mod_ssl

Introduction Compatibility

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         SSLCertificateChainFile         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 pretty much occur 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

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 two arguments (the first is of the form ``servername:portnumber'', the second is either ``RSA'' or ``DSA''), which indicate for which server and algorithm 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 you like. Mod_ssl just defines the interface: an executable program which provides the Pass Phrase on stdout. Nothing more or less! So, if 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 so 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 (under Unix) 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. Note: 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 worry about conflicts yourself. Notice that this type of mutex is not available under the Win32 environment. There you have to use the semaphore mutex.

• 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 underlying 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 (and bytes is given to /path/to/source as the first argument). When bytes is not specified the whole file forms the entropy (and 0 is given to /path/to/source as the first argument). 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) or use egd:/path/to/egd-socket (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 its stdout contents form the entropy. When bytes is not specified, the entirety of the 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 in the connection context slows down the server too dramatically, of course. So usually you should avoid using external programs in that context.

• egd:/path/to/egd-socket (Unix only)

  This variant uses the Unix domain socket of the external Entropy Gathering Daemon (EGD) (see http://www.lothar.com/tech /crypto/) to seed the PRNG. Use this if no random device exists on your platform.

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

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. 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 slight increase in I/O on the server results in a visible request speedup for your clients, so this type of storage is generally recommended.

• 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

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

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. By 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

This directive can be used to control the SSL protocol flavors mod_ssl should use when establishing its 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 almost 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

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.

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, TLSv1, 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)

$ 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

Example:

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

SSLCertificateFile

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

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 discourage this practice. Instead we recommend you 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

SSLCertificateChainFile

This directive sets the optional all-in-one file where you can assemble the certificates of Certification Authorities (CA) which form the certificate chain of the server certificate. This starts with the issuing CA certificate of of the server certificate and can range up to the root CA certificate. Such a file is simply the concatenation of the various PEM-encoded CA Certificate files, usually in certificate chain order.

This should be used alternatively and/or additionally to SSLCACertificatePath for explicitly constructing the server certificate chain which is sent to the browser in addition to the server certificate. It is especially useful to avoid conflicts with CA certificates when using client authentication. Because although placing a CA certificate of the server certificate chain into SSLCACertificatePath has the same effect for the certificate chain construction, it has the side-effect that client certificates issued by this same CA certificate are also accepted on client authentication. That's usually not one expect.

But be careful: Providing the certificate chain works only if you are using a single (either RSA or DSA) based server certificate. If you are using a coupled RSA+DSA certificate pair, this will work only if actually both certificates use the same certificate chain. Else the browsers will be confused in this situation.

Example:

SSLCertificateChainFile /usr/local/apache/conf/ssl.crt/ca.crt

SSLCACertificatePath

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 can't just place the Certificate files there: you also 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