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

Apache HTTP Server Version 2.4

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

# SSL/TLS Strong Encryption: How-To

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This documented 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.

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## 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>

## 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.

Available Languages: en | fr

## Comments

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