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| author |   hongbotian <hongbo.tianhongbo@huawei.com> | 2015-11-30 01:45:08 -0500 |
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| committer | hongbotian <hongbo.tianhongbo@huawei.com> | 2015-11-30 01:45:08 -0500 |
| commit | e8ec7aa8e38a93f5b034ac74cebce5de23710317 (patch) | |
| tree | aa031937bf856c1f8d6ad7877b8d2cb0224da5ef /rubbos/app/httpd-2.0.64/docs/manual/dns-caveats.html.en | |
| parent | cc40af334e619bb549038238507407866f774f8f (diff) | |
upload http
JIRA: BOTTLENECK-10
Change-Id: I7598427ff904df438ce77c2819ee48ac75ffa8da
Signed-off-by: hongbotian <hongbo.tianhongbo@huawei.com>
Diffstat (limited to 'rubbos/app/httpd-2.0.64/docs/manual/dns-caveats.html.en')
| -rw-r--r-- | rubbos/app/httpd-2.0.64/docs/manual/dns-caveats.html.en | 245 |
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diff --git a/rubbos/app/httpd-2.0.64/docs/manual/dns-caveats.html.en b/rubbos/app/httpd-2.0.64/docs/manual/dns-caveats.html.en new file mode 100644 index 00000000..12e87973 --- /dev/null +++ b/rubbos/app/httpd-2.0.64/docs/manual/dns-caveats.html.en @@ -0,0 +1,245 @@ +<?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="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> +<link href="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" /> +<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="<-" alt="<-" src="./images/left.gif" /></a></div> +<div id="path"> +<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 2.0</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache</h1> +<div class="toplang"> +<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> | +<a href="./es/dns-caveats.html" hreflang="es" rel="alternate" title="Español"> es </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 + 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> + </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> + <VirtualHost www.abc.dom> <br /> + ServerAdmin webgirl@abc.dom <br /> + DocumentRoot /www/abc <br /> + </VirtualHost> + </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 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 + 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> + <VirtualHost 10.0.0.1> <br /> + ServerAdmin webgirl@abc.dom <br /> + DocumentRoot /www/abc <br /> + </VirtualHost> + </code></p></div> + + <p>This time 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> + <VirtualHost 10.0.0.1> <br /> + ServerName www.abc.dom <br /> + ServerAdmin webgirl@abc.dom <br /> + DocumentRoot /www/abc <br /> + </VirtualHost> + </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, 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 /> + ServerAdmin webgirl@abc.dom <br /> + DocumentRoot /www/abc <br /> + </VirtualHost> <br /> + <br /> + <VirtualHost www.def.dom> <br /> + ServerAdmin webguy@def.dom <br /> + DocumentRoot /www/def <br /> + </VirtualHost> + </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.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 + 10.0.0.1. 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 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.dom</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 <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> + (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><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> +<div class="bottomlang"> +<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> | +<a href="./es/dns-caveats.html" hreflang="es" rel="alternate" title="Español"> es </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><div id="footer"> +<p class="apache">Copyright 2009 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</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>
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