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-<title>Issues Regarding DNS and Apache - Apache HTTP Server</title>
+<title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server</title>
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-<p class="apache">Apache HTTP Server Version 2.3</p>
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+<p class="apache">Apache HTTP Server Version 2.5</p>
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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 2.3</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache</h1>
+<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 2.5</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache HTTP Server</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </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
+ configure Apache HTTP Server in such a way that it relies on DNS resolution
+ for parsing of the configuration files. If httpd 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>
+ may be subject to reliability problems (ie. it might not start up),
+ or denial and theft of service attacks (including virtual hosts able
+ to steal hits from other virtual hosts).</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>
+</ul><ul class="seealso"><li><a href="#comments_section">Comments</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>
+ <pre class="prettyprint lang-config">
+# This is a misconfiguration example, do not use on your server
+<VirtualHost www.example.dom>
+ ServerAdmin webgirl@example.dom
+ DocumentRoot /www/example
+</VirtualHost>
+ </pre>
+
- <p>In order for Apache to function properly, it absolutely needs
+ <p>In order for the server 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
+ example does not include the IP address, so httpd must use DNS
+ to find the address of <code>www.example.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>
+ to this virtual host.</p>
- <p>Suppose that <code>www.abc.dom</code> has address 192.0.2.1.
+ <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
Then consider this configuration snippet:</p>
- <div class="example"><p><code>
- <VirtualHost 192.0.2.1> <br />
- ServerAdmin webgirl@abc.dom <br />
- DocumentRoot /www/abc <br />
- </VirtualHost>
- </code></p></div>
+ <pre class="prettyprint lang-config">
+# This is a misconfiguration example, do not use on your server
+<VirtualHost 192.0.2.1>
+ ServerAdmin webgirl@example.dom
+ DocumentRoot /www/example
+</VirtualHost>
+ </pre>
+
- <p>This time Apache needs to use reverse DNS to find the
+ <p>This time httpd 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).
+ lookup fails then it will partially disable the virtualhost.
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>
+ work. However, if httpd should ever have to generate a full
+ URL for the server which includes the server name (such as when a
+ Redirect is issued), 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 192.0.2.1> <br />
- ServerName www.abc.dom <br />
- ServerAdmin webgirl@abc.dom <br />
- DocumentRoot /www/abc <br />
- </VirtualHost>
- </code></p></div>
+ <pre class="prettyprint lang-config">
+<VirtualHost 192.0.2.1>
+ ServerName www.example.dom
+ ServerAdmin webgirl@example.dom
+ DocumentRoot /www/example
+</VirtualHost>
+ </pre>
+
</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 />
- <span class="indent">
- ServerAdmin webgirl@abc.dom<br />
- DocumentRoot /www/abc<br />
- </span>
- </VirtualHost><br />
- <br />
- <VirtualHost www.def.dom><br />
- <span class="indent">
- ServerAdmin webguy@def.dom<br />
- DocumentRoot /www/def<br />
- </span>
- </VirtualHost>
- </code></p></div>
+ <p>Consider this configuration snippet:</p>
+
+ <pre class="prettyprint lang-config">
+<VirtualHost www.example1.dom>
+ ServerAdmin webgirl@example1.dom
+ DocumentRoot /www/example1
+</VirtualHost>
+<VirtualHost www.example2.dom>
+ ServerAdmin webguy@example2.dom
+ DocumentRoot /www/example2
+</VirtualHost>
+ </pre>
+
<p>Suppose that you've assigned 192.0.2.1 to
- <code>www.abc.dom</code> and 192.0.2.2 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
- 192.0.2.1. Since they control their own DNS you can't stop them
- from pointing the <code>www.def.dom</code> record wherever they
+ <code>www.example1.dom</code> and 192.0.2.2 to
+ <code>www.example2.dom</code>. Furthermore, suppose that
+ <code>example1.dom</code> has control of their own DNS. With this
+ config you have put <code>example1.dom</code> into a position where
+ they can steal all traffic destined to <code>example2.dom</code>. To
+ do so, all they have to do is set <code>www.example1.dom</code> to
+ 192.0.2.2. Since they control their own DNS you can't stop them
+ from pointing the <code>www.example1.dom</code> record wherever they
wish.</p>
- <p>Requests coming in to 192.0.2.1 (including all those where
+ <p>Requests coming in to 192.0.2.2 (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
+ <code>http://www.example2.dom/whatever</code>) will all be served by
+ the <code>example1.dom</code> virtual host. To better understand why
+ this happens requires a more in-depth discussion of how httpd
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>
<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
+ <p><a href="vhosts/name-based.html">Name-based
+ virtual host support</a> requires httpd 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>
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
+ reason then you might be able to get away with running httpd
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
+ 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">
<ul>
<li>
- use IP addresses in
+ use IP addresses in
<code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
</li>
<li>
- use IP addresses in
+ use IP addresses in
<code class="directive"><a href="./mod/mpm_common.html#listen">Listen</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>
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<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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+</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. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div>
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