And luckily, someone else has written a nice little UDP library for it as well: http://code.google.com/p/cocoaasyncsocket

Using that, in some of my code (which this post was also written for), sending a UDP broadcast is as simple as:


// initialization
socket = [[AsyncUdpSocket alloc] initWithDelegate:self];
NSError *error = nil;
if (![socket enableBroadcast:YES error:&error]) {
NSLog(@"Failed to enable broadcast: '%@'", error);
}


and


// use
unsigned char bytes[] = {0};
NSData *data = [NSData dataWithBytes:bytes length:sizeof(bytes)];
if (![socket sendData:data toHost:broadcastHost port:33848 withTimeout:2.0 tag:DISCOVERY_TAG]) {
NSLog(@"Failed to send data to broadcast host '%@'", broadcastHost);
return;
}

// casually do other stuff, the UDP is asynchronous
[socket receiveWithTimeout:10.0 tag:DISCOVERY_TAG];


Finally, a callback like:


- (BOOL)onUdpSocket:(AsyncUdpSocket *)sock didReceiveData:(NSData *)data withTag:(long)tag fromHost:(NSString *)host port:(UInt16)port {
if (DISCOVERY_TAG != tag) {
return NO;
}

// queue up work unit for data received
// ....
[[NSNotificationCenter defaultCenter] postNotificationName:@"udp.work.recv" object:self];
[socket receiveWithTimeout:10.0 tag:DISCOVERY_TAG];
return YES;
}


The other part is, I need to actually send a UDP broadcast message to the WiFi broadcast address, from my iPhone, and listen for a response. I’ve found a bunch of examples, mostly linux-based, but all of them fail on the iPhone. Here’s my code – what fails is listed below it:

{
struct sockaddr_in saddr = { 0 };
struct sockaddr_in raddr = { 0 };

int sock;
int on = 1;

// open socket
if ((sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1)
{
return ( TEL_SOCKET_OPEN_ERROR );
}

// enable udp broadcast
if ( setsockopt ( sock, SOL_SOCKET, SO_BROADCAST, &on, sizeof on ) < 0 )
{
close ( sock );
return ( TEL_SOCKET_OPEN_ERROR );
}

// setup source port / destination address

saddr.sin_len = sizeof ( saddr );
saddr.sin_family = AF_INET;
saddr.sin_port = htons(4031);
saddr.sin_addr.s_addr = inet_addr ( "10.255.255.255" ); // htonl(INADDR_BROADCAST);

// bind outgoing port

if (bind(sock, (sockaddr *) &saddr, sizeof(saddr)) < 0)
{
perror ( "bind" );
// return ( TEL_SOCKET_OPEN_ERROR );
}

raddr.sin_len = sizeof ( raddr );
raddr.sin_family = AF_INET;
raddr.sin_port = htons(4031);
raddr.sin_addr.s_addr = inet_addr ( "10.255.255.255" ); // htonl(INADDR_BROADCAST);

const char *buf = "skyfi?";
if (sendto(sock, buf, strlen(buf), 0, (sockaddr *) &raddr, sizeof(raddr)) < 0)
{
perror ( "sendto" );
return ( TEL_SOCKET_WRITE_ERROR );
}

sleep ( 1 );

char reply[256] = { 0 };
int replen = 0;

replen = recvfrom ( sock, reply, sizeof ( reply ), 0, NULL, 0 );
if ( replen < 1 )
{
return ( TEL_SOCKET_READ_ERROR );
}

return ( TEL_NO_ERROR );
}

The above code always fails at the sendto() call, and errno is set to 49 (EADDRNOTAVAIL). perror() gives me sendto: Can't assign requested address. I happen to know that the WiFi network broadcast address is 10.255.255.255 – that's why that value is hard coded. But I still can't send to it. Arrrgh!

Incidentally, your code (if it works) would solve the other part of my problem, which is finding the broadcast address in the first place.

Thanks in advance,

-Tim

However much I hope to never again need this information, sadly I’m sure I will.

Thanks for this article it’s just saved me some work and messing around – that is always welcome

I think possibly the reason that you need to mount the share manually is nothing to do with the fact the backup directory is a Samba share, but more to do with the fact that you’re not announcing it via ZeroConf/Bonjour.

I’ve come to this conclusion because I’ve just had a similar problem trying to restore a backup from an AirPort Extreme to a Hackintosh. The problem though, as far as I can tell, is that the Apple boot DVD doesn’t properly enable Bonjour on the Hackintosh Macs (something to do with the UUID string for the host not being determined properly which breaks Bonjour).

I’m reasonably sure if the share were to be announced via Bonjour the Mac would pick it up and happily restore from it, regardless of whether it’s delivered over SMB/CIFS or AFP.

It’s a hunch!