Apache > HTTP Server > Documentation > Version 2.4 > SSL/TLS

SSL/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.

See also


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"

Cipher Suites and Enforcing Strong Security

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):

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

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

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The Online Certificate Status Protocol (OCSP) is a mechanism for determining whether or not a server certificate has been revoked, and OCSP Stapling is a special form of this in which the server, such as httpd and mod_ssl, maintains current OCSP responses for its certificates and sends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