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-<title>Issues Regarding DNS and Apache - Apache HTTP Server</title>
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-<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>
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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 2.0</a></div><div id="page-content"><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> |
-<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>
-
- <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">
-<p class="apache">Copyright 2009 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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