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in particular, the non-english files, to make sure I did not screw anything up. They look fine to me. git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91112 13f79535-47bb-0310-9956-ffa450edef68
218 lines
8.6 KiB
HTML
218 lines
8.6 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Issues Regarding DNS and Apache</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<!--#include virtual="header.html" -->
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<h1 align="CENTER">Issues Regarding DNS and Apache</h1>
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<p>This page could be summarized with the statement: <em>don't
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require Apache to use DNS for any parsing of the configuration
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files</em>. If Apache has to use DNS to parse the configuration
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files then your server may be subject to reliability problems
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(it might not boot), or denial and theft of service attacks
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(including users able to steal hits from other users).</p>
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<h3>A Simple Example</h3>
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Consider this configuration snippet:
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<blockquote>
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<pre>
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<VirtualHost www.abc.dom>
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ServerAdmin webgirl@abc.dom
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DocumentRoot /www/abc
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</VirtualHost>
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</pre>
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</blockquote>
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<p>In order for Apache to function properly it absolutely needs
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to have two pieces of information about each virtual host: the
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<a href="mod/core.html#servername"><code>ServerName</code></a>
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and at least one IP address that the server responds to. This
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example does not include the IP address, so Apache must use DNS
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to find the address of <code>www.abc.dom</code>. If for some
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reason DNS is not available at the time your server is parsing
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its config file, then this virtual host <strong>will not be
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configured</strong>. It won't be able to respond to any hits to
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this virtual host (prior to Apache version 1.2 the server would
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not even boot).</p>
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<p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.
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Then consider this configuration snippet:</p>
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<blockquote>
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<pre>
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<VirtualHost 10.0.0.1>
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ServerAdmin webgirl@abc.dom
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DocumentRoot /www/abc
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</VirtualHost>
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</pre>
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</blockquote>
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<p>Now Apache needs to use reverse DNS to find the
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<code>ServerName</code> for this virtualhost. If that reverse
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lookup fails then it will partially disable the virtualhost
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(prior to Apache version 1.2 the server would not even boot).
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If the virtual host is name-based then it will effectively be
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totally disabled, but if it is IP-based then it will mostly
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work. However if Apache should ever have to generate a full URL
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for the server which includes the server name then it will fail
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to generate a valid URL.</p>
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<p>Here is a snippet that avoids both of these problems.</p>
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<blockquote>
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<pre>
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<VirtualHost 10.0.0.1>
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ServerName www.abc.dom
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ServerAdmin webgirl@abc.dom
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DocumentRoot /www/abc
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</VirtualHost>
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</pre>
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</blockquote>
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<h3>Denial of Service</h3>
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<p>There are (at least) two forms that denial of service can
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come in. If you are running a version of Apache prior to
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version 1.2 then your server will not even boot if one of the
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two DNS lookups mentioned above fails for any of your virtual
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hosts. In some cases this DNS lookup may not even be under your
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control. For example, if <code>abc.dom</code> is one of your
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customers and they control their own DNS then they can force
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your (pre-1.2) server to fail while booting simply by deleting
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the <code>www.abc.dom</code> record.</p>
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<p>Another form is far more insidious. Consider this
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configuration snippet:</p>
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<blockquote>
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<pre>
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<VirtualHost www.abc.dom>
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ServerAdmin webgirl@abc.dom
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DocumentRoot /www/abc
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</VirtualHost>
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</pre>
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</blockquote>
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<blockquote>
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<pre>
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<VirtualHost www.def.dom>
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ServerAdmin webguy@def.dom
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DocumentRoot /www/def
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</VirtualHost>
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</pre>
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</blockquote>
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<p>Suppose that you've assigned 10.0.0.1 to
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<code>www.abc.dom</code> and 10.0.0.2 to
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<code>www.def.dom</code>. Furthermore, suppose that
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<code>def.com</code> has control of their own DNS. With this
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config you have put <code>def.com</code> into a position where
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they can steal all traffic destined to <code>abc.com</code>. To
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do so, all they have to do is set <code>www.def.dom</code> to
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10.0.0.1. Since they control their own DNS you can't stop them
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from pointing the <code>www.def.com</code> record wherever they
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wish.</p>
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<p>Requests coming in to 10.0.0.1 (including all those where
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users typed in URLs of the form
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<code>http://www.abc.dom/whatever</code>) will all be served by
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the <code>def.com</code> virtual host. To better understand why
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this happens requires a more in-depth discussion of how Apache
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matches up incoming requests with the virtual host that will
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serve it. A rough document describing this <a
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href="vhosts/details.html">is available</a>.</p>
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<h3>The "main server" Address</h3>
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<p>The addition of <a href="vhosts/name-based.html">name-based
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virtual host support</a> in Apache 1.1 requires Apache to know
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the IP address(es) of the host that httpd is running on. To get
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this address it uses either the global <code>ServerName</code>
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(if present) or calls the C function <code>gethostname</code>
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(which should return the same as typing "hostname" at the
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command prompt). Then it performs a DNS lookup on this address.
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At present there is no way to avoid this lookup.</p>
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<p>If you fear that this lookup might fail because your DNS
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server is down then you can insert the hostname in
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<code>/etc/hosts</code> (where you probably already have it so
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that the machine can boot properly). Then ensure that your
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machine is configured to use <code>/etc/hosts</code> in the
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event that DNS fails. Depending on what OS you are using this
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might be accomplished by editing <code>/etc/resolv.conf</code>,
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or maybe <code>/etc/nsswitch.conf</code>.</p>
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<p>If your server doesn't have to perform DNS for any other
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reason then you might be able to get away with running Apache
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with the <code>HOSTRESORDER</code> environment variable set to
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"local". This all depends on what OS and resolver libraries you
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are using. It also affects CGIs unless you use <a
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href="mod/mod_env.html"><code>mod_env</code></a> to control the
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environment. It's best to consult the man pages or FAQs for
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your OS.</p>
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<h3><a id="tips" name="tips">Tips to Avoid these
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problems</a></h3>
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<ul>
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<li>use IP addresses in <code><VirtualHost></code></li>
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<li>use IP addresses in <code>Listen</code></li>
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<li>use IP addresses in <code>BindAddress</code></li>
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<li>ensure all virtual hosts have an explicit
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<code>ServerName</code></li>
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<li>create a <code><VirtualHost _default_:*></code>
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server that has no pages to serve</li>
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</ul>
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<h3>Appendix: Future Directions</h3>
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<p>The situation regarding DNS is highly undesirable. For
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Apache 1.2 we've attempted to make the server at least continue
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booting in the event of failed DNS, but it might not be the
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best we can do. In any event requiring the use of explicit IP
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addresses in configuration files is highly undesirable in
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today's Internet where renumbering is a necessity.</p>
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<p>A possible work around to the theft of service attack
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described above would be to perform a reverse DNS lookup on the
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ip address returned by the forward lookup and compare the two
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names. In the event of a mismatch the virtualhost would be
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disabled. This would require reverse DNS to be configured
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properly (which is something that most admins are familiar with
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because of the common use of "double-reverse" DNS lookups by
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FTP servers and TCP wrappers).</p>
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<p>In any event it doesn't seem possible to reliably boot a
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virtual-hosted web server when DNS has failed unless IP
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addresses are used. Partial solutions such as disabling
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portions of the configuration might be worse than not booting
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at all depending on what the webserver is supposed to
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accomplish.</p>
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<p>As HTTP/1.1 is deployed and browsers and proxies start
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issuing the <code>Host</code> header it will become possible to
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avoid the use of IP-based virtual hosts entirely. In this event
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a webserver has no requirement to do DNS lookups during
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configuration. But as of March 1997 these features have not
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been deployed widely enough to be put into use on critical
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webservers. <!--#include virtual="footer.html" -->
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</p>
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</body>
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</html>
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