Apache Module mod_ssl

SSLCACertificateFile Directive

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

SSLCACertificatePath Directive

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

The files in this directory have to be PEM-encoded and are accessed throughhash filenames. So usually you can't just place the Certificate filesthere: you also have to create symbolic links namedhash-value.N. And you should always make sure this directorycontains the appropriate symbolic links. Use the Makefile whichcomes with mod_ssl to accomplish this task.

SSLCADNRequestFile Directive

When a client certificate is requested by mod_ssl, a list ofacceptable Certificate Authority names is sent to the clientin the SSL handshake. These CA names can be used by the client toselect an appropriate client certificate out of those it hasavailable.

If neither of the directives SSLCADNRequestPath or SSLCADNRequestFile are given, then theset of acceptable CA names sent to the client is the names of all theCA certificates given by the SSLCACertificateFile and SSLCACertificatePath directives; in otherwords, the names of the CAs which will actually be used to verify theclient certificate.

In some circumstances, it is useful to be able to send a set ofacceptable CA names which differs from the actual CAs used to verifythe client certificate - for example, if the client certificates aresigned by intermediate CAs. In such cases, SSLCADNRequestPath and/or SSLCADNRequestFile can be used; theacceptable CA names are then taken from the complete set ofcertificates in the directory and/or file specified by this pair ofdirectives.

SSLCADNRequestFile mustspecify an all-in-one file containing a concatenation ofPEM-encoded CA certificates.

SSLCADNRequestPath Directive

This optional directive can be used to specify the set ofacceptable CA names which will be sent to the client when aclient certificate is requested. See the SSLCADNRequestFile directive for moredetails.

The files in this directory have to be PEM-encoded and are accessedthrough hash filenames. So usually you can't just place theCertificate files there: you also have to create symbolic links namedhash-value.N. And you should always make surethis directory contains the appropriate symbolic links. Use theMakefile which comes with mod_ssl to accomplish thistask.

SSLCARevocationFile Directive

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

SSLCARevocationPath Directive

This directive sets the directory where you keep the Certificate RevocationLists (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 throughhash filenames. So usually you have not only to place the CRL files there.Additionally you have to create symbolic links namedhash-value.rN. And you should always make sure this directorycontains the appropriate symbolic links. Use the Makefile whichcomes with mod_ssl to accomplish this task.

SSLCertificateChainFile Directive

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

This should be used alternatively and/or additionally to SSLCACertificatePath for explicitlyconstructing the server certificate chain which is sent to the browserin addition to the server certificate. It is especially useful toavoid conflicts with CA certificates when using clientauthentication. Because although placing a CA certificate of theserver certificate chain into SSLCACertificatePath has the same effectfor the certificate chain construction, it has the side-effect thatclient certificates issued by this same CA certificate are alsoaccepted on client authentication.

But be careful: Providing the certificate chain works only if you are using asingle RSA or DSA based server certificate. If you areusing a coupled RSA+DSA certificate pair, this will work only if actually bothcertificates use the same certificate chain. Else the browsers will beconfused in this situation.

SSLCertificateFile Directive

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

SSLCertificateKeyFile Directive

This directive points to the PEM-encoded Private Key file for theserver. If the Private Key is not combined with the Certificate in theSSLCertificateFile, use this additional directive topoint to the file with the stand-alone Private Key. WhenSSLCertificateFile is used and the filecontains both the Certificate and the Private Key this directive neednot be used. But we strongly discourage this practice. Instead werecommend you to separate the Certificate and the Private Key. If thecontained Private Key is encrypted, the Pass Phrase dialog is forcedat startup time. This directive can be used up to two times(referencing different filenames) when both a RSA and a DSA basedprivate key is used in parallel.

SSLCipherSuite Directive

This complex directive uses a colon-separated cipher-spec stringconsisting of OpenSSL cipher specifications to configure the Cipher Suite theclient is permitted to negotiate in the SSL handshake phase. Notice that thisdirective can be used both in per-server and per-directory context. Inper-server context it applies to the standard SSL handshake when a connectionis established. In per-directory context it forces a SSL renegotation with thereconfigured Cipher Suite after the HTTP request was read but before the HTTPresponse is sent.

An SSL cipher specification in cipher-spec is composed of 4 majorattributes 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/TLSv1cipher (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 tospecify the preference and order for the ciphers (see Table1).

Now where this becomes interesting is that these can be put togetherto specify the order and ciphers you wish to use. To speed this upthere are also aliases (SSLv2, SSLv3, TLSv1, EXP, LOW, MEDIUM,HIGH) for certain groups of ciphers. These tags can be joinedtogether with prefixes to form the cipher-spec. Availableprefixes 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 thecorrect cipher-spec string. The default cipher-spec stringis ``ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP'' whichmeans the following: first, remove from consideration any ciphers that do notauthenticate, 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 lowsecurity ciphers. Finally pull all SSLv2 and export ciphers to theend 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=SHA1NULL-MD5 SSLv3 Kx=RSA  Au=RSA  Enc=None  Mac=MD5EDH-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  exportEXP-RC2-CBC-MD5 SSLv2 Kx=RSA(512) Au=RSA  Enc=RC2(40)   Mac=MD5  exportEXP-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.

SSLCryptoDevice Directive

This directive enables use of a cryptographic hardware acceleratorboard to offload some of the SSL processing overhead. This directivecan only be used if the SSL toolkit is built with "engine" support;OpenSSL 0.9.7 and later releases have "engine" support by default, theseparate "-engine" releases of OpenSSL 0.9.6 must be used.

To discover which engine names are supported, run the command"openssl engine".

SSLEngine Directive

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

In Apache 2.1 and later, SSLEngine can be set to optional. This enables support for RFC 2817, Upgrading to TLS Within HTTP/1.1. At this time no web browsers support RFC 2817.

SSLHonorCipherOrder Directive

When choosing a cipher during an SSLv3 or TLSv1 handshake, normallythe client's preference is used. If this directive is enabled, theserver's preference will be used instead.

SSLMutex Directive

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

The following Mutex types are available:

• none | no

  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.

• posixsem

  This is an elegant Mutex variant where a Posix Semaphore is used when possible. It is only available when the underlying platform and APR supports it.

• sysvsem

  This is a somewhat elegant Mutex variant where a SystemV IPC Semaphore is used when possible. It is possible to "leak" SysV semaphores if processes crash before the semaphore is removed. It is only available when the underlying platform and APR supports it.

• sem

  This directive tells the SSL Module to pick the "best" semaphore implementation available to it, choosing between Posix and SystemV IPC, in that order. It is only available when the underlying platform and APR supports at least one of the 2.

• pthread

  This directive tells the SSL Module to use Posix thread mutexes. It is only available if the underlying platform and APR supports it.

• fcntl:/path/to/mutex

  This is a portable Mutex variant where a physical (lock-)file and the fcntl() function are 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. It is only available when the underlying platform and APR supports it. 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.

• flock:/path/to/mutex

  This is similar to the fcntl:/path/to/mutex method with the exception that the flock() function is used to provide file locking. It is only available when the underlying platform and APR supports it.

• file:/path/to/mutex

  This directive tells the SSL Module to pick the "best" file locking implementation available to it, choosing between fcntl and flock, in that order. It is only available when the underlying platform and APR supports at least one of the 2.

• default | yes

  This directive tells the SSL Module to pick the default locking implementation as determined by the platform and APR.

SSLOptions Directive

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

The available options are:

• StdEnvVars

  When this option is enabled, the standard set of SSL related CGI/SSI environment variables are created. This per default is disabled for performance reasons, because the information extraction step is a rather expensive operation. So one usually enables this option for CGI and SSI requests only.

• 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 particular variables generated.

• ExportCertData

  When this option is enabled, additional CGI/SSI environment variables are created: SSL_SERVER_CERT, SSL_CLIENT_CERT and SSL_CLIENT_CERT_CHAIN_n (with n = 0,1,2,..). These contain the PEM-encoded X.509 Certificates of server and client for the current HTTPS connection and can be used by CGI scripts for deeper Certificate checking. Additionally all other certificates of the client certificate chain are provided, too. 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 DES-encrypted version of the word `password''. Those who live under MD5-based encryption (for instance under FreeBSD or BSD/OS, etc.) should use the following MD5 hash of the same word: ``$1$OXLyS...$Owx8s2/m9/gfkcRVXzgoE/''.

• StrictRequire

  This forces forbidden access when SSLRequireSSL or SSLRequire successfully decided that access should be forbidden. Usually the default is that in the case where a ``Satisfy any'' directive is used, and other access restrictions are passed, denial of access due to SSLRequireSSL or SSLRequire is overridden (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.

• OptRenegotiate

  This enables optimized SSL connection renegotiation handling when SSL directives are used in per-directory context. By default a strict scheme is enabled where every per-directory reconfiguration of SSL parameters causes a full SSL renegotiation handshake. When this option is used mod_ssl tries to avoid unnecessary handshakes by doing more granular (but still safe) parameter checks. Nevertheless these granular checks sometimes maybe not what the user expects, so enable this on a per-directory basis only, please.

SSLPassPhraseDialog Directive

When Apache starts up it has to read the various Certificate (seeSSLCertificateFile) andPrivate Key (see SSLCertificateKeyFile) files of theSSL-enabled virtual servers. Because for security reasons the PrivateKey files are usually encrypted, mod_ssl needs to query theadministrator for a Pass Phrase in order to decrypt those files. Thisquery can be done in two ways which can be configured bytype:

• 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).

• |/path/to/program [args...]

  This mode allows an external program to be used which acts as a pipe to a particular input device; the program is sent the standard prompt text used for the builtin mode on stdin, and is expected to write password strings on stdout. If several passwords are needed (or an incorrect password is entered), additional prompt text will be written subsequent to the first password being returned, and more passwords must then be written back.

• 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.

SSLProtocol Directive

This directive can be used to control the SSL protocol flavors mod_ssl shoulduse when establishing its server environment. Clients then can only connectwith 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. Though it's use has been deprecated, because of weaknesses in the security of the protocol.

• SSLv3

  This is the Secure Sockets Layer (SSL) protocol, version 3.0, from the Netscape Corporation. It is the successor to SSLv2 and the predecessor to TLSv1. 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 is defined in RFC2246. Which has been obsoleted by RFC4346.

• All

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

SSLProxyCACertificateFile Directive

This directive sets the all-in-one file where you can assemble theCertificates of Certification Authorities (CA) whose remote servers you dealwith. These are used for Remote Server Authentication. Such a file is simply theconcatenation of the various PEM-encoded Certificate files, in order ofpreference. This can be used alternatively and/or additionally to SSLProxyCACertificatePath.

SSLProxyCACertificatePath Directive

This directive sets the directory where you keep the Certificates ofCertification Authorities (CAs) whose remote servers you deal with. These are used toverify the remote server certificate on Remote Server Authentication.

The files in this directory have to be PEM-encoded and are accessed throughhash filenames. So usually you can't just place the Certificate filesthere: you also have to create symbolic links namedhash-value.N. And you should always make sure this directorycontains the appropriate symbolic links. Use the Makefile whichcomes with mod_ssl to accomplish this task.

SSLProxyCARevocationFile Directive

This directive sets the all-in-one file where you canassemble the Certificate Revocation Lists (CRL) of CertificationAuthorities (CA) whose remote servers you deal with. These are usedfor Remote Server Authentication. Such a file is simply the concatenation ofthe various PEM-encoded CRL files, in order of preference. This can beused alternatively and/or additionally to SSLProxyCARevocationPath.

SSLProxyCARevocationPath Directive

This directive sets the directory where you keep the Certificate RevocationLists (CRL) of Certification Authorities (CAs) whose remote servers you deal with.These are used to revoke the remote server certificate on Remote Server Authentication.

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

SSLProxyCheckPeerCN Directive

This directive sets whether the remote server certificates CN field iscompared against the hostname of the request URL. If both are not equala 502 status code (Bad Gateway) is sent.

SSLProxyCheckPeerExpire Directive

This directive sets whether it is checked if the remote server certificateis expired or not. If the check fails a 502 status code (Bad Gateway) issent.

SSLProxyCipherSuite Directive

Equivalent to SSLCipherSuite, but for the proxy connection.Please refer to SSLCipherSuitefor additional information.

SSLProxyEngine Directive

This directive toggles the usage of the SSL/TLS Protocol Engine for proxy. Thisis usually used inside a <VirtualHost> section to enable SSL/TLS for proxyusage in a particular virtual host. By default the SSL/TLS Protocol Engine isdisabled for proxy image both for the main server and all configured virtual hosts.

SSLProxyMachineCertificateFile Directive

This directive sets the all-in-one file where you keep the certificates andkeys used for authentication of the proxy server to remote servers.

This referenced file is simply the concatenation of the various PEM-encodedcertificate files, in order of preference. Use this directive alternativelyor additionally to SSLProxyMachineCertificatePath.

SSLProxyMachineCertificatePath Directive

This directive sets the directory where you keep the certificates andkeys used for authentication of the proxy server to remote servers.

The files in this directory must be PEM-encoded and are accessed throughhash filenames. Additionally, you must create symbolic links namedhash-value.N. And you should always make sure thisdirectory contains the appropriate symbolic links. Use the Makefile whichcomes with mod_ssl to accomplish this task.

SSLProxyProtocol Directive

This directive can be used to control the SSL protocol flavors mod_ssl shoulduse when establishing its server environment for proxy . It will only connectto servers using one of the provided protocols.

Please refer to SSLProtocolfor additional information.

SSLProxyVerify Directive

When a proxy is configured to forward requests to a remote SSLserver, this directive can be used to configure certificateverification of the remote server. Notice that this directive can beused both in per-server and per-directory context. In per-servercontext it applies to the remote server authentication process used inthe standard SSL handshake when a connection is established by theproxy. In per-directory context it forces a SSL renegotation with thereconfigured remote server verification level after the HTTP requestwas read but before the HTTP response is sent.

The following levels are available for level:

• none: no remote server Certificate is required at all
• optional: the remote server may present a valid Certificate
• require: the remote server has to present a valid Certificate
• optional_no_ca: the remote server may present a valid Certificate
  but it need not to be (successfully) verifiable.

In practice only levels none andrequire are really interesting, because leveloptional doesn't work with all servers and leveloptional_no_ca is actually against the idea ofauthentication (but can be used to establish SSL test pages, etc.)

SSLProxyVerifyDepth Directive

This directive sets how deeply mod_ssl should verify before deciding that theremote server does not have a valid certificate. Notice that this directive can beused both in per-server and per-directory context. In per-server context itapplies to the client authentication process used in the standard SSLhandshake when a connection is established. In per-directory context it forcesa SSL renegotation with the reconfigured remote server verification depth after theHTTP 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 whileverifying the remote server certificate. A depth of 0 means that self-signedremote server certificates are accepted only, the default depth of 1 meansthe remote server certificate can be self-signed or has to be signed by a CAwhich is directly known to the server (i.e. the CA's certificate is underSSLProxyCACertificatePath), etc.

SSLRandomSeed Directive

This configures one or more sources for seeding the Pseudo Random NumberGenerator (PRNG) in OpenSSL at startup time (context isstartup) and/or just before a new SSL connection is established(context is connect). This directive can only be usedin 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.

SSLRenegBufferSize Directive

If an SSL renegotiation is required in per-location context, forexample, any use of SSLVerifyClient in a Directory orLocation block, then mod_ssl must buffer any HTTPrequest body into memory until the new SSL handshake can be performed.This directive can be used to set the amount of memory that will beused for this buffer.

SSLRequire Directive

This directive specifies a general access requirement which has to befulfilled in order to allow access. It is a very powerful directive because therequirement specification is an arbitrarily complex boolean expressioncontaining any number of access checks.

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

expr ::= "true" | "false"   | "!" expr   | expr "&&" expr   | expr "||" expr   | "(" expr ")"   | compcomp ::= 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 "in" "OID(" word ")"   | word "=~" regex   | word "!~" regexwordlist ::= word   | wordlist "," wordword ::= digit   | cstring   | variable   | functiondigit ::= [0-9]+cstring  ::= "..."variable ::= "%{" varname "}"function ::= funcname "(" funcargs ")"

while for varname any variable from Table 3 can be used. Finally forfuncname 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 machinerepresentation and then evaluated in a second step. Actually, in Global andPer-Server Class context expression is parsed at startup time andat runtime only the machine representation is executed. For Per-Directorycontext this is different: here expression has to be parsed andimmediately executed for every request.

The OID() function expects to find zero or more instancesof the given OID in the client certificate, and compares the left-hand sidestring against the value of matching OID attributes. Every matching OID ischecked, until a match is found.

Standard CGI/1.0 and Apache variables:

HTTP_USER_AGENT PATH_INFO AUTH_TYPEHTTP_REFERER   QUERY_STRING  SERVER_SOFTWAREHTTP_COOKIE REMOTE_HOST   API_VERSIONHTTP_FORWARDED REMOTE_IDENT  TIME_YEARHTTP_HOST  IS_SUBREQ TIME_MONHTTP_PROXY_CONNECTION  DOCUMENT_ROOT TIME_DAYHTTP_ACCEPT SERVER_ADMIN  TIME_HOURHTTP:headername SERVER_NAME   TIME_MINTHE_REQUEST SERVER_PORT   TIME_SECREQUEST_METHOD SERVER_PROTOCOL   TIME_WDAYREQUEST_SCHEME REMOTE_ADDR   TIMEREQUEST_URI REMOTE_USER   ENV:variablenameREQUEST_FILENAME

SSL-related variables:

HTTPS  SSL_CLIENT_M_VERSION   SSL_SERVER_M_VERSION   SSL_CLIENT_M_SERIAL SSL_SERVER_M_SERIALSSL_PROTOCOL   SSL_CLIENT_V_START SSL_SERVER_V_STARTSSL_SESSION_ID SSL_CLIENT_V_END   SSL_SERVER_V_ENDSSL_CIPHER SSL_CLIENT_S_DN SSL_SERVER_S_DNSSL_CIPHER_EXPORT  SSL_CLIENT_S_DN_C  SSL_SERVER_S_DN_CSSL_CIPHER_ALGKEYSIZE  SSL_CLIENT_S_DN_ST SSL_SERVER_S_DN_STSSL_CIPHER_USEKEYSIZE  SSL_CLIENT_S_DN_L  SSL_SERVER_S_DN_LSSL_VERSION_LIBRARY SSL_CLIENT_S_DN_O  SSL_SERVER_S_DN_OSSL_VERSION_INTERFACE  SSL_CLIENT_S_DN_OU SSL_SERVER_S_DN_OU   SSL_CLIENT_S_DN_CN SSL_SERVER_S_DN_CN   SSL_CLIENT_S_DN_T  SSL_SERVER_S_DN_T   SSL_CLIENT_S_DN_I  SSL_SERVER_S_DN_I   SSL_CLIENT_S_DN_G  SSL_SERVER_S_DN_G   SSL_CLIENT_S_DN_S  SSL_SERVER_S_DN_S   SSL_CLIENT_S_DN_D  SSL_SERVER_S_DN_D   SSL_CLIENT_S_DN_UID SSL_SERVER_S_DN_UID   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_ST SSL_SERVER_I_DN_ST   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_T  SSL_SERVER_I_DN_T   SSL_CLIENT_I_DN_I  SSL_SERVER_I_DN_I   SSL_CLIENT_I_DN_G  SSL_SERVER_I_DN_G   SSL_CLIENT_I_DN_S  SSL_SERVER_I_DN_S   SSL_CLIENT_I_DN_D  SSL_SERVER_I_DN_D   SSL_CLIENT_I_DN_UID SSL_SERVER_I_DN_UID   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   SSL_CLIENT_CERT_CHAIN_n   SSL_CLIENT_VERIFY

SSLRequireSSL Directive

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

SSLSessionCache Directive

This configures the storage type of the global/inter-process SSL SessionCache. This cache is an optional facility which speeds up parallel requestprocessing. For requests to the same server process (via HTTP keep-alive),OpenSSL already caches the SSL session information locally. But because modernclients request inlined images and other data via parallel requests (usuallyup to four parallel requests are common) those requests are served bydifferent pre-forked server processes. Here an inter-process cachehelps to avoid unneccessary session handshakes.

The following four storage types are currently supported:

• none

  This disables the global/inter-process Session Cache. This will incur a noticeable speed penalty and may cause problems if using certain browsers, particularly if client certificates are enabled. This setting is not recommended.

• nonenotnull

  This disables any global/inter-process Session Cache. However it does force OpenSSL to send a non-null session ID to accommodate buggy clients that require one.

• 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. This session cache may suffer reliability issues under high load.

• shm:/path/to/datafile[(size)]

  This makes use of a high-performance cyclic buffer (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 is the recommended session cache.

• dc:UNIX:/path/to/socket

  This makes use of the distcache distributed session caching libraries. The argument should specify the location of the server or proxy to be used using the distcache address syntax; for example, UNIX:/path/to/socket specifies a UNIX domain socket (typically a local dc_client proxy); IP:server.example.com:9001 specifies an IP address.

SSLSessionCacheTimeout Directive

This directive sets the timeout in seconds for the information stored in theglobal/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 highervalues like 300 in real life.

SSLStrictSNIVHostCheck Directive

This directive sets whether a non SNI client is allowed to access a name basedvirtual host. If set to on in the non default name based virtualhost, non SNI clients are not allowed to access this particular virtual host.If set to on in the default name based virtual host, non SNIclients are not allowed to access any name based virtual host belonging tothis IP / port combination.

SSLUserName Directive

This directive sets the "user" field in the Apache request object.This is used by lower modules to identify the user with a characterstring. In particular, this may cause the environment variableREMOTE_USER to be set. The varname can beany of the SSL environment variables.

Note that this directive has no effect if theFakeBasic option is used (see SSLOptions).

SSLVerifyClient Directive

This directive sets the Certificate verification level for the ClientAuthentication. Notice that this directive can be used both in per-server andper-directory context. In per-server context it applies to the clientauthentication process used in the standard SSL handshake when a connection isestablished. In per-directory context it forces a SSL renegotation with thereconfigured client verification level after the HTTP request was read butbefore 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 it need not to be (successfully) verifiable.

In practice only levels none andrequire are really interesting, because leveloptional doesn't work with all browsers and leveloptional_no_ca is actually against the idea ofauthentication (but can be used to establish SSL test pages, etc.)

SSLVerifyDepth Directive

This directive sets how deeply mod_ssl should verify before deciding that theclients don't have a valid certificate. Notice that this directive can beused both in per-server and per-directory context. In per-server context itapplies to the client authentication process used in the standard SSLhandshake when a connection is established. In per-directory context it forcesa SSL renegotation with the reconfigured client verification depth after theHTTP 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 whileverifying the client certificate. A depth of 0 means that self-signed clientcertificates are accepted only, the default depth of 1 means the clientcertificate can be self-signed or has to be signed by a CA which is directlyknown to the server (i.e. the CA's certificate is underSSLCACertificatePath), etc.