GCDasyncUdpSocket and wifi Interface

Question

en0 seems to be the wifi interface for Apple iOS devices, but in my code, small multicast client when I specify the interface en0 I'm not receiving anything. Any clue of what could be wrong ? GDCasyncUdpSocket logs don't show any error => Binding socket to port(1234) interface((en0))

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
if (self) {
    //log test
    [DDLog addLogger:[DDTTYLogger sharedInstance]];

    // Create multicast High Priotity queue
    mcastQueue = dispatch_queue_create("mcastQueue", NULL);
    dispatch_queue_t high = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

    dispatch_set_target_queue(mcastQueue, high);

    // Create UDP Socket
    mcastSocket=[[GCDAsyncUdpSocket alloc] initWithDelegate:self     delegateQueue:mcastQueue];

    [mcastSocket setPreferIPv4];
    return self;
}

- (void)viewDidLoad {
[super viewDidLoad];

  NSError *socketError=nil;

if (![mcastSocket bindToPort:1234 interface:@"en0" error:&socketError]) {
    NSLog(@"Failed binding socket to port: %@" ,socketError);
    return;
}

if (![mcastSocket enableBroadcast:YES error:&socketError]) {
    NSLog(@"Failed enabling broadcast: %@" ,socketError);
    return;
}

if (![mcastSocket joinMulticastGroup:@"239.0.0.1" error:&socketError]) {
    NSLog(@"Failed joining multicast group: %@" ,socketError);
    return;
}

//start receiving multicast data
if (![mcastSocket beginReceiving:&socketError]) {
    [mcastSocket close];
    NSLog(@"Failed to start receiving: %@" ,socketError);

} else {
    NSLog(@"Multicast start receiving");
}
}

Regards

Too little code to see anything. en0 is indeed wifi interface. You should show what delegate methods do, and also the mcastSocket creation. — DarthMike, Aug 13 '12 at 14:36
Hi DarthMike, I have added the mcastSocket creation. The code works ok if you don't specify the interface. — tony590, Aug 13 '12 at 15:37

score 2 · Accepted Answer · edited May 23 '17 at 12:07

After a year or so of intermittently trying to solve this and giving up, I finally discovered what I believe is the answer to this.

The answer is that you should not bind to an interface when receiving UDP packets. (See https://stackoverflow.com/a/10739443/179216)

Instead you should specify the interface when joining the multicast group:

// Do *not* specify the interface here
if (![mcastSocket bindToPort:1234 error:&socketError]) {
    NSLog(@"Failed binding socket to port: %@" ,socketError);
    return;
}

if (![mcastSocket enableBroadcast:YES error:&socketError]) {
    NSLog(@"Failed enabling broadcast: %@" ,socketError);
    return;
}

// This is where you specify the interface
if (![mcastSocket joinMulticastGroup:@"239.0.0.1" onInterface:@"en0" error:&socketError]) {
    NSLog(@"Failed joining multicast group: %@" ,socketError);
    return;
}

GCDasyncUdpSocket and wifi Interface

1 Answers1