how to broadcast in LAN with GCDAsyncUdpSocket?

Question

I want to discover whether there is a server, which is listening to a specific port, in the LAN with unknown IP address with GCDAsyncUdpSocket. I will broadcast some message in the LAN, and if the server exists, it will feedback a message then i will know the server's IP address. Now I try to use the the following code to do the work: ``` udpSocket = [[GCDAsyncUdpSocket alloc] initWithDelegate:self delegateQueue:dispatch_get_main_queue()]; if (![udpSocket bindToPort:18686 error:&error]) { [self logError:FORMAT(@"Error binding: %@", error)]; return; } if (![udpSocket beginReceiving:&error]) { [self logError:FORMAT(@"Error receiving: %@", error)]; return; } NSString *host = @"192.168.2.139"; // server IP i know int port = 8585; NSString *message = @"Hello"; NSData *data = [msg dataUsingEncoding:NSUTF8StringEncoding]; [udpSocket sendData:data toHost:host port:port withTimeout:-1 tag:tag]; ``` If i set the host IP to server IP, its all right, server will response. But i want to broadcast…

Accepted Answer

I checked the `GCDAsyncUdpSocket.h` and find the function named `- (BOOL)enableBroadcast:(BOOL)flag error:(NSError **)errPtr;` I add the following to the setupSocket

```
if (![udpSocket enableBroadcast:YES error:&error]) {
        [self logError:FORMAT(@"Error enableBroadcast:%@",error)];
        return;
}
```

Then broadcast works...

to my question, i checked the 192.168.2.* LAN broadcast IP is 192.168.2.255, and the unknown LAN broadcast IP is 255.255.255.255.

I don't know why we need to enable the broadcast in GCDAsyncUdpSocket to use a broadcast IP. Hope this will help others someday.

how to broadcast in LAN with GCDAsyncUdpSocket?

Solution

Problem