title: iptables: how to drop packets by their first bytes
body: <p>I want all packets whose content starts with <code>&quot;\x18\x00&quot;</code> to be dropped.
  And so, I wrote the following config:</p>
  <pre><code>iptables -N LIMIT
  iptables -I INPUT -j LIMIT
  iptables -A LIMIT -p udp -m string --string &quot;\x18\x00&quot; --algo kmp -j DROP
  </code></pre>
  <p>But iptables turns my bytes into a string and no packets are dropped.</p>