Annotation of apache-modules/mod_ip_forwarding/mod_ip_forwarding.html, revision 1.11
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6: <title>IP Forwarding Module: Compilation and Configuration</title>
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10: <p><code>Created: 24/August/1998</code></p>
1.11 ! kahan 11:
! 12: <p>Latest release: 2/Feb/2006</p>
1.2 kahan 13: <hr>
1.1 kahan 14:
1.2 kahan 15: <h1 style="text-align: center"><span style="color: #E55500">mod_ip_forwarding
1.8 kahan 16: v2.0</span></h1>
1.2 kahan 17: <hr>
18:
19: <h2>Outline of the document</h2>
1.1 kahan 20: <ul>
1.4 kahan 21: <li>1. <a href="#1">Purpose</a></li>
1.9 kahan 22: <li>2. <a href="#2">Compiling the IP Forwarding module into Apache</a>
23: <ul>
24: <li>2.1 <a href="#s2.1">Apache 2.4.x</a></li>
25: <li>2.2 <a href="#s2.2">Debian Sarge package for Apache 2.4.x</a></li>
26: <li>2.3 <a href="#s2.3">Apache 1.3.x</a></li>
27: </ul>
28: </li>
1.4 kahan 29: <li>3. <a href="#3">Configuring the IP Forwarding module</a>
30: <ul>
31: <li>3.1 <a href="#3.1">New directives</a></li>
32: <li>3.2 <a href="#3.2">Example</a></li>
33: </ul>
34: </li>
35: <li>4. <a href="#4.">Security Notes</a></li>
1.1 kahan 36: </ul>
37:
38: <h3>Special considerations</h3>
1.2 kahan 39:
1.4 kahan 40: <p>This module is dependent on the Apache API. If you'd like to use the
1.11 ! kahan 41: module on an Apache version higher than v/2.x / v1.3.x, please <a
1.2 kahan 42: href="mailto:jose@w3.org">mail me</a>.</p>
1.1 kahan 43:
44: <h3>Notation</h3>
45: <dl>
1.4 kahan 46: <dt>apache/</dt>
47: <dd>The apache source directory</dd>
1.1 kahan 48: </dl>
49:
50: <h2><a name="1"></a>1. Purpose<a name="2."></a></h2>
1.2 kahan 51:
52: <p>The motivation for this module was to be able to forward the IP address of
53: the source of a request between a proxy and the final destination server.
54: Under normal circumstances, this information is lost and the server only sees
55: the proxy's IP address. This hampers access control based on IP addresses.
56: This module answers this limitation in a transparent and secure way.</p>
57:
58: <p>Proxies forward the IP address inside a custom HTTP header. A server only
1.1 kahan 59: accepts this new header from a list of user-defined authorized proxies. Once
60: the header is accepted, it's internally substituted with the proxy's IP
61: address, so that this value is used for all access control and CGI modules.
62: This doesn't affect the actual IP address used to answer the proxy (they are
63: stored in different places).</p>
64:
1.9 kahan 65: <h2><a name="s2" id="s2">2.</a> Compiling the IP forwarding module into
66: Apache</h2>
1.2 kahan 67:
1.10 kahan 68: <h3><a name="s2.1" id="s2.1">2.1</a> Apache 2.x</h3>
1.4 kahan 69:
1.10 kahan 70: <p>This is what I do to compile the module under Apache 2.0.54. I compile it
71: as a shared library under Debian Sarge. You don't need to compile apache
72: yourself anymore. Isn't life beautiful? These steps may change or not under
73: newer versions of Apache.</p>
1.1 kahan 74: <ol>
1.5 kahan 75: <li>Copy the latest apache 2 file <a
1.10 kahan 76: href="http://dev.w3.org/cvsweb/~checkout~/apache-modules/mod_ip_forwarding/mod_ip_forwarding.c?content-type=text/plain&only_with_tag=HEAD">mod_ip_forwarding.c</a>
1.5 kahan 77: to a local directory</li>
1.4 kahan 78: <li>Compile and install the module as follows:
79: <p><code># /path/to/this/tool/apxs2 -i -a -c
80: /path/to/mod_ip_forwarding.c</code></p>
81: </li>
82: </ol>
1.2 kahan 83:
1.4 kahan 84: <p>Note that module has to have the highest priority among the access control
85: modules. If you're using other proprietary access control modules, this
86: module should correspond to the last <code>activate-module </code>option.
87: Alternatively, you may use the Apache <code>ClearModuleList</code> and
88: <code>AddModule</code> configuration directives to reorder the modules. Also
89: note that you need to enable the <code>proxy</code> module too, as you want
90: to proxy requests.</p>
1.1 kahan 91:
1.10 kahan 92: <h3><a name="s2.2" id="s2.2">2.2</a> Debian Sarge package for Apache 2.x (or
93: superior)</h3>
1.9 kahan 94:
1.10 kahan 95: <p>This is what I do to make a Debian Sarge package of this module for Apache
96: 2.0.54.</p>
1.9 kahan 97: <ol>
98: <li>Make a cvs checkout of the entire mod_ip_forwarding directory:
99: <p><code>$ cvs -d dev.w3.org:/sources/public co
100: apache-modules/mod_ip_forwarding</code></p>
101: </li>
102: <li><code>$ cd apache-modules/mod_ip_forwarding</code></li>
103: <li>As root, type:
104: <p># <code>dpkg-buildpackage -us -us</code></p>
105: <p><code>(obmit the last two parameters if you want to sign the
106: module)</code></p>
107: </li>
108: <li>Once the module is created, it will be stored on the parent directory,
109: with a name such as
110: <code>libapache2-mod-ip-forward_20051129-1_i386.deb</code>. You can
111: install
112: <p>it using the usual debian command (as root):</p>
113: <p><code># dpkg --install
114: libapache2-mod-ip-forward_20051129-1_i386.deb</code></p>
115: </li>
116: </ol>
117:
118: <h3><a name="s2.3" id="s2.3">2.3</a> Apache 1.3.x</h3>
1.1 kahan 119:
1.7 kahan 120: <p>This is what I do to compile the module under Apache 1.3.4. These steps
121: may change or not under newer versions of Apache.</p>
1.4 kahan 122: <ol>
123: <li>Copy the apache (1) file <a
1.10 kahan 124: href="http://dev.w3.org/cvsweb/~checkout~/apache-modules/mod_ip_forwarding/mod_ip_forwarding.c?rev=1.4">mod_ip_forwarding.c</a>
1.4 kahan 125: to the apache/src/modules/extra directory.
126: <p><em><strong>Note that you must follow this link to get the correct
127: version of this file. The latest version will only compile on apache
128: 2.x..</strong></em></p>
129: </li>
130: <li>Compile the module as follows:
131: <p><code># cd apache</code></p>
132: <p><code># ./configure --prefix=/usr/local/apache \</code></p>
133: <p><code>--enable-module=proxy \</code></p>
134: <p><code>--activate-module=src/modules/extra/mod_ip_forwarding.c</code></p>
135: <p><code># make</code></p>
136: <p><code># make install</code></p>
137: </li>
1.1 kahan 138: </ol>
1.2 kahan 139: Note that module has to have the highest priority among the access control
1.4 kahan 140: modules. If you're using other proprietary access control modules, this
141: module should correspond to the last <code>activate-module </code>option.
1.2 kahan 142: Alternatively, you may use the Apache <code>ClearModuleList</code> and
143: <code>AddModule</code> configuration directives to reorder the modules. Also
1.4 kahan 144: note that you need to enable the <code>proxy</code> module too, as you want
145: to proxy requests.
146:
1.9 kahan 147: <h2><a name="2">3</a>. Configuring the IP forwarding module</h2>
1.1 kahan 148:
149: <h3><a name="2.1"></a>3.1 New directives</h3>
1.2 kahan 150:
151: <p>The IP Forwarding module introduces three directives to Apache:</p>
1.1 kahan 152: <dl>
1.4 kahan 153: <dt>ForwardClientIPAddress [on/off default = off]</dt>
154: <dd>Controls forwarding of the X_Client_Address header</dd>
155: <dt>AcceptForwardedClientIPAddress [on/off default = off]</dt>
156: <dd>Authorizes accepting an X_Client_Address header</dd>
157: <dt>LogUnauthorizedIPForwarding [on/off default = on]</dt>
158: <dd>Logs any unauthorized attemp to forward an IP address</dd>
159: <dt>X_ClientIPAddrHeader string [default = X_Fwd_IP_Addr]</dt>
160: <dd>Customizable header string for sending the client ip _addr</dd>
161: <dt>AuthorizedProxies [space separated list of IP addresses]</dt>
162: <dd>List of authorized proxies who can send an X_Client_Address
163: header</dd>
1.1 kahan 164: </dl>
1.2 kahan 165:
166: <p>Note that you need to set up this module on both the proxy server and the
1.1 kahan 167: final destination server. In addition, both servers must use the same
1.4 kahan 168: <code>X_ClientIPAddrHeader</code> value. Finally, the final destination
169: server must add the address of the proxy server using the
1.1 kahan 170: <code>AuthorizedProxies</code> directive. This protects against intruders
171: spoofing this header (unless they also spoof the IP address).</p>
172:
1.3 kahan 173: <p><em>The following is a tip contributed by Paul Roe. Thanks!</em></p>
174:
1.4 kahan 175: <p>If you're using this module as an Apache precompiled module (dynamic
1.3 kahan 176: modules that are downloaded at runtime), you need to add the following
177: configuration directive:</p>
178: <dl>
1.4 kahan 179: <dt>LoadModule ip_forwarding_module
180: /path_to_apache_modules/mod_ip_forwarding.so</dt>
1.3 kahan 181: </dl>
1.4 kahan 182:
1.3 kahan 183: <p>Note that the Debian apache-common-1.3.12-1 package seems to have renamed
1.4 kahan 184: this module as w3c_ip_forwarding_module. In this case, you need to load the
1.3 kahan 185: precompiled module as follows:</p>
186: <dl>
1.4 kahan 187: <dt>LoadModule w3c_ip_forwarding_module
188: /path_to_apache_modules/mod_ip_forwarding.so</dt>
1.3 kahan 189: </dl>
190:
1.1 kahan 191: <h3><a name="3.2"></a>3.2 Example</h3>
1.2 kahan 192:
193: <p>Let's suppose I have a proxy on 134.129.20.116 and a server on
194: 134.129.20.130. In addition, my client is at 123.123.123.123. Here's a
195: fragment of the proxy and final server's configuration:</p>
196: <pre>## Proxy configuration (may also be setup using mod_rewrite)
197: ProxyPass / http://134.129.20.130/
1.1 kahan 198: ForwardClientIPAddress On</pre>
199: <pre>## Server Configuration
200: AcceptForwardedClientIPAddress On
201: AuthorizedProxies 132.129.20.116</pre>
1.2 kahan 202:
203: <p>When the proxy contacts the server, it includes the following header:</p>
204: <pre>X_Forward_IP_Addr: 123.123.123.123</pre>
205:
206: <p>When the server calls a CGI-script, it sends the following environment
1.1 kahan 207: variable:</p>
1.2 kahan 208: <pre>REMOTE_ADDR: 123.123.123.123</pre>
209:
210: <p>Note that the value of the REMOTE_ADDR environment variable corresponds to
211: the one in the X_FWD_IP_ADDR header. Also, the X_ header was removed, as the
212: final server is not configured to forward the X_ header. To continue
1.4 kahan 213: forwarding the same header, you need to set the ForwardClientIPAddress in
214: that server too.</p>
1.2 kahan 215:
216: <p>If a non-authorized proxy (say,<code> 132.129.20.116</code>) sends the
217: custom header, the module will remove it from the headers and add the
218: following entry in the error log:</p>
1.1 kahan 219: <pre>[Mon Aug 24 15:57:48 1998] Unauthorized Proxy (132.129.20.116) tried to forward
1.2 kahan 220: a client IP address (123.123.123.123)</pre>
221:
222: <h2><a name="4.">4.</a> Security notes</h2>
223:
224: <p>This module does not implement a full-blown secure ip@ forwarding
225: mechanism. In particular, there are two weak security points:</p>
226: <ul>
1.4 kahan 227: <li><strong>IP spoofing.</strong> There's no protection against an
228: intruder's spoofing of the IP@ of an authorized proxy. Under such an
229: attack, the intruder may send any IP@ he wants in the<code>
230: X_ClientIPAddrHeader</code> heeader. This attack is less important when
231: the authorized proxy is on the border of a firewall and the destination
232: server is behind the firewall.</li>
233: <li><strong>Denial of service</strong>. The module generates a log message
1.9 kahan 234: each time that it detects an an IP@ forwarding by an unauthorized proxy.
235: An intruder may attempt filling up a server's logs with a flood of
1.4 kahan 236: unauthorized IP@ forwarding requests. If this attacks occurs, you can
237: temporarily turn off the logging by means of the module's
238: <code>LogUnauthorizedIPForwarding</code> directive.</li>
1.2 kahan 239: </ul>
240:
241: <p></p>
242: <hr>
1.1 kahan 243: <address>
1.4 kahan 244: <a href="mailto:jose@w3.org">Jose</a>
245: </address>
1.11 ! kahan 246: $Author: kahan $ $Date: Friday 03 February 2006 - 16:59:38$</body>
1.1 kahan 247: </html>
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