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[apache] / docs / manual / dns-caveats.xml

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<manualpage metafile="dns-caveats.xml.meta">

  <title>Issues Regarding DNS and Apache HTTP Server</title>

  <summary>
    <p>This page could be summarized with the statement: don't
    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>
  </summary>

  <section id="example">
    <title>A Simple Example</title>

    <example>
      # This is a misconfiguration example, do not use on your server <br />
      &lt;VirtualHost www.abc.dom&gt; <br />
      ServerAdmin webgirl@abc.dom <br />
      DocumentRoot /www/abc <br />
      &lt;/VirtualHost&gt;
    </example>

    <p>In order for the server 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 will bind and respond to. The above
    example does not include the IP address, so httpd 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 httpd version 1.2 the server
    would not even boot).</p>

    <p>Suppose that <code>www.abc.dom</code> has address 192.0.2.1.
    Then consider this configuration snippet:</p>

    <example>
      # This is a misconfiguration example, do not use on your server <br />
      &lt;VirtualHost 192.0.2.1&gt; <br />
      ServerAdmin webgirl@abc.dom <br />
      DocumentRoot /www/abc <br />
      &lt;/VirtualHost&gt;
    </example>

    <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.
    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 httpd 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 192.0.2.1&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 httpd 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>

    <example>
      &lt;VirtualHost www.abc.dom&gt;<br />
      <indent>
        ServerAdmin webgirl@abc.dom<br />
        DocumentRoot /www/abc<br />
      </indent>
      &lt;/VirtualHost&gt;<br />
      <br />
      &lt;VirtualHost www.def.dom&gt;<br />
      <indent>
        ServerAdmin webguy@def.dom<br />
        DocumentRoot /www/def<br />
      </indent>
      &lt;/VirtualHost&gt;
    </example>

    <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
    wish.</p>

    <p>Requests coming in to 192.0.2.1 (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 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>
  </section>

  <section id="main">
    <title>The "main server" Address</title>

    <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 <program>httpd</program>
    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 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
    <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. Although
    we've attempted to make the server at least continue
    booting in the event of failed DNS, 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>
  </section>
</manualpage>