macOS Dual Stack UDP Socket Bug

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <pthread.h>

void *send_udp_packet(void *arg) {
    int sock;
    struct sockaddr_in dest_addr;
    char buffer[1000] = {0};  // Buffer filled with zeros
    int port = *(int *)arg;

    // Create an IPv4 UDP socket
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock == -1) {
        perror("Socket creation failed");
        return NULL;
    }

    // Set up the destination address structure for IPv4
    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_port = htons(port);
    inet_pton(AF_INET, "127.0.0.1", &dest_addr.sin_addr);

    // Send the packet
    if (sendto(sock, buffer, sizeof(buffer), 0, (struct sockaddr *)&dest_addr, sizeof(dest_addr)) == -1) {
        perror("sendto failed");
    }

    close(sock);
    return NULL;
}

int main() {
    int sock;
    struct sockaddr_in6 addr6, local_addr;
    struct sockaddr_storage src_addr;
    socklen_t src_addr_len = sizeof(src_addr), local_addr_len = sizeof(local_addr);
    char addr_str[INET6_ADDRSTRLEN];
    char buffer[1024];
    int received;
    pthread_t thread_id;
    int port;

    // Create a UDP IPv6 socket
    sock = socket(AF_INET6, SOCK_DGRAM, 0);
    if (sock == -1) {
        perror("Socket creation failed");
        return 1;
    }

    // Enable dual-stack (IPv4 and IPv6) mode
    int no = 0;
    if (setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, (void *)&no, sizeof(no)) == -1) {
        perror("setsockopt IPV6_V6ONLY");
        close(sock);
        return 1;
    }

    // Bind the socket to the loopback address and let kernel choose the port
    memset(&addr6, 0, sizeof(addr6));
    addr6.sin6_family = AF_INET6;
    addr6.sin6_addr = in6addr_any;
    addr6.sin6_port = htons(0);

    if (bind(sock, (struct sockaddr *)&addr6, sizeof(addr6)) == -1) {
        perror("Bind failed");
        close(sock);
        return 1;
    }

    // Get the assigned port
    if (getsockname(sock, (struct sockaddr *)&local_addr, &local_addr_len) == -1) {
        perror("getsockname failed");
        close(sock);
        return 1;
    }

    port = ntohs(local_addr.sin6_port);
    printf("Listening for UDP packet on port %d...\n", port);

    // Start the sender thread
    pthread_create(&thread_id, NULL, send_udp_packet, &port);

    // Wait to receive one packet
    received = recvfrom(sock, buffer, sizeof(buffer), 0, (struct sockaddr *)&src_addr, &src_addr_len);
    if (received == -1) {
        perror("recvfrom failed");
        close(sock);
        return 1;
    }

    // Print the source address and port
    if (src_addr.ss_family == AF_INET6) {
        struct sockaddr_in6 *s = (struct sockaddr_in6 *)&src_addr;
        inet_ntop(AF_INET6, &s->sin6_addr, addr_str, sizeof(addr_str));
        printf("Received packet from [%s]:%d\n", addr_str, ntohs(s->sin6_port));
    } else { // AF_INET
        struct sockaddr_in *s = (struct sockaddr_in *)&src_addr;
        inet_ntop(AF_INET, &s->sin_addr, addr_str, sizeof(addr_str));
        printf("Received packet from %s:%d\n", addr_str, ntohs(s->sin_port));
    }

    // Clean up
    close(sock);
    pthread_join(thread_id, NULL);
    return 0;
}

Run repeatedly: gcc -pthread -o main main.c && while true; do; ./main; done

The dual-stack socket fails to receive the packet after a few hundred / thousand runs on macOS, but runs reliably on Linux.

It also runs reliably if a specific port number is defined, and the kernel doesn't get to choose the port number.