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cpp-httplib
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    <h1>HTTPS Client</h1>
    <p>前章でOpenSSLのセットアップが済んだので、さっそくHTTPSクライアントを使ってみましょう。2章で使った <code>httplib::Client</code> がそのまま使えます。コンストラクタに <code>https://</code> 付きのURLを渡すだけです。</p>
<h2>GETリクエスト</h2>
<p>実在するHTTPSサイトにアクセスしてみましょう。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#cc99cc;">#define </span><span style="color:#d3d0c8;">CPPHTTPLIB_OPENSSL_SUPPORT
</span><span style="color:#cc99cc;">#include </span><span style="color:#d3d0c8;">&quot;</span><span style="color:#99cc99;">httplib.h</span><span style="color:#d3d0c8;">&quot;
</span><span style="color:#cc99cc;">#include </span><span style="color:#d3d0c8;">&lt;</span><span style="color:#99cc99;">iostream</span><span style="color:#d3d0c8;">&gt;
</span><span style="color:#d3d0c8;">
</span><span style="color:#cc99cc;">int </span><span style="color:#6699cc;">main</span><span style="color:#d3d0c8;">() {
</span><span style="color:#d3d0c8;">    httplib::Client </span><span style="color:#6699cc;">cli</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">https://nghttp2.org</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">
</span><span style="color:#d3d0c8;">    </span><span style="color:#cc99cc;">auto</span><span style="color:#d3d0c8;"> res = cli.</span><span style="color:#6699cc;">Get</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">/</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">    </span><span style="color:#cc99cc;">if </span><span style="color:#d3d0c8;">(res) {
</span><span style="color:#d3d0c8;">        std::cout &lt;&lt; res-&gt;</span><span style="color:#f2777a;">status </span><span style="color:#d3d0c8;">&lt;&lt; std::endl;           </span><span style="color:#747369;">// 200
</span><span style="color:#d3d0c8;">        std::cout &lt;&lt; res-&gt;</span><span style="color:#f2777a;">body</span><span style="color:#d3d0c8;">.</span><span style="color:#6699cc;">substr</span><span style="color:#d3d0c8;">(</span><span style="color:#f99157;">0</span><span style="color:#d3d0c8;">, </span><span style="color:#f99157;">100</span><span style="color:#d3d0c8;">) &lt;&lt; std::endl;  </span><span style="color:#747369;">// HTMLの先頭部分
</span><span style="color:#d3d0c8;">    } </span><span style="color:#cc99cc;">else </span><span style="color:#d3d0c8;">{
</span><span style="color:#d3d0c8;">        std::cout &lt;&lt; &quot;</span><span style="color:#99cc99;">Error: </span><span style="color:#d3d0c8;">&quot; &lt;&lt; </span><span style="color:#6699cc;">httplib::to_string</span><span style="color:#d3d0c8;">(res.</span><span style="color:#6699cc;">error</span><span style="color:#d3d0c8;">()) &lt;&lt; std::endl;
</span><span style="color:#d3d0c8;">    }
</span><span style="color:#d3d0c8;">}
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#b48ead;">#define </span><span style="color:#4f5b66;">CPPHTTPLIB_OPENSSL_SUPPORT
</span><span style="color:#b48ead;">#include </span><span style="color:#4f5b66;">&quot;</span><span style="color:#a3be8c;">httplib.h</span><span style="color:#4f5b66;">&quot;
</span><span style="color:#b48ead;">#include </span><span style="color:#4f5b66;">&lt;</span><span style="color:#a3be8c;">iostream</span><span style="color:#4f5b66;">&gt;
</span><span style="color:#4f5b66;">
</span><span style="color:#b48ead;">int </span><span style="color:#8fa1b3;">main</span><span style="color:#4f5b66;">() {
</span><span style="color:#4f5b66;">    httplib::Client </span><span style="color:#8fa1b3;">cli</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">https://nghttp2.org</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">
</span><span style="color:#4f5b66;">    </span><span style="color:#b48ead;">auto</span><span style="color:#4f5b66;"> res = cli.</span><span style="color:#8fa1b3;">Get</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">/</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">    </span><span style="color:#b48ead;">if </span><span style="color:#4f5b66;">(res) {
</span><span style="color:#4f5b66;">        std::cout &lt;&lt; res-&gt;</span><span style="color:#bf616a;">status </span><span style="color:#4f5b66;">&lt;&lt; std::endl;           </span><span style="color:#a7adba;">// 200
</span><span style="color:#4f5b66;">        std::cout &lt;&lt; res-&gt;</span><span style="color:#bf616a;">body</span><span style="color:#4f5b66;">.</span><span style="color:#8fa1b3;">substr</span><span style="color:#4f5b66;">(</span><span style="color:#d08770;">0</span><span style="color:#4f5b66;">, </span><span style="color:#d08770;">100</span><span style="color:#4f5b66;">) &lt;&lt; std::endl;  </span><span style="color:#a7adba;">// HTMLの先頭部分
</span><span style="color:#4f5b66;">    } </span><span style="color:#b48ead;">else </span><span style="color:#4f5b66;">{
</span><span style="color:#4f5b66;">        std::cout &lt;&lt; &quot;</span><span style="color:#a3be8c;">Error: </span><span style="color:#4f5b66;">&quot; &lt;&lt; </span><span style="color:#8fa1b3;">httplib::to_string</span><span style="color:#4f5b66;">(res.</span><span style="color:#8fa1b3;">error</span><span style="color:#4f5b66;">()) &lt;&lt; std::endl;
</span><span style="color:#4f5b66;">    }
</span><span style="color:#4f5b66;">}
</span></pre>
</div>
<p>2章では <code>httplib::Client cli("http://localhost:8080")</code> と書きましたよね。スキームを <code>https://</code> に変えるだけです。<code>Get()</code> や <code>Post()</code> など、2章で学んだAPIはすべてそのまま使えます。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#6699cc;">curl</span><span style="color:#d3d0c8;"> https://nghttp2.org/
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#8fa1b3;">curl</span><span style="color:#4f5b66;"> https://nghttp2.org/
</span></pre>
</div>
<h2>ポートの指定</h2>
<p>HTTPSのデフォルトポートは443です。別のポートを使いたい場合は、URLにポートを含めます。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#d3d0c8;">httplib::Client </span><span style="color:#6699cc;">cli</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">https://localhost:8443</span><span style="color:#d3d0c8;">&quot;);
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#4f5b66;">httplib::Client </span><span style="color:#8fa1b3;">cli</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">https://localhost:8443</span><span style="color:#4f5b66;">&quot;);
</span></pre>
</div>
<h2>CA証明書の検証</h2>
<p><code>httplib::Client</code> はHTTPS接続時、デフォルトでサーバー証明書を検証します。信頼できるCA（認証局）が発行した証明書を持つサーバーにしか接続しません。</p>
<p>CA証明書は、macOSならKeychain、LinuxならシステムのCA証明書ストア、WindowsならWindowsの証明書ストアから自動で読み込みます。ほとんどの場合、追加の設定は要りません。</p>
<h3>CA証明書ファイルの指定</h3>
<p>環境によってはシステムのCA証明書が見つからないこともあります。そのときは <code>set_ca_cert_path()</code> でパスを直接指定してください。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#d3d0c8;">httplib::Client </span><span style="color:#6699cc;">cli</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">https://nghttp2.org</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">cli.</span><span style="color:#6699cc;">set_ca_cert_path</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">/etc/ssl/certs/ca-certificates.crt</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">
</span><span style="color:#cc99cc;">auto</span><span style="color:#d3d0c8;"> res = cli.</span><span style="color:#6699cc;">Get</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">/</span><span style="color:#d3d0c8;">&quot;);
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#4f5b66;">httplib::Client </span><span style="color:#8fa1b3;">cli</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">https://nghttp2.org</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">cli.</span><span style="color:#8fa1b3;">set_ca_cert_path</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">/etc/ssl/certs/ca-certificates.crt</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">
</span><span style="color:#b48ead;">auto</span><span style="color:#4f5b66;"> res = cli.</span><span style="color:#8fa1b3;">Get</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">/</span><span style="color:#4f5b66;">&quot;);
</span></pre>
</div><div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#6699cc;">curl</span><span style="color:#f2777a;"> --cacert</span><span style="color:#d3d0c8;"> /etc/ssl/certs/ca-certificates.crt https://nghttp2.org/
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#8fa1b3;">curl</span><span style="color:#bf616a;"> --cacert</span><span style="color:#4f5b66;"> /etc/ssl/certs/ca-certificates.crt https://nghttp2.org/
</span></pre>
</div>
<h3>証明書検証の無効化</h3>
<p>開発中、自己署名証明書のサーバーに接続したいときは、検証を無効にできます。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#d3d0c8;">httplib::Client </span><span style="color:#6699cc;">cli</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">https://localhost:8443</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">cli.</span><span style="color:#6699cc;">enable_server_certificate_verification</span><span style="color:#d3d0c8;">(</span><span style="color:#f99157;">false</span><span style="color:#d3d0c8;">);
</span><span style="color:#d3d0c8;">
</span><span style="color:#cc99cc;">auto</span><span style="color:#d3d0c8;"> res = cli.</span><span style="color:#6699cc;">Get</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">/</span><span style="color:#d3d0c8;">&quot;);
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#4f5b66;">httplib::Client </span><span style="color:#8fa1b3;">cli</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">https://localhost:8443</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">cli.</span><span style="color:#8fa1b3;">enable_server_certificate_verification</span><span style="color:#4f5b66;">(</span><span style="color:#d08770;">false</span><span style="color:#4f5b66;">);
</span><span style="color:#4f5b66;">
</span><span style="color:#b48ead;">auto</span><span style="color:#4f5b66;"> res = cli.</span><span style="color:#8fa1b3;">Get</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">/</span><span style="color:#4f5b66;">&quot;);
</span></pre>
</div><div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#6699cc;">curl</span><span style="color:#f2777a;"> -k</span><span style="color:#d3d0c8;"> https://localhost:8443/
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#8fa1b3;">curl</span><span style="color:#bf616a;"> -k</span><span style="color:#4f5b66;"> https://localhost:8443/
</span></pre>
</div>
<p>本番では絶対に無効にしないでください。中間者攻撃のリスクがあります。</p>
<h2>リダイレクトの追跡</h2>
<p>HTTPSサイトへのアクセスでは、リダイレクトに遭遇することがよくあります。たとえば <code>http://</code> から <code>https://</code> へ、あるいは <code>www</code> なしから <code>www</code> ありへ転送されるケースです。</p>
<p>デフォルトではリダイレクトを追跡しません。リダイレクト先は <code>Location</code> ヘッダーで確認できます。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#d3d0c8;">httplib::Client </span><span style="color:#6699cc;">cli</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">https://nghttp2.org</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">
</span><span style="color:#cc99cc;">auto</span><span style="color:#d3d0c8;"> res = cli.</span><span style="color:#6699cc;">Get</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">/httpbin/redirect/3</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#cc99cc;">if </span><span style="color:#d3d0c8;">(res) {
</span><span style="color:#d3d0c8;">    std::cout &lt;&lt; res-&gt;</span><span style="color:#f2777a;">status </span><span style="color:#d3d0c8;">&lt;&lt; std::endl;  </span><span style="color:#747369;">// 302
</span><span style="color:#d3d0c8;">    std::cout &lt;&lt; res-&gt;</span><span style="color:#6699cc;">get_header_value</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">Location</span><span style="color:#d3d0c8;">&quot;) &lt;&lt; std::endl;
</span><span style="color:#d3d0c8;">}
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#4f5b66;">httplib::Client </span><span style="color:#8fa1b3;">cli</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">https://nghttp2.org</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">
</span><span style="color:#b48ead;">auto</span><span style="color:#4f5b66;"> res = cli.</span><span style="color:#8fa1b3;">Get</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">/httpbin/redirect/3</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#b48ead;">if </span><span style="color:#4f5b66;">(res) {
</span><span style="color:#4f5b66;">    std::cout &lt;&lt; res-&gt;</span><span style="color:#bf616a;">status </span><span style="color:#4f5b66;">&lt;&lt; std::endl;  </span><span style="color:#a7adba;">// 302
</span><span style="color:#4f5b66;">    std::cout &lt;&lt; res-&gt;</span><span style="color:#8fa1b3;">get_header_value</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">Location</span><span style="color:#4f5b66;">&quot;) &lt;&lt; std::endl;
</span><span style="color:#4f5b66;">}
</span></pre>
</div><div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#6699cc;">curl</span><span style="color:#d3d0c8;"> https://nghttp2.org/httpbin/redirect/3
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#8fa1b3;">curl</span><span style="color:#4f5b66;"> https://nghttp2.org/httpbin/redirect/3
</span></pre>
</div>
<p><code>set_follow_location(true)</code> を設定すると、リダイレクトを自動で追跡して、最終的なレスポンスを返してくれます。</p>
<div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#d3d0c8;">httplib::Client </span><span style="color:#6699cc;">cli</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">https://nghttp2.org</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#d3d0c8;">cli.</span><span style="color:#6699cc;">set_follow_location</span><span style="color:#d3d0c8;">(</span><span style="color:#f99157;">true</span><span style="color:#d3d0c8;">);
</span><span style="color:#d3d0c8;">
</span><span style="color:#cc99cc;">auto</span><span style="color:#d3d0c8;"> res = cli.</span><span style="color:#6699cc;">Get</span><span style="color:#d3d0c8;">(&quot;</span><span style="color:#99cc99;">/httpbin/redirect/3</span><span style="color:#d3d0c8;">&quot;);
</span><span style="color:#cc99cc;">if </span><span style="color:#d3d0c8;">(res) {
</span><span style="color:#d3d0c8;">    std::cout &lt;&lt; res-&gt;</span><span style="color:#f2777a;">status </span><span style="color:#d3d0c8;">&lt;&lt; std::endl;  </span><span style="color:#747369;">// 200（最終的なレスポンス）
</span><span style="color:#d3d0c8;">}
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#4f5b66;">httplib::Client </span><span style="color:#8fa1b3;">cli</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">https://nghttp2.org</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#4f5b66;">cli.</span><span style="color:#8fa1b3;">set_follow_location</span><span style="color:#4f5b66;">(</span><span style="color:#d08770;">true</span><span style="color:#4f5b66;">);
</span><span style="color:#4f5b66;">
</span><span style="color:#b48ead;">auto</span><span style="color:#4f5b66;"> res = cli.</span><span style="color:#8fa1b3;">Get</span><span style="color:#4f5b66;">(&quot;</span><span style="color:#a3be8c;">/httpbin/redirect/3</span><span style="color:#4f5b66;">&quot;);
</span><span style="color:#b48ead;">if </span><span style="color:#4f5b66;">(res) {
</span><span style="color:#4f5b66;">    std::cout &lt;&lt; res-&gt;</span><span style="color:#bf616a;">status </span><span style="color:#4f5b66;">&lt;&lt; std::endl;  </span><span style="color:#a7adba;">// 200（最終的なレスポンス）
</span><span style="color:#4f5b66;">}
</span></pre>
</div><div class="code-dark"><pre style="background-color:#2d2d2d;">
<span style="color:#6699cc;">curl</span><span style="color:#f2777a;"> -L</span><span style="color:#d3d0c8;"> https://nghttp2.org/httpbin/redirect/3
</span></pre>
</div><div class="code-light"><pre style="background-color:#eff1f5;">
<span style="color:#8fa1b3;">curl</span><span style="color:#bf616a;"> -L</span><span style="color:#4f5b66;"> https://nghttp2.org/httpbin/redirect/3
</span></pre>
</div>
<h2>次のステップ</h2>
<p>HTTPSクライアントの使い方がわかりましたね。次は自分でHTTPSサーバーを立ててみましょう。自己署名証明書の作り方から始めます。</p>
<p><strong>次:</strong> <a href="../07-https-server">HTTPS Server</a></p>

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