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8 <title>Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server</title>
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18 <p class="apache">Apache HTTP Server Version 2.5</p>
19 <img alt="" src="./images/feather.gif" /></div>
20 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
22 <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 2.5</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache HTTP Server</h1>
24 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
25 <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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31 <p>This page could be summarized with the statement: don't
32 configure Apache HTTP Server in such a way that it relies on DNS resolution
33 for parsing of the configuration files. If httpd requires DNS
34 resolution to parse the configuration files then your server
35 may be subject to reliability problems (ie. it might not start up),
36 or denial and theft of service attacks (including virtual hosts able
37 to steal hits from other virtual hosts).</p>
39 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#example">A Simple Example</a></li>
40 <li><img alt="" src="./images/down.gif" /> <a href="#denial">Denial of Service</a></li>
41 <li><img alt="" src="./images/down.gif" /> <a href="#main">The "main server" Address</a></li>
42 <li><img alt="" src="./images/down.gif" /> <a href="#tips">Tips to Avoid These Problems</a></li>
44 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
46 <h2><a name="example" id="example">A Simple Example</a></h2>
49 <pre class="prettyprint lang-config">
50 # This is a misconfiguration example, do not use on your server
51 <VirtualHost www.example.dom>
52 ServerAdmin webgirl@example.dom
53 DocumentRoot /www/example
58 <p>In order for the server to function properly, it absolutely needs
59 to have two pieces of information about each virtual host: the
60 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
61 IP address that the server will bind and respond to. The above
62 example does not include the IP address, so httpd must use DNS
63 to find the address of <code>www.example.dom</code>. If for some
64 reason DNS is not available at the time your server is parsing
65 its config file, then this virtual host <strong>will not be
66 configured</strong>. It won't be able to respond to any hits
67 to this virtual host.</p>
69 <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
70 Then consider this configuration snippet:</p>
72 <pre class="prettyprint lang-config">
73 # This is a misconfiguration example, do not use on your server
74 <VirtualHost 192.0.2.1>
75 ServerAdmin webgirl@example.dom
76 DocumentRoot /www/example
81 <p>This time httpd needs to use reverse DNS to find the
82 <code>ServerName</code> for this virtualhost. If that reverse
83 lookup fails then it will partially disable the virtualhost.
84 If the virtual host is name-based then it will effectively be
85 totally disabled, but if it is IP-based then it will mostly
86 work. However, if httpd should ever have to generate a full
87 URL for the server which includes the server name (such as when a
88 Redirect is issued), then it will fail to generate a valid URL.</p>
90 <p>Here is a snippet that avoids both of these problems:</p>
92 <pre class="prettyprint lang-config">
93 <VirtualHost 192.0.2.1>
94 ServerName www.example.dom
95 ServerAdmin webgirl@example.dom
96 DocumentRoot /www/example
100 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
101 <div class="section">
102 <h2><a name="denial" id="denial">Denial of Service</a></h2>
105 <p>Consider this configuration snippet:</p>
107 <pre class="prettyprint lang-config">
108 <VirtualHost www.example1.dom>
109 ServerAdmin webgirl@example1.dom
110 DocumentRoot /www/example1
112 <VirtualHost www.example2.dom>
113 ServerAdmin webguy@example2.dom
114 DocumentRoot /www/example2
119 <p>Suppose that you've assigned 192.0.2.1 to
120 <code>www.example1.dom</code> and 192.0.2.2 to
121 <code>www.example2.dom</code>. Furthermore, suppose that
122 <code>example1.dom</code> has control of their own DNS. With this
123 config you have put <code>example1.dom</code> into a position where
124 they can steal all traffic destined to <code>example2.dom</code>. To
125 do so, all they have to do is set <code>www.example1.dom</code> to
126 192.0.2.2. Since they control their own DNS you can't stop them
127 from pointing the <code>www.example1.dom</code> record wherever they
130 <p>Requests coming in to 192.0.2.2 (including all those where
131 users typed in URLs of the form
132 <code>http://www.example2.dom/whatever</code>) will all be served by
133 the <code>example1.dom</code> virtual host. To better understand why
134 this happens requires a more in-depth discussion of how httpd
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><a href="vhosts/name-based.html">Name-based
143 virtual host support</a> requires httpd to know
144 the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
145 is running on. To get 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 httpd
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>
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194 <p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
195 <a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
196 <a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
197 <a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
198 <a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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