title: Qt program only receiving UDP from certain IP addresses
body: <p>I have a problem with a Qt program only receiving UDP [Artnet] datagrams from my STM32 modules when the STM32 is set to some seemingly random, but specific IP addresses. </p>
  
  <p>So, I have the following:</p>
  
  <ol>
  <li>Linux Machine Running Qt program with dynamic IP address</li>
  <li>STM32 Running FreeRTOS Module manually set to address A </li>
  <li>STM32 Running FreeRTOS Module manually set to address B</li>
  </ol>
  
  <p>I have written the Qt software and used uIP with FreeRTOS on the STM32 boards - modifying uIP to work with UDP properly and added simple Artnet code. </p>
  
  <p>All are on 192.168.0.x subnet. </p>
  
  <p>I've written the Qt program to send out an Artnet poll to the subnet's broadcast address. So, it sends the standard Artnet poll out on UDP port 6454 to address 192.168.0.255. Wireshark, shows my two STM32 modules return Artnet Poll Replies just fine. So far, so good. </p>
  
  <p>However, where it starts to get really weird is that if I, for example, set one STM32 to have an IP address of 192.168.0.177 and the other to 192.168.0.176, the 177 node is seen and processed by the Qt program, however, despite the 176 node sending back a correct Artnet poll reply, the Qt program flatly refuses to read the reply packet. If I change the IP address of the 176 node to be .44, then the Qt program processes the reply. If I change the [working] .177 node to be, say, .43, it does not work. </p>
  
  <p>I should stress that whatever IP address I set the STM32s to, Wireshark indicates all is well with the replies.</p>
  
  <p>Can anyone suggest anything that might shed light on this please? I've played around with netcat, but that doesn't seem to read any of these Artnet replies, no matter what address they come from, so I might misunderstand what netcat can do. I've tried netcat with my Qt program only opening and outbound port, not an inbound and it makes no difference, but again, I might completely misunderstand netcat or something about UDP. Maybe if you open an outbound UDP port, the same inbound is automatically opened?  </p>
  
  <p>There are no IP address clashes going on and I have no firewall turned on, on my Linux machine.</p>
  
  <p>Many thanks.</p>
  
  <p>Edit: Added code as requested.</p>
  
  <pre><code>void MainWindow::processPendingDatagrams(void)
  {
      struct ArtNetPollReplyStruct *newReply;
      QHostAddress sendingAddress;
      quint16 sendingUdpPort;
      QString versionString;
      QByteArray datagram;
  
      while (udpReceiveSocket-&gt;hasPendingDatagrams())
      {       
          datagram.resize(udpReceiveSocket-&gt;pendingDatagramSize());
          udpReceiveSocket-&gt;readDatagram(datagram.data(), datagram.size(), &amp;sendingAddress,&amp;sendingUdpPort);
          newReply = (struct ArtNetPollReplyStruct*)(datagram.data());
          if (newReply-&gt;OpCode == OP_POLL_REPLY)
              {
              if (sendingAddress != QHostAddress("192.168.0.18"))
                  {
                  if (checkAndAddAddress(sendingAddress))
                      {
                      versionString = QString::number(newReply-&gt;VersionInfoH,10) + "." + QString::number(newReply-&gt;VersionInfo,10);
                      addNodeToList(sendingAddress.toString(), versionString);
                      ui-&gt;textEdit-&gt;append( QString::fromUtf8(newReply-&gt;LongName));
                      }
                  }
              }
          }
  }
  </code></pre>
  
  <p>The code for initialising the UDP ports is here:</p>
  
  <pre><code>udpSendSocket = new QUdpSocket(this); udpReceiveSocket = new QUdpSocket(this);    
  udpSendSocket-&gt;bind(6454, QUdpSocket::ShareAddress); 
  udpReceiveSocket-&gt;bind(QHostAddress::Any,6454);
  connect(udpReceiveSocket, SIGNAL(readyRead()),this, SLOT(processPendingDatagrams()));
  connect(ui-&gt;innoLEDListTable,SIGNAL(itemChanged(QTableWidgetItem*)),this,SLOT(tableItemClicked(QTableWidgetItem*)));
  </code></pre>
  
  
  
  <pre><code>$ ifconfig
  enp5s0    Link encap:Ethernet  HWaddr 14:DA:E9:30:36:22  
            inet addr:192.168.0.18  Bcast:192.168.0.255  Mask:255.255.255.0
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:265615 errors:0 dropped:0 overruns:0 frame:0
            TX packets:190104 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:1000 
            RX bytes:306893983 (292.6 MiB)  TX bytes:20997451 (20.0 MiB)
  
  lo        Link encap:Local Loopback  
            inet addr:127.0.0.1  Mask:255.0.0.0
            UP LOOPBACK RUNNING  MTU:65536  Metric:1
            RX packets:25205 errors:0 dropped:0 overruns:0 frame:0
            TX packets:25205 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:0 
            RX bytes:6063842 (5.7 MiB)  TX bytes:6063842 (5.7 MiB)
  
  $ route -n
  Kernel IP routing table
  Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
  0.0.0.0         192.168.0.2     0.0.0.0         UG    10     0        0 enp5s0
  169.254.0.0     0.0.0.0         255.255.0.0     U     10     0        0 enp5s0
  192.168.0.0     0.0.0.0         255.255.255.0   U     10     0        0 enp5s0
  </code></pre>