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