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8 <title>Issues Regarding DNS and Apache - Apache HTTP Server</title>
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14 <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>
15 <p class="apache">Apache HTTP Server Version 2.1</p>
16 <img alt="" src="./images/feather.gif" /></div>
17 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
19 <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 2.1</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache</h1>
20 <p>This page could be summarized with the statement: don't require Apache
21 to use DNS for any parsing of the configuration files. If Apache has to
22 use DNS to parse the configuration files then your server may be subject
23 to reliability problems (it might not boot), or denial and theft of
24 service attacks (including users able to steal hits from other users).</p>
26 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
27 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
28 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
29 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
30 <li><img alt="" src="./images/down.gif" /> <a href="#appendix">Appendix: Future Directions</a></li>
32 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
34 <h2><a name="example" id="example">A Simple Example</a></h2>
37 <div class="example"><p><code>
38 <VirtualHost www.abc.dom> <br />
39 ServerAdmin webgirl@abc.dom <br />
40 DocumentRoot /www/abc <br />
44 <p>In order for Apache to function properly it absolutely needs
45 to have two pieces of information about each virtual host: the
46 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
47 and at least one IP address that the server responds to. This
48 example does not include the IP address, so Apache must use DNS
49 to find the address of <code>www.abc.dom</code>. If for some
50 reason DNS is not available at the time your server is parsing
51 its config file, then this virtual host <strong>will not be
52 configured</strong>. It won't be able to respond to any hits to
53 this virtual host (prior to Apache version 1.2 the server would
56 <p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.
57 Then consider this configuration snippet:</p>
59 <div class="example"><p><code>
60 <VirtualHost 10.0.0.1> <br />
61 ServerAdmin webgirl@abc.dom <br />
62 DocumentRoot /www/abc <br />
66 <p>Now Apache needs to use reverse DNS to find the
67 <code>ServerName</code> for this virtualhost. If that reverse
68 lookup fails then it will partially disable the virtualhost
69 (prior to Apache version 1.2 the server would not even boot).
70 If the virtual host is name-based then it will effectively be
71 totally disabled, but if it is IP-based then it will mostly
72 work. However if Apache should ever have to generate a full URL
73 for the server which includes the server name then it will fail
74 to generate a valid URL.</p>
76 <p>Here is a snippet that avoids both of these problems.</p>
78 <div class="example"><p><code>
79 <VirtualHost 10.0.0.1> <br />
80 ServerName www.abc.dom <br />
81 ServerAdmin webgirl@abc.dom <br />
82 DocumentRoot /www/abc <br />
85 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
87 <h2><a name="denial" id="denial">Denial of Service</a></h2>
90 <p>There are (at least) two forms that denial of service can
91 come in. If you are running a version of Apache prior to
92 version 1.2 then your server will not even boot if one of the
93 two DNS lookups mentioned above fails for any of your virtual
94 hosts. In some cases this DNS lookup may not even be under your
95 control. For example, if <code>abc.dom</code> is one of your
96 customers and they control their own DNS then they can force
97 your (pre-1.2) server to fail while booting simply by deleting
98 the <code>www.abc.dom</code> record.</p>
100 <p>Another form is far more insidious. Consider this
101 configuration snippet:</p>
103 <div class="example"><p><code>
104 <VirtualHost www.abc.dom> <br />
105 ServerAdmin webgirl@abc.dom <br />
106 DocumentRoot /www/abc <br />
107 </VirtualHost> <br />
109 <VirtualHost www.def.dom> <br />
110 ServerAdmin webguy@def.dom <br />
111 DocumentRoot /www/def <br />
115 <p>Suppose that you've assigned 10.0.0.1 to
116 <code>www.abc.dom</code> and 10.0.0.2 to
117 <code>www.def.dom</code>. Furthermore, suppose that
118 <code>def.dom</code> has control of their own DNS. With this
119 config you have put <code>def.dom</code> into a position where
120 they can steal all traffic destined to <code>abc.dom</code>. To
121 do so, all they have to do is set <code>www.def.dom</code> to
122 10.0.0.1. Since they control their own DNS you can't stop them
123 from pointing the <code>www.def.dom</code> record wherever they
126 <p>Requests coming in to 10.0.0.1 (including all those where
127 users typed in URLs of the form
128 <code>http://www.abc.dom/whatever</code>) will all be served by
129 the <code>def.dom</code> virtual host. To better understand why
130 this happens requires a more in-depth discussion of how Apache
131 matches up incoming requests with the virtual host that will
132 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
133 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
134 <div class="section">
135 <h2><a name="main" id="main">The "main server" Address</a></h2>
138 <p>The addition of <a href="vhosts/name-based.html">name-based
139 virtual host support</a> in Apache 1.1 requires Apache to know
140 the IP address(es) of the host that httpd is running on. To get
141 this address it uses either the global
142 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
143 (if present) or calls the C function <code>gethostname</code>
144 (which should return the same as typing "hostname" at the
145 command prompt). Then it performs a DNS lookup on this address.
146 At present there is no way to avoid this lookup.</p>
148 <p>If you fear that this lookup might fail because your DNS
149 server is down then you can insert the hostname in
150 <code>/etc/hosts</code> (where you probably already have it so
151 that the machine can boot properly). Then ensure that your
152 machine is configured to use <code>/etc/hosts</code> in the
153 event that DNS fails. Depending on what OS you are using this
154 might be accomplished by editing <code>/etc/resolv.conf</code>,
155 or maybe <code>/etc/nsswitch.conf</code>.</p>
157 <p>If your server doesn't have to perform DNS for any other
158 reason then you might be able to get away with running Apache
159 with the <code>HOSTRESORDER</code> environment variable set to
160 "local". This all depends on what OS and resolver libraries you
161 are using. It also affects CGIs unless you use
162 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
163 to consult the man pages or FAQs for your OS.</p>
164 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
165 <div class="section">
166 <h2><a name="tips" id="tips">Tips to Avoid These Problems</a></h2>
172 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
177 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
181 ensure all virtual hosts have an explicit
182 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
185 <li>create a <code><VirtualHost _default_:*></code>
186 server that has no pages to serve</li>
188 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
189 <div class="section">
190 <h2><a name="appendix" id="appendix">Appendix: Future Directions</a></h2>
193 <p>The situation regarding DNS is highly undesirable. For
194 Apache 1.2 we've attempted to make the server at least continue
195 booting in the event of failed DNS, but it might not be the
196 best we can do. In any event requiring the use of explicit IP
197 addresses in configuration files is highly undesirable in
198 today's Internet where renumbering is a necessity.</p>
200 <p>A possible work around to the theft of service attack
201 described above would be to perform a reverse DNS lookup on the
202 ip address returned by the forward lookup and compare the two
203 names. In the event of a mismatch the virtualhost would be
204 disabled. This would require reverse DNS to be configured
205 properly (which is something that most admins are familiar with
206 because of the common use of "double-reverse" DNS lookups by
207 FTP servers and TCP wrappers).</p>
209 <p>In any event it doesn't seem possible to reliably boot a
210 virtual-hosted web server when DNS has failed unless IP
211 addresses are used. Partial solutions such as disabling
212 portions of the configuration might be worse than not booting
213 at all depending on what the webserver is supposed to
216 <p>As HTTP/1.1 is deployed and browsers and proxies start
217 issuing the <code>Host</code> header it will become possible to
218 avoid the use of IP-based virtual hosts entirely. In this event
219 a webserver has no requirement to do DNS lookups during
220 configuration. But as of March 1997 these features have not
221 been deployed widely enough to be put into use on critical
225 <p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p>
226 <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>