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SSL/TLS Strong Encryption: How-To - Apache HTTP Server Version 2.4








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Apache HTTP Server Version 2.4



Apache > HTTP Server > Documentation > Version 2.4 > SSL/TLSSSL/TLS Strong Encryption: How-To

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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.

 Basic Configuration Example
 Cipher Suites and Enforcing Strong Security
 OCSP Stapling
 Client Authentication and Access Control
 Logging
See alsoComments


Basic Configuration Example


Your SSL configuration will need to contain, at minimum, the
following directives.

LoadModule ssl_module modules/mod_ssl.so

Listen 443
<VirtualHost *:443>
    ServerName www.example.com
    SSLEngine on
    SSLCertificateFile "/path/to/www.example.com.cert"
    SSLCertificateKeyFile "/path/to/www.example.com.key"
</VirtualHost>




Cipher Suites and Enforcing Strong Security


How can I create an SSL server which accepts strong encryption only?
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?


How can I create an SSL server which accepts strong encryption
only?

    The following enables only the strongest ciphers:
    SSLCipherSuite HIGH:!aNULL:!MD5


    While with the following configuration you specify a preference
    for specific speed-optimized ciphers (which will be selected by
    mod_ssl, provided that they are supported by the client):

    SSLCipherSuite RC4-SHA:AES128-SHA:HIGH:!aNULL:!MD5
SSLHonorCipherOrder on



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?

    Obviously, a server-wide SSLCipherSuite which restricts
    ciphers to the strong variants, isn't the answer here. However,
    mod_ssl can be reconfigured within Location
    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:
    # be liberal in general
SSLCipherSuite ALL:!aNULL:RC4+RSA:+HIGH:+MEDIUM:+LOW:+EXP:+eNULL

<Location "/strong/area">
# but https://hostname/strong/area/ and below
# requires strong ciphers
SSLCipherSuite HIGH:!aNULL:!MD5
</Location>




OCSP Stapling


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.

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.

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.

Once general SSL support has been configured properly, enabling OCSP
Stapling generally requires only very minor modifications to the httpd
configuration — the addition of these two directives:

    SSLUseStapling On
SSLStaplingCache "shmcb:logs/ssl_stapling(32768)"


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 conf/extra/httpd-ssl.conf for normal 
open source builds of httpd, /etc/apache2/mods-enabled/ssl.conf
for the Ubuntu or Debian-bundled httpd, etc.

The path on the SSLStaplingCache directive
(e.g., logs/) should match the one on the 
SSLSessionCache directive.  This path is relative
to ServerRoot.

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

    SSLSessionCache "dbm:logs/ssl_scache"
SSLStaplingCache "dbm:logs/ssl_stapling"


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

$ 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
...

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

If more than a few SSL certificates are used for the server

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.


If the certificate does not point to an OCSP responder, or if a
different address must be used

Refer to the 
SSLStaplingForceURL directive.

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

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

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.

If no OCSP URI is provided, contact your Certificate Authority to
determine if one is available; if so, configure it with
SSLStaplingForceURL in the virtual
host that uses the certificate.


If multiple SSL-enabled virtual hosts are configured and OCSP
Stapling should be disabled for some


Add SSLUseStapling Off to the virtual hosts for which OCSP
Stapling should be disabled.


If the OCSP responder is slow or unreliable

Several directives are available to handle timeouts and errors.  Refer
to the documentation for the
SSLStaplingFakeTryLater,
SSLStaplingResponderTimeout, and
SSLStaplingReturnResponderErrors
directives.


If mod_ssl logs error AH02217

AH02217: ssl_stapling_init_cert: Can't retrieve issuer certificate!
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.

Refer to the SSLCertificateChainFile
and SSLCertificateFile for instructions
for configuring the certificate chain.




Client Authentication and Access Control


How can I force clients to authenticate using certificates?
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?
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?
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?


How can I force clients to authenticate using certificates?


    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 (ca.crt) and then verify the clients against this
    certificate.
    # 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"



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?


    To force clients to authenticate using certificates for a particular URL,
    you can use the per-directory reconfiguration features of
    mod_ssl:

    SSLVerifyClient none
SSLCACertificateFile "conf/ssl.crt/ca.crt"

<Location "/secure/area">
SSLVerifyClient require
SSLVerifyDepth 1
</Location>



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?


    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 mod_auth_basic or
    SSLRequire.

    The mod_auth_basic 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 all
    clients allowed, as follows:

    SSLVerifyClient      none
SSLCACertificateFile "conf/ssl.crt/ca.crt"
SSLCACertificatePath "conf/ssl.crt"

<Directory "/usr/local/apache2/htdocs/secure/area">
    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
</Directory>


    The password used in this example is the DES encrypted string "password".
    See the SSLOptions docs for more
    information.

    httpd.passwd/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

    When your clients are all part of a common hierarchy, which is encoded
    into the DN, you can match them more easily using SSLRequire, as follows:


    SSLVerifyClient      none
SSLCACertificateFile "conf/ssl.crt/ca.crt"
SSLCACertificatePath "conf/ssl.crt"

<Directory "/usr/local/apache2/htdocs/secure/area">
  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"}
</Directory>



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.


   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 /usr/local/apache2/htdocs/subarea.
   This configuration should remain outside of your HTTPS virtual host, so
   that it applies to both HTTPS and HTTP.

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

<Directory "/usr/local/apache2/htdocs">
    #   Outside the subarea only Intranet access is granted
    Require              ip 192.168.1.0/24
</Directory>

<Directory "/usr/local/apache2/htdocs/subarea">
    #   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} >= 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
</Directory>




Logging
    

    mod_ssl can log extremely verbose debugging information
    to the error log, when its LogLevel is
    set to the higher trace levels. On the other hand, on a very busy server,
    level info may already be too much. Remember that you can
    configure the LogLevel per module to
    suite your needs.


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