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From dpej...@apache.org
Subject cvs commit: httpd-2.0/docs/manual dns-caveats.html.en dns-caveats.xml dns-caveats.html
Date Fri, 06 Sep 2002 05:27:07 GMT
dpejesh     2002/09/05 22:27:07

  Added:       docs/manual dns-caveats.html.en dns-caveats.xml
  Removed:     docs/manual dns-caveats.html
  Log:
  New XML.
  
  Revision  Changes    Path
  1.1                  httpd-2.0/docs/manual/dns-caveats.html.en
  
  Index: dns-caveats.html.en
  ===================================================================
  <?xml version="1.0" encoding="ISO-8859-1"?>
  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
                This file is generated from xml source: DO NOT EDIT
          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        --><title>Issues Regarding DNS and Apache - Apache HTTP Server</title><link
href="./style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet"
/><link href="./images/favicon.ico" rel="shortcut icon" /></head><body id="manual-page"><div
id="page-header"><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="&lt;-" alt="&lt;-"
src="./images/left.gif" /></a></div><div id="path"><a href="http://www.apache.org/">Apache</a>
&gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs-project/">Documentation</a>
&gt; <a href="./">Version 2.0</a></div><div id="page-content"><div
id="preamble"><h1>Issues Regarding DNS and Apache</h1>
      <p>This page could be summarized with the statement: don't require Apache 
      to use DNS for any parsing of the configuration files. If Apache has to 
      use DNS to parse the configuration files then your server may be subject 
      to reliability problems (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>
        &lt;VirtualHost www.abc.dom&gt; <br />
        ServerAdmin webgirl@abc.dom <br />
        DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt;
      </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 responds to. This
      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>
        &lt;VirtualHost 10.0.0.1&gt; <br />
        ServerAdmin webgirl@abc.dom <br />
        DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt;
      </code></p></div>
      
      <p>Now 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>
        &lt;VirtualHost 10.0.0.1&gt; <br />
        ServerName www.abc.dom <br />
        ServerAdmin webgirl@abc.dom <br />
        DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt;
      </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 then 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>
        &lt;VirtualHost www.abc.dom&gt; <br />
          ServerAdmin webgirl@abc.dom <br />
          DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt; <br />
        <br />
        &lt;VirtualHost www.def.dom&gt; <br />
          ServerAdmin webguy@def.dom <br />
          DocumentRoot /www/def <br />
        &lt;/VirtualHost&gt;
      </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.com</code> has control of their own DNS. With this
      config you have put <code>def.com</code> into a position where
      they can steal all traffic destined to <code>abc.com</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.com</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.com</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 httpd 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>&lt;VirtualHost _default_:*&gt;</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 event
      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 id="footer"><p class="apache">Maintained by the
<a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p><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></div></body></html>
  
  
  1.1                  httpd-2.0/docs/manual/dns-caveats.xml
  
  Index: dns-caveats.xml
  ===================================================================
  <?xml version="1.0" encoding="UTF-8" ?>
  <!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
  <?xml-stylesheet type="text/xsl" href="./style/manual.en.xsl"?>
  
  <manualpage>
    <relativepath href="." />
    
    <title>Issues Regarding DNS and Apache</title>
    
    <summary>
      <p>This page could be summarized with the statement: don't require Apache 
      to use DNS for any parsing of the configuration files. If Apache has to 
      use DNS to parse the configuration files then your server may be subject 
      to reliability problems (it might not boot), or denial and theft of 
      service attacks (including users able to steal hits from other users).</p>
    </summary>
    
    <section id="example">
      <title>A Simple Example</title>
      
      <example>
        &lt;VirtualHost www.abc.dom&gt; <br />
        ServerAdmin webgirl@abc.dom <br />
        DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt;
      </example>
      
      <p>In order for Apache to function properly it absolutely needs
      to have two pieces of information about each virtual host: the
      <directive module="core">ServerName</directive>
      and at least one IP address that the server responds to. This
      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>
      
      <example>
        &lt;VirtualHost 10.0.0.1&gt; <br />
        ServerAdmin webgirl@abc.dom <br />
        DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt;
      </example>
      
      <p>Now 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>
      
      <example>
        &lt;VirtualHost 10.0.0.1&gt; <br />
        ServerName www.abc.dom <br />
        ServerAdmin webgirl@abc.dom <br />
        DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt;
      </example>
    </section>
    
    <section id="denial">
      <title>Denial of Service</title>
      
      <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 then 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>
      
      <example>
        &lt;VirtualHost www.abc.dom&gt; <br />
        &nbsp;&nbsp;ServerAdmin webgirl@abc.dom <br />
        &nbsp;&nbsp;DocumentRoot /www/abc <br />
        &lt;/VirtualHost&gt; <br />
        <br />
        &lt;VirtualHost www.def.dom&gt; <br />
        &nbsp;&nbsp;ServerAdmin webguy@def.dom <br />
        &nbsp;&nbsp;DocumentRoot /www/def <br />
        &lt;/VirtualHost&gt;
      </example>
      
      <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.com</code> has control of their own DNS. With this
      config you have put <code>def.com</code> into a position where
      they can steal all traffic destined to <code>abc.com</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.com</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.com</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>
    </section>
    
    <section id="main">
      <title>The "main server" Address</title>
      
      <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 httpd is running on. To get
      this address it uses either the global 
      <directive module="core">ServerName</directive>
      (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 
      <module>mod_env</module> to control the environment. It's best 
      to consult the man pages or FAQs for your OS.</p>
    </section>
    
    <section id="tips">
      <title>Tips to Avoid These Problems</title>
      
      <ul>
        <li>
          use IP addresses in 
          <directive module="core">VirtualHost</directive>
        </li>
      
        <li>
          use IP addresses in 
          <directive module="mpm_common">Listen</directive>
        </li>
      
        <li>
          ensure all virtual hosts have an explicit
          <directive module="core">ServerName</directive>
        </li>
      
        <li>create a <code>&lt;VirtualHost _default_:*&gt;</code>
        server that has no pages to serve</li>
      </ul>
    </section>
    
    <section id="appendix">
      <title>Appendix: Future Directions</title>
      
      <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 event
      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>
    </section>
  </manualpage>
  
  

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