Created
June 23, 2024 14:46
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macOS Dual Stack UDP Socket Bug
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#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; | |
} |
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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.