mod_ssl Chapter 3
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Reference
``Try to understand everything, but believe nothing!''
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T his chapter provides a reference to all configuration directives and additional user visible features mod_ssl provides. It's intended as the official resource when you want to know how a particilar mod_ssl functionality is actually configured or activated. Each directive is documented similar to the way standard Apache directives are documented in the official Apache documentation set, i.e. for each directive especially the syntax, default and context where applicable is given.

Notice that there are three major classes of directives which are used by mod_ssl: First Global Directives (i.e. directives with context ``server config''), which can occur inside the server config files but only outside of any sectioning commands like <VirtualHost>. Second Per-Server Directives (i.e. those with context ``server config, virtual host''), which can occur inside the server config files both outside (for the main/default server) and inside <VirtualHost> sections.

  
Table Of Contents
Configuration Directives
        SSLPassPhraseDialog
        SSLMutex
        SSLRandomSeed
        SSLSessionCache
        SSLSessionCacheTimeout
        SSLEngine
        SSLProtocol
        SSLCipherSuite
        SSLCertificateFile
        SSLCertificateKeyFile
        SSLCACertificatePath
        SSLCACertificateFile
        SSLCARevocationPath
        SSLCARevocationFile
        SSLVerifyClient
        SSLVerifyDepth
        SSLLog
        SSLLogLevel
        SSLOptions
        SSLRequireSSL
        SSLRequire
Additional Features
        Environment Variables
        Custom Log Formats

And third Per-Directory Directives (i.e. those with context ``server config, virtual host, directory, .htaccess''), which can occur mostly everywhere. Especially both inside the server config files and the per-directory .htaccess files. The three classes are subsets of each other, i.e. directives from the per-directory class can also be used in the per-server and global context, and directives from the per-server class can also be used the in the global context.

Additional directives and environment variables provided by mod_ssl (via on-the-fly mapping) for backward compatiblity to other Apache SSL solutions are documented in the Compatibility chapter.

Configuration Directives

The most visible and error-prone things of mod_ssl are the configuration directives it provides. So we document them in great detail here to assist you in setting up the best possible configuration of your SSL-aware webserver.


SSLPassPhraseDialog

Name: SSLPassPhraseDialog
Description: Type of pass phrase dialog for encrypted private keys
Syntax: SSLPassPhraseDialog type
Default: SSLPassPhraseDialog builtin
Context: server config
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

When Apache starts up it has to read the various Certificate (see SSLCertificateFile) and Private Key (see SSLCertificateKeyFile) files of the SSL-enabled virtual servers. Because for security reasons the Private Key files are usually encrypted, mod_ssl needs to query the administrator for a Pass Phrase in order to decrypt those files. This query can be done in two ways which can be configured by type:

  • builtin

    This is the default where an interactive terminal dialog occurs at startup time just before Apache detaches from the terminal. Here the administrator has to manually enter the Pass Phrase for each encrypted Private Key file. Because a lot of SSL-enabled virtual hosts can be configured, the following reuse-scheme is used to minimize the dialog: When a Private Key file is encrypted, all known Pass Phrases (at the beginning there are none, of course) are tried. If one of those known Pass Phrases succeeds no dialog pops up for this particular Private Key file. If none succeeded, another Pass Phrase is queried on the terminal and remembered for the next round (where it perhaps can be reused).

    This scheme allows mod_ssl to be maximally flexible (because for N encrypted Private Key files you can use N different Pass Phrases - but then you have to enter all of them, of course) while minimizing the terminal dialog (i.e. when you use a single Pass Phrase for all N Private Key files this Pass Phrase is queried only once).

  • exec:/path/to/program

    Here an external program is configured which is called at startup for each encrypted Private Key file. It is called with an argument of ``servername:portnumber'' for which it has to print the corresponding Pass Phrase to stdout. The intent is that this external program first runs security checks to make sure that the system is not compromised by an attacker, and only when these checks were passed successfully it provides the Pass Phrase.

    Both these security checks and the way the Pass Phrase is determined can be as complex as one could think about it. mod_ssl just defines the interface: an executable program which provides the Pass Phrase on stdout. Nothing more or less! So, when you're really paranoid about security, here is your interface. Anything else has to be left as an exercise to the administrator because local security requirements are too different.

    The reuse-algorithm above is used here, too. In other words: The external program is called only once per unique Pass Phrase.

Example:

SSLPassPhraseDialog exec:/usr/local/apache/sbin/pp-filter


SSLMutex

Name: SSLMutex
Description: Semaphore for internal mutual exclusion of operations
Syntax: SSLMutex type
Default: SSLMutex none
Context: server config
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This configures the SSL engine's semaphore (aka. lock) which is used for mutual exclusion of operations which have to be done in a synchronized way between the pre-forked Apache server processes. This directive can only be used in the global server context because it's only useful to have one global mutex.

The following Mutex types are available:

  • none

    This is the default where no Mutex is used at all. Use it at your own risk. But because currently the Mutex is mainly used for synchronizing write access to the SSL Session Cache you can live without it as long as you accept a sometimes garbled Session Cache. So it's not recommended to leave this the default. Instead configure a real Mutex.

  • file:/path/to/mutex

    This is the portable and always provided Mutex variant where a physical (lock-)file is used as the Mutex. Always use a local disk filesystem for /path/to/mutex and never a file residing on a NFS- or AFS-filesystem. Notice: Internally the Process ID (PID) of the Apache parent process is automatically appended to /path/to/mutex to make it unique, so you don't have to care about conflicts yourself.

  • sem

    This is the most elegant but also most non-portable Mutex variant where a SysV IPC Semaphore (under Unix) and a Windows Mutex (under Win32) is used when possible. It is only available when the underlaying platform supports it.

Example:

SSLMutex file:/usr/local/apache/logs/ssl_mutex


SSLRandomSeed

Name: SSLRandomSeed
Description: Pseudo Random Number Generator (PRNG) seeding source
Syntax: SSLRandomSeed context source [bytes]
Default: none
Context: server config
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.2

This configures one or more sources for seeding the Pseudo Random Number Generator (PRNG) in OpenSSL at startup time (context is startup) and/or just before a new SSL connection is established (context is connect). This directive can only be used in the global server context because the PRNG is a global facility.

The following source variants are available:

  • builtin

    This is the always available builtin seeding source. It's usage consumes minimum CPU cycles under runtime and hence can be always used without drawbacks. The source used for seeding the PRNG contains of the current time, the current process id and (when applicable) a randomly choosen 1KB extract of the inter-process scoreboard structure of Apache. The drawback is that this is not really a strong source and at startup time (where the scoreboard is still not available) this source just produces a few bytes of entropy. So you should always, at least for the startup, use an additional seeding source.

  • file:/path/to/source

    This variant uses an external file /path/to/source as the source for seeding the PRNG. When bytes is specified only the first bytes number of bytes of the file form the entropy. When bytes is not specified the whole file forms the entropy. Use this especially at startup time, for instance with an available /dev/random and/or /dev/urandom devices (which usually exist on modern Unix derivates like FreeBSD and Linux).

    But be careful: Usually /dev/random provides only as much entropy data as it actually has, i.e. when you request 512 bytes of entropy, but the device currently has only 100 bytes available two things can happen: On some platforms you receive only the 100 bytes while on other platforms the read blocks until enough bytes are available (which can take a long time). Here using an existing /dev/urandom is better, because it never blocks and actually gives the amount of requested data. The drawback is just that the quality of the received data may not be the best.

    On some platforms like FreeBSD one can even control how the entropy is actually generated, i.e. by which system interrupts. More details one can find under rndcontrol(8) on those platforms. Alternatively, when your system lacks such a random device, you can use tool like EGD (Entropy Gathering Daemon) and run it's client program with the exec:/path/to/program/ variant (see below).

  • exec:/path/to/program

    This variant uses an external executable /path/to/program as the source for seeding the PRNG. When bytes is specified only the first bytes number of bytes of it's stdout contents form the entropy. When bytes is not specified the whole data produced on stdout form the entropy. Use this only at startup time when you need a very strong seeding with the help of an external program (for instance as in the example above with the truerand utility you can find in the mod_ssl distribution which is based on the AT&T truerand library). Using this at the connection context slows down the server too dramatically, of course. So usually you should avoid using external programs at this context.

Example:

SSLRandomSeed startup builtin
SSLRandomSeed startup file:/dev/random
SSLRandomSeed startup file:/dev/urandom 1024
SSLRandomSeed startup exec:/usr/local/bin/truerand 16
SSLRandomSeed connect builtin
SSLRandomSeed connect file:/dev/random
SSLRandomSeed connect file:/dev/urandom 1024


SSLSessionCache

Name: SSLSessionCache
Description: Type of the global/inter-process SSL Session Cache
Syntax: SSLSessionCache type
Default: SSLSessionCache none
Context: server config
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This configures the storage type of the global/inter-process SSL Session Cache. This cache is an optional facility which speeds up parallel request processing. Because for requests to the same server process (via HTTP keep-alive) OpenSSL already caches the SSL session information locally. But because modern clients request inlined images and other data via parallel requests (usually up to four parallel requests are common) those requests are served by different pre-forked server processes. Here an inter-process cache helps to avoid unneccessary session handshakes.

The following two storage types are currently supported:

  • none

    This is the default and just disables the global/inter-process Session Cache. There is no drawback in functionality, but a noticeable speed penalty can be observed.

  • dbm:/path/to/datafile

    This makes use of a DBM hashfile on the local disk to synchronize the local OpenSSL memory caches of the server processes. The little more amount of I/O on the server results in a visible request speedup for your clients. So it's recommended to use this storage.

  • shm:/path/to/datafile[(size)]

    This makes use of a high-performance hash table (approx. size bytes in size) inside a shared memory segment in RAM (established via /path/to/datafile) to synchronize the local OpenSSL memory caches of the server processes. This storage type is not available on all platforms. See the mod_ssl INSTALL document for details on how to build Apache+EAPI with shared memory support.

Examples:

SSLSessionCache dbm:/usr/local/apache/logs/ssl_gcache_data
SSLSessionCache shm:/usr/local/apache/logs/ssl_gcache_data(512000)


SSLSessionCacheTimeout

Name: SSLSessionCacheTimeout
Description: Number of seconds before an SSL session expires in the Session Cache
Syntax: SSLSessionCacheTimeout seconds
Default: SSLSessionCacheTimeout 300
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive sets the timeout in seconds for the information stored in the global/inter-process SSL Session Cache and the OpenSSL internal memory cache. It can be set as low as 15 for testing, but should be set to higher values like 300 in real life.

Example:

SSLSessionCacheTimeout 600


SSLEngine

Name: SSLEngine
Description: SSL Engine Operation Switch
Syntax: SSLEngine on|off
Default: SSLEngine off
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This directive toggles the usage of the SSL/TLS Protocol Engine. This is usually used inside a <VirtualHost> section to enable SSL/TLS for a particular virtual host. Per default the SSL/TLS Protocol Engine is disabled for both the main server and all configured virtual hosts.

Example:

<VirtualHost _default_:443>
SSLEngine on
...
</VirtualHost>


SSLProtocol

Name: SSLProtocol
Description: Configure useable SSL protocol flavors
Syntax: SSLProtocol [+-]protocol ...
Default: SSLProtocol all
Context: server config, virtual host
Override: Options
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.2

This directive can be used to control the SSL protocol flavors mod_ssl should use when establishing it's server environment. Clients then can only connect with one of the provided protocols.

The available (case-insensitive) protocols are:

  • SSLv2

    This is the Secure Sockets Layer (SSL) protocol, version 2.0. It is the original SSL protocol as designed by Netscape Corporation.

  • SSLv3

    This is the Secure Sockets Layer (SSL) protocol, version 3.0. It is the successor to SSLv2 and the currently (as of February 1999) de-facto standardized SSL protocol from Netscape Corporation. It's supported by mostly all popular browsers.

  • TLSv1

    This is the Transport Layer Security (TLS) protocol, version 1.0. It is the successor to SSLv3 and currently (as of February 1999) still under construction by the Internet Engineering Task Force (IETF). It's still not supported by any popular browsers.

  • All

    This is a shortcut for ``+SSLv2 +SSLv3 +TLSv1'' and a convinient way for enabling all protocols except one when used in combination with the minus sign on a protocol as the example above shows.

Example:

#   enable SSLv3 and TLSv1, but not SSLv2
SSLProtocol all -SSLv2


SSLCipherSuite

Name: SSLCipherSuite
Description: Cipher Suite available for negotiation in SSL handshake
Syntax: SSLCipherSuite cipher-spec
Default: SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP
Context: server config, virtual host, directory, .htaccess
Override: AuthConfig
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This complex directive uses a colon-separated cipher-spec string consisting of OpenSSL cipher specifications to configure the Cipher Suite the client is permitted to negotiate in the SSL handshake phase. Notice that this directive can be used both in per-server and per-directory context. In per-server context it applies to the standard SSL handshake when a connection is established. In per-directory context it forces a SSL renegotation with the reconfigured Cipher Suite after the HTTP request was read but before the HTTP response is sent.

An SSL cipher specification in cipher-spec is composed of 4 major attributes plus a few extra minor ones:

  • Key Exchange Algorithm:
    RSA or Diffie-Hellman variants.

  • Authentication Algorithm:
    RSA, Diffie-Hellman, DSS or none.

  • Cipher/Encryption Algorithm:
    DES, Triple-DES, RC4, RC2, IDEA or none.

  • MAC Digest Algorithm:
    MD5, SHA or SHA1.
An SSL cipher can also be an export cipher and is either a SSLv2 or SSLv3/TLSv1 cipher (here TLSv1 is equivalent to SSLv3). To specify which ciphers to use, one can either specify all the Ciphers, one at a time, or use aliases to specify the preference and order for the ciphers (see Table 1).

Table 1: OpenSSL Cipher Specification Tags
Tag Description
Key Exchange Algorithm:
kRSA RSA key exchange
kDHr Diffie-Hellman key exchange with RSA key
kDHd Diffie-Hellman key exchange with DSA key
kEDH Ephemeral (temp.key) Diffie-Hellman key exchange (no cert)
Authentication Algorithm:
aNULL No authentication
aRSA RSA authentication
aDSS DSS authentication
aDH Diffie-Hellman authentication
Cipher Encoding Algorithm:
eNULL No encoding
DES DES encoding
3DES Triple-DES encoding
RC4 RC4 encoding
RC2 RC2 encoding
IDEA IDEA encoding
MAC Digest Algorithm:
MD5 MD5 hash function
SHA1 SHA1 hash function
SHA SHA hash function
Aliases:
SSLv2 all SSL version 2.0 ciphers
SSLv3 all SSL version 3.0 ciphers
EXP all export ciphers
LOW all low strength ciphers (no export, single DES)
MEDIUM all ciphers with 128 bit encryption
HIGH all ciphers using Triple-DES
RSA all ciphers using RSA key exchange
DH all ciphers using Diffie-Hellman key exchange
EDH all ciphers using Ephemeral Diffie-Hellman key exchange
ADH all ciphers using Anonymous Diffie-Hellman key exchange
DSS all ciphers using DSS authentication
NULL all ciphers using no encryption

Now where this becomes interesting is that these can be put together to specify the order and ciphers you wish to use. To speed this up there are also aliases (SSLv2, SSLv3, EXP, LOW, MEDIUM, HIGH) for certain groups of ciphers. These tags can be joined together with prefixes to form the cipher-spec. Available prefixes are:

  • none: add cipher to list
  • +: add ciphers to list and pull them to current location in list
  • -: remove cipher from list (can be added later again)
  • !: kill cipher from list completely (can not be added later again)
A simpler way to look at all of this is to use the ``openssl ciphers -v'' command which provides a nice way to successively create the correct cipher-spec string. The default cipher-spec string is ``ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP'' which means the following: first, remove from consideration any ciphers that do not authenticate, i.e. for SSL only the Anonymous Diffie-Hellman ciphers. Next, use ciphers using RC4 and RSA. Next include the high, medium and then the low security ciphers. Finally pull all SSLv2 and export ciphers to the end of the list.
$ openssl ciphers -v 'ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP'
NULL-SHA                SSLv3 Kx=RSA      Au=RSA  Enc=None      Mac=SHA1
NULL-MD5                SSLv3 Kx=RSA      Au=RSA  Enc=None      Mac=MD5
EDH-RSA-DES-CBC3-SHA    SSLv3 Kx=DH       Au=RSA  Enc=3DES(168) Mac=SHA1
...                     ...               ...     ...           ...
EXP-RC4-MD5             SSLv3 Kx=RSA(512) Au=RSA  Enc=RC4(40)   Mac=MD5  export
EXP-RC2-CBC-MD5         SSLv2 Kx=RSA(512) Au=RSA  Enc=RC2(40)   Mac=MD5  export
EXP-RC4-MD5             SSLv2 Kx=RSA(512) Au=RSA  Enc=RC4(40)   Mac=MD5  export
The complete list of particular RSA & DH ciphers for SSL is given in Table 2.

Example:

SSLCipherSuite RSA:!EXP:!NULL:+HIGH:+MEDIUM:-LOW

Table 2: Particular SSL Ciphers
Cipher-Tag Protocol Key Ex. Auth. Enc. MAC Type
RSA Ciphers:
DES-CBC3-SHA SSLv3 RSA RSA 3DES(168) SHA1  
DES-CBC3-MD5 SSLv2 RSA RSA 3DES(168) MD5  
IDEA-CBC-SHA SSLv3 RSA RSA IDEA(128) SHA1  
RC4-SHA SSLv3 RSA RSA RC4(128) SHA1  
RC4-MD5 SSLv3 RSA RSA RC4(128) MD5  
IDEA-CBC-MD5 SSLv2 RSA RSA IDEA(128) MD5  
RC2-CBC-MD5 SSLv2 RSA RSA RC2(128) MD5  
RC4-MD5 SSLv2 RSA RSA RC4(128) MD5  
DES-CBC-SHA SSLv3 RSA RSA DES(56) SHA1  
RC4-64-MD5 SSLv2 RSA RSA RC4(64) MD5  
DES-CBC-MD5 SSLv2 RSA RSA DES(56) MD5  
EXP-DES-CBC-SHA SSLv3 RSA(512) RSA DES(40) SHA1 export
EXP-RC2-CBC-MD5 SSLv3 RSA(512) RSA RC2(40) MD5 export
EXP-RC4-MD5 SSLv3 RSA(512) RSA RC4(40) MD5 export
EXP-RC2-CBC-MD5 SSLv2 RSA(512) RSA RC2(40) MD5 export
EXP-RC4-MD5 SSLv2 RSA(512) RSA RC4(40) MD5 export
NULL-SHA SSLv3 RSA RSA None SHA1  
NULL-MD5 SSLv3 RSA RSA None MD5  
Diffie-Hellman Ciphers:
ADH-DES-CBC3-SHA SSLv3 DH None 3DES(168) SHA1  
ADH-DES-CBC-SHA SSLv3 DH None DES(56) SHA1  
ADH-RC4-MD5 SSLv3 DH None RC4(128) MD5  
EDH-RSA-DES-CBC3-SHA SSLv3 DH RSA 3DES(168) SHA1  
EDH-DSS-DES-CBC3-SHA SSLv3 DH DSS 3DES(168) SHA1  
EDH-RSA-DES-CBC-SHA SSLv3 DH RSA DES(56) SHA1  
EDH-DSS-DES-CBC-SHA SSLv3 DH DSS DES(56) SHA1  
EXP-EDH-RSA-DES-CBC-SHA SSLv3 DH(512) RSA DES(40) SHA1 export
EXP-EDH-DSS-DES-CBC-SHA SSLv3 DH(512) DSS DES(40) SHA1 export
EXP-ADH-DES-CBC-SHA SSLv3 DH(512) None DES(40) SHA1 export
EXP-ADH-RC4-MD5 SSLv3 DH(512) None RC4(40) MD5 export


SSLCertificateFile

Name: SSLCertificateFile
Description: Server PEM-encoded X.509 Certificate file
Syntax: SSLCertificateFile filename
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive points to the PEM-encoded Certificate file for the server and optionally also to the corresponding RSA or DSA Private Key file for it (contained in the same file). If the contained Private Key is encrypted the Pass Phrase dialog is forced at startup time. This directive can be used up to two times (referencing different filenames) when both a RSA and a DSA based server certificate is used in parallel.

Example:

SSLCertificateFile /usr/local/apache/conf/ssl.crt/server.crt


SSLCertificateKeyFile

Name: SSLCertificateKeyFile
Description: Server PEM-encoded RSA Private Key file
Syntax: SSLCertificateKeyFile filename
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive points to the PEM-encoded Private Key file for the server. If the Private Key is not combined with the Certificate in the SSLCertificateFile, use this additional directive to point to the file with the stand-alone Private Key. When SSLCertificateFile is used and the file contains both the Certificate and the Private Key this directive need not be used. But we strongly dissuade from this practice. Instead it is recommended to separate the Certificate and the Private Key. If the contained Private Key is encrypted, the Pass Phrase dialog is forced at startup time. This directive can be used up to two times (referencing different filenames) when both a RSA and a DSA based private key is used in parallel.

Example:

SSLCertificateKeyFile /usr/local/apache/conf/ssl.key/server.key


SSLCACertificatePath

Name: SSLCACertificatePath
Description: Directory of PEM-encoded CA Certificates for Client Auth.
Syntax: SSLCACertificatePath directory
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive sets the directory where you keep the Certificates of Certification Authorities (CAs) whose clients you deal with. These are used to verify the client certificate on Client Authentication.

The files in this directory have to be PEM-encoded and are accessed through hash filenames. So usually you have not only to place the Certificate files there. Additionally you have to create symbolic links named hash-value.N. And you should always make sure this directory contains the appropriate symbolic links. Use the Makefile which comes with mod_ssl to accomplish this task.

Example:

SSLCACertificatePath /usr/local/apache/conf/ssl.crt/


SSLCACertificateFile

Name: SSLCACertificateFile
Description: File of concatenated PEM-encoded CA Certificates for Client Auth.
Syntax: SSLCACertificateFile filename
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive sets the all-in-one file where you can assemble the Certificates of Certification Authorities (CA) whose clients you deal with. These are used for Client Authentication. Such a file is simply the concatenation of the various PEM-encoded Certificate files, in order of preference. This can be used alternatively and/or additionally to SSLCACertificatePath.

Example:

SSLCACertificateFile /usr/local/apache/conf/ssl.crt/ca-bundle-client.crt


SSLCARevocationPath

Name: SSLCARevocationPath
Description: Directory of PEM-encoded CA CRLs for Client Auth.
Syntax: SSLCARevocationPath directory
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.3

This directive sets the directory where you keep the Certificate Revocation Lists (CRL) of Certification Authorities (CAs) whose clients you deal with. These are used to revoke the client certificate on Client Authentication.

The files in this directory have to be PEM-encoded and are accessed through hash filenames. So usually you have not only to place the CRL files there. Additionally you have to create symbolic links named hash-value.rN. And you should always make sure this directory contains the appropriate symbolic links. Use the Makefile which comes with mod_ssl to accomplish this task.

Example:

SSLCARevocationPath /usr/local/apache/conf/ssl.crl/


SSLCARevocationFile

Name: SSLCARevocationFile
Description: File of concatenated PEM-encoded CA CRLs for Client Auth.
Syntax: SSLCARevocationFile filename
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.3

This directive sets the all-in-one file where you can assemble the Certificate Revocation Lists (CRL) of Certification Authorities (CA) whose clients you deal with. These are used for Client Authentication. Such a file is simply the concatenation of the various PEM-encoded CRL files, in order of preference. This can be used alternatively and/or additionally to SSLCARevocationPath.

Example:

SSLCARevocationFile /usr/local/apache/conf/ssl.crl/ca-bundle-client.crl


SSLVerifyClient

Name: SSLVerifyClient
Description: Type of Client Certificate verification
Syntax: SSLVerifyClient level
Default: SSLVerifyClient none
Context: server config, virtual host, directory, .htaccess
Override: AuthConfig
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive sets the Certificate verification level for the Client Authentication. Notice that this directive can be used both in per-server and per-directory context. In per-server context it applies to the client authentication process used in the standard SSL handshake when a connection is established. In per-directory context it forces a SSL renegotation with the reconfigured client verification level after the HTTP request was read but before the HTTP response is sent.

The following levels are available for level:

  • none: no client Certificate is required at all
  • optional: the client may present a valid Certificate
  • require: the client has to present a valid Certificate
  • optional_no_ca: the client may present a valid Certificate
    but has not to be (successfully) verifyable.
In practice only levels none and require are really interesting. Because level optional doesn't work with all browsers and level optional_no_ca is actually against the idea of authentication (but can be used to establish SSL test pages, etc.)

Example:

SSLVerifyClient require


SSLVerifyDepth

Name: SSLVerifyDepth
Description: Maximum depth of CA Certificates in Client Certificate verification
Syntax: SSLVerifyDepth number
Default: SSLVerifyDepth 1
Context: server config, virtual host, directory, .htaccess
Override: AuthConfig
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive sets how deeply mod_ssl should verify before deciding that the clients don't have a valid certificate. Notice that this directive can be used both in per-server and per-directory context. In per-server context it applies to the client authentication process used in the standard SSL handshake when a connection is established. In per-directory context it forces a SSL renegotation with the reconfigured client verification depth after the HTTP request was read but before the HTTP response is sent.

The depth actually is the maximum number of intermediate certificate issuers, i.e. the number of CA certificates which are max allowed to be followed while verifying the client certificate. A depth of 0 means that self-signed client certificates are accepted only, the default depth of 1 means the client certificate can be self-signed or has to be signed by a CA which is directly known to the server (i.e. the CA's certificate is under SSLCACertificatePath), etc.

Example:

SSLVerifyDepth 10


SSLLog

Name: SSLLog
Description: Where to write the dedicated SSL engine logfile
Syntax: SSLLog filename
Default: None
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This directive sets the name of the dedicated SSL protocol engine logfile. Error type messages are additionally duplicated to the general Apache error log file (directive ErrorLog). Put this somewhere where it cannot be used for symlink attacks on a real server (i.e. somewhere where only root can write). If the filename does not begin with a slash ('/') then it is assumed to be relative to the Server Root. If filename begins with a bar ('|') then the following string is assumed to be a path to an executable program to which a reliable pipe can be established. The directive should occur only once per virtual server config.

Example:

SSLLog /usr/local/apache/logs/ssl_engine_log


SSLLogLevel

Name: SSLLogLevel
Description: Logging level for the dedicated SSL engine logfile
Syntax: SSLLogLevel level
Default: SSLLogLevel none
Context: server config, virtual host
Override: Not applicable
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This directive sets the verbosity degree of the dedicated SSL protocol engine logfile. The level is one of the following (in ascending order where higher levels include lower levels):

  • none
    no dedicated SSL logging is done, but messages of level ``error'' are still written to the general Apache error logfile.

  • error
    log messages of error type only, i.e. messages which show fatal situations (processing is stopped). Those messages are also duplicated to the general Apache error logfile.

  • warn
    log also warning messages, i.e. messages which show non-fatal problems (processing is continued).

  • info
    log also informational messages, i.e. messages which show major processing steps.

  • trace
    log also trace messages, i.e. messages which show minor processing steps.

  • debug
    log also debugging messages, i.e. messages which show development and low-level I/O information.

Example:

SSLLogLevel warn


SSLOptions

Name: SSLOptions
Description: Configure various SSL engine run-time options
Syntax: SSLOptions [+-]option ...
Default: None
Context: server config, virtual host, directory, .htaccess
Override: Options
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This directive can be used to control various run-time options on a per-directory basis. Normally, if multiple SSLOptions could apply to a directory, then the most specific one is taken completely; the options are not merged. However if all the options on the SSLOptions directive are preceded by a plus (+) or minus (-) symbol, the options are merged. Any options preceded by a + are added to the options currently in force, and any options preceded by a - are removed from the options currently in force.

The available options are:

  • CompatEnvVars

    When this option is enabled, additional CGI/SSI environment variables are created for backward compatibility to other Apache SSL solutions. Look in the Compatibility chapter for details on the actually generated variables.

  • ExportCertData

    When this option is enabled, two additional CGI/SSI environment variables are created: SSL_CLIENT_CERT and SSL_SERVER_CERT. These contain the PEM-encoded X.509 Certificates of client and server for the current HTTPS connection and can be used by CGI scripts for deeper Certificate checking. This bloats up the environment a little bit which is why you have to use this option to enable it on demand.

  • FakeBasicAuth

    When this option is enabled, the Subject Distinguished Name (DN) of the Client X509 Certificate is translated into a HTTP Basic Authorization username. This means that the standard Apache authentication methods can be used for access control. The user name is just the Subject of the Client's X509 Certificate (can be determined by running OpenSSL's openssl x509 command: openssl x509 -noout -subject -in certificate.crt). Note that no password is obtained from the user. Every entry in the user file needs this password: ``xxj31ZMTZzkVA'', which is the encrypted version of the word ``password''.

  • StrictRequire

    This forces forbidden access when SSLRequireSSL or SSLRequire successfully decided that access should be forbidden. Usually the default is that at least a used ``Satisfy any'' can cancel such access denies (when other access restrictions were passed), because that's how the Apache Satisfy mechanism should work. But for strict access restriction you can use SSLRequireSSL and/or SSLRequire in combination with an ``SSLOptions +StrictRequire''. Then an additional ``Satisfy Any'' has no chance once mod_ssl has decided to deny access.

Example:

SSLOptions +FakeBasicAuth -CompatEnvVars


SSLRequireSSL

Name: SSLRequireSSL
Description: Deny access when SSL is not used for the HTTP request
Syntax: SSLRequireSSL
Default: None
Context: directory, .htaccess
Override: AuthConfig
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.0

This directive forbids access unless HTTP over SSL (i.e. HTTPS) is enabled for the current connection. This is very handy inside the SSL-enabled virtual host or directories for defending against configuration errors that expose stuff that should be protected. When this directive is present all requests are denied which are not using SSL.

Example:

SSLRequireSSL


SSLRequire

Name: SSLRequire
Description: Allow access only when an arbitrarily complex boolean expression is true
Syntax: SSLRequire expression
Default: None
Context: directory, .htaccess
Override: AuthConfig
Status: Extension
Module: mod_ssl
Compatibility: mod_ssl 2.1

This directive specifies a general access requirement which has to be fulfilled in order to allow access. It's a very powerful directive because the requirement specification is an arbitrarily complex boolean expression containing any number of access checks.

The expression must match the following syntax (given as a BNF grammar notation):

expr     ::= "true" | "false"
           | "!" expr
           | expr "&&" expr
           | expr "||" expr
           | "(" expr ")"
           | comp

comp     ::= word "==" word | word "eq" word
           | word "!=" word | word "ne" word
           | word "<"  word | word "lt" word
           | word "<=" word | word "le" word
           | word ">"  word | word "gt" word
           | word ">=" word | word "ge" word
           | word "in" "{" wordlist "}"
           | word "=~" regex
           | word "!~" regex

wordlist ::= word
           | wordlist "," word

word     ::= digit
           | cstring
           | variable
           | function

digit    ::= [0-9]+
cstring  ::= "..."
variable ::= "%{" varname "}"
function ::= funcname "(" funcargs ")"
while for varname any variable from Table 3 can be used. Finally for funcname the following functions are available:
  • file(filename)

    This function takes one string argument and expands to the contents of the file. This is especially useful for matching this contents against a regular expression, etc.

Notice that expression is first parsed into an internal machine representation and then evaluated in a second step. Actually in Global and Per-Server Class context expression is parsed at startup time and at runtime the machine representation is executed only. For Per-Directory context this is different: Here expression has to be parsed and immediately executed for every request.

Example:

SSLRequire (    %{SSL_CIPHER} !~ m/^(EXP|NULL)-/ \
            and %{SSL_CLIENT_S_DN_O} eq "Snake Oil, Ltd." \
            and %{SSL_CLIENT_S_DN_OU} in {"Staff", "CA", "Dev"} \
            and %{TIME_WDAY} >= 1 and %{TIME_WDAY} <= 5 \
            and %{TIME_HOUR} >= 8 and %{TIME_HOUR} <= 20       ) \
           or %{REMOTE_ADDR} =~ m/^192\.76\.162\.[0-9]+$/
Table 3: Available Variables for SSLRequire
Standard CGI/1.0 and Apache variables:
HTTP_USER_AGENT        PATH_INFO             AUTH_TYPE
HTTP_REFERER           QUERY_STRING          SERVER_SOFTWARE
HTTP_COOKIE            REMOTE_HOST           API_VERSION
HTTP_FORWARDED         REMOTE_IDENT          TIME_YEAR
HTTP_HOST              IS_SUBREQ             TIME_MON
HTTP_PROXY_CONNECTION  DOCUMENT_ROOT         TIME_DAY
HTTP_ACCEPT            SERVER_ADMIN          TIME_HOUR
HTTP:headername        SERVER_NAME           TIME_MIN
THE_REQUEST            SERVER_PORT           TIME_SEC
REQUEST_METHOD         SERVER_PROTOCOL       TIME_WDAY
REQUEST_SCHEME         REMOTE_ADDR           TIME
REQUEST_URI            REMOTE_USER           ENV:variablename
REQUEST_FILENAME
SSL-related variables:
HTTPS                  SSL_CLIENT_M_VERSION  SSL_SERVER_M_VERSION
                       SSL_CLIENT_M_SERIAL   SSL_SERVER_M_SERIAL
SSL_PROTOCOL           SSL_CLIENT_V_START    SSL_SERVER_V_START
SSL_SESSION_ID         SSL_CLIENT_V_END      SSL_SERVER_V_END
SSL_CIPHER             SSL_CLIENT_S_DN       SSL_SERVER_S_DN
SSL_CIPHER_ALGKEYSIZE  SSL_CLIENT_S_DN_C     SSL_SERVER_S_DN_C
SSL_CIPHER_USEKEYSIZE  SSL_CLIENT_S_DN_SP    SSL_SERVER_S_DN_SP
SSL_VERSION_LIBRARY    SSL_CLIENT_S_DN_L     SSL_SERVER_S_DN_L
SSL_VERSION_INTERFACE  SSL_CLIENT_S_DN_O     SSL_SERVER_S_DN_O
                       SSL_CLIENT_S_DN_OU    SSL_SERVER_S_DN_OU
                       SSL_CLIENT_S_DN_CN    SSL_SERVER_S_DN_CN
                       SSL_CLIENT_S_DN_Email SSL_SERVER_S_DN_Email
                       SSL_CLIENT_I_DN       SSL_SERVER_I_DN
                       SSL_CLIENT_I_DN_C     SSL_SERVER_I_DN_C
                       SSL_CLIENT_I_DN_SP    SSL_SERVER_I_DN_SP
                       SSL_CLIENT_I_DN_L     SSL_SERVER_I_DN_L
                       SSL_CLIENT_I_DN_O     SSL_SERVER_I_DN_O
                       SSL_CLIENT_I_DN_OU    SSL_SERVER_I_DN_OU
                       SSL_CLIENT_I_DN_CN    SSL_SERVER_I_DN_CN
                       SSL_CLIENT_I_DN_Email SSL_SERVER_I_DN_Email
                       SSL_CLIENT_A_SIG      SSL_SERVER_A_SIG
                       SSL_CLIENT_A_KEY      SSL_SERVER_A_KEY
                       SSL_CLIENT_CERT       SSL_SERVER_CERT


Additional Features

Environment Variables

This module provides a lot of SSL information as additional environment variables to the SSI and CGI namespace. The generated variables are listed in Table 4. For backward compatibility the information can be made available under different names, too. Look in the Compatibility chapter for details on the compatibility variables.

Table 4: SSI/CGI Environment Variables
Variable Name: Value Type: Description:
HTTPS flag HTTPS is being used.
SSL_PROTOCOL string The SSL protocol version (SSLv2, SSLv3, TLSv1)
SSL_SESSION_ID string The hex-encoded SSL session id
SSL_CIPHER string The cipher specification name
SSL_CIPHER_USEKEYSIZE number Number of cipher bits (actually used)
SSL_CIPHER_ALGKEYSIZE number Number of cipher bits (possible)
SSL_VERSION_INTERFACE string The mod_ssl program version
SSL_VERSION_LIBRARY string The OpenSSL program version
SSL_CLIENT_M_VERSION string The version of the client certificate
SSL_CLIENT_M_SERIAL string The serial of the client certificate
SSL_CLIENT_S_DN string Subject DN in client's certificate
SSL_CLIENT_S_DN_x509 string Component of client's Subject DN
SSL_CLIENT_I_DN string Issuer DN of client's certificate
SSL_CLIENT_I_DN_x509 string Component of client's Issuer DN
SSL_CLIENT_V_START string Validity of client's certificate (start time)
SSL_CLIENT_V_END string Validity of client's certificate (end time)
SSL_CLIENT_A_SIG string Algorithm used for the signature of client's certificate
SSL_CLIENT_A_KEY string Algorithm used for the public key of client's certificate
SSL_SERVER_M_VERSION string The version of the server certificate
SSL_SERVER_M_SERIAL string The serial of the server certificate
SSL_SERVER_S_DN string Subject DN in server's certificate
SSL_SERVER_S_DN_x509 string Component of server's Subject DN
SSL_SERVER_I_DN string Issuer DN of server's certificate
SSL_SERVER_I_DN_x509 string Component of server's Issuer DN
SSL_SERVER_V_START string Validity of server's certificate (start time)
SSL_SERVER_V_END string Validity of server's certificate (end time)
SSL_SERVER_A_SIG string Algorithm used for the signature of server's certificate
SSL_SERVER_A_KEY string Algorithm used for the public key of server's certificate
[ where x509 is a component of a X.509 DN: C, SP, L, O, OU, CN, Email ]


Custom Log Formats

When mod_ssl is built into Apache or at least loaded (under DSO situation) additional functions exist for the Custom Log Format of mod_log_config. First there is an additional ``%{varname}x'' eXtension format function which can be used to expand any variables provided by any module, especially those provided by mod_ssl which can you find in Table 4.

For backward compatibility there is additionally a special ``%{name}c'' cryptography format function provided. Information about this function is provided in the Compatibility chapter.

Example:

CustomLog logs/ssl_request_log \
          "%t %h %{SSL_PROTOCOL}x %{SSL_CIPHER}x \"%r\" %b"


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