAM;
  hints.ai_protocol = 0;
  hints.ai_flags    = AI_V4MAPPED|AI_ALL;
  struct addrinfo* res = {0};
  struct addrinfo* list;

  int exit_status = getaddrinfo("www.google.fr","8080", &hints, &res);

  if (exit_status != 0){
    perror("getaddrinfo:");
    return EXIT_FAILURE;
  }

  for (list = res; list != NULL; list = list->ai_next) {
    if (list->ai_family == AF_INET6) {
      printf("AF_INET6\n");
      int rc = sendto(s, test, 4, MSG_MORE, list->ai_addr, sizeof(*list->ai_addr));
      if (rc < 0) {
         perror("PROBLEME ... ");
      }

    }

  }

  return EXIT_SUCCESS; 
}

I Expect no perror printing but I get network is unreacheable. I would like to know why this dont work someone any idea ?

jxh jxh · Accepted Answer · 2019-04-23T16:54:01

One obvious problem is that you are not passing enough bytes for the address length in the sendto call. The the ai_addr field is a struct sockaddr *, but a struct sockaddr is merely a header structure, and the real address structures will allocate larger structures.

Use list->ai_addrlen instead.

How can we send data to ipv4 and ipv6 google interface using non connected socket udp in C?

2 Answers