diff options
| author | David Shane Holden <dpejesh@apache.org> | 2002-09-06 05:27:07 +0000 |
|---|---|---|
| committer | David Shane Holden <dpejesh@apache.org> | 2002-09-06 05:27:07 +0000 |
| commit | 3ff18531eb9ec41d58d190a3955421847cfc6b57 (patch) | |
| tree | 6e31b595807793efde255d5305919b93af63ae98 /docs/manual/dns-caveats.html.en | |
| parent | cd3576cd5c6ccc456a5df963dbb611c133d5f1cf (diff) | |
| download | httpd-3ff18531eb9ec41d58d190a3955421847cfc6b57.tar.gz | |
New XML.
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96669 13f79535-47bb-0310-9956-ffa450edef68
Diffstat (limited to 'docs/manual/dns-caveats.html.en')
| -rw-r--r-- | docs/manual/dns-caveats.html.en | 194 |
1 files changed, 194 insertions, 0 deletions
diff --git a/docs/manual/dns-caveats.html.en b/docs/manual/dns-caveats.html.en new file mode 100644 index 0000000000..5a681794dc --- /dev/null +++ b/docs/manual/dns-caveats.html.en @@ -0,0 +1,194 @@ +<?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="<-" 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-project/">Documentation</a> > <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> + <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 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> + <VirtualHost 10.0.0.1> <br /> + ServerAdmin webgirl@abc.dom <br /> + DocumentRoot /www/abc <br /> + </VirtualHost> + </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> + <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 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> + <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.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><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 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>
\ No newline at end of file |