Question

I’m connecting via UDP to a server on a different device which is advertised by Bonjour. When both the iOS device which this code is running on, and the server, are on our wifi network it works just fine because the bonjour service resolves to a 192.168.0.xxx address which our dhcp server hands out. However when it is advertised by bluetooth, sometimes the service resolves to 169.254.xxx.xxx (IPv4) in which case it works just fine. But sometimes it resolves to fe80::xxxx:xxxx:xxxx:xxxx (IPv6) in which case the socket connects (I receive the udpSocket:didConnectToAddress callback) but immediately closes when I try sending data (I receive the udpSocketDidClose:withError callback immediately upon calling send).

- (BOOL) setupConnection: (DNSSDService*) service
{
    NSString *host = [service resolvedHost];
    NSUInteger port = [service resolvedPort];
    NSLog(@"in setupConnection: host %@ port %u",
          host, port);

    self.sock = [[GCDAsyncUdpSocket alloc]initWithDelegate:self 
                delegateQueue:dispatch_get_main_queue() ];
    NSError *err = nil;
    if (![self.sock connectToHost:host onPort:port error:&err]) {
        NSLog(@"we goofed: %@", err);
        return NO;
    }
    return YES;
}

My udpSocket:didConnectToAddress method calls a send, and my other callbacks are basically just informational (NSLog) at this point. This is the NSError passed to udpSocketDidClose:withError:

Error Domain=GCDAsyncUdpSocketErrorDomain Code=4 "Socket closed" UserInfo=0x2630c0 {NSLocalizedDescription=Socket closed}

Less than useful.

In fixing this I’d like to make it work with IPv6 instead of force IPv4… forcing IPv4 just seems fragile to me.

Answer 1

What I did was call setPreferIPv4 and setIPv6Enabled:FALSE on the socket, which would make connecting fail if the DNS lookup only returned an IPv6 address. Then, in udpSocket:didNotConnect: i checked for that specific error (IPv6 has been disabled and DNS lookup found no IPv4 address(es).) and if the connect failed for that reason, went back into my setupConnection method and tried again. Eventually the DNS lookup returns an IPv4 address and things proceed smoothly from there.

This isn’t the most elegant solution, but it works.