diff options
| author | Jason McIntyre <jmc@cvs.openbsd.org> | 2005-07-29 23:55:41 +0000 |
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| committer | Jason McIntyre <jmc@cvs.openbsd.org> | 2005-07-29 23:55:41 +0000 |
| commit | 4b50c27ec5ec786ab4022e61f94c0e58e969c883 (patch) | |
| tree | 5d9ad9804b385464c6c70e2f061f916929bc8ed5 /usr.sbin/httpd/htdocs/manual/dns-caveats.html | |
| parent | 4654fe49edf1731316a76df08f77c5ebe63b4b0b (diff) | |
from tamas tevesz:
==> de-multiviewification complete.
(rename *.html.html -> *.html)
Diffstat (limited to 'usr.sbin/httpd/htdocs/manual/dns-caveats.html')
| -rw-r--r-- | usr.sbin/httpd/htdocs/manual/dns-caveats.html | 231 |
1 files changed, 231 insertions, 0 deletions
diff --git a/usr.sbin/httpd/htdocs/manual/dns-caveats.html b/usr.sbin/httpd/htdocs/manual/dns-caveats.html new file mode 100644 index 00000000000..0d47f605fe0 --- /dev/null +++ b/usr.sbin/httpd/htdocs/manual/dns-caveats.html @@ -0,0 +1,231 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> + +<html xmlns="http://www.w3.org/1999/xhtml"> + <head> + <meta name="generator" content="HTML Tidy, see www.w3.org" /> + + <title>Issues Regarding DNS and Apache</title> + </head> + <!-- Background white, links blue (unvisited), navy (visited), red (active) --> + + <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" + vlink="#000080" alink="#FF0000"> + <div align="CENTER"> + <img src="images/sub.gif" alt="[APACHE DOCUMENTATION]" /> + + <h3>Apache HTTP Server</h3> + </div> + + + + <h1 align="CENTER">Issues Regarding DNS and Apache</h1> + + <p>This page could be summarized with the statement: <em>don't + require Apache to use DNS for any parsing of the configuration + files</em>. 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> + + <h3>A Simple Example</h3> + Consider this configuration snippet: + + <blockquote> +<pre> + <VirtualHost www.abc.dom> + ServerAdmin webgirl@abc.dom + DocumentRoot /www/abc + </VirtualHost> +</pre> + </blockquote> + + <p>In order for Apache to function properly it absolutely needs + to have two pieces of information about each virtual host: the + <a href="mod/core.html#servername"><code>ServerName</code></a> + 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> + + <blockquote> +<pre> + <VirtualHost 10.0.0.1> + ServerAdmin webgirl@abc.dom + DocumentRoot /www/abc + </VirtualHost> +</pre> + </blockquote> + + <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> + + <blockquote> +<pre> + <VirtualHost 10.0.0.1> + ServerName www.abc.dom + ServerAdmin webgirl@abc.dom + DocumentRoot /www/abc + </VirtualHost> +</pre> + </blockquote> + + <h3>Denial of Service</h3> + + <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> + + <blockquote> +<pre> + <VirtualHost www.abc.dom> + ServerAdmin webgirl@abc.dom + DocumentRoot /www/abc + </VirtualHost> +</pre> + </blockquote> + + <blockquote> +<pre> + <VirtualHost www.def.dom> + ServerAdmin webguy@def.dom + DocumentRoot /www/def + </VirtualHost> +</pre> + </blockquote> + + <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> + + <h3>The "main server" Address</h3> + + <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>ServerName</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 <a + href="mod/mod_env.html"><code>mod_env</code></a> to control the + environment. It's best to consult the man pages or FAQs for + your OS.</p> + + <h3><a id="tips" name="tips">Tips to Avoid these + problems</a></h3> + + <ul> + <li>use IP addresses in <code><VirtualHost></code></li> + + <li>use IP addresses in <code>Listen</code></li> + + <li>use IP addresses in <code>BindAddress</code></li> + + <li>ensure all virtual hosts have an explicit + <code>ServerName</code></li> + + <li>create a <code><VirtualHost _default_:*></code> + server that has no pages to serve</li> + </ul> + + <h3>Appendix: Future Directions</h3> + + <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. <hr /> + + <h3 align="CENTER">Apache HTTP Server</h3> + <a href="./"><img src="images/index.gif" alt="Index" /></a> + + </p> + </body> +</html> + + + |