AF_UNSPEC cause UDP packet losses - c

Question

Firstly please bear me with the long question - I'm writing a UDP client-server program and odd enough I had this packet loss problem whenever I use this flag for protocol family. Symptom is packets never left the sender host (no captures from tcpdump) but sendto() returns the correct positive value, which is never bigger than 200 bytes. It took me a ridiculous amount of time to figure out this can be resolved by forcing IPv4 but I don't understand why? It's been tested numerous times with server and client running on the same system (scientific linux 6) so the common unreliable UDP transmission is probably not the cause here, nor is there any firewall rule that drops anything at all. I'm wondering if anyone happens to know the cause? Also I thought about posting some code but it doesn't seem to be necessary.. Simply put, AF_UNSPEC results in all UDP packets never leaving the sender but no error, and with AF_INET everything is perfect. This might be a weird question but any insight is appreciated. Thanks a lot!Code for server initialization:
int udp_srv_init(char * port)
{
struct addrinfo hints, *servinfo, *p;
int rv;
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE;
if ((rv = getaddrinfo(NULL, port, &hints, &servinfo)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
return EXIT_FAILURE;
}
for(p = servinfo; p != NULL; p = p->ai_next) {
if ((self_udp = socket(p->ai_family, p->ai_socktype,
p->ai_protocol)) == -1) {
perror("socket");
continue;
}
if (bind(self_udp, p->ai_addr, p->ai_addrlen) == -1) {
close(self_udp);
fprintf(stderr, "Port %s: ",port);
perror("bind");
continue;
}
break;
}
if (p == NULL) {
fprintf(stderr, "server: failed to bind socket\n");
return EXIT_FAILURE;
}
freeaddrinfo(servinfo);
int buffsize = 65536; // I added this because I thought it was some buffer problem, but didn't change a thing
setsockopt(self_udp, SOL_SOCKET, SO_RCVBUF, (void*)&buffsize, sizeof(buffsize));
return EXIT_SUCCESS;
}
and for sending:
//...
for(i=0; i<num_neighbors; i++)
{
if(neighbors[i].cost == -1)
continue;
int bytes_sent = 0, ret;
while(bytes_sent < packet_size)
{
if(-1 == (ret = sendto(neighbors[i].skt, (void*)&my_neighs +
(ptrdiff_t)bytes_sent, packet_size-bytes_sent,
0, neighbors[i].p->ai_addr, neighbors[i].p->ai_addrlen)))
{
perror("sendto1");
return EXIT_FAILURE;
}
bytes_sent += ret;
}
}
//....
neighbors is a structure array with elements that have member fields:
struct addrinfo *servinfo, *p;
They are obtained from socket initialization, and freeaddrinfo() is only called at the very end of program execution so they are all valid.