217 lines
12 KiB
XML
217 lines
12 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head>
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<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
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<!--
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<title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server Version 2.4</title>
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<link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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</script>
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<link href="./images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.4</p>
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<img alt="" src="./images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
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<div id="path">
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="./">Version 2.4</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache HTTP Server</h1>
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<div class="toplang">
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<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
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<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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</div>
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<p>This page could be summarized with the statement: don't
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configure Apache HTTP Server in such a way that it relies on DNS resolution
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for parsing of the configuration files. If httpd requires DNS
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resolution to parse the configuration files then your server
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may be subject to reliability problems (ie. it might not start up),
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or denial and theft of service attacks (including virtual hosts able
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to steal hits from other virtual hosts).</p>
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</div>
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<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
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<li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
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</ul><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div>
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<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="example" id="example">A Simple Example</a></h2>
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<pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server
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<VirtualHost www.example.dom>
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ServerAdmin webgirl@example.dom
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DocumentRoot "/www/example"
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</VirtualHost></pre>
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<p>In order for the server to function properly, it absolutely needs
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to have two pieces of information about each virtual host: the
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<code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
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IP address that the server will bind and respond to. The above
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example does not include the IP address, so httpd must use DNS
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to find the address of <code>www.example.dom</code>. If for some
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reason DNS is not available at the time your server is parsing
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its config file, then this virtual host <strong>will not be
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configured</strong>. It won't be able to respond to any hits
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to this virtual host.</p>
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<p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
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Then consider this configuration snippet:</p>
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<pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server
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<VirtualHost 192.0.2.1>
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ServerAdmin webgirl@example.dom
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DocumentRoot "/www/example"
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</VirtualHost></pre>
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<p>This time httpd needs to use reverse DNS to find the
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<code>ServerName</code> for this virtualhost. If that reverse
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lookup fails then it will partially disable the virtualhost.
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If the virtual host is name-based then it will effectively be
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totally disabled, but if it is IP-based then it will mostly
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work. However, if httpd should ever have to generate a full
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URL for the server which includes the server name (such as when a
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Redirect is issued), then it will fail to generate a valid URL.</p>
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<p>Here is a snippet that avoids both of these problems:</p>
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<pre class="prettyprint lang-config"><VirtualHost 192.0.2.1>
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ServerName www.example.dom
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ServerAdmin webgirl@example.dom
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DocumentRoot "/www/example"
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</VirtualHost></pre>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="denial" id="denial">Denial of Service</a></h2>
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<p>Consider this configuration snippet:</p>
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<pre class="prettyprint lang-config"><VirtualHost www.example1.dom>
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ServerAdmin webgirl@example1.dom
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DocumentRoot "/www/example1"
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</VirtualHost>
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<VirtualHost www.example2.dom>
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ServerAdmin webguy@example2.dom
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DocumentRoot "/www/example2"
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</VirtualHost></pre>
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<p>Suppose that you've assigned 192.0.2.1 to
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<code>www.example1.dom</code> and 192.0.2.2 to
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<code>www.example2.dom</code>. Furthermore, suppose that
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<code>example1.dom</code> has control of their own DNS. With this
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config you have put <code>example1.dom</code> into a position where
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they can steal all traffic destined to <code>example2.dom</code>. To
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do so, all they have to do is set <code>www.example1.dom</code> to
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192.0.2.2. Since they control their own DNS you can't stop them
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from pointing the <code>www.example1.dom</code> record wherever they
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wish.</p>
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<p>Requests coming in to 192.0.2.2 (including all those where
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users typed in URLs of the form
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<code>http://www.example2.dom/whatever</code>) will all be served by
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the <code>example1.dom</code> virtual host. To better understand why
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this happens requires a more in-depth discussion of how httpd
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matches up incoming requests with the virtual host that will
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serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="main" id="main">The "main server" Address</a></h2>
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<p><a href="vhosts/name-based.html">Name-based
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virtual host support</a> requires httpd to know
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the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
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is running on. To get this address it uses either the global
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<code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
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(if present) or calls the C function <code>gethostname</code>
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(which should return the same as typing "hostname" at the
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command prompt). Then it performs a DNS lookup on this address.
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At present there is no way to avoid this lookup.</p>
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<p>If you fear that this lookup might fail because your DNS
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server is down then you can insert the hostname in
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<code>/etc/hosts</code> (where you probably already have it so
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that the machine can boot properly). Then ensure that your
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machine is configured to use <code>/etc/hosts</code> in the
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event that DNS fails. Depending on what OS you are using this
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might be accomplished by editing <code>/etc/resolv.conf</code>,
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or maybe <code>/etc/nsswitch.conf</code>.</p>
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<p>If your server doesn't have to perform DNS for any other
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reason then you might be able to get away with running httpd
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with the <code>HOSTRESORDER</code> environment variable set to
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"local". This all depends on what OS and resolver libraries you
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are using. It also affects CGIs unless you use
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<code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
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to consult the man pages or FAQs for your OS.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
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<ul>
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<li>
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use IP addresses in
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<code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
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</li>
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<li>
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use IP addresses in
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<code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
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</li>
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<li>
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ensure all virtual hosts have an explicit
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<code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
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</li>
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<li>create a <code><VirtualHost _default_:*></code>
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server that has no pages to serve</li>
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</ul>
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</div></div>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
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<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
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<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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</div><div class="top"><a href="#page-header"><img src="./images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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