title: what is "additional section" in DNS and how it works?
body: <p>When i use <code>dig</code></p>
  
  <pre><code>$ dig www.google.com
  
  ; &lt;&lt;&gt;&gt; DiG 9.10.6 &lt;&lt;&gt;&gt; www.google.com
  ;; global options: +cmd
  ;; Got answer:
  ;; -&gt;&gt;HEADER&lt;&lt;- opcode: QUERY, status: NOERROR, id: 59489
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 9
  
  ;; OPT PSEUDOSECTION:
  ; EDNS: version: 0, flags:; udp: 4096
  ;; QUESTION SECTION:
  ;www.google.com.            IN  A
  
  ;; ANSWER SECTION:
  www.google.com.     23  IN  A   69.171.247.71
  
  ;; AUTHORITY SECTION:
  google.com.     124333  IN  NS  ns4.GOoGLE.cOM.
  google.com.     124333  IN  NS  ns1.GOoGLE.cOM.
  google.com.     124333  IN  NS  ns3.GOoGLE.cOM.
  google.com.     124333  IN  NS  ns2.GOoGLE.cOM.
  
  ;; ADDITIONAL SECTION:
  ns1.google.com.     300146  IN  A   216.239.32.10
  ns1.google.com.     308505  IN  AAAA    2001:4860:4802:32::a
  ns2.google.com.     303470  IN  A   216.239.34.10
  ns2.google.com.     124333  IN  AAAA    2001:4860:4802:34::a
  ns3.google.com.     303470  IN  A   216.239.36.10
  ns3.google.com.     124333  IN  AAAA    2001:4860:4802:36::a
  ns4.google.com.     302836  IN  A   216.239.38.10
  ns4.google.com.     302716  IN  AAAA    2001:4860:4802:38::a
  </code></pre>
  
  <p>There is a <code>ADDITIONAL SECTION</code>.</p>
  
  <p>I know the structure of DNS packet is:</p>
  
  <pre><code>+---------------------+
  | Header              |
  +---------------------+
  | Question            | the question for the name server
  +---------------------+
  | Answer              | Answers to the question
  +---------------------+
  | Authority           | Not used in this project
  +---------------------+
  | Additional          | Not used in this project
  +---------------------+
  </code></pre>
  
  <p>But i don't know the detail of <code>ADDITIONAL SECTION</code>.</p>
  
  <p>I mean, where <code>ADDITIONAL SECTION</code> comes from(authority server?) and what's it used for?</p>