Triggering the Local Network Privacy Alert

Code Block  #include <ifaddrs.h>#include <sys/socket.h>#include <net/if.h>#include <netinet/in.h>/// Returns the addresses of the discard service (port 9) on every/// broadcast-capable interface.////// Each array entry contains either a `sockaddr_in` or `sockaddr_in6`.static NSArray<NSData *> * addressesOfDiscardServiceOnBroadcastCapableInterfaces(void) {    struct ifaddrs * addrList = NULL;    int err = getifaddrs(&addrList);    if (err != 0) {        return @[];    }    NSMutableArray<NSData *> * result = [NSMutableArray array];    for (struct ifaddrs * cursor = addrList; cursor != NULL; cursor = cursor->ifa_next) {        if ( (cursor->ifa_flags & IFF_BROADCAST) &&             (cursor->ifa_addr != NULL)           ) {            switch (cursor->ifa_addr->sa_family) {            case AF_INET: {                struct sockaddr_in sin = *(struct sockaddr_in *) cursor->ifa_addr;                sin.sin_port = htons(9);                NSData * addr = [NSData dataWithBytes:&sin length:sizeof(sin)];                [result addObject:addr];            } break;            case AF_INET6: {                struct sockaddr_in6 sin6 = *(struct sockaddr_in6 *) cursor->ifa_addr;                sin6.sin6_port = htons(9);                NSData * addr = [NSData dataWithBytes:&sin6 length:sizeof(sin6)];                [result addObject:addr];            } break;            default: {                // do nothing            } break;            }        }    }    freeifaddrs(addrList);    return result;}/// Does a best effort attempt to trigger the local network privacy alert.////// It works by sending a UDP datagram to the discard service (port 9) of every/// IP address associated with a broadcast-capable interface interface. This/// should trigger the local network privacy alert, assuming the alert hasn’t/// already been displayed for this app.////// This code takes a ‘best effort’. It handles errors by ignoring them. As/// such, there’s guarantee that it’ll actually trigger the alert.////// - note: iOS devices don’t actually run the discard service. I’m using it/// here because I need a port to send the UDP datagram to and port 9 is/// always going to be safe (either the discard service is running, in which/// case it will discard the datagram, or it’s not, in which case the TCP/IP/// stack will discard it).////// There should be a proper API for this (r. 69157424).////// For more background on this, see [Triggering the Local Network Privacy Alert](https://developer.apple.com/forums/thread/663768).extern void triggerLocalNetworkPrivacyAlertObjC(void) {    int sock4 = socket(AF_INET, SOCK_DGRAM, 0);    int sock6 = socket(AF_INET6, SOCK_DGRAM, 0);        if ((sock4 >= 0) && (sock6 >= 0)) {        char message = '!';        NSArray<NSData *> * addresses = addressesOfDiscardServiceOnBroadcastCapableInterfaces();        for (NSData * address in addresses) {            int sock = ((const struct sockaddr *) address.bytes)->sa_family == AF_INET ? sock4 : sock6;            (void) sendto(sock, &message, sizeof(message), MSG_DONTWAIT, address.bytes, (socklen_t) address.length);        }    }        // If we failed to open a socket, the descriptor will be -1 and it’s safe to    // close that (it’s guaranteed to fail with `EBADF`).    close(sock4);    close(sock6);}