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<a class="dropdown-item" tabindex="-1" href="Extended5DNS.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item" href="SequenceModels.html"> 1.6. Sequence Models in UML</a>
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<a class="dropdown-item" href="StateModelImplementation.html"> 1.7. Extended Example: State Model Implementation</a>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="Syscall.html"> 2.4. System Call Interface</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<a class="dropdown-item disabled"><b>Chapter 4</b></a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="UDPSockets.html"> 4.6. UDP Socket Programming: DNS</a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="ReadWrite.html"> 8.4. Readers-Writers Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<a id="nextmod" href="ThreadsOverview.html">6.1. Concurrency with Multithreading</a>  »
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "Extended5DNS";ODSA.SETTINGS.MODULE_LONG_NAME = "Extended Example: DNS Client";ODSA.SETTINGS.MODULE_CHAPTER = "The Internet and Connectivity"; ODSA.SETTINGS.BUILD_DATE = "2021-06-14 17:15:25"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="extended-example-dns-client">
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<h1>5.8. Extended Example: DNS Client<a class="headerlink" href="Extended5DNS.html#extended-example-dns-client" title="Permalink to this headline">¶</a></h1>
|
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<p>This Extended Example is a minimal client for performing a DNS query for IPv4
|
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addresses. Given a domain name (such as <code class="docutils literal notranslate"><span class="pre">example.com</span></code>), the client sets up
|
||
the DNS question in <code class="docutils literal notranslate"><span class="pre">setup_dns_request()</span></code> (lines 200 – 234). The <code class="docutils literal notranslate"><span class="pre">packed</span></code>
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attribute of the <code class="docutils literal notranslate"><span class="pre">dns_record_a_t</span></code> (lines 31 – 39) ensures that the compiler
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does not add any unnecessary padding that would make the question improperly
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structured. The <code class="docutils literal notranslate"><span class="pre">build_domain_qname()</span></code> function (lines 95 – 134) replaces the
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dots in the domain name with an integer to denote the length of the next field.
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Lines 65 – 82 perform the actual query, sending the request to OpenDNS using a
|
||
UDP socket. The client is only designed to support DNS <code class="docutils literal notranslate"><span class="pre">A</span></code> type records for
|
||
IPv4. I.e., this client does not support <code class="docutils literal notranslate"><span class="pre">CNAME</span></code>, <code class="docutils literal notranslate"><span class="pre">NS</span></code>, or <code class="docutils literal notranslate"><span class="pre">MX</span></code> records.
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The <code class="docutils literal notranslate"><span class="pre">print_dns_response()</span></code> function (lines 153 – 198) will stop if any other
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record types are encountered.</p>
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234</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="cp">#include</span> <span class="cpf"><arpa/inet.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><assert.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><inttypes.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><netdb.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><netinet/in.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><stdint.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><stdint.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><stdlib.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><string.h></span><span class="cp"></span>
|
||
<span class="cp">#include</span> <span class="cpf"><unistd.h></span><span class="cp"></span>
|
||
|
||
<span class="cm">/* Structure of the bytes for a DNS header */</span>
|
||
<span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
|
||
<span class="kt">uint16_t</span> <span class="n">xid</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">flags</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">qdcount</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">ancount</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">nscount</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">arcount</span><span class="p">;</span>
|
||
<span class="p">}</span> <span class="n">dns_header_t</span><span class="p">;</span>
|
||
|
||
<span class="cm">/* Structure of the bytes for a DNS question */</span>
|
||
<span class="k">typedef</span> <span class="k">struct</span>
|
||
<span class="p">{</span>
|
||
<span class="kt">char</span> <span class="o">*</span><span class="n">name</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">dnstype</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">dnsclass</span><span class="p">;</span>
|
||
<span class="p">}</span> <span class="n">dns_question_t</span><span class="p">;</span>
|
||
|
||
<span class="cm">/* Structure of the bytes for an IPv4 answer */</span>
|
||
<span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
|
||
<span class="kt">uint16_t</span> <span class="n">compression</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">type</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">class</span><span class="p">;</span>
|
||
<span class="kt">uint32_t</span> <span class="n">ttl</span><span class="p">;</span>
|
||
<span class="kt">uint16_t</span> <span class="n">length</span><span class="p">;</span>
|
||
<span class="k">struct</span> <span class="n">in_addr</span> <span class="n">addr</span><span class="p">;</span>
|
||
<span class="p">}</span> <span class="n">__attribute__</span><span class="p">((</span><span class="n">packed</span><span class="p">))</span> <span class="n">dns_record_a_t</span><span class="p">;</span>
|
||
|
||
<span class="kt">char</span> <span class="o">*</span> <span class="nf">build_domain_qname</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">);</span>
|
||
<span class="kt">void</span> <span class="nf">print_byte_block</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">,</span> <span class="kt">size_t</span><span class="p">);</span>
|
||
<span class="kt">void</span> <span class="nf">print_dns_response</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">);</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span> <span class="nf">setup_dns_request</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">,</span> <span class="kt">size_t</span> <span class="o">*</span><span class="p">);</span>
|
||
|
||
<span class="kt">int</span>
|
||
<span class="nf">main</span> <span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
|
||
<span class="p">{</span>
|
||
<span class="k">if</span> <span class="p">(</span><span class="n">argc</span> <span class="o">!=</span> <span class="mi">2</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">fprintf</span> <span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"ERROR: Must pass a domain name</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
|
||
<span class="k">return</span> <span class="mi">1</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="kt">char</span> <span class="o">*</span><span class="n">hostname</span> <span class="o">=</span> <span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">];</span>
|
||
|
||
<span class="cm">/* Set up the packet and get the length */</span>
|
||
<span class="kt">size_t</span> <span class="n">packetlen</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span><span class="n">packet</span> <span class="o">=</span> <span class="n">setup_dns_request</span> <span class="p">(</span><span class="n">hostname</span><span class="p">,</span> <span class="o">&</span><span class="n">packetlen</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Print the raw bytes formatted as 0000 0000 0000 ... */</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"Lookup %s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">hostname</span><span class="p">);</span>
|
||
<span class="n">print_byte_block</span> <span class="p">(</span><span class="n">packet</span><span class="p">,</span> <span class="n">packetlen</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Send the packet to OpenDNS. Create an IPv4 UDP socket to</span>
|
||
<span class="cm"> 208.67.222.222 (0xd043dede), the IP address for OpenDNS.</span>
|
||
<span class="cm"> DNS servers listen on port 53. */</span>
|
||
<span class="kt">int</span> <span class="n">socketfd</span> <span class="o">=</span> <span class="n">socket</span> <span class="p">(</span><span class="n">AF_INET</span><span class="p">,</span> <span class="n">SOCK_DGRAM</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span>
|
||
<span class="k">struct</span> <span class="n">sockaddr_in</span> <span class="n">address</span><span class="p">;</span>
|
||
<span class="n">address</span><span class="p">.</span><span class="n">sin_family</span> <span class="o">=</span> <span class="n">AF_INET</span><span class="p">;</span>
|
||
<span class="n">address</span><span class="p">.</span><span class="n">sin_addr</span><span class="p">.</span><span class="n">s_addr</span> <span class="o">=</span> <span class="n">htonl</span> <span class="p">(</span><span class="mh">0xd043dede</span><span class="p">);</span>
|
||
<span class="n">address</span><span class="p">.</span><span class="n">sin_port</span> <span class="o">=</span> <span class="n">htons</span> <span class="p">(</span><span class="mi">53</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Send the request and get the response */</span>
|
||
<span class="n">sendto</span> <span class="p">(</span><span class="n">socketfd</span><span class="p">,</span> <span class="n">packet</span><span class="p">,</span> <span class="n">packetlen</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="p">(</span><span class="k">struct</span> <span class="n">sockaddr</span> <span class="o">*</span><span class="p">)</span><span class="o">&</span><span class="n">address</span><span class="p">,</span>
|
||
<span class="p">(</span><span class="kt">socklen_t</span><span class="p">)</span><span class="k">sizeof</span> <span class="p">(</span><span class="n">address</span><span class="p">));</span>
|
||
|
||
<span class="kt">socklen_t</span> <span class="n">length</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
||
<span class="kt">uint8_t</span> <span class="n">response</span><span class="p">[</span><span class="mi">512</span><span class="p">];</span>
|
||
<span class="n">memset</span> <span class="p">(</span><span class="o">&</span><span class="n">response</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">512</span><span class="p">);</span>
|
||
<span class="kt">ssize_t</span> <span class="n">bytes</span> <span class="o">=</span> <span class="n">recvfrom</span> <span class="p">(</span><span class="n">socketfd</span><span class="p">,</span> <span class="n">response</span><span class="p">,</span> <span class="mi">512</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span>
|
||
<span class="p">(</span><span class="k">struct</span> <span class="n">sockaddr</span> <span class="o">*</span><span class="p">)</span><span class="o">&</span><span class="n">address</span><span class="p">,</span> <span class="o">&</span><span class="n">length</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Print the raw bytes formatted as 0000 0000 0000 ... */</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"Received %zd bytes from %s:</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">bytes</span><span class="p">,</span>
|
||
<span class="n">inet_ntoa</span> <span class="p">(</span><span class="n">address</span><span class="p">.</span><span class="n">sin_addr</span><span class="p">));</span>
|
||
<span class="n">print_byte_block</span> <span class="p">(</span><span class="n">response</span><span class="p">,</span> <span class="n">bytes</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Parse the DNS response into a struct and print the result */</span>
|
||
<span class="n">print_dns_response</span> <span class="p">(</span><span class="n">response</span><span class="p">);</span>
|
||
|
||
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="kt">char</span> <span class="o">*</span>
|
||
<span class="nf">build_domain_qname</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">hostname</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">assert</span> <span class="p">(</span><span class="n">hostname</span> <span class="o">!=</span> <span class="nb">NULL</span><span class="p">);</span>
|
||
|
||
<span class="kt">char</span> <span class="o">*</span><span class="n">name</span> <span class="o">=</span> <span class="n">calloc</span> <span class="p">(</span><span class="n">strlen</span> <span class="p">(</span><span class="n">hostname</span><span class="p">)</span> <span class="o">+</span> <span class="mi">2</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="kt">uint8_t</span><span class="p">));</span>
|
||
|
||
<span class="cm">/* Leave the first byte blank for the first field length */</span>
|
||
<span class="n">memcpy</span> <span class="p">(</span><span class="n">name</span> <span class="o">+</span> <span class="mi">1</span><span class="p">,</span> <span class="n">hostname</span><span class="p">,</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">hostname</span><span class="p">));</span>
|
||
|
||
<span class="cm">/* Example:</span>
|
||
<span class="cm"> +---+---+---+---+---+---+---+---+---+---+---+</span>
|
||
<span class="cm"> | a | b | c | . | d | e | . | c | o | m | \0|</span>
|
||
<span class="cm"> +---+---+---+---+---+---+---+---+---+---+---+</span>
|
||
|
||
<span class="cm"> becomes:</span>
|
||
<span class="cm"> +---+---+---+---+---+---+---+---+---+---+---+---+</span>
|
||
<span class="cm"> | 3 | a | b | c | 2 | d | e | 3 | c | o | m | 0 |</span>
|
||
<span class="cm"> +---+---+---+---+---+---+---+---+---+---+---+---+</span>
|
||
<span class="cm"> */</span>
|
||
|
||
<span class="kt">uint8_t</span> <span class="n">count</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span><span class="n">prev</span> <span class="o">=</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">)</span><span class="n">name</span><span class="p">;</span>
|
||
<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">hostname</span><span class="p">);</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="cm">/* Look for the next ., then copy the length back to the</span>
|
||
<span class="cm"> location of the previous . */</span>
|
||
<span class="k">if</span> <span class="p">(</span><span class="n">hostname</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">==</span> <span class="sc">'.'</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="o">*</span><span class="n">prev</span> <span class="o">=</span> <span class="n">count</span><span class="p">;</span>
|
||
<span class="n">prev</span> <span class="o">=</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">)</span><span class="n">name</span> <span class="o">+</span> <span class="n">i</span> <span class="o">+</span> <span class="mi">1</span><span class="p">;</span>
|
||
<span class="n">count</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
<span class="k">else</span>
|
||
<span class="n">count</span><span class="o">++</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
<span class="o">*</span><span class="n">prev</span> <span class="o">=</span> <span class="n">count</span><span class="p">;</span>
|
||
|
||
<span class="k">return</span> <span class="n">name</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="kt">void</span>
|
||
<span class="nf">print_byte_block</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="n">bytes</span><span class="p">,</span> <span class="kt">size_t</span> <span class="n">length</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" "</span><span class="p">);</span>
|
||
<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">length</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"%02x"</span><span class="p">,</span> <span class="n">bytes</span><span class="p">[</span><span class="n">i</span><span class="p">]);</span>
|
||
<span class="k">if</span> <span class="p">(</span><span class="n">i</span> <span class="o">==</span> <span class="n">length</span> <span class="o">-</span> <span class="mi">1</span><span class="p">)</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
|
||
<span class="k">else</span> <span class="k">if</span> <span class="p">((</span><span class="n">i</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span> <span class="o">%</span> <span class="mi">16</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"</span><span class="se">\n</span><span class="s"> "</span><span class="p">);</span>
|
||
<span class="k">else</span> <span class="k">if</span> <span class="p">((</span><span class="n">i</span> <span class="o">%</span> <span class="mi">2</span><span class="p">)</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" "</span><span class="p">);</span>
|
||
<span class="p">}</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="kt">void</span>
|
||
<span class="nf">print_dns_response</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="n">response</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="cm">/* First, check the header for an error response code */</span>
|
||
<span class="n">dns_header_t</span> <span class="o">*</span><span class="n">response_header</span> <span class="o">=</span> <span class="p">(</span><span class="n">dns_header_t</span> <span class="o">*</span><span class="p">)</span><span class="n">response</span><span class="p">;</span>
|
||
<span class="k">if</span> <span class="p">((</span><span class="n">ntohs</span> <span class="p">(</span><span class="n">response_header</span><span class="o">-></span><span class="n">flags</span><span class="p">)</span> <span class="o">&</span> <span class="mh">0xf</span><span class="p">)</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">fprintf</span> <span class="p">(</span><span class="n">stderr</span><span class="p">,</span> <span class="s">"Failed to get response</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
|
||
<span class="k">return</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="cm">/* Reconstruct the question */</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span><span class="n">start_of_question</span> <span class="o">=</span> <span class="n">response</span> <span class="o">+</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">dns_header_t</span><span class="p">);</span>
|
||
<span class="n">dns_question_t</span> <span class="o">*</span><span class="n">questions</span>
|
||
<span class="o">=</span> <span class="n">calloc</span> <span class="p">(</span><span class="k">sizeof</span> <span class="p">(</span><span class="n">dns_question_t</span><span class="p">),</span> <span class="n">response_header</span><span class="o">-></span><span class="n">ancount</span><span class="p">);</span>
|
||
<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">ntohs</span> <span class="p">(</span><span class="n">response_header</span><span class="o">-></span><span class="n">ancount</span><span class="p">);</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">questions</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">name</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span><span class="n">start_of_question</span><span class="p">;</span>
|
||
<span class="kt">uint8_t</span> <span class="n">total</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span><span class="n">field_length</span> <span class="o">=</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">)</span><span class="n">questions</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">name</span><span class="p">;</span>
|
||
<span class="k">while</span> <span class="p">(</span><span class="o">*</span><span class="n">field_length</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">total</span> <span class="o">+=</span> <span class="o">*</span><span class="n">field_length</span> <span class="o">+</span> <span class="mi">1</span><span class="p">;</span>
|
||
<span class="o">*</span><span class="n">field_length</span> <span class="o">=</span> <span class="sc">'.'</span><span class="p">;</span>
|
||
<span class="n">field_length</span> <span class="o">=</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">)</span><span class="n">questions</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">name</span> <span class="o">+</span> <span class="n">total</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
<span class="n">questions</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">name</span><span class="o">++</span><span class="p">;</span>
|
||
<span class="cm">/* Skip null byte, qtype, and qclass */</span>
|
||
<span class="n">start_of_question</span> <span class="o">=</span> <span class="n">field_length</span> <span class="o">+</span> <span class="mi">5</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="cm">/* The records start right after the question section. For each record,</span>
|
||
<span class="cm"> confirm that it is an A record (only type supported). If any are not</span>
|
||
<span class="cm"> an A-type, then return. */</span>
|
||
<span class="n">dns_record_a_t</span> <span class="o">*</span><span class="n">records</span> <span class="o">=</span> <span class="p">(</span><span class="n">dns_record_a_t</span> <span class="o">*</span><span class="p">)</span><span class="n">start_of_question</span><span class="p">;</span>
|
||
<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">ntohs</span> <span class="p">(</span><span class="n">response_header</span><span class="o">-></span><span class="n">ancount</span><span class="p">);</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">"Record for %s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">questions</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">name</span><span class="p">);</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" TYPE: %"</span> <span class="n">PRId16</span> <span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">ntohs</span> <span class="p">(</span><span class="n">records</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">type</span><span class="p">));</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" CLASS: %"</span> <span class="n">PRId16</span> <span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">ntohs</span> <span class="p">(</span><span class="n">records</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">class</span><span class="p">));</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" TTL: %"</span> <span class="n">PRIx32</span> <span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">ntohl</span> <span class="p">(</span><span class="n">records</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">ttl</span><span class="p">));</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" IPv4: %08"</span> <span class="n">PRIx32</span> <span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span>
|
||
<span class="n">ntohl</span> <span class="p">((</span><span class="kt">uint32_t</span><span class="p">)</span><span class="n">records</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">addr</span><span class="p">.</span><span class="n">s_addr</span><span class="p">));</span>
|
||
<span class="n">printf</span> <span class="p">(</span><span class="s">" IPv4: %s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">inet_ntoa</span> <span class="p">(</span><span class="n">records</span><span class="p">[</span><span class="n">i</span><span class="p">].</span><span class="n">addr</span><span class="p">));</span>
|
||
<span class="p">}</span>
|
||
<span class="p">}</span>
|
||
|
||
<span class="kt">uint8_t</span> <span class="o">*</span>
|
||
<span class="nf">setup_dns_request</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">hostname</span><span class="p">,</span> <span class="kt">size_t</span> <span class="o">*</span><span class="n">packetlen</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="cm">/* Set up the DNS header */</span>
|
||
<span class="n">dns_header_t</span> <span class="n">header</span><span class="p">;</span>
|
||
<span class="n">memset</span> <span class="p">(</span><span class="o">&</span><span class="n">header</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">dns_header_t</span><span class="p">));</span>
|
||
<span class="n">header</span><span class="p">.</span><span class="n">xid</span><span class="o">=</span> <span class="n">htons</span> <span class="p">(</span><span class="mh">0x1234</span><span class="p">);</span> <span class="cm">/* Randomly chosen ID */</span>
|
||
<span class="n">header</span><span class="p">.</span><span class="n">flags</span> <span class="o">=</span> <span class="n">htons</span> <span class="p">(</span><span class="mh">0x0100</span><span class="p">);</span> <span class="cm">/* Q=0, RD=1 */</span>
|
||
<span class="n">header</span><span class="p">.</span><span class="n">qdcount</span> <span class="o">=</span> <span class="n">htons</span> <span class="p">(</span><span class="mi">1</span><span class="p">);</span> <span class="cm">/* Sending 1 question */</span>
|
||
|
||
<span class="cm">/* Set up the DNS question */</span>
|
||
<span class="n">dns_question_t</span> <span class="n">question</span><span class="p">;</span>
|
||
<span class="n">question</span><span class="p">.</span><span class="n">dnstype</span> <span class="o">=</span> <span class="n">htons</span> <span class="p">(</span><span class="mi">1</span><span class="p">);</span> <span class="cm">/* QTYPE 1=A */</span>
|
||
<span class="n">question</span><span class="p">.</span><span class="n">dnsclass</span> <span class="o">=</span> <span class="n">htons</span> <span class="p">(</span><span class="mi">1</span><span class="p">);</span> <span class="cm">/* QCLASS 1=IN */</span>
|
||
<span class="n">question</span><span class="p">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">build_domain_qname</span> <span class="p">(</span><span class="n">hostname</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Copy all fields into a single, concatenated packet */</span>
|
||
<span class="o">*</span><span class="n">packetlen</span> <span class="o">=</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">header</span><span class="p">)</span> <span class="o">+</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">hostname</span><span class="p">)</span> <span class="o">+</span> <span class="mi">2</span>
|
||
<span class="o">+</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">question</span><span class="p">.</span><span class="n">dnstype</span><span class="p">)</span> <span class="o">+</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">question</span><span class="p">.</span><span class="n">dnsclass</span><span class="p">);</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span><span class="n">packet</span> <span class="o">=</span> <span class="n">calloc</span> <span class="p">(</span><span class="o">*</span><span class="n">packetlen</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="kt">uint8_t</span><span class="p">));</span>
|
||
<span class="kt">uint8_t</span> <span class="o">*</span><span class="n">p</span> <span class="o">=</span> <span class="p">(</span><span class="kt">uint8_t</span> <span class="o">*</span><span class="p">)</span><span class="n">packet</span><span class="p">;</span>
|
||
|
||
<span class="cm">/* Copy the header first */</span>
|
||
<span class="n">memcpy</span> <span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="o">&</span><span class="n">header</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">header</span><span class="p">));</span>
|
||
<span class="n">p</span> <span class="o">+=</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">header</span><span class="p">);</span>
|
||
|
||
<span class="cm">/* Copy the question name, QTYPE, and QCLASS fields */</span>
|
||
<span class="n">memcpy</span> <span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="n">question</span><span class="p">.</span><span class="n">name</span><span class="p">,</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">hostname</span><span class="p">)</span> <span class="o">+</span> <span class="mi">2</span><span class="p">);</span>
|
||
<span class="n">p</span> <span class="o">+=</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">hostname</span><span class="p">)</span> <span class="o">+</span> <span class="mi">2</span><span class="p">;</span>
|
||
<span class="n">memcpy</span> <span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="o">&</span><span class="n">question</span><span class="p">.</span><span class="n">dnstype</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">question</span><span class="p">.</span><span class="n">dnstype</span><span class="p">));</span>
|
||
<span class="n">p</span> <span class="o">+=</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">question</span><span class="p">.</span><span class="n">dnstype</span><span class="p">);</span>
|
||
<span class="n">memcpy</span> <span class="p">(</span><span class="n">p</span><span class="p">,</span> <span class="o">&</span><span class="n">question</span><span class="p">.</span><span class="n">dnsclass</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="n">question</span><span class="p">.</span><span class="n">dnsclass</span><span class="p">));</span>
|
||
|
||
<span class="k">return</span> <span class="n">packet</span><span class="p">;</span>
|
||
<span class="p">}</span>
|
||
</pre></div>
|
||
</td></tr></table></div>
|
||
</div>
|
||
|
||
|
||
</div>
|
||
|
||
|
||
|
||
<div class="container">
|
||
|
||
<div class="mt-4 container center">
|
||
«  <a id="prevmod1" href="Wireless.html">5.7. Wireless Connectivity: Wi-Fi, Bluetooth, and Zigbee</a>
|
||
  ::  
|
||
<a class="uplink" href="index.html">Contents</a>
|
||
  ::  
|
||
<a id="nextmod1" href="ThreadsOverview.html">6.1. Concurrency with Multithreading</a>  »
|
||
</div>
|
||
|
||
|
||
</div>
|
||
|
||
<br />
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||
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|
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</center>
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