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<parentdocument href="./">SSL/TLS</parentdocument>

<title>SSL/TLS Strong Encryption: How-To</title>

<summary>

<blockquote>

<p>The solution to this problem is trivial

and is left as an exercise for the reader.</p>

<p class="cite">-- <cite>Standard textbook cookie</cite></p>

</blockquote>

<p>How to solve particular security problems for an SSL-aware

webserver is not always obvious because of the interactions between SSL,

HTTP and Apache's way of processing requests. This chapter gives

instructions on how to solve some typical situations. Treat it as a first

step to find out the final solution, but always try to understand the

stuff before you use it. Nothing is worse than using a security solution

without knowing its restrictions and how it interacts with other systems.</p>

</summary>

<section id="configexample">

<title>Basic Configuration Example</title>

<p>Your SSL configuration will need to contain, at a minumum, the

following directives.</p>

<example>

Listen 443<br />

&lt;VirtualHost _default_:443&gt;<br />

<indent>

ServerName www.example.com<br />

SSLEngine on<br />

SSLCertificateFile /path/to/www.example.com.cert<br />

SSLCertificateKeyFile /path/to/www.example.com.key<br />

</indent>

&lt;/VirtualHost&gt;

</example>

</section>

<section id="ciphersuites">

<title>Cipher Suites and Enforcing Strong Security</title>

<ul>

<li><a href="#onlystrong">How can I create an SSL server which accepts strong encryption only?</a></li>

<li><a href="#strongurl">How can I create an SSL server which accepts all types of ciphers in general, but

requires a strong cipher for access to a particular URL?</a></li>

</ul>

<section id="onlystrong">

<title>How can I create an SSL server which accepts strong encryption

only?</title>

<p>The following enables only the strongest ciphers:</p>

<example><title>httpd.conf</title>

SSLProtocol all -SSLv2<br />

SSLCipherSuite HIGH:!ADH:!EXP:!MD5:!NULL<br />

</example>

<p>While with the following configuration you enable two ciphers

which are resonably secure, and fast:</p>

<example><title>httpd.conf</title>

SSLProtocol all -SSLv2<br />

SSLCipherSuite RC4-SHA:AES128-SHA:HIGH:!ADH:!EXP:!MD5:!NULL<br />

SSLHonorCipherOrder on

</example>

<p>This strongly reflects the default value of <directive module="mod_ssl"

>SSLCipherSuite</directive> and is the recommanded way to configure it.</p>

</section>

<section id="strongurl">

<title>How can I create an SSL server which accepts all types of ciphers

in general, but requires a strong ciphers for access to a particular

URL?</title>

<p>Obviously, a server-wide <directive

module="mod_ssl">SSLCipherSuite</directive> which restricts

ciphers to the strong variants, isn't the answer here. However,

<module>mod_ssl</module> can be reconfigured within <code>Location</code>

blocks, to give a per-directory solution, and can automatically force

a renegotiation of the SSL parameters to meet the new configuration.

This can be done as follows:</p>

<example>

# be liberal in general<br />

SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL<br />

<br />

&lt;Location /strong/area&gt;<br />

# but https://hostname/strong/area/ and below<br />

# requires strong ciphers<br />

SSLCipherSuite HIGH:!ADH:!EXP:!MD5:!NULL<br />

&lt;/Location&gt;

</example>

</section>

</section>

<section id="accesscontrol">

<title>Client Authentication and Access Control</title>

<ul>

<li><a href="#allclients">How can I force clients to authenticate using certificates?</a></li>

<li><a href="#arbitraryclients">How can I force clients to authenticate using certificates for a

particular URL, but still allow arbitrary clients to access the rest of the server?</a></li>

<li><a href="#certauthenticate">How can I allow only clients who have certificates to access a

particular URL, but allow all clients to access the rest of the server?</a></li>

<li><a href="#intranet">How can I require HTTPS with strong ciphers, and either

basic authentication or client certificates, for access to part of the

Intranet website, for clients coming from the Internet?</a></li>

</ul>

<section id="allclients">

<title>How can I force clients to authenticate using certificates?</title>

<p>When you know all of your users (eg, as is often the case on a corporate

Intranet), you can require plain certificate authentication. All you

need to do is to create client certificates signed by your own CA

certificate (<code>ca.crt</code>) and then verify the clients against this

certificate.</p>

<example><title>httpd.conf</title>

# require a client certificate which has to be directly<br />

# signed by our CA certificate in ca.crt<br />

SSLVerifyClient require<br />

SSLVerifyDepth 1<br />

SSLCACertificateFile conf/ssl.crt/ca.crt

</example>

</section>

<section id="arbitraryclients">

<title>How can I force clients to authenticate using certificates for a

particular URL, but still allow arbitrary clients to access the rest of the server?</title>

<p>To force clients to authenticate using certificates for a particular URL,

you can use the per-directory reconfiguration features of

<module>mod_ssl</module>:</p>

<example><title>httpd.conf</title>

SSLVerifyClient none<br />

SSLCACertificateFile conf/ssl.crt/ca.crt<br />

<br />

&lt;Location /secure/area&gt;<br />

SSLVerifyClient require<br />

SSLVerifyDepth 1<br />

&lt;/Location&gt;<br />

</example>

</section>

<section id="certauthenticate">

<title>How can I allow only clients who have certificates to access a

particular URL, but allow all clients to access the rest of the server?</title>

<p>The key to doing this is checking that part of the client certificate

matches what you expect. Usually this means checking all or part of the

Distinguished Name (DN), to see if it contains some known string.

There are two ways to do this, using either <module>mod_auth_basic</module> or

<directive module="mod_ssl">SSLRequire</directive>.</p>

<p>The <module>mod_auth_basic</module> method is generally required when

the certificates are completely arbitrary, or when their DNs have

no common fields (usually the organisation, etc.). In this case,

you should establish a password database containing <em>all</em>

clients allowed, as follows:</p>

<example><title>httpd.conf</title><pre>

SSLVerifyClient none

&lt;Directory /usr/local/apache2/htdocs/secure/area&gt;

SSLVerifyClient require

SSLVerifyDepth 5

SSLCACertificateFile conf/ssl.crt/ca.crt

SSLCACertificatePath conf/ssl.crt

SSLOptions +FakeBasicAuth

SSLRequireSSL

AuthName "Snake Oil Authentication"

AuthType Basic

AuthBasicProvider file

AuthUserFile /usr/local/apache2/conf/httpd.passwd

Require valid-user

&lt;/Directory&gt;</pre>

</example>

<p>The password used in this example is the DES encrypted string "password".

See the <directive module="mod_ssl">SSLOptions</directive> docs for more

information.</p>

<example><title>httpd.passwd</title><pre>

/C=DE/L=Munich/O=Snake Oil, Ltd./OU=Staff/CN=Foo:xxj31ZMTZzkVA

/C=US/L=S.F./O=Snake Oil, Ltd./OU=CA/CN=Bar:xxj31ZMTZzkVA

/C=US/L=L.A./O=Snake Oil, Ltd./OU=Dev/CN=Quux:xxj31ZMTZzkVA</pre>

</example>

<p>When your clients are all part of a common hierarchy, which is encoded

into the DN, you can match them more easily using <directive module="mod_ssl"

>SSLRequire</directive>, as follows:</p>

<example><title>httpd.conf</title><pre>

SSLVerifyClient none

&lt;Directory /usr/local/apache2/htdocs/secure/area&gt;

SSLVerifyClient require

SSLVerifyDepth 5

SSLCACertificateFile conf/ssl.crt/ca.crt

SSLCACertificatePath conf/ssl.crt

SSLOptions +FakeBasicAuth

SSLRequireSSL

SSLRequire %{SSL_CLIENT_S_DN_O} eq "Snake Oil, Ltd." \

and %{SSL_CLIENT_S_DN_OU} in {"Staff", "CA", "Dev"}

&lt;/Directory&gt;</pre>

</example>

</section>

<section id="intranet">

<title>How can I require HTTPS with strong ciphers, and either basic

authentication or client certificates, for access to part of the

Intranet website, for clients coming from the Internet? I still want to allow

plain HTTP access for clients on the Intranet.</title>

<p>These examples presume that clients on the Intranet have IPs in the range

192.168.1.0/24, and that the part of the Intranet website you want to allow

internet access to is <code>/usr/local/apache2/htdocs/subarea</code>.

This configuration should remain outside of your HTTPS virtual host, so

that it applies to both HTTPS and HTTP.</p>

<example><title>httpd.conf</title><pre>

SSLCACertificateFile conf/ssl.crt/company-ca.crt

&lt;Directory /usr/local/apache2/htdocs&gt;

# Outside the subarea only Intranet access is granted

Order deny,allow

Deny from all

Allow from 192.168.1.0/24

&lt;/Directory&gt;

&lt;Directory /usr/local/apache2/htdocs/subarea&gt;

# Inside the subarea any Intranet access is allowed

# but from the Internet only HTTPS + Strong-Cipher + Password

# or the alternative HTTPS + Strong-Cipher + Client-Certificate

# If HTTPS is used, make sure a strong cipher is used.

# Additionally allow client certs as alternative to basic auth.

SSLVerifyClient optional

SSLVerifyDepth 1

SSLOptions +FakeBasicAuth +StrictRequire

SSLRequire %{SSL_CIPHER_USEKEYSIZE} &gt;= 128

# Force clients from the Internet to use HTTPS

RewriteEngine on

RewriteCond %{REMOTE_ADDR} !^192\.168\.1\.[0-9]+$

RewriteCond %{HTTPS} !=on

RewriteRule .* - [F]

# Allow Network Access and/or Basic Auth

Satisfy any

# Network Access Control

Order deny,allow

Deny from all

Allow 192.168.1.0/24

# HTTP Basic Authentication

AuthType basic

AuthName "Protected Intranet Area"

AuthBasicProvider file

AuthUserFile conf/protected.passwd

Require valid-user

&lt;/Directory&gt;</pre>

</example>

</section>

</section>

<section id="logging">

<title>Logging</title>

<p><module>mod_ssl</module> can log extremely verbose debugging information

to the error log, when its <directive module="core">LogLevel</directive> is

set to the higher trace levels. On the other hand, on a very busy server,

level <code>info</code> may already be too much. Remember that you can

configure the <directive module="core">LogLevel</directive> per module to

suite your needs.</p>

</section>

</manualpage>