1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
|
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
This file is generated from xml source: DO NOT EDIT
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-->
<title>Issues Regarding DNS and Apache HTTP Server - 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="./style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
<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 2.3</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/">Documentation</a> > <a href="./">Version 2.3</a></div><div id="page-content"><div id="preamble"><h1>Issues Regarding DNS and Apache HTTP Server</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
</div>
<p>This page could be summarized with the statement: don't
configure Apache HTTP Server in such a way that it relies on DNS resolution
for parsing of the configuration files. If httpd requires DNS
resolution to parse the configuration files then your server
may be subject to reliability problems (ie. it might not start up),
or denial and theft of service attacks (including virtual hosts able
to steal hits from other virtual hosts).</p>
</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>
</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>
<div class="example"><p><code>
# This is a misconfiguration example, do not use on your server <br />
<VirtualHost www.example.dom> <br />
ServerAdmin webgirl@example.dom <br />
DocumentRoot /www/example <br />
</VirtualHost>
</code></p></div>
<p>In order for the server to function properly, it absolutely needs
to have two pieces of information about each virtual host: the
<code class="directive"><a href="./mod/core.html#servername">ServerName</a></code> and at least one
IP address that the server will bind and respond to. The above
example does not include the IP address, so httpd must use DNS
to find the address of <code>www.example.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.</p>
<p>Suppose that <code>www.example.dom</code> has address 192.0.2.1.
Then consider this configuration snippet:</p>
<div class="example"><p><code>
# This is a misconfiguration example, do not use on your server <br />
<VirtualHost 192.0.2.1> <br />
ServerAdmin webgirl@example.dom <br />
DocumentRoot /www/example <br />
</VirtualHost>
</code></p></div>
<p>This time httpd 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.
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 httpd should ever have to generate a full
URL for the server which includes the server name (such as when a
Redirect is issued), then it will fail to generate a valid URL.</p>
<p>Here is a snippet that avoids both of these problems:</p>
<div class="example"><p><code>
<VirtualHost 192.0.2.1> <br />
ServerName www.example.dom <br />
ServerAdmin webgirl@example.dom <br />
DocumentRoot /www/example <br />
</VirtualHost>
</code></p></div>
</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>
<p>Consider this configuration snippet:</p>
<div class="example"><p><code>
<VirtualHost www.example1.dom><br />
<span class="indent">
ServerAdmin webgirl@example1.dom<br />
DocumentRoot /www/example1<br />
</span>
</VirtualHost><br />
<br />
<VirtualHost www.example2.dom><br />
<span class="indent">
ServerAdmin webguy@example2.dom<br />
DocumentRoot /www/example2<br />
</span>
</VirtualHost>
</code></p></div>
<p>Suppose that you've assigned 192.0.2.1 to
<code>www.example1.dom</code> and 192.0.2.2 to
<code>www.example2.dom</code>. Furthermore, suppose that
<code>example1.dom</code> has control of their own DNS. With this
config you have put <code>example1.dom</code> into a position where
they can steal all traffic destined to <code>example2.dom</code>. To
do so, all they have to do is set <code>www.example1.dom</code> to
192.0.2.2. Since they control their own DNS you can't stop them
from pointing the <code>www.example1.dom</code> record wherever they
wish.</p>
<p>Requests coming in to 192.0.2.2 (including all those where
users typed in URLs of the form
<code>http://www.example2.dom/whatever</code>) will all be served by
the <code>example1.dom</code> virtual host. To better understand why
this happens requires a more in-depth discussion of how httpd
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>
</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>
<p><a href="vhosts/name-based.html">Name-based
virtual host support</a> requires httpd to know
the IP address(es) of the host that <code class="program"><a href="./programs/httpd.html">httpd</a></code>
is running on. To get this address it uses either the global
<code class="directive"><a href="./mod/core.html#servername">ServerName</a></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 httpd
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
<code class="module"><a href="./mod/mod_env.html">mod_env</a></code> to control the environment. It's best
to consult the man pages or FAQs for your OS.</p>
</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>
<ul>
<li>
use IP addresses in
<code class="directive"><a href="./mod/core.html#virtualhost">VirtualHost</a></code>
</li>
<li>
use IP addresses in
<code class="directive"><a href="./mod/mpm_common.html#listen">Listen</a></code>
</li>
<li>
ensure all virtual hosts have an explicit
<code class="directive"><a href="./mod/core.html#servername">ServerName</a></code>
</li>
<li>create a <code><VirtualHost _default_:*></code>
server that has no pages to serve</li>
</ul>
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="./en/dns-caveats.html" title="English"> en </a> |
<a href="./fr/dns-caveats.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
<a href="./ja/dns-caveats.html" hreflang="ja" rel="alternate" title="Japanese"> ja </a> |
<a href="./ko/dns-caveats.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
<a href="./tr/dns-caveats.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
</div><div id="footer">
<p class="apache">Copyright 2011 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
<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>
</body></html>
|
