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<!DOCTYPE html>
<html lang="en" data-content_root="./" >
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Case Study: DNS — Fuzzing with Fandango</title>
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<h1>Case Study: DNS</h1>
<!-- Table of contents -->
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<h2> Contents </h2>
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<ul class="visible nav section-nav flex-column">
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#a-dns-exchange">A DNS Exchange</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#dns-requests">DNS Requests</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#request-headers">Request Headers</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#request-questions">Request Questions</a><ul class="nav section-nav flex-column">
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#request-names">Request Names</a></li>
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#request-types">Request Types</a></li>
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#request-class">Request Class</a></li>
</ul>
</li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#dns-responses">DNS Responses</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#dns-response-headers">DNS Response Headers</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#dns-response-question-section">DNS Response Question Section</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#dns-response-answer-section">DNS Response Answer Section</a><ul class="nav section-nav flex-column">
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#an-answers">AN Answers</a><ul class="nav section-nav flex-column">
<li class="toc-h5 nav-item toc-entry"><a class="reference internal nav-link" href="#type-a-answers">Type A Answers</a></li>
<li class="toc-h5 nav-item toc-entry"><a class="reference internal nav-link" href="#type-cname-answers">Type CNAME Answers</a></li>
<li class="toc-h5 nav-item toc-entry"><a class="reference internal nav-link" href="#type-ns-answers">Type NS Answers</a></li>
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<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#opt-answers">OPT Answers</a></li>
<li class="toc-h4 nav-item toc-entry"><a class="reference internal nav-link" href="#consistency-in-responses">Consistency in Responses</a></li>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#compression-and-decompression">Compression and Decompression</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#compression">Compression</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#decompression">Decompression</a></li>
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<article class="bd-article">
<section class="tex2jax_ignore mathjax_ignore" id="case-study-dns">
<span id="sec-dns"></span><h1>Case Study: DNS<a class="headerlink" href="#case-study-dns" title="Link to this heading">#</a></h1>
<p>This Fandango specification allows testing clients and name servers of the Internet <a class="reference external" href="https://en.wikipedia.org/wiki/Domain_Name_System">Domain Name System</a> (DNS, <a class="reference external" href="https://datatracker.ietf.org/doc/html/rfc1035">RFC 1035</a>).
It demonstrates</p>
<ul class="simple">
<li><p>how a nontrivial protocol with bit-level <em>binary</em> commands is implemented;</p></li>
<li><p>how to use constraints to <em>validate</em> responses; and</p></li>
<li><p>how to embed <em>compression</em> and <em>decompression</em> functions.</p></li>
</ul>
<p>The Fandango DNS protocol spec <a class="reference download internal" download="" href="_downloads/bce816cb933f81a9b89ac912092effb0/dns.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">dns.fan</span></code></span></a> is available for download.
To test it with Fandango as client querying the public Cloudflare DNS server 1.1.1.1 on the default DNS port 53, invoke Fandango as</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>-v<span class="w"> </span>talk<span class="w"> </span>-n<span class="w"> </span><span class="m">1</span><span class="w"> </span>-f<span class="w"> </span>dns.fan<span class="w"> </span>--client<span class="w"> </span>udp://1.1.1.1:53
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 1.1: </span>These features are available in Fandango 1.1 and later.</p>
</div>
<section id="a-dns-exchange">
<h2>A DNS Exchange<a class="headerlink" href="#a-dns-exchange" title="Link to this heading">#</a></h2>
<p>At an abstract level, the DNS <em>protocol</em> is pretty simple.
A <em>Client</em> sends a DNS request to the server (via UDP, by default on port 53), and gets a DNS response.
For instance, as of January 2026, the host <code class="docutils literal notranslate"><span class="pre">example.com</span></code> had the IP address <code class="docutils literal notranslate"><span class="pre">104.18.27.120</span></code>:</p>
<pre class="mermaid">
sequenceDiagram
Client ->> Server: example.com
Server ->> Client: 104.18.27.120
</pre><p>This is also how the exchange is modeled in <code class="docutils literal notranslate"><span class="pre">dns.fan</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">start</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">exchange</span><span class="o">></span>
<span class="o"><</span><span class="n">exchange</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">Client</span><span class="p">:</span><span class="n">dns_req</span><span class="o">></span> <span class="o"><</span><span class="n">Server</span><span class="p">:</span><span class="n">dns_resp</span><span class="o">></span>
</pre></div>
</div>
<p>Alas, the devil is in the details.
Let’s dig a bit deeper.</p>
</section>
<section id="dns-requests">
<h2>DNS Requests<a class="headerlink" href="#dns-requests" title="Link to this heading">#</a></h2>
<p>A DNS request consists of a <em>header</em> and a number of <em>questions</em>.
The actual number of questions is transmitted in a header field <code class="docutils literal notranslate"><span class="pre"><req_qd_count></span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">dns_req</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">header_req</span><span class="o">></span> <span class="o"><</span><span class="n">question</span><span class="o">></span><span class="p">{</span><span class="n">byte_to_int</span><span class="p">(</span><span class="o"><</span><span class="n">req_qd_count</span><span class="o">></span><span class="p">)}</span>
</pre></div>
</div>
<p>We decode <code class="docutils literal notranslate"><span class="pre"><req_qd_count></span></code> using a function <code class="docutils literal notranslate"><span class="pre">byte_to_int()</span></code>:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># Convert a 2-byte byte array to an integer</span>
<span class="k">def</span><span class="w"> </span><span class="nf">byte_to_int</span><span class="p">(</span><span class="n">byte_val</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">int</span><span class="p">(</span><span class="n">unpack</span><span class="p">(</span><span class="s1">'>H'</span><span class="p">,</span> <span class="nb">bytes</span><span class="p">(</span><span class="n">byte_val</span><span class="p">))[</span><span class="mi">0</span><span class="p">])</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>We do not model DNSSEC and other DNS extensions.</p>
</div>
<section id="request-headers">
<h3>Request Headers<a class="headerlink" href="#request-headers" title="Link to this heading">#</a></h3>
<p>The header consists of a number of fields as defined below.
Note the mix of bits and bytes, which is common in DNS messages.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">header_req</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">h_id</span><span class="o">></span> <span class="mi">0</span> <span class="o"><</span><span class="n">h_opcode_standard</span><span class="o">></span> <span class="mi">0</span> <span class="mi">0</span> <span class="o"><</span><span class="n">h_rd</span><span class="o">></span> <span class="mi">0</span> <span class="mi">0</span> <span class="o"><</span><span class="n">bit</span><span class="o">></span> <span class="mi">0</span> <span class="o"><</span><span class="n">h_rcode_noerror</span><span class="o">></span> <span class="o"><</span><span class="n">req_qd_count</span><span class="o">></span> <span class="mi">0</span><span class="p">{</span><span class="mi">16</span><span class="p">}</span> <span class="mi">0</span><span class="p">{</span><span class="mi">16</span><span class="p">}</span> <span class="mi">0</span><span class="p">{</span><span class="mi">16</span><span class="p">}</span>
</pre></div>
</div>
<p>The ID is an identifier of two bytes.
When sending the request, we choose a random value:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">h_id</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">byte</span><span class="o">><</span><span class="n">byte</span><span class="o">></span>
</pre></div>
</div>
<p>The remaining fields are set to fixed values:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">h_opcode_standard</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="mi">0</span> <span class="mi">0</span> <span class="mi">0</span> <span class="mi">0</span>
<span class="o"><</span><span class="n">h_rd</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="mi">1</span> <span class="c1"># 0 causes server failure with cname</span>
<span class="o"><</span><span class="n">req_qd_count</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">byte</span><span class="o">></span><span class="p">{</span><span class="mi">2</span><span class="p">}</span> <span class="o">:=</span> <span class="n">pack</span><span class="p">(</span><span class="s2">">H"</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="request-questions">
<h3>Request Questions<a class="headerlink" href="#request-questions" title="Link to this heading">#</a></h3>
<p>Now for the actual DNS questions.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">question</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">q_name</span><span class="o">></span> <span class="o"><</span><span class="n">q_type</span><span class="o">></span> <span class="o"><</span><span class="n">rr_class</span><span class="o">></span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p><code class="docutils literal notranslate"><span class="pre"><q_name></span></code> is where the host name (say, <code class="docutils literal notranslate"><span class="pre">example.com</span></code>) is sent to the DNS server.</p>
</div>
<section id="request-names">