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<h2 class="post-title" itemprop="name headline">[Kubernetes] DNS Setting with Dockerd(原始碼分析上)
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<div class="post-description">在前篇文章有跟大家分享過實際部屬上遇到的 DNS 問題，並且透過實驗佐證去觀察結果, 本篇文章則是透過另外一種觀點來觀察結果,透過閱讀原始碼的方式來觀察到底 kubernetes 再創建相關的 Pod 時會如何去處理 DNS 相關的設定，由於整個過程牽扯到 kubernetes 以及 CRI(Container Runtime Interface)的操作，而我們預設使用的則是 Docker 作為底層容器的操作. 由於篇幅過長，所以本文會著重於 kubernetes 原始碼的部分，而 Docker 相關的部分則會餘下篇文章來研究.</div>
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<h1 id="Preface"><a href="#Preface" class="headerlink" title="Preface"></a>Preface</h1><p>此篇文章是 Kubernetes Pod-DNS 系列文章第三篇<br>此系列文會從使用者的用法到一些問題的發掘，最後透過閱讀程式碼的方式去分析這些問題</p>
<p>相關的文章連結如下</p>
<ul>
<li><a href="https://www.hwchiu.com/kubernetes-dns.html">[Kubernetes] DNS setting in your Pod</a></li>
<li><a href="https://www.hwchiu.com/kubernetes-dns-ii.html">[Kubernetes] DNS Setting with Dockerd</a></li>
<li><a href="https://www.hwchiu.com/kubernetes-dns-iiii.html">[Kubernetes] DNS Setting with Dockerd(原始碼分析下)</a></li>
</ul>
<h1 id="正文"><a href="#正文" class="headerlink" title="正文"></a>正文</h1><p>在前篇文章<br><a href="https://www.hwchiu.com/kubernetes-dns-ii.html">[Kubernetes] DNS Setting with Dockerd</a> 中已經詳細介紹了整個問題的流程以及觀察結果。</p>
<p>再次重申一次結論<br><code>kubernetes</code> 會先嘗試使用節點上 <code>/etc/resolv.conf</code> 的資料，但是若發現 <code>/etc/resolv.conf</code> 是空的，這時候就會去依賴 <code>dockerd</code> 幫忙產生的 <code>/etc/resolv.conf</code></p>
<p>本篇會直接從 <code>kubernetes</code> 以及 <code>docker</code> 的原始碼來研究這個問題，並且佐證上篇文章的觀察結果。</p>
<h1 id="Kubernetes"><a href="#Kubernetes" class="headerlink" title="Kubernetes"></a>Kubernetes</h1><p>在觀察這個現象前，我們必須要先思考方向，該怎麼去追這段程式碼?<br>到底程式碼的開頭應該從哪邊開始追起?</p>
<p>要講起這個問題實在不是一言兩語可以解決的，這部份除非本身已經對 <code>kubernetes</code>原始碼很瞭解，否則就是要倚賴關鍵字加上本身對程式語言的經驗來判斷，趕緊找到一個正確的進入點。</p>
<p>這邊直接先使用一個簡單的流程圖，來描述整個運作的邏輯，然後接下來會針對這段流程進行更進一步的分析來解決我們的疑問</p>
<h1 id="CRI-流程圖"><a href="#CRI-流程圖" class="headerlink" title="CRI 流程圖"></a>CRI 流程圖</h1><p>這邊我們先簡單的知道， <code>kubernetes</code> 本身有非常多的插件，包含了 <code>Storage</code>, <code>Network</code>, <code>Device</code> 以及 <code>Runtime</code>。<br>這就是你會常常聽到 <code>CNI</code>, <code>CRI</code>, <code>CSI</code> 以及 <code>Device Plugin</code> 等這些名詞出現的理由</p>
<p><code>Kubernetes</code> 藉由這些插件把部分功能都給模組化，讓任何符合該標準的第三方套件都能夠與 <code>kubernetes</code> 緊密合作。</p>
<blockquote>
<p>未來將會撰寫一系列的文章來介紹 CNI，從 CNI 概念到自己撰寫一個 CNI，敬請期待</p>
</blockquote>
<p>因為今天的問題主要是運行容器內的設定問題，這部份則是會跟 <code>CRI(Contaienr Runtime Interface)</code> 有關連, 因此我透過下圖稍微說明一下整個流程。</p>
<p>在每個 <code>kubernetes</code> 的節點上都會運行一隻守護程序 <code>kubelet</code>，該 <code>kubelet</code> 內部有一個稱為 <code>kube-runtime</code> 的套件會用來管理在該節點上所有跟容器有關的操作。</p>
<p><code>kube-runtime</code> 本身會遵守 <code>CRI</code> 的規範來管理容器，而 <code>CRI</code> 則是建立在 <code>gRPC</code> Client/Server 的架構下運行的。</p>
<p>因此 <code>kube runtime</code> 本身會扮演者 <code>gRPC</code> client 的角色，對 gRPC server <code>dockershim</code> 發送管理容器的請求。</p>
<p>而 <code>dockershim</code> 則是一個 <code>kubernetes</code> 內部自行設計實作的一個基於 <code>docker container</code> 且相容 <code>CRI</code> 的 gRPC server.</p>
<p>當 <code>dockershim</code> 收到請求後，就會透過 <code>Docker Container</code> 本身的 <code>API</code> 進行 <code>Docker Container</code> 相關的資源處理。</p>
<p><img src="https://i.imgur.com/t5jQGmd.png" alt="Imgur"></p>
<h1 id="程式碼追蹤"><a href="#程式碼追蹤" class="headerlink" title="程式碼追蹤"></a>程式碼追蹤</h1><p>藉由上述流程圖的幫忙後，我們可以有更進一步的方向去思考程式的進入點。</p>
<p>首先我們知道透過對 <code>Pod</code> 設定 <code>dnsPolicy:default</code> 才會有這個問題，同時也觀察到 <code>dockerd</code> 本身的設定也會影響整個 <code>/etc/resolve.conf</code> 的結果。</p>
<p>所以 <code>kube runtime</code> 這邊是一個可能線索，如何解析 <code>Pod</code> 本身的參數並處理是一個方向<br>同時 <code>dockershim</code> 這邊如何針對 <code>docker container</code> 則會是另外一個線索來追尋。</p>
<p>針對這兩個方向，經過仔細的追尋後，我們可以得到類似下圖的流程<br><div class="note danger">
<p>圖中藍色區域都是真實的函式名稱</p>
</div></p>
<p><img src="https://i.imgur.com/6xsRAD1.png" alt="Imgur"></p>
<p>當 <strong>1</strong> 號發出<code>Create Pod</code>的請求出來後， <code>kubelet</code> 裡面於 <code>kube runtime</code> 模組內則會呼叫到 <code>createPodSandbox</code> 的函式。<br>在 <code>createPodSandbox</code> 則會進行下列事情</p>
<ol>
<li>呼叫 <code>generatePodSandboxConfig</code></li>
<li>透過 <code>CRI</code> 要求遠方執行 <code>RunPodSandbox</code></li>
</ol>
<h2 id="kube-runtime"><a href="#kube-runtime" class="headerlink" title="kube runtime"></a>kube runtime</h2><h3 id="createPodSandbox"><a href="#createPodSandbox" class="headerlink" title="createPodSandbox"></a>createPodSandbox</h3><p>整個函式如下<br><figure class="highlight go"><figcaption><span>createPodSandbox</span><a href="https://github.com/kubernetes/kubernetes/blob/f634f7dae4a49bb513ae8f5c53e5d115308231d3/pkg/kubelet/kuberuntime/kuberuntime_sandbox.go#L37" target="_blank" rel="noopener">kuberuntime_sandbox.go</a></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// createPodSandbox creates a pod sandbox and returns (podSandBoxID, message, error).</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *kubeGenericRuntimeManager)</span> <span class="title">createPodSandbox</span><span class="params">(pod *v1.Pod, attempt <span class="keyword">uint32</span>)</span> <span class="params">(<span class="keyword">string</span>, <span class="keyword">string</span>, error)</span></span> {</span><br><span class="line"> podSandboxConfig, err := m.generatePodSandboxConfig(pod, attempt)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> message := fmt.Sprintf(<span class="string">"GeneratePodSandboxConfig for pod %q failed: %v"</span>, format.Pod(pod), err)</span><br><span class="line"> glog.Error(message)</span><br><span class="line"> <span class="keyword">return</span> <span class="string">""</span>, message, err</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// Create pod logs directory</span></span><br><span class="line"> err = m.osInterface.MkdirAll(podSandboxConfig.LogDirectory, <span class="number">0755</span>)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> message := fmt.Sprintf(<span class="string">"Create pod log directory for pod %q failed: %v"</span>, format.Pod(pod), err)</span><br><span class="line"> glog.Errorf(message)</span><br><span class="line"> <span class="keyword">return</span> <span class="string">""</span>, message, err</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> podSandBoxID, err := m.runtimeService.RunPodSandbox(podSandboxConfig)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> message := fmt.Sprintf(<span class="string">"CreatePodSandbox for pod %q failed: %v"</span>, format.Pod(pod), err)</span><br><span class="line"> glog.Error(message)</span><br><span class="line"> <span class="keyword">return</span> <span class="string">""</span>, message, err</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> podSandBoxID, <span class="string">""</span>, <span class="literal">nil</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<h3 id="generatePodSandboxConfig"><a href="#generatePodSandboxConfig" class="headerlink" title="generatePodSandboxConfig"></a>generatePodSandboxConfig</h3><p><code>generatePodSandboxConfig</code> 這個函式很簡單，基本上就是從 <code>Pod</code> 本身 <code>yaml</code> 檔案裡面去讀取設定，根據這些設定來處理相關的資訊。<br>我們在意的部份就是 <code>DNS</code> 相關，所以以下就擷取 <code>DNS</code> 相關的函式內容</p>
<figure class="highlight go"><figcaption><span>generatePodSandboxConfig</span><a href="https://github.com/kubernetes/kubernetes/blob/f634f7dae4a49bb513ae8f5c53e5d115308231d3/pkg/kubelet/kuberuntime/kuberuntime_sandbox.go#L64" target="_blank" rel="noopener">kuberuntime_sandbox.go</a></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// createPodSandbox creates a pod sandbox and returns (podSandBoxID, message, error).</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *kubeGenericRuntimeManager)</span> <span class="title">generatePodSandboxConfig</span><span class="params">(pod *v1.Pod, attempt <span class="keyword">uint32</span>)</span> <span class="params">(*runtimeapi.PodSandboxConfig, error)</span></span> {</span><br><span class="line">....</span><br><span class="line"> dnsConfig, err := m.runtimeHelper.GetPodDNS(pod)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nil</span>, err</span><br><span class="line"> }</span><br><span class="line"> podSandboxConfig.DnsConfig = dnsConfig</span><br><span class="line">.....</span><br></pre></td></tr></table></figure>
<p>這邊可以看到呼叫了 <code>GetPodDNS</code> 來取得對應的設定，然後我們就一路往下追</p>
<h3 id="GetPodDNS"><a href="#GetPodDNS" class="headerlink" title="GetPodDNS"></a>GetPodDNS</h3><figure class="highlight go"><figcaption><span>GetPodDNS</span><a href="https://github.com/kubernetes/kubernetes/blob/1ca851baec6a245bbb2bd3aea284e6cb4f364348/pkg/kubelet/network/dns/dns.go#L325" target="_blank" rel="noopener">dns.go</a></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(c *Configurer)</span> <span class="title">GetPodDNS</span><span class="params">(pod *v1.Pod)</span> <span class="params">(*runtimeapi.DNSConfig, error)</span></span> {</span><br><span class="line"> dnsConfig, err := c.getHostDNSConfig(pod)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nil</span>, err</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> dnsType, err := getPodDNSType(pod)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> glog.Errorf(<span class="string">"Failed to get DNS type for pod %q: %v. Falling back to DNSClusterFirst policy."</span>, format.Pod(pod), err)</span><br><span class="line"> dnsType = podDNSCluster</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">switch</span> dnsType {</span><br><span class="line"> <span class="keyword">case</span> podDNSNone:</span><br><span class="line"> <span class="comment">// DNSNone should use empty DNS settings as the base.</span></span><br><span class="line"> dnsConfig = &runtimeapi.DNSConfig{}</span><br><span class="line"> <span class="keyword">case</span> podDNSCluster:</span><br><span class="line"> ...</span><br><span class="line"> <span class="keyword">case</span> podDNSHost:</span><br><span class="line"> <span class="comment">// When the kubelet --resolv-conf flag is set to the empty string, use</span></span><br><span class="line"> <span class="comment">// DNS settings that override the docker default (which is to use</span></span><br><span class="line"> <span class="comment">// /etc/resolv.conf) and effectively disable DNS lookups. According to</span></span><br><span class="line"> <span class="comment">// the bind documentation, the behavior of the DNS client library when</span></span><br><span class="line"> <span class="comment">// "nameservers" are not specified is to "use the nameserver on the</span></span><br><span class="line"> <span class="comment">// local machine". A nameserver setting of localhost is equivalent to</span></span><br><span class="line"> <span class="comment">// this documented behavior.</span></span><br><span class="line"> <span class="keyword">if</span> c.ResolverConfig == <span class="string">""</span> {</span><br><span class="line"> <span class="keyword">switch</span> {</span><br><span class="line"> <span class="keyword">case</span> c.nodeIP == <span class="literal">nil</span> || c.nodeIP.To4() != <span class="literal">nil</span>:</span><br><span class="line"> dnsConfig.Servers = []<span class="keyword">string</span>{<span class="string">"127.0.0.1"</span>}</span><br><span class="line"> <span class="keyword">case</span> c.nodeIP.To16() != <span class="literal">nil</span>:</span><br><span class="line"> dnsConfig.Servers = []<span class="keyword">string</span>{<span class="string">"::1"</span>}</span><br><span class="line"> }</span><br><span class="line"> dnsConfig.Searches = []<span class="keyword">string</span>{<span class="string">"."</span>}</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">...</span><br><span class="line"> <span class="keyword">return</span> c.formDNSConfigFitsLimits(dnsConfig, pod), <span class="literal">nil</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>這邊的邏輯非常簡單</p>
<ol>
<li>先透過 <code>getHostDNSConfig</code> 取得當前 <code>/etc/resolv.conf</code> 的內容，並且存於變數 <strong>dnsCOnfig</strong></li>
<li>接下來透過 <code>getPodDNSType</code> 該函式從 <code>Pod Yaml</code> 內讀取對應的 <code>dnsPolicy</code> 資訊，來決定當前 <code>Pod</code> 的 <code>dnsType</code></li>
<li>第三步驟就是最重要的，根據當前 <code>dnsType</code> 來決定要如何處理 <code>dnsConfig</code>.<br>由於 <code>podDNSCluster</code> 的程式碼太多了，我這邊就先忽略掉，我們只要專注看 <code>DNSHost</code> 的案例。<br>裡面非常簡單，如果你當初運行 <code>kubelet</code> 的時候有特別設定不要使用 <code>/etc/resolv.conf</code> 的話，就會把 <code>dnsConfig</code> 補上一個 <code>127.0.0.1</code> 的資訊，不過這個 <code>Case</code> 我暫時還沒想到，可能就是機器本身就是一個 <code>DNSServer</code> 的情況吧。</li>
<li>將 <code>dnsConfig</code> 回傳出去<br>所以在正常情況下，我們的 <code>dnsConfig</code> 理論上就會是節點上 <code>/etc/resolv.conf</code> 內的資料。</li>
</ol>
<p>到這邊為止，就是 <code>kube-runtime</code> 自行處理的部份，並且將得到的 <code>dnsConfig</code> 放置到變數 <code>podSandboxConfig.DnsConfig</code> 之中。</p>
<p>接下來我們來看一下 <code>dockershim</code> 裡面的 <code>RunPodSandbox</code> 會怎麼處理這些 <code>DNS</code> 的資訊。</p>
<h2 id="DocekrShim"><a href="#DocekrShim" class="headerlink" title="DocekrShim"></a>DocekrShim</h2><h3 id="RunPodSandbox"><a href="#RunPodSandbox" class="headerlink" title="RunPodSandbox"></a>RunPodSandbox</h3><p>由於 <code>RunPodSandbox</code> 的函式非常長，這邊就擷取跟 <code>DNS</code> 相關的部份來觀察</p>
<figure class="highlight go"><figcaption><span>RunPodSandbox</span><a href="https://github.com/kubernetes/kubernetes/blob/67ebbc675aac2b58abc95b186749a81236242625/pkg/kubelet/dockershim/docker_sandbox.go#L81" target="_blank" rel="noopener">docker_sandbox.go</a></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(ds *dockerService)</span> <span class="title">RunPodSandbox</span><span class="params">(ctx context.Context, r *runtimeapi.RunPodSandboxRequest)</span> <span class="params">(*runtimeapi.RunPodSandboxResponse, error)</span></span> {</span><br><span class="line"> config := r.GetConfig()</span><br><span class="line">....</span><br><span class="line"> <span class="comment">// Rewrite resolv.conf file generated by docker.</span></span><br><span class="line"> <span class="comment">// <span class="doctag">NOTE:</span> cluster dns settings aren't passed anymore to docker api in all cases,</span></span><br><span class="line"> <span class="comment">// not only for pods with host network: the resolver conf will be overwritten</span></span><br><span class="line"> <span class="comment">// after sandbox creation to override docker's behaviour. This resolv.conf</span></span><br><span class="line"> <span class="comment">// file is shared by all containers of the same pod, and needs to be modified</span></span><br><span class="line"> <span class="comment">// only once per pod.</span></span><br><span class="line"> <span class="keyword">if</span> dnsConfig := config.GetDnsConfig(); dnsConfig != <span class="literal">nil</span> {</span><br><span class="line"> containerInfo, err := ds.client.InspectContainer(createResp.ID)</span><br><span class="line"> <span class="keyword">if</span> err != <span class="literal">nil</span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nil</span>, fmt.Errorf(<span class="string">"failed to inspect sandbox container for pod %q: %v"</span>, config.Metadata.Name, err)</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> err := rewriteResolvFile(containerInfo.ResolvConfPath, dnsConfig.Servers, dnsConfig.Searches, dnsConfig.Options); err != <span class="literal">nil</span> {</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nil</span>, fmt.Errorf(<span class="string">"rewrite resolv.conf failed for pod %q: %v"</span>, config.Metadata.Name, err)</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">....</span><br><span class="line"> <span class="keyword">return</span> resp, <span class="literal">nil</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>其實這邊的程式碼已經有了滿不錯的註解來解釋相關的行為。<br>1.之前透過 <code>kube run-time</code> 得到的 <code>dns</code> 設定都只是一個存在於記憶體內的一堆資料而已，其實都還沒有跟真正的容器有任何關係。<br>2.透過 <code>dockershim</code> 的操作，這時候其實已經將對應的容器創見完畢，而且該容器內<code>/etc/resolv.conf</code>的內容是完全依賴 <code>docker container</code> 決定<br>3.<code>kubernetes</code> 這邊的邏輯非常簡單，如果我之前有產生過任何 <code>dns</code> 的設定，就直接將 <code>docker</code> 產生出來的 <code>/etc/resolv.conf</code> 給直接覆蓋掉。</p>
<p>所以我們可以看一下 <code>rewriteResolvFile</code> 這個函式</p>
<h3 id="rewriteResolvFile"><a href="#rewriteResolvFile" class="headerlink" title="rewriteResolvFile"></a>rewriteResolvFile</h3><figure class="highlight go"><figcaption><span>RunPodSandbox</span><a href="https://github.com/kubernetes/kubernetes/blob/67ebbc675aac2b58abc95b186749a81236242625/pkg/kubelet/dockershim/docker_sandbox.go#L666" target="_blank" rel="noopener">docker_sandbox.go</a></figcaption><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// rewriteResolvFile rewrites resolv.conf file generated by docker.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">rewriteResolvFile</span><span class="params">(resolvFilePath <span class="keyword">string</span>, dns []<span class="keyword">string</span>, dnsSearch []<span class="keyword">string</span>, dnsOptions []<span class="keyword">string</span>)</span> <span class="title">error</span></span> {</span><br><span class="line">...</span><br><span class="line"> <span class="keyword">var</span> resolvFileContent []<span class="keyword">string</span></span><br><span class="line"> <span class="keyword">for</span> _, srv := <span class="keyword">range</span> dns {</span><br><span class="line"> resolvFileContent = <span class="built_in">append</span>(resolvFileContent, <span class="string">"nameserver "</span>+srv)</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> <span class="built_in">len</span>(dnsSearch) > <span class="number">0</span> {</span><br><span class="line"> resolvFileContent = <span class="built_in">append</span>(resolvFileContent, <span class="string">"search "</span>+strings.Join(dnsSearch, <span class="string">" "</span>))</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> <span class="built_in">len</span>(dnsOptions) > <span class="number">0</span> {</span><br><span class="line"> resolvFileContent = <span class="built_in">append</span>(resolvFileContent, <span class="string">"options "</span>+strings.Join(dnsOptions, <span class="string">" "</span>))</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> <span class="built_in">len</span>(resolvFileContent) > <span class="number">0</span> {</span><br><span class="line"> resolvFileContentStr := strings.Join(resolvFileContent, <span class="string">"\n"</span>)</span><br><span class="line"> resolvFileContentStr += <span class="string">"\n"</span></span><br><span class="line"></span><br><span class="line"> glog.V(<span class="number">4</span>).Infof(<span class="string">"Will attempt to re-write config file %s with: \n%s"</span>, resolvFilePath, resolvFileContent)</span><br><span class="line"> <span class="keyword">if</span> err := rewriteFile(resolvFilePath, resolvFileContentStr); err != <span class="literal">nil</span> {</span><br><span class="line"> glog.Errorf(<span class="string">"resolv.conf could not be updated: %v"</span>, err)</span><br><span class="line"> <span class="keyword">return</span> err</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> <span class="literal">nil</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>這邊也不能理解，就是根據之前存放於 <code>podSandboxConfig.DnsConfig</code> 內各式各樣關於 <code>DNS</code> 的設定組合起來，然後直接覆蓋掉本來的 <code>/etc/resolv.conf</code>.</p>
<h1 id="結論"><a href="#結論" class="headerlink" title="結論"></a>結論</h1><p>到這邊，我們已經把整個問題給釐清一半了</p>
<p>重新看一次之前的結論</p>
<div class="note success">
<p><code>kubernetes</code> 會先嘗試使用節點上 <code>/etc/resolv.conf</code> 的資料，但是若發現 <code>/etc/resolv.conf</code> 是空的，這時候就會去依賴 <code>dockerd</code> 幫忙產生的 <code>/etc/resolv.conf</code></p>
</div>
<p>我們的推論跟我們程式碼觀察的結果是完全吻合的，再 <code>dnsPolicy=default</code> 的前提下，只要 <code>kubernetes</code> 只要能夠獲得合法的 <code>/etc/resolv.conf</code> 就會使用，否則直接使用 <code>docekr container</code> 所創造的 <code>/etc/resolv.conf</code>.</p>
<p>但是這邊還有留下一個謎點，到底 <code>dockerd</code> 的設定是如何影響 <code>/etc/resolv.conf</code> 以及 <code>8.8.8.8/8.8.4.4</code> 是如何出現的?</p>
<p>由於再寫下去本篇文章會愈來愈長，所以決定將 <code>docker container</code> 相關的程式碼分析再寫一篇文章來處理。</p>
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