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7 --><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="./style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /><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>
8 <p>This page could be summarized with the statement: don't require Apache
9 to use DNS for any parsing of the configuration files. If Apache has to
10 use DNS to parse the configuration files then your server may be subject
11 to reliability problems (it might not boot), or denial and theft of
12 service attacks (including users able to steal hits from other users).</p>
13 </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>
16 <div class="example"><p><code>
17 <VirtualHost www.abc.dom> <br />
18 ServerAdmin webgirl@abc.dom <br />
19 DocumentRoot /www/abc <br />
23 <p>In order for Apache to function properly it absolutely needs
24 to have two pieces of information about each virtual host: the
25 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
26 and at least one IP address that the server responds to. This
27 example does not include the IP address, so Apache must use DNS
28 to find the address of <code>www.abc.dom</code>. If for some
29 reason DNS is not available at the time your server is parsing
30 its config file, then this virtual host <strong>will not be
31 configured</strong>. It won't be able to respond to any hits to
32 this virtual host (prior to Apache version 1.2 the server would
35 <p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.
36 Then consider this configuration snippet:</p>
38 <div class="example"><p><code>
39 <VirtualHost 10.0.0.1> <br />
40 ServerAdmin webgirl@abc.dom <br />
41 DocumentRoot /www/abc <br />
45 <p>Now Apache needs to use reverse DNS to find the
46 <code>ServerName</code> for this virtualhost. If that reverse
47 lookup fails then it will partially disable the virtualhost
48 (prior to Apache version 1.2 the server would not even boot).
49 If the virtual host is name-based then it will effectively be
50 totally disabled, but if it is IP-based then it will mostly
51 work. However if Apache should ever have to generate a full URL
52 for the server which includes the server name then it will fail
53 to generate a valid URL.</p>
55 <p>Here is a snippet that avoids both of these problems.</p>
57 <div class="example"><p><code>
58 <VirtualHost 10.0.0.1> <br />
59 ServerName www.abc.dom <br />
60 ServerAdmin webgirl@abc.dom <br />
61 DocumentRoot /www/abc <br />
64 </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>
67 <p>There are (at least) two forms that denial of service can
68 come in. If you are running a version of Apache prior to
69 version 1.2 then your server will not even boot if one of the
70 two DNS lookups mentioned above fails for any of your virtual
71 hosts. In some cases this DNS lookup may not even be under your
72 control. For example, if <code>abc.dom</code> is one of your
73 customers and they control their own DNS then they can force
74 your (pre-1.2) server to fail while booting simply by deleting
75 the <code>www.abc.dom</code> record.</p>
77 <p>Another form is far more insidious. Consider this
78 configuration snippet:</p>
80 <div class="example"><p><code>
81 <VirtualHost www.abc.dom> <br />
82 ServerAdmin webgirl@abc.dom <br />
83 DocumentRoot /www/abc <br />
84 </VirtualHost> <br />
86 <VirtualHost www.def.dom> <br />
87 ServerAdmin webguy@def.dom <br />
88 DocumentRoot /www/def <br />
92 <p>Suppose that you've assigned 10.0.0.1 to
93 <code>www.abc.dom</code> and 10.0.0.2 to
94 <code>www.def.dom</code>. Furthermore, suppose that
95 <code>def.com</code> has control of their own DNS. With this
96 config you have put <code>def.com</code> into a position where
97 they can steal all traffic destined to <code>abc.com</code>. To
98 do so, all they have to do is set <code>www.def.dom</code> to
99 10.0.0.1. Since they control their own DNS you can't stop them
100 from pointing the <code>www.def.com</code> record wherever they
103 <p>Requests coming in to 10.0.0.1 (including all those where
104 users typed in URLs of the form
105 <code>http://www.abc.dom/whatever</code>) will all be served by
106 the <code>def.com</code> virtual host. To better understand why
107 this happens requires a more in-depth discussion of how Apache
108 matches up incoming requests with the virtual host that will
109 serve it. A rough document describing this <a href="vhosts/details.html">is available</a>.</p>
110 </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>
113 <p>The addition of <a href="vhosts/name-based.html">name-based
114 virtual host support</a> in Apache 1.1 requires Apache to know
115 the IP address(es) of the host that httpd is running on. To get
116 this address it uses either the global
117 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
118 (if present) or calls the C function <code>gethostname</code>
119 (which should return the same as typing "hostname" at the
120 command prompt). Then it performs a DNS lookup on this address.
121 At present there is no way to avoid this lookup.</p>
123 <p>If you fear that this lookup might fail because your DNS
124 server is down then you can insert the hostname in
125 <code>/etc/hosts</code> (where you probably already have it so
126 that the machine can boot properly). Then ensure that your
127 machine is configured to use <code>/etc/hosts</code> in the
128 event that DNS fails. Depending on what OS you are using this
129 might be accomplished by editing <code>/etc/resolv.conf</code>,
130 or maybe <code>/etc/nsswitch.conf</code>.</p>
132 <p>If your server doesn't have to perform DNS for any other
133 reason then you might be able to get away with running Apache
134 with the <code>HOSTRESORDER</code> environment variable set to
135 "local". This all depends on what OS and resolver libraries you
136 are using. It also affects CGIs unless you use
137 <code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
138 to consult the man pages or FAQs for your OS.</p>
139 </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>
145 <code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
150 <code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
154 ensure all virtual hosts have an explicit
155 <code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
158 <li>create a <code><VirtualHost _default_:*></code>
159 server that has no pages to serve</li>
161 </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>
164 <p>The situation regarding DNS is highly undesirable. For
165 Apache 1.2 we've attempted to make the server at least continue
166 booting in the event of failed DNS, but it might not be the
167 best we can do. In any event requiring the use of explicit IP
168 addresses in configuration files is highly undesirable in
169 today's Internet where renumbering is a necessity.</p>
171 <p>A possible work around to the theft of service attack
172 described above would be to perform a reverse DNS lookup on the
173 ip address returned by the forward lookup and compare the two
174 names. In the event of a mismatch the virtualhost would be
175 disabled. This would require reverse DNS to be configured
176 properly (which is something that most admins are familiar with
177 because of the common use of "double-reverse" DNS lookups by
178 FTP servers and TCP wrappers).</p>
180 <p>In any event it doesn't seem possible to reliably boot a
181 virtual-hosted web server when DNS has failed unless IP
182 addresses are used. Partial solutions such as disabling
183 portions of the configuration might be worse than not booting
184 at all depending on what the webserver is supposed to
187 <p>As HTTP/1.1 is deployed and browsers and proxies start
188 issuing the <code>Host</code> header it will become possible to
189 avoid the use of IP-based virtual hosts entirely. In this event
190 a webserver has no requirement to do DNS lookups during
191 configuration. But as of March 1997 these features have not
192 been deployed widely enough to be put into use on critical
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