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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>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<img alt="" src="./images/feather.gif" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
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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 2.2</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache</h1>
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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="./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></p>
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<p>This page could be summarized with the statement: don't
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configure Apache in such a way that it relies on DNS resolution
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for parsing of the configuration files. If Apache 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 boot),
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or denial and theft of service attacks (including users able
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to steal hits from other users).</p>
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<div id="quickview"><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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<li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
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<h2><a name="example" id="example">A Simple Example</a></h2>
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<div class="example"><p><code>
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<VirtualHost www.abc.dom> <br />
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ServerAdmin webgirl@abc.dom <br />
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DocumentRoot /www/abc <br />
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<p>In order for Apache 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 Apache must use DNS
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to find the address of <code>www.abc.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 (prior to Apache version 1.2 the server
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would not even boot).</p>
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<p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.
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Then consider this configuration snippet:</p>
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<div class="example"><p><code>
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<VirtualHost 10.0.0.1> <br />
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ServerAdmin webgirl@abc.dom <br />
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DocumentRoot /www/abc <br />
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<p>This time Apache 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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(prior to Apache version 1.2 the server would not even boot).
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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 Apache should ever have to generate a full
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URL for the server which includes the server name, then it will
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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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<div class="example"><p><code>
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<VirtualHost 10.0.0.1> <br />
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ServerName www.abc.dom <br />
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ServerAdmin webgirl@abc.dom <br />
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DocumentRoot /www/abc <br />
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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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<h2><a name="denial" id="denial">Denial of Service</a></h2>
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<p>There are (at least) two forms that denial of service
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can come in. If you are running a version of Apache prior to
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version 1.2 then your server will not even boot if one of the
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two DNS lookups mentioned above fails for any of your virtual
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hosts. In some cases this DNS lookup may not even be under your
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control; for example, if <code>abc.dom</code> is one of your
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customers and they control their own DNS, they can force your
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(pre-1.2) server to fail while booting simply by deleting the
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<code>www.abc.dom</code> record.</p>
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<p>Another form is far more insidious. Consider this
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configuration snippet:</p>
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<div class="example"><p><code>
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<VirtualHost www.abc.dom> <br />
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ServerAdmin webgirl@abc.dom <br />
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DocumentRoot /www/abc <br />
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</VirtualHost> <br />
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<VirtualHost www.def.dom> <br />
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ServerAdmin webguy@def.dom <br />
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DocumentRoot /www/def <br />
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<p>Suppose that you've assigned 10.0.0.1 to
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<code>www.abc.dom</code> and 10.0.0.2 to
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<code>www.def.dom</code>. Furthermore, suppose that
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<code>def.dom</code> has control of their own DNS. With this
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config you have put <code>def.dom</code> into a position where
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they can steal all traffic destined to <code>abc.dom</code>. To
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do so, all they have to do is set <code>www.def.dom</code> to
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10.0.0.1. Since they control their own DNS you can't stop them
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from pointing the <code>www.def.dom</code> record wherever they
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<p>Requests coming in to 10.0.0.1 (including all those where
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users typed in URLs of the form
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<code>http://www.abc.dom/whatever</code>) will all be served by
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the <code>def.dom</code> virtual host. To better understand why
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this happens requires a more in-depth discussion of how Apache
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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>The addition of <a href="vhosts/name-based.html">name-based
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virtual host support</a> in Apache 1.1 requires Apache 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 Apache
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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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<code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
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<code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
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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>create a <code><VirtualHost _default_:*></code>
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server that has no pages to serve</li>
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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="appendix" id="appendix">Appendix: Future Directions</a></h2>
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<p>The situation regarding DNS is highly undesirable. For
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Apache 1.2 we've attempted to make the server at least continue
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booting in the event of failed DNS, but it might not be the
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best we can do. In any event, requiring the use of explicit IP
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addresses in configuration files is highly undesirable in
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today's Internet where renumbering is a necessity.</p>
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<p>A possible work around to the theft of service attack
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described above would be to perform a reverse DNS lookup on the
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IP address returned by the forward lookup and compare the two
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names -- in the event of a mismatch, the virtualhost would be
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disabled. This would require reverse DNS to be configured
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properly (which is something that most admins are familiar with
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because of the common use of "double-reverse" DNS lookups by
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FTP servers and TCP wrappers).</p>
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<p>In any event, it doesn't seem possible to reliably boot a
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virtual-hosted web server when DNS has failed unless IP
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addresses are used. Partial solutions such as disabling
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portions of the configuration might be worse than not booting
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at all depending on what the webserver is supposed to
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<p>As HTTP/1.1 is deployed and browsers and proxies start
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issuing the <code>Host</code> header it will become possible to
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avoid the use of IP-based virtual hosts entirely. In this case,
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a webserver has no requirement to do DNS lookups during
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configuration. But as of March 1997 these features have not
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been deployed widely enough to be put into use on critical
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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="./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></p>
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</div><div id="footer">
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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