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| <title>Issues Regarding DNS and Apache - Apache HTTP Server</title> |
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| <p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p> |
| <p class="apache">Apache HTTP Server Version 2.0</p> |
| <img alt="" src="./images/feather.gif" /></div> |
| <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div> |
| <div id="path"> |
| <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 2.0</a></div><div id="page-content"><div class="retired"><h4>Please note</h4> |
| <p>This document refers to the <strong>2.0</strong> version of Apache httpd, which <strong>is no longer maintained</strong>. Upgrade, and refer to the current version of httpd instead, documented at:</p> |
| <ul><li><a href="http://httpd.apache.org/docs/current/">Current release version of Apache HTTP Server documentation</a></li></ul><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> |
| <div class="toplang"> |
| <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> | |
| <a href="./es/dns-caveats.html" hreflang="es" rel="alternate" title="Español"> es </a> | |
| <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | |
| <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | |
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| <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> |
| </div> |
| |
| <p>This page could be summarized with the statement: don't |
| configure Apache in such a way that it relies on DNS resolution |
| for parsing of the configuration files. If Apache requires DNS |
| resolution to parse the configuration files then your server |
| may be subject to reliability problems (ie. it might not boot), |
| or denial and theft of service attacks (including users able |
| to steal hits from other users).</p> |
| </div> |
| <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li> |
| <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li> |
| <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li> |
| <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li> |
| <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li> |
| </ul></div> |
| <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="example" id="example">A Simple Example</a></h2> |
| |
| |
| <div class="example"><p><code> |
| <VirtualHost www.abc.dom> <br /> |
| ServerAdmin webgirl@abc.dom <br /> |
| DocumentRoot /www/abc <br /> |
| </VirtualHost> |
| </code></p></div> |
| |
| <p>In order for Apache to function properly, it absolutely needs |
| to have two pieces of information about each virtual host: the |
| <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one |
| IP address that the server will bind and respond to. The above |
| example does not include the IP address, so Apache must use DNS |
| to find the address of <code>www.abc.dom</code>. If for some |
| reason DNS is not available at the time your server is parsing |
| its config file, then this virtual host <strong>will not be |
| configured</strong>. It won't be able to respond to any hits |
| to this virtual host (prior to Apache version 1.2 the server |
| would not even boot).</p> |
| |
| <p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1. |
| Then consider this configuration snippet:</p> |
| |
| <div class="example"><p><code> |
| <VirtualHost 10.0.0.1> <br /> |
| ServerAdmin webgirl@abc.dom <br /> |
| DocumentRoot /www/abc <br /> |
| </VirtualHost> |
| </code></p></div> |
| |
| <p>This time Apache needs to use reverse DNS to find the |
| <code>ServerName</code> for this virtualhost. If that reverse |
| lookup fails then it will partially disable the virtualhost |
| (prior to Apache version 1.2 the server would not even boot). |
| If the virtual host is name-based then it will effectively be |
| totally disabled, but if it is IP-based then it will mostly |
| work. However, if Apache should ever have to generate a full |
| URL for the server which includes the server name, then it will |
| fail to generate a valid URL.</p> |
| |
| <p>Here is a snippet that avoids both of these problems:</p> |
| |
| <div class="example"><p><code> |
| <VirtualHost 10.0.0.1> <br /> |
| ServerName www.abc.dom <br /> |
| ServerAdmin webgirl@abc.dom <br /> |
| DocumentRoot /www/abc <br /> |
| </VirtualHost> |
| </code></p></div> |
| </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="denial" id="denial">Denial of Service</a></h2> |
| |
| |
| <p>There are (at least) two forms that denial of service |
| can come in. If you are running a version of Apache prior to |
| version 1.2 then your server will not even boot if one of the |
| two DNS lookups mentioned above fails for any of your virtual |
| hosts. In some cases this DNS lookup may not even be under your |
| control; for example, if <code>abc.dom</code> is one of your |
| customers and they control their own DNS, they can force your |
| (pre-1.2) server to fail while booting simply by deleting the |
| <code>www.abc.dom</code> record.</p> |
| |
| <p>Another form is far more insidious. Consider this |
| configuration snippet:</p> |
| |
| <div class="example"><p><code> |
| <VirtualHost www.abc.dom> <br /> |
| ServerAdmin webgirl@abc.dom <br /> |
| DocumentRoot /www/abc <br /> |
| </VirtualHost> <br /> |
| <br /> |
| <VirtualHost www.def.dom> <br /> |
| ServerAdmin webguy@def.dom <br /> |
| DocumentRoot /www/def <br /> |
| </VirtualHost> |
| </code></p></div> |
| |
| <p>Suppose that you've assigned 10.0.0.1 to |
| <code>www.abc.dom</code> and 10.0.0.2 to |
| <code>www.def.dom</code>. Furthermore, suppose that |
| <code>def.dom</code> has control of their own DNS. With this |
| config you have put <code>def.dom</code> into a position where |
| they can steal all traffic destined to <code>abc.dom</code>. To |
| do so, all they have to do is set <code>www.def.dom</code> to |
| 10.0.0.1. Since they control their own DNS you can't stop them |
| from pointing the <code>www.def.dom</code> record wherever they |
| wish.</p> |
| |
| <p>Requests coming in to 10.0.0.1 (including all those where |
| users typed in URLs of the form |
| <code>http://www.abc.dom/whatever</code>) will all be served by |
| the <code>def.dom</code> virtual host. To better understand why |
| this happens requires a more in-depth discussion of how Apache |
| matches up incoming requests with the virtual host that will |
| serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p> |
| </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="main" id="main">The "main server" Address</a></h2> |
| |
| |
| <p>The addition of <a href="vhosts/name-based.html">name-based |
| virtual host support</a> in Apache 1.1 requires Apache to know |
| the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code> is running |
| on. To get this address it uses either the global |
| <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> |
| (if present) or calls the C function <code>gethostname</code> |
| (which should return the same as typing "hostname" at the |
| command prompt). Then it performs a DNS lookup on this address. |
| At present there is no way to avoid this lookup.</p> |
| |
| <p>If you fear that this lookup might fail because your DNS |
| server is down then you can insert the hostname in |
| <code>/etc/hosts</code> (where you probably already have it so |
| that the machine can boot properly). Then ensure that your |
| machine is configured to use <code>/etc/hosts</code> in the |
| event that DNS fails. Depending on what OS you are using this |
| might be accomplished by editing <code>/etc/resolv.conf</code>, |
| or maybe <code>/etc/nsswitch.conf</code>.</p> |
| |
| <p>If your server doesn't have to perform DNS for any other |
| reason then you might be able to get away with running Apache |
| with the <code>HOSTRESORDER</code> environment variable set to |
| "local". This all depends on what OS and resolver libraries you |
| are using. It also affects CGIs unless you use |
| <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best |
| to consult the man pages or FAQs for your OS.</p> |
| </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2> |
| |
| |
| <ul> |
| <li> |
| use IP addresses in |
| <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code> |
| </li> |
| |
| <li> |
| use IP addresses in |
| <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code> |
| </li> |
| |
| <li> |
| ensure all virtual hosts have an explicit |
| <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> |
| </li> |
| |
| <li>create a <code><VirtualHost _default_:*></code> |
| server that has no pages to serve</li> |
| </ul> |
| </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div> |
| <div class="section"> |
| <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2> |
| |
| |
| <p>The situation regarding DNS is highly undesirable. For |
| Apache 1.2 we've attempted to make the server at least continue |
| booting in the event of failed DNS, but it might not be the |
| best we can do. In any event, requiring the use of explicit IP |
| addresses in configuration files is highly undesirable in |
| today's Internet where renumbering is a necessity.</p> |
| |
| <p>A possible work around to the theft of service attack |
| described above would be to perform a reverse DNS lookup on the |
| IP address returned by the forward lookup and compare the two |
| names -- in the event of a mismatch, the virtualhost would be |
| disabled. This would require reverse DNS to be configured |
| properly (which is something that most admins are familiar with |
| because of the common use of "double-reverse" DNS lookups by |
| FTP servers and TCP wrappers).</p> |
| |
| <p>In any event, it doesn't seem possible to reliably boot a |
| virtual-hosted web server when DNS has failed unless IP |
| addresses are used. Partial solutions such as disabling |
| portions of the configuration might be worse than not booting |
| at all depending on what the webserver is supposed to |
| accomplish.</p> |
| |
| <p>As HTTP/1.1 is deployed and browsers and proxies start |
| issuing the <code>Host</code> header it will become possible to |
| avoid the use of IP-based virtual hosts entirely. In this case, |
| a webserver has no requirement to do DNS lookups during |
| configuration. But as of March 1997 these features have not |
| been deployed widely enough to be put into use on critical |
| webservers.</p> |
| </div></div> |
| <div class="bottomlang"> |
| <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> | |
| <a href="./es/dns-caveats.html" hreflang="es" rel="alternate" title="Español"> es </a> | |
| <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> | |
| <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> | |
| <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> | |
| <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p> |
| </div><div id="footer"> |
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