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  <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>