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306 lines
12 KiB
XML
306 lines
12 KiB
XML
<?xml version='1.0' encoding='UTF-8' ?>
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<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
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<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
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<!-- $LastChangedRevision$ -->
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<!--
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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<manualpage metafile="ssl_howto.xml.meta">
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<parentdocument href="./">SSL/TLS</parentdocument>
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<title>SSL/TLS Strong Encryption: How-To</title>
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<summary>
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<blockquote>
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<p>The solution to this problem is trivial
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and is left as an exercise for the reader.</p>
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<p class="cite">-- <cite>Standard textbook cookie</cite></p>
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</blockquote>
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<p>How to solve particular security problems for an SSL-aware
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webserver is not always obvious because of the interactions between SSL,
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HTTP and Apache's way of processing requests. This chapter gives
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instructions on how to solve some typical situations. Treat it as a first
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step to find out the final solution, but always try to understand the
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stuff before you use it. Nothing is worse than using a security solution
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without knowing its restrictions and how it interacts with other systems.</p>
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</summary>
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<section id="ciphersuites">
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<title>Cipher Suites and Enforcing Strong Security</title>
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<ul>
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<li><a href="#realssl">How can I create a real SSLv2-only server?</a></li>
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<li><a href="#onlystrong">How can I create an SSL server which accepts strong encryption only?</a></li>
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<li><a href="#upgradeenc">How can I create an SSL server which accepts strong encryption only, but allows
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export browsers to upgrade to stronger encryption?</a></li>
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<li><a href="#strongurl">How can I create an SSL server which accepts all types of ciphers in general, but
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requires a strong cipher for access to a particular URL?</a></li>
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</ul>
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<section id="realssl">
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<title>How can I create a real SSLv2-only server?</title>
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<p>The following creates an SSL server which speaks only the SSLv2 protocol and
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its ciphers.</p>
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<example><title>httpd.conf</title>
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SSLProtocol -all +SSLv2<br />
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SSLCipherSuite SSLv2:+HIGH:+MEDIUM:+LOW:+EXP<br />
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</example>
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</section>
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<section id="onlystrong">
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<title>How can I create an SSL server which accepts strong encryption
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only?</title>
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<p>The following enables only the seven strongest ciphers:</p>
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<example><title>httpd.conf</title>
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SSLProtocol all<br />
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SSLCipherSuite HIGH:MEDIUM<br />
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</example>
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</section>
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<section id="upgradeenc">
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<title>How can I create an SSL server which accepts strong encryption
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only, but allows export browsers to upgrade to stronger encryption?</title>
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<p>This facility is called Server Gated Cryptography (SGC) and requires
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a Global ID server certificate, signed by a special CA certificate
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from Verisign. This enables strong encryption in 'export' versions of
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browsers, which traditionally could not support it (because of US export
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restrictions).</p>
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<p>When a browser connects with an export cipher, the server sends its Global
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ID certificate. The browser verifies this, and can then upgrade its
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cipher suite before any HTTP communication takes place. The problem
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lies in allowing browsers to upgrade in this fashion, but still requiring
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strong encryption. In other words, we want browsers to either start a
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connection with strong encryption, or to start with export ciphers but
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upgrade to strong encryption before beginning HTTP communication.</p>
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<p>This can be done as follows:</p>
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<example><title>httpd.conf</title>
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# allow all ciphers for the initial handshake,<br />
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# so export browsers can upgrade via SGC facility<br />
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SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL<br />
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<br />
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<Directory /usr/local/apache2/htdocs><br />
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# but finally deny all browsers which haven't upgraded<br />
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SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128<br />
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</Directory>
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</example>
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</section>
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<section id="strongurl">
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<title>How can I create an SSL server which accepts all types of ciphers
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in general, but requires a strong ciphers for access to a particular
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URL?</title>
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<p>Obviously, a server-wide <directive
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module="mod_ssl">SSLCipherSuite</directive> which restricts
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ciphers to the strong variants, isn't the answer here. However,
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<module>mod_ssl</module> can be reconfigured within <code>Location</code>
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blocks, to give a per-directory solution, and can automatically force
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a renegotiation of the SSL parameters to meet the new configuration.
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This can be done as follows:</p>
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<example>
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# be liberal in general<br />
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SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL<br />
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<br />
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<Location /strong/area><br />
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# but https://hostname/strong/area/ and below<br />
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# requires strong ciphers<br />
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SSLCipherSuite HIGH:MEDIUM<br />
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</Location>
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</example>
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</section>
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</section>
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<!-- /ciphersuites -->
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<section id="accesscontrol">
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<title>Client Authentication and Access Control</title>
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<ul>
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<li><a href="#allclients">How can I force clients to authenticate using certificates?</a></li>
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<li><a href="#arbitraryclients">How can I force clients to authenticate using certificates for a
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particular URL, but still allow arbitrary clients to access the rest of the server?</a></li>
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<li><a href="#certauthenticate">How can I allow only clients who have certificates to access a
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particular URL, but allow all clients to access the rest of the server?</a></li>
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<li><a href="#intranet">How can I require HTTPS with strong ciphers, and either
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basic authentication or client certificates, for access to part of the
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Intranet website, for clients coming from the Internet?</a></li>
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</ul>
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<section id="allclients">
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<title>How can I force clients to authenticate using certificates?</title>
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<p>When you know all of your users (eg, as is often the case on a corporate
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Intranet), you can require plain certificate authentication. All you
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need to do is to create client certificates signed by your own CA
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certificate (<code>ca.crt</code>) and then verify the clients against this
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certificate.</p>
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<example><title>httpd.conf</title>
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# require a client certificate which has to be directly<br />
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# signed by our CA certificate in ca.crt<br />
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SSLVerifyClient require<br />
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SSLVerifyDepth 1<br />
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SSLCACertificateFile conf/ssl.crt/ca.crt
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</example>
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</section>
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<section id="arbitraryclients">
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<title>How can I force clients to authenticate using certificates for a
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particular URL, but still allow arbitrary clients to access the rest of the server?</title>
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<p>To force clients to authenticate using certificates for a particular URL,
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you can use the per-directory reconfiguration features of
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<module>mod_ssl</module>:</p>
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<example><title>httpd.conf</title>
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SSLVerifyClient none<br />
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SSLCACertificateFile conf/ssl.crt/ca.crt<br />
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<br />
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<Location /secure/area><br />
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SSLVerifyClient require<br />
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SSLVerifyDepth 1<br />
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</Location><br />
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</example>
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</section>
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<section id="certauthenticate">
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<title>How can I allow only clients who have certificates to access a
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particular URL, but allow all clients to access the rest of the server?</title>
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<p>The key to doing this is checking that part of the client certificate
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matches what you expect. Usually this means checking all or part of the
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Distinguished Name (DN), to see if it contains some known string.
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There are two ways to do this, using either <module>mod_auth_basic</module> or
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<directive module="mod_ssl">SSLRequire</directive>.</p>
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<p>The <module>mod_auth_basic</module> method is generally required when
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the certificates are completely arbitrary, or when their DNs have
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no common fields (usually the organisation, etc.). In this case,
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you should establish a password database containing <em>all</em>
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clients allowed, as follows:</p>
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<example><title>httpd.conf</title><pre>
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SSLVerifyClient none
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<Directory /usr/local/apache2/htdocs/secure/area>
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SSLVerifyClient require
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SSLVerifyDepth 5
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SSLCACertificateFile conf/ssl.crt/ca.crt
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SSLCACertificatePath conf/ssl.crt
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SSLOptions +FakeBasicAuth
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SSLRequireSSL
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AuthName "Snake Oil Authentication"
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AuthType Basic
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AuthBasicProvider file
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AuthUserFile /usr/local/apache2/conf/httpd.passwd
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Require valid-user
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</Directory></pre>
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</example>
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<p>The password used in this example is the DES encrypted string "password".
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See the <directive module="mod_ssl">SSLOptions</directive> docs for more
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information.</p>
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<example><title>httpd.passwd</title><pre>
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/C=DE/L=Munich/O=Snake Oil, Ltd./OU=Staff/CN=Foo:xxj31ZMTZzkVA
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/C=US/L=S.F./O=Snake Oil, Ltd./OU=CA/CN=Bar:xxj31ZMTZzkVA
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/C=US/L=L.A./O=Snake Oil, Ltd./OU=Dev/CN=Quux:xxj31ZMTZzkVA</pre>
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</example>
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<p>When your clients are all part of a common hierarchy, which is encoded
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into the DN, you can match them more easily using <directive module="mod_ssl"
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>SSLRequire</directive>, as follows:</p>
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<example><title>httpd.conf</title><pre>
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SSLVerifyClient none
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<Directory /usr/local/apache2/htdocs/secure/area>
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SSLVerifyClient require
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SSLVerifyDepth 5
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SSLCACertificateFile conf/ssl.crt/ca.crt
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SSLCACertificatePath conf/ssl.crt
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SSLOptions +FakeBasicAuth
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SSLRequireSSL
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SSLRequire %{SSL_CLIENT_S_DN_O} eq "Snake Oil, Ltd." \
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and %{SSL_CLIENT_S_DN_OU} in {"Staff", "CA", "Dev"}
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</Directory></pre>
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</example>
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</section>
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<section id="intranet">
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<title>How can I require HTTPS with strong ciphers, and either basic
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authentication or client certificates, for access to part of the
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Intranet website, for clients coming from the Internet? I still want to allow
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plain HTTP access for clients on the Intranet.</title>
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<p>These examples presume that clients on the Intranet have IPs in the range
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192.168.1.0/24, and that the part of the Intranet website you want to allow
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internet access to is <code>/usr/local/apache2/htdocs/subarea</code>.
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This configuration should remain outside of your HTTPS virtual host, so
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that it applies to both HTTPS and HTTP.</p>
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<example><title>httpd.conf</title><pre>
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SSLCACertificateFile conf/ssl.crt/company-ca.crt
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<Directory /usr/local/apache2/htdocs>
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# Outside the subarea only Intranet access is granted
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Order deny,allow
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Deny from all
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Allow from 192.168.1.0/24
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</Directory>
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<Directory /usr/local/apache2/htdocs/subarea>
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# Inside the subarea any Intranet access is allowed
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# but from the Internet only HTTPS + Strong-Cipher + Password
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# or the alternative HTTPS + Strong-Cipher + Client-Certificate
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# If HTTPS is used, make sure a strong cipher is used.
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# Additionally allow client certs as alternative to basic auth.
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SSLVerifyClient optional
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SSLVerifyDepth 1
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SSLOptions +FakeBasicAuth +StrictRequire
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SSLRequire %{SSL_CIPHER_USEKEYSIZE} >= 128
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# Force clients from the Internet to use HTTPS
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RewriteEngine on
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RewriteCond %{REMOTE_ADDR} !^192\.168\.1\.[0-9]+$
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RewriteCond %{HTTPS} !=on
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RewriteRule .* - [F]
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# Allow Network Access and/or Basic Auth
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Satisfy any
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# Network Access Control
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Order deny,allow
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Deny from all
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Allow 192.168.1.0/24
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# HTTP Basic Authentication
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AuthType basic
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AuthName "Protected Intranet Area"
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AuthBasicProvider file
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AuthUserFile conf/protected.passwd
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Require valid-user
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</Directory></pre>
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</example>
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</section>
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</section>
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<!-- /access control -->
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</manualpage>
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