Chapter 3

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Reference

``Try to understand everything,
but believe nothing!''
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This 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

1. Configuration Directives
   
2. SSLPassPhraseDialog
   
3. SSLMutex
   
4. SSLRandomSeed
   
5. SSLSessionCache
   
6. SSLSessionCacheTimeout
   
7. SSLEngine
   
8. SSLCipherSuite
   
9. SSLCertificateFile
   
10. SSLCertificateKeyFile
    
11. SSLCACertificatePath
    
12. SSLCACertificateFile
    
13. SSLVerifyClient
    
14. SSLVerifyDepth
    
15. SSLLog
    
16. SSLLogLevel
    
17. SSLOptions
    
18. SSLRequireSSL
    
19. SSLRequire
    
20. Additional Features
    
21. Environment Variables
    
22. 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

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 SSLeay 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).

• 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

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) SSLeay 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 SSLeay 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.

Example:

SSLSessionCache dbm:/usr/local/apache/logs/ssl_gcache_data

SSLSessionCacheTimeout

This directive sets the timeout in seconds for the information stored in the global/inter-process SSL Session Cache and the SSLeay 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. 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>

SSLCipherSuite

This complex directive uses a colon-separated cipher-spec string consisting of SSLeay 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, 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)

$ ssleay 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 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.

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 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.

Example:

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

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 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

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

SSLVerifyClient

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.

Example:

SSLVerifyClient require

SSLVerifyDepth

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

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

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 tace 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

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 SSLeay's ssleay x509 command: ssleay 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''.

Example:

SSLOptions +FakeBasicAuth -CompatEnvVars

SSLRequireSSL

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

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 ")"

• 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.

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]+$/

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

HTTPS                  SSL_CLIENT_M_VERSION  SSL_SERVER_M_VERSION
                       SSL_CLIENT_M_SERIAL   SSL_SERVER_M_SERIAL
SSL_VERSION_LIBRARY    SSL_CLIENT_V_START    SSL_SERVER_V_START
SSL_VERSION_INTERFACE  SSL_CLIENT_V_END      SSL_SERVER_V_END
SSL_CIPHER             SSL_CLIENT_S_DN       SSL_SERVER_S_DN
SSL_CIPHER_USEKEYSIZE  SSL_CLIENT_S_DN_C     SSL_SERVER_S_DN_C
SSL_CIPHER_ALGKEYSIZE  SSL_CLIENT_S_DN_SP    SSL_SERVER_S_DN_SP
                       SSL_CLIENT_S_DN_L     SSL_SERVER_S_DN_L
                       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

Custom Log Formats

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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mod_ssl 2.2, User Manual. The Apache Interface to SSLeay
Copyright © 1998 Ralf S. Engelschall. All Rights Reserved