title: What are the reasons for UDP packets to be dropped by the network stack
body: <p>I see UDP packets arriving on my Linux box (via tcpdump) with destination port set to 25555. However:</p>
  
  <pre><code>nc -l -u 25555
  </code></pre>
  
  <p>shows no traffic.</p>
  
  <p>I've already checked:</p>
  
  <ol>
  <li>iptables is off</li>
  <li>destination MAC address matches the incoming interface</li>
  <li>destination IP address matches the incoming interface</li>
  <li>IP checksum is OK</li>
  <li>UDP checksum is OK</li>
  </ol>
  
  <p>Also, <em>all</em> the packets are being dropped, thus it's not a problem with overlowing rx buffers.</p>
  
  <p>Any ideas what else may cause the pakcets to be dropped?</p>