github.com/krum110487/go-htaccess@v0.0.0-20240316004156-60641c8e7598/tests/data/apache_2_2_34/manual/dns-caveats.html.en (about) 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 <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> 5 <!-- 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 This file is generated from xml source: DO NOT EDIT 8 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 9 --> 10 <title>Issues Regarding DNS and Apache - Apache HTTP Server Version 2.2</title> 11 <link href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 12 <link href="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 13 <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" /> 14 <script src="./style/scripts/prettify.min.js" type="text/javascript"> 15 </script> 16 17 <link href="./images/favicon.ico" rel="shortcut icon" /><link href="http://httpd.apache.org/docs/current/dns-caveats.html" rel="canonical" /></head> 18 <body id="manual-page"><div id="page-header"> 19 <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> 20 <p class="apache">Apache HTTP Server Version 2.2</p> 21 <img alt="" src="./images/feather.gif" /></div> 22 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div> 23 <div id="path"> 24 <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 2.2</a></div><div id="page-content"><div class="retired"><h4>Please note</h4> 25 <p> This document refers to a legacy release (<strong>2.2</strong>) of Apache httpd. The active release (<strong>2.4</strong>) is documented <a href="http://httpd.apache.org/docs/current">here</a>. If you have not already upgraded, please follow <a href="http://httpd.apache.org/docs/current/upgrading.html">this link</a> for more information.</p> 26 <p>You may follow <a href="http://httpd.apache.org/docs/current/dns-caveats.html">this link</a> to go to the current version of this document.</p></div><div id="preamble"><h1>Issues Regarding DNS and Apache</h1> 27 <div class="toplang"> 28 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> | 29 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 30 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 31 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> 32 </div> 33 34 <p>This page could be summarized with the statement: don't 35 configure Apache in such a way that it relies on DNS resolution 36 for parsing of the configuration files. If Apache requires DNS 37 resolution to parse the configuration files then your server 38 may be subject to reliability problems (ie. it might not boot), 39 or denial and theft of service attacks (including users able 40 to steal hits from other users).</p> 41 </div> 42 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li> 43 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li> 44 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li> 45 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li> 46 <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li> 47 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 48 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 49 <div class="section"> 50 <h2><a name="example" id="example">A Simple Example</a></h2> 51 52 53 <div class="example"><p><code> 54 <VirtualHost www.abc.dom> <br /> 55 ServerAdmin webgirl@abc.dom <br /> 56 DocumentRoot /www/abc <br /> 57 </VirtualHost> 58 </code></p></div> 59 60 <p>In order for Apache to function properly, it absolutely needs 61 to have two pieces of information about each virtual host: the 62 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one 63 IP address that the server will bind and respond to. The above 64 example does not include the IP address, so Apache must use DNS 65 to find the address of <code>www.abc.dom</code>. If for some 66 reason DNS is not available at the time your server is parsing 67 its config file, then this virtual host <strong>will not be 68 configured</strong>. It won't be able to respond to any hits 69 to this virtual host (prior to Apache version 1.2 the server 70 would not even boot).</p> 71 72 <p>Suppose that <code>www.abc.dom</code> has address 192.0.2.1. 73 Then consider this configuration snippet:</p> 74 75 <div class="example"><p><code> 76 <VirtualHost 192.0.2.1> <br /> 77 ServerAdmin webgirl@abc.dom <br /> 78 DocumentRoot /www/abc <br /> 79 </VirtualHost> 80 </code></p></div> 81 82 <p>This time Apache needs to use reverse DNS to find the 83 <code>ServerName</code> for this virtualhost. If that reverse 84 lookup fails then it will partially disable the virtualhost 85 (prior to Apache version 1.2 the server would not even boot). 86 If the virtual host is name-based then it will effectively be 87 totally disabled, but if it is IP-based then it will mostly 88 work. However, if Apache should ever have to generate a full 89 URL for the server which includes the server name, then it will 90 fail to generate a valid URL.</p> 91 92 <p>Here is a snippet that avoids both of these problems:</p> 93 94 <div class="example"><p><code> 95 <VirtualHost 192.0.2.1> <br /> 96 ServerName www.abc.dom <br /> 97 ServerAdmin webgirl@abc.dom <br /> 98 DocumentRoot /www/abc <br /> 99 </VirtualHost> 100 </code></p></div> 101 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 102 <div class="section"> 103 <h2><a name="denial" id="denial">Denial of Service</a></h2> 104 105 106 <p>There are (at least) two forms that denial of service 107 can come in. If you are running a version of Apache prior to 108 version 1.2 then your server will not even boot if one of the 109 two DNS lookups mentioned above fails for any of your virtual 110 hosts. In some cases this DNS lookup may not even be under your 111 control; for example, if <code>abc.dom</code> is one of your 112 customers and they control their own DNS, they can force your 113 (pre-1.2) server to fail while booting simply by deleting the 114 <code>www.abc.dom</code> record.</p> 115 116 <p>Another form is far more insidious. Consider this 117 configuration snippet:</p> 118 119 <div class="example"><p><code> 120 <VirtualHost www.abc.dom><br /> 121 <span class="indent"> 122 ServerAdmin webgirl@abc.dom<br /> 123 DocumentRoot /www/abc<br /> 124 </span> 125 </VirtualHost><br /> 126 <br /> 127 <VirtualHost www.def.dom><br /> 128 <span class="indent"> 129 ServerAdmin webguy@def.dom<br /> 130 DocumentRoot /www/def<br /> 131 </span> 132 </VirtualHost> 133 </code></p></div> 134 135 <p>Suppose that you've assigned 192.0.2.1 to 136 <code>www.abc.dom</code> and 192.0.2.2 to 137 <code>www.def.dom</code>. Furthermore, suppose that 138 <code>abc.dom</code> has control of their own DNS. With this 139 config you have put <code>abc.dom</code> into a position where 140 they can steal all traffic destined to <code>def.dom</code>. To 141 do so, all they have to do is set <code>www.abc.dom</code> to 142 192.0.2.2. Since they control their own DNS you can't stop them 143 from pointing the <code>www.abc.dom</code> record wherever they 144 wish.</p> 145 146 <p>Requests coming in to 192.0.2.2 (including all those where 147 users typed in URLs of the form 148 <code>http://www.def.dom/whatever</code>) will all be served by 149 the <code>abc.dom</code> virtual host. To better understand why 150 this happens requires a more in-depth discussion of how Apache 151 matches up incoming requests with the virtual host that will 152 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p> 153 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 154 <div class="section"> 155 <h2><a name="main" id="main">The "main server" Address</a></h2> 156 157 158 <p>The addition of <a href="vhosts/name-based.html">name-based 159 virtual host support</a> in Apache 1.1 requires Apache to know 160 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code> 161 is running on. To get this address it uses either the global 162 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> 163 (if present) or calls the C function <code>gethostname</code> 164 (which should return the same as typing "hostname" at the 165 command prompt). Then it performs a DNS lookup on this address. 166 At present there is no way to avoid this lookup.</p> 167 168 <p>If you fear that this lookup might fail because your DNS 169 server is down then you can insert the hostname in 170 <code>/etc/hosts</code> (where you probably already have it so 171 that the machine can boot properly). Then ensure that your 172 machine is configured to use <code>/etc/hosts</code> in the 173 event that DNS fails. Depending on what OS you are using this 174 might be accomplished by editing <code>/etc/resolv.conf</code>, 175 or maybe <code>/etc/nsswitch.conf</code>.</p> 176 177 <p>If your server doesn't have to perform DNS for any other 178 reason then you might be able to get away with running Apache 179 with the <code>HOSTRESORDER</code> environment variable set to 180 "local". This all depends on what OS and resolver libraries you 181 are using. It also affects CGIs unless you use 182 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best 183 to consult the man pages or FAQs for your OS.</p> 184 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 185 <div class="section"> 186 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2> 187 188 189 <ul> 190 <li> 191 use IP addresses in 192 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code> 193 </li> 194 195 <li> 196 use IP addresses in 197 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code> 198 </li> 199 200 <li> 201 ensure all virtual hosts have an explicit 202 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> 203 </li> 204 205 <li>create a <code><VirtualHost _default_:*></code> 206 server that has no pages to serve</li> 207 </ul> 208 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> 209 <div class="section"> 210 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2> 211 212 213 <p>The situation regarding DNS is highly undesirable. For 214 Apache 1.2 we've attempted to make the server at least continue 215 booting in the event of failed DNS, but it might not be the 216 best we can do. In any event, requiring the use of explicit IP 217 addresses in configuration files is highly undesirable in 218 today's Internet where renumbering is a necessity.</p> 219 220 <p>A possible work around to the theft of service attack 221 described above would be to perform a reverse DNS lookup on the 222 IP address returned by the forward lookup and compare the two 223 names -- in the event of a mismatch, the virtualhost would be 224 disabled. This would require reverse DNS to be configured 225 properly (which is something that most admins are familiar with 226 because of the common use of "double-reverse" DNS lookups by 227 FTP servers and TCP wrappers).</p> 228 229 <p>In any event, it doesn't seem possible to reliably boot a 230 virtual-hosted web server when DNS has failed unless IP 231 addresses are used. Partial solutions such as disabling 232 portions of the configuration might be worse than not booting 233 at all depending on what the webserver is supposed to 234 accomplish.</p> 235 236 <p>As HTTP/1.1 is deployed and browsers and proxies start 237 issuing the <code>Host</code> header it will become possible to 238 avoid the use of IP-based virtual hosts entirely. In this case, 239 a webserver has no requirement to do DNS lookups during 240 configuration. But as of March 1997 these features have not 241 been deployed widely enough to be put into use on critical 242 webservers.</p> 243 </div></div> 244 <div class="bottomlang"> 245 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> | 246 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | 247 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | 248 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> 249 </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. 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