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[apache] / docs / manual / ssl / ssl_howto.xml

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<manualpage metafile="ssl_howto.xml.meta">
<parentdocument href="./">SSL/TLS</parentdocument>

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

<summary>

<p>This document is intended to get you started, and get a few things
working. You are strongly encouraged to read the rest of the SSL
documentation, and arrive at a deeper understanding of the material,
before progressing to the advanced techniques.</p>
</summary>

<section id="configexample">
<title>Basic Configuration Example</title>

<p>Your SSL configuration will need to contain, at minimum, the
following directives.</p>

<highlight language="config">
Listen 443
&lt;VirtualHost *:443&gt;
    ServerName www.example.com
    SSLEngine on
    SSLCertificateFile "/path/to/www.example.com.cert"
    SSLCertificateKeyFile "/path/to/www.example.com.key"
&lt;/VirtualHost&gt;
</highlight>

</section>

<section id="ciphersuites">
<title>Cipher Suites and Enforcing Strong Encryption</title>

<note type="warning">
<p>"Strong encryption" is, and has always been, a moving target. Furthermore,
the definition of "strong" depends on your desired use cases, your threat
models, and your acceptable levels of risk. The Apache HTTP Server team cannot
determine these things for you.</p>
<p>For the purposes of this document, which was last updated in mid-2016,
"strong encryption" refers to a TLS implementation which provides all of the
following, in addition to the basic confidentiality, integrity, and authenticity
protection that most users already expect:</p>
<ul>
<li>Perfect Forward Secrecy, which ensures that a compromise to a server's
private key in the present does not compromise the confidentiality of past TLS
communication.</li>
<li>Protection from known attacks on older SSL and TLS implementations, such
as <a href="https://en.wikipedia.org/wiki/POODLE">POODLE</a> and
<a href="https://en.wikipedia.org/wiki/Transport_Layer_Security#BEAST_attack">BEAST</a>.</li>
<li>Support for the strongest ciphers available to modern (and up-to-date) web
browsers and other HTTP clients.</li>
<li><strong>Rejection</strong> of clients that cannot meet these requirements.
In other words, "strong encryption" requires that out-of-date clients be
completely unable to connect to the server, to prevent them from endangering
their users. Whether or not this is appropriate for your situation is a decision
that only you can make.</li>
</ul>
<p>Please note that strong <em>encryption</em> does not, by itself, ensure
strong <em>security</em>. (As an example, HTTP compression oracle attacks such
as <a href="https://en.wikipedia.org/wiki/BREACH_(security_exploit)">BREACH</a>
may require further steps to mitigate.)</p>
</note>

<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 many 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 configuration enables "strong encryption", as defined
    above, and is derived from the Mozilla Foundation's
    <a href="https://wiki.mozilla.org/Security/Server_Side_TLS">Server Side
    TLS</a> requirements:</p>

    <highlight language="config">
# "Modern" configuration, defined by the Mozilla Foundation's SSL Configuration
# Generator as of August 2016. This tool is available at
# https://mozilla.github.io/server-side-tls/ssl-config-generator/
SSLProtocol         all -SSLv3 -TLSv1 -TLSv1.1
# Many ciphers defined here require a modern version (1.0.1+) of OpenSSL. Some
# require OpenSSL 1.1.0, which as of this writing was in pre-release.
SSLCipherSuite      ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256
SSLHonorCipherOrder on
SSLCompression      off
SSLSessionTickets   off
    </highlight>

    <ul>
    <li>SSL 3.0 and TLS 1.0 are susceptible to known attacks on the protocol;
    they are disabled entirely.</li>
    <li>Disabling TLS 1.1 is (as of August 2016) mostly optional; TLS 1.2
    provides stronger encryption options, but 1.1 is not yet known to be broken.
    Disabling 1.1 may mitigate attacks against some broken TLS
    implementations.</li>
    <li>Enabling <directive module="mod_ssl">SSLHonorCipherOrder</directive>
    ensures that the server's cipher preferences are followed instead of the
    client's.</li>
    <li>Disabling <directive module="mod_ssl">SSLCompression</directive>
    prevents TLS compression oracle attacks (e.g.
    <a href="https://en.wikipedia.org/wiki/CRIME">CRIME</a>).</li>
    <li>Disabling <directive module="mod_ssl">SSLSessionTickets</directive>
    ensures Perfect Forward Secrecy is not compromised if the server is not
    restarted regularly.</li>
    </ul>

    <p>The exact ciphersuites supported in the
    <directive module="mod_ssl">SSLCipherSuite</directive> line are determined
    by your OpenSSL installation, not the server. You may need to upgrade to a
    modern version of OpenSSL in order to use them.</p>
</section>

<section id="strongurl">
<title>How can I create an SSL server which accepts many types of ciphers
in general, but requires a strong cipher 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>
    <highlight language="config">
# be liberal in general -- use Mozilla's "Intermediate" ciphersuites (weaker
# ciphersuites may also be used, but will not be documented here)
SSLCipherSuite ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA:!DSS

&lt;Location "/strong/area"&gt;
# but https://hostname/strong/area/ and below requires strong ciphersuites
SSLCipherSuite ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256
&lt;/Location&gt;
    </highlight>
</section>
</section>
<!-- /ciphersuites -->

<section id="ocspstapling">
<title>OCSP Stapling</title>

<p>The Online Certificate Status Protocol (OCSP) is a mechanism for
determining whether or not a server certificate has been revoked, and OCSP
Stapling is a special form of this in which the server, such as httpd and
mod_ssl, maintains current OCSP responses for its certificates and sends
them to clients which communicate with the server.  Most certificates
contain the address of an OCSP responder maintained by the issuing
Certificate Authority, and mod_ssl can communicate with that responder to
obtain a signed response that can be sent to clients communicating with
the server.</p>

<p>Because the client can obtain the certificate revocation status from
the server, without requiring an extra connection from the client to the
Certificate Authority, OCSP Stapling is the preferred way for the
revocation status to be obtained.  Other benefits of eliminating the
communication between clients and the Certificate Authority are that the
client browsing history is not exposed to the Certificate Authority and
obtaining status is more reliable by not depending on potentially heavily
loaded Certificate Authority servers.</p>

<p>Because the response obtained by the server can be reused for all clients
using the same certificate during the time that the response is valid, the
overhead for the server is minimal.</p>

<p>Once general SSL support has been configured properly, enabling OCSP
Stapling generally requires only very minor modifications to the httpd
configuration &mdash; the addition of these two directives:</p>

    <highlight language="config">
SSLUseStapling On
SSLStaplingCache "shmcb:ssl_stapling(32768)"
    </highlight>

<p>These directives are placed at global scope (i.e., not within a virtual
host definition) wherever other global SSL configuration directives are
placed, such as in <code>conf/extra/httpd-ssl.conf</code> for normal
open source builds of httpd, <code>/etc/apache2/mods-enabled/ssl.conf</code>
for the Ubuntu or Debian-bundled httpd, etc.</p>

<p>This particular <directive>SSLStaplingCache</directive> directive requires
<module>mod_socache_shmcb</module> (from the <code>shmcb</code> prefix on the
directive's argument).  This module is usually enabled already for
<directive>SSLSessionCache</directive> or on behalf of some module other than
<module>mod_ssl</module>.  If you enabled an SSL session cache using a
mechanism other than <module>mod_socache_shmcb</module>, use that alternative
mechanism for <directive>SSLStaplingCache</directive> as well.  For example:</p>

    <highlight language="config">
SSLSessionCache "dbm:ssl_scache"
SSLStaplingCache "dbm:ssl_stapling"
    </highlight>

<p>You can use the openssl command-line program to verify that an OCSP response
is sent by your server:</p>

<pre>
$ openssl s_client -connect www.example.com:443 -status -servername www.example.com
...
OCSP response:
======================================
OCSP Response Data:
    OCSP Response Status: successful (0x0)
    Response Type: Basic OCSP Response
...
    Cert Status: Good
...
</pre>

<p>The following sections highlight the most common situations which require
further modification to the configuration.  Refer also to the
<module>mod_ssl</module> reference manual.</p>

<section>
<title>If more than a few SSL certificates are used for the server</title>
<p>OCSP responses are stored in the SSL stapling cache.  While the responses
are typically a few hundred to a few thousand bytes in size, mod_ssl
supports OCSP responses up to around 10K bytes in size.  With more than a
few certificates, the stapling cache size (32768 bytes in the example above)
may need to be increased.  Error message AH01929 will be logged in case of
an error storing a response.</p>
</section>

<section>
<title>If the certificate does not point to an OCSP responder, or if a
different address must be used</title>
<p>Refer to the
<directive module="mod_ssl">SSLStaplingForceURL</directive> directive.</p>

<p>You can confirm that a server certificate points to an OCSP responder
using the openssl command-line program, as follows:</p>

<pre>
$ openssl x509 -in ./www.example.com.crt -text | grep 'OCSP.*http'
OCSP - URI:http://ocsp.example.com
</pre>

<p>If the OCSP URI is provided and the web server can communicate to it
directly without using a proxy, no configuration is required.  Note that
firewall rules that control outbound connections from the web server may
need to be adjusted.</p>

<p>If no OCSP URI is provided, contact your Certificate Authority to
determine if one is available; if so, configure it with
<directive module="mod_ssl">SSLStaplingForceURL</directive> in the virtual
host that uses the certificate.</p>
</section>

<section>
<title>If multiple SSL-enabled virtual hosts are configured and OCSP
Stapling should be disabled for some</title>

<p>Add <code>SSLUseStapling Off</code> to the virtual hosts for which OCSP
Stapling should be disabled.</p>
</section>

<section>
<title>If the OCSP responder is slow or unreliable</title>
<p>Several directives are available to handle timeouts and errors.  Refer
to the documentation for the
<directive module="mod_ssl">SSLStaplingFakeTryLater</directive>,
<directive module="mod_ssl">SSLStaplingResponderTimeout</directive>, and
<directive module="mod_ssl">SSLStaplingReturnResponderErrors</directive>
directives.</p>
</section>

<section>
<title>If mod_ssl logs error AH02217</title>
<pre>
AH02217: ssl_stapling_init_cert: Can't retrieve issuer certificate!
</pre>
<p>In order to support OCSP Stapling when a particular server certificate is
used, the certificate chain for that certificate must be configured.  If it
was not configured as part of enabling SSL, the AH02217 error will be issued
when stapling is enabled, and an OCSP response will not be provided for clients
using the certificate.</p>

<p>Refer to the <directive module="mod_ssl">SSLCertificateChainFile</directive>
and <directive module="mod_ssl">SSLCertificateFile</directive> for instructions
for configuring the certificate chain.</p>
</section>

</section>
<!-- /ocspstapling -->

<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>
    <highlight language="config">
# require a client certificate which has to be directly
# signed by our CA certificate in ca.crt
SSLVerifyClient require
SSLVerifyDepth 1
SSLCACertificateFile "conf/ssl.crt/ca.crt"
    </highlight>
</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>

    <highlight language="config">
SSLVerifyClient none
SSLCACertificateFile "conf/ssl.crt/ca.crt"

&lt;Location "/secure/area"&gt;
SSLVerifyClient require
SSLVerifyDepth 1
&lt;/Location&gt;
    </highlight>
</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>

    <highlight language="config">
SSLVerifyClient      none
SSLCACertificateFile "conf/ssl.crt/ca.crt"
SSLCACertificatePath "conf/ssl.crt"

&lt;Directory "/usr/local/apache2/htdocs/secure/area"&gt;
    SSLVerifyClient      require
    SSLVerifyDepth       5
    SSLOptions           +FakeBasicAuth
    SSLRequireSSL
    AuthName             "Snake Oil Authentication"
    AuthType             Basic
    AuthBasicProvider    file
    AuthUserFile         "/usr/local/apache2/conf/httpd.passwd"
    Require              valid-user
&lt;/Directory&gt;
    </highlight>

    <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>


    <highlight language="config">
SSLVerifyClient      none
SSLCACertificateFile "conf/ssl.crt/ca.crt"
SSLCACertificatePath "conf/ssl.crt"

&lt;Directory "/usr/local/apache2/htdocs/secure/area"&gt;
  SSLVerifyClient      require
  SSLVerifyDepth       5
  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;
    </highlight>
</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>

    <highlight language="config">
SSLCACertificateFile "conf/ssl.crt/company-ca.crt"

&lt;Directory "/usr/local/apache2/htdocs"&gt;
    #   Outside the subarea only Intranet access is granted
    Require              ip 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
    Require              ip 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;
    </highlight>
</section>
</section>
<!-- /access control -->

<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>