Retrieving data from a UDP server in an iOS application does not work with a Linux server, but works with macbook pro

Prerequisites: I made a UDP based IPhone application. It works as a client and server when it communicates with a Linux server [Ubuntu]. Im using GCDAsyncUdpSocket to send and receive data. I also have a backup server / client application on my Macbook to check / verify socket communication application. It just acts as a Linux server [Ubuntu]. It works great.

PROBLEM: I cannot get any data from the Linux server [Ubuntu].

DETAILS: I can successfully send data to a Linux server and it responds / processes this data. But when the server sends responses or echoes, my iPhone application does not see / read. A similar application developed by me with an Android colleague reads / sees data from the server. See code below. Hooray!

This is the code for it: .m file:

-(void)viewDidLoad
{
    [super viewDidLoad];
    backGround.userInteractionEnabled=YES;  
    udpSocket = [[GCDAsyncUdpSocket alloc] initWithDelegate:self delegateQueue:dispatch_get_main_queue()];

    NSError *error = nil;
    if (![udpSocket bindToPort:0 error:&error]) //check ff of dit werkt!
    {
        return;
    }
    if (![udpSocket beginReceiving:&error])
    {
        return;
    }
}


- (IBAction)sendData:(id)sender 
{
    NSString *msg = messageField.text;
    NSData *data = [msg dataUsingEncoding:NSUTF8StringEncoding];
    [self sendingRealData:data];
}

-(void)sendingRealData:(NSData *) Content
{
    NSString *msg = [[NSString alloc] initWithData:Content encoding:NSUTF8StringEncoding];
    NSLog(@"msg is: %@", msg);
    NSString *host = [self getHost];
    int port = [self getPort]; 

    if ((port == 65536) && (host == @"-1"))
    {
        NSLog(@"Port and IP are not filled in!");
        [self errorManag:2]; 
    }

    else if ((port == 65536) && (host != @"-1"))
    {
        NSLog(@"return was -2");
        [self errorManag:1];
    }

    else if (host == @"-1")
    {
        NSLog(@"return was -1");
        [self errorManag:0]; 
    }

    else 
    {
        [udpSocket sendData:Content toHost:host port:port withTimeout:1 tag:tag];
        NSLog(@"Gestuurd");
    }
}

- (void)udpSocket:(GCDAsyncUdpSocket *)sock didReceiveData:(NSData *)data
  fromAddress:(NSData *)address
withFilterContext:(id)filterContext
{
    NSLog(@"niets."); 
    NSString *msg = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
if (msg)
{
        NSLog(@"iets gekregen");
}
else
{
    NSLog(@"Error convertion");
    //[self logError:@"Error converting received data into UTF-8 String"];
}
//NSString *test = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
//NSLog(@"%@",test); 
//[udpSocket sendData:data toAddress:address withTimeout:-1 tag:0];
    NSLog(@"HMMMM");
    messageField.text = [[NSString alloc] initWithFormat:@"RE: %@", msg]; 
}