1<?xml version="1.0" encoding="ISO-8859-1"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- 4 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 5 This file is generated from xml source: DO NOT EDIT 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 --> 8<title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server</title> 9<link href="/style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 10<link href="/style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 11<link href="/style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /><link rel="stylesheet" type="text/css" href="/style/css/prettify.css" /> 12<script src="/style/scripts/prettify.min.js" type="text/javascript"> 13</script> 14 15<link href="/images/favicon.ico" rel="shortcut icon" /></head> 16<body id="manual-page"><div id="page-header"> 17<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> 18<p class="apache">Apache HTTP Server Version 2.4</p> 19<img alt="" src="/images/feather.gif" /></div> 20<div class="up"><a href="./"><img title="<-" alt="<-" src="/images/left.gif" /></a></div> 21<div id="path"> 22<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> 23<div class="toplang"> 24<p><span>Available Languages: </span><a href="/en/dns-caveats.html" title="English"> en </a> | 25<a href="/fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a> | 26<a href="/ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 27<a href="/ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 28<a href="/tr/dns-caveats.html" hreflang="tr" rel="alternate" title="T�rk�e"> tr </a></p> 29</div> 30 31 <p>This page could be summarized with the statement: don't 32 configure Apache HTTP Server in such a way that it relies on DNS resolution 33 for parsing of the configuration files. If httpd requires DNS 34 resolution to parse the configuration files then your server 35 may be subject to reliability problems (ie. it might not start up), 36 or denial and theft of service attacks (including virtual hosts able 37 to steal hits from other virtual hosts).</p> 38 </div> 39<div id="quickview"><ul id="toc"><li><img alt="" src="/images/down.gif" /> <a href="#example">A Simple Example</a></li> 40<li><img alt="" src="/images/down.gif" /> <a href="#denial">Denial of Service</a></li> 41<li><img alt="" src="/images/down.gif" /> <a href="#main">The "main server" Address</a></li> 42<li><img alt="" src="/images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li> 43</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 44<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 45<div class="section"> 46<h2><a name="example" id="example">A Simple Example</a></h2> 47 48 49 <pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server 50<VirtualHost www.example.dom> 51 ServerAdmin webgirl@example.dom 52 DocumentRoot /www/example 53</VirtualHost></pre> 54 55 56 <p>In order for the server to function properly, it absolutely needs 57 to have two pieces of information about each virtual host: the 58 <code class="directive"><a href="/mod/core.html#servername">ServerName</a></code> and at least one 59 IP address that the server will bind and respond to. The above 60 example does not include the IP address, so httpd must use DNS 61 to find the address of <code>www.example.dom</code>. If for some 62 reason DNS is not available at the time your server is parsing 63 its config file, then this virtual host <strong>will not be 64 configured</strong>. It won't be able to respond to any hits 65 to this virtual host.</p> 66 67 <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1. 68 Then consider this configuration snippet:</p> 69 70 <pre class="prettyprint lang-config"># This is a misconfiguration example, do not use on your server 71<VirtualHost 192.0.2.1> 72 ServerAdmin webgirl@example.dom 73 DocumentRoot /www/example 74</VirtualHost></pre> 75 76 77 <p>This time httpd needs to use reverse DNS to find the 78 <code>ServerName</code> for this virtualhost. If that reverse 79 lookup fails then it will partially disable the virtualhost. 80 If the virtual host is name-based then it will effectively be 81 totally disabled, but if it is IP-based then it will mostly 82 work. However, if httpd should ever have to generate a full 83 URL for the server which includes the server name (such as when a 84 Redirect is issued), then it will fail to generate a valid URL.</p> 85 86 <p>Here is a snippet that avoids both of these problems:</p> 87 88 <pre class="prettyprint lang-config"><VirtualHost 192.0.2.1> 89 ServerName www.example.dom 90 ServerAdmin webgirl@example.dom 91 DocumentRoot /www/example 92</VirtualHost></pre> 93 94 </div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 95<div class="section"> 96<h2><a name="denial" id="denial">Denial of Service</a></h2> 97 98 99 <p>Consider this configuration snippet:</p> 100 101 <pre class="prettyprint lang-config"><VirtualHost www.example1.dom> 102 ServerAdmin webgirl@example1.dom 103 DocumentRoot /www/example1 104</VirtualHost> 105<VirtualHost www.example2.dom> 106 ServerAdmin webguy@example2.dom 107 DocumentRoot /www/example2 108</VirtualHost></pre> 109 110 111 <p>Suppose that you've assigned 192.0.2.1 to 112 <code>www.example1.dom</code> and 192.0.2.2 to 113 <code>www.example2.dom</code>. Furthermore, suppose that 114 <code>example1.dom</code> has control of their own DNS. With this 115 config you have put <code>example1.dom</code> into a position where 116 they can steal all traffic destined to <code>example2.dom</code>. To 117 do so, all they have to do is set <code>www.example1.dom</code> to 118 192.0.2.2. Since they control their own DNS you can't stop them 119 from pointing the <code>www.example1.dom</code> record wherever they 120 wish.</p> 121 122 <p>Requests coming in to 192.0.2.2 (including all those where 123 users typed in URLs of the form 124 <code>http://www.example2.dom/whatever</code>) will all be served by 125 the <code>example1.dom</code> virtual host. To better understand why 126 this happens requires a more in-depth discussion of how httpd 127 matches up incoming requests with the virtual host that will 128 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p> 129 </div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 130<div class="section"> 131<h2><a name="main" id="main">The "main server" Address</a></h2> 132 133 134 <p><a href="vhosts/name-based.html">Name-based 135 virtual host support</a> requires httpd to know 136 the IP address(es) of the host that <code class="program"><a href="/programs/httpd.html">httpd</a></code> 137 is running on. To get this address it uses either the global 138 <code class="directive"><a href="/mod/core.html#servername">ServerName</a></code> 139 (if present) or calls the C function <code>gethostname</code> 140 (which should return the same as typing "hostname" at the 141 command prompt). Then it performs a DNS lookup on this address. 142 At present there is no way to avoid this lookup.</p> 143 144 <p>If you fear that this lookup might fail because your DNS 145 server is down then you can insert the hostname in 146 <code>/etc/hosts</code> (where you probably already have it so 147 that the machine can boot properly). Then ensure that your 148 machine is configured to use <code>/etc/hosts</code> in the 149 event that DNS fails. Depending on what OS you are using this 150 might be accomplished by editing <code>/etc/resolv.conf</code>, 151 or maybe <code>/etc/nsswitch.conf</code>.</p> 152 153 <p>If your server doesn't have to perform DNS for any other 154 reason then you might be able to get away with running httpd 155 with the <code>HOSTRESORDER</code> environment variable set to 156 "local". This all depends on what OS and resolver libraries you 157 are using. It also affects CGIs unless you use 158 <code class="module"><a href="/mod/mod_env.html">mod_env</a></code> to control the environment. It's best 159 to consult the man pages or FAQs for your OS.</p> 160 </div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 161<div class="section"> 162<h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2> 163 164 165 <ul> 166 <li> 167 use IP addresses in 168 <code class="directive"><a href="/mod/core.html#virtualhost">VirtualHost</a></code> 169 </li> 170 171 <li> 172 use IP addresses in 173 <code class="directive"><a href="/mod/mpm_common.html#listen">Listen</a></code> 174 </li> 175 176 <li> 177 ensure all virtual hosts have an explicit 178 <code class="directive"><a href="/mod/core.html#servername">ServerName</a></code> 179 </li> 180 181 <li>create a <code><VirtualHost _default_:*></code> 182 server that has no pages to serve</li> 183 </ul> 184 </div></div> 185<div class="bottomlang"> 186<p><span>Available Languages: </span><a href="/en/dns-caveats.html" title="English"> en </a> | 187<a href="/fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a> | 188<a href="/ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 189<a href="/ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 190<a href="/tr/dns-caveats.html" hreflang="tr" rel="alternate" title="T�rk�e"> tr </a></p> 191</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 again by our moderators if they are either implemented or considered invalid/off-topic. 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