Apache Module mod_ssl

SSLCACertificateFile Directive ¶

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.

SSLCACertificateFile "/usr/local/apache2/conf/ssl.crt/ca-bundle-client.crt"

SSLCACertificatePath Directive ¶

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.

SSLCACertificatePath "/usr/local/apache2/conf/ssl.crt/"

SSLCADNRequestFile Directive ¶

When a client certificate is requested by mod_ssl, a list of acceptable Certificate Authority names is sent to the client in the SSL handshake. These CA names can be used by the client to select an appropriate client certificate out of those it has available.

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

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

SSLCADNRequestFile must specify an all-in-one file containing a concatenation of PEM-encoded CA certificates.

SSLCADNRequestFile "/usr/local/apache2/conf/ca-names.crt"

SSLCADNRequestPath Directive ¶

This optional directive can be used to specify the set of acceptable CA names which will be sent to the client when a client certificate is requested. See the SSLCADNRequestFile directive for more details.

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.

SSLCADNRequestPath "/usr/local/apache2/conf/ca-names.crt/"

SSLCARevocationCheck Directive ¶

Enables certificate revocation list (CRL) checking. At least one of SSLCARevocationFile or SSLCARevocationPath must be configured. When set to chain (recommended setting), CRL checks are applied to all certificates in the chain, while setting it to leaf limits the checks to the end-entity cert.

The available flags are:

• no_crl_for_cert_ok

  Prior to version 2.3.15, CRL checking in mod_ssl also succeeded when no CRL(s) for the checked certificate(s) were found in any of the locations configured with SSLCARevocationFile or SSLCARevocationPath.

  With the introduction of SSLCARevocationFile, the behavior has been changed: by default with chain or leaf, CRLs must be present for the validation to succeed - otherwise it will fail with an "unable to get certificate CRL" error.

  The flag no_crl_for_cert_ok allows to restore previous behaviour.

SSLCARevocationCheck chain

SSLCARevocationCheck chain no_crl_for_cert_ok

SSLCARevocationFile Directive ¶

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.

SSLCARevocationFile "/usr/local/apache2/conf/ssl.crl/ca-bundle-client.crl"

SSLCARevocationPath Directive ¶

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.

SSLCARevocationPath "/usr/local/apache2/conf/ssl.crl/"

SSLCertificateChainFile Directive ¶

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 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.

But be careful: Providing the certificate chain works only if you are using a single 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.

SSLCertificateChainFile "/usr/local/apache2/conf/ssl.crt/ca.crt"

SSLCertificateFile Directive ¶

This directive points to a file with certificate data in PEM format, or the certificate identifier through a configured cryptographic token. If using a PEM file, at minimum, the file must include an end-entity (leaf) certificate. The directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication - typically RSA, DSA, and ECC. The number of supported algorithms depends on the OpenSSL version being used for mod_ssl: with version 1.0.0 or later, openssl list-public-key-algorithms will output a list of supported algorithms, see also the note below about limitations of OpenSSL versions prior to 1.0.2 and the ways to work around them.

The files may also include intermediate CA certificates, sorted from leaf to root. This is supported with version 2.4.8 and later, and obsoletes SSLCertificateChainFile. When running with OpenSSL 1.0.2 or later, this allows to configure the intermediate CA chain on a per-certificate basis.

Custom DH parameters and an EC curve name for ephemeral keys, can also be added to end of the first file configured using SSLCertificateFile. This is supported in version 2.4.7 or later. Such parameters can be generated using the commands openssl dhparam and openssl ecparam. The parameters can be added as-is to the end of the first certificate file. Only the first file can be used for custom parameters, as they are applied independently of the authentication algorithm type.

Finally the end-entity certificate's private key can also be added to the certificate file instead of using a separate SSLCertificateKeyFile directive. This practice is highly discouraged. If it is used, the certificate files using such an embedded key must be configured after the certificates using a separate key file. If the private key is encrypted, the pass phrase dialog is forced at startup time.

As an alternative to storing certificates and private keys in files, a certificate identifier can be used to identify a certificate stored in a token. Currently, only PKCS#11 URIs are recognized as certificate identifiers, and can be used in conjunction with the OpenSSL pkcs11 engine. If SSLCertificateKeyFile is omitted, the certificate and private key can be loaded through the single identifier specified with SSLCertificateFile.

# Example using a PEM-encoded file.
SSLCertificateFile "/usr/local/apache2/conf/ssl.crt/server.crt"
# Example use of a certificate and private key from a PKCS#11 token:
SSLCertificateFile "pkcs11:token=My%20Token%20Name;id=45"

SSLCertificateKeyFile Directive ¶

This directive points to the PEM-encoded private key file for the server, or the key ID through a configured cryptographic token. If the contained private key is encrypted, the pass phrase dialog is forced at startup time.

The directive can be used multiple times (referencing different filenames) to support multiple algorithms for server authentication. For each SSLCertificateKeyFile directive, there must be a matching SSLCertificateFile directive.

The private key may also be combined with the certificate in the file given by SSLCertificateFile, but this practice is highly discouraged. If it is used, the certificate files using such an embedded key must be configured after the certificates using a separate key file.

As an alternative to storing private keys in files, a key identifier can be used to identify a private key stored in a token. Currently, only PKCS#11 URIs are recognized as private key identifiers, and can be used in conjunction with the OpenSSL pkcs11 engine.

# To use a private key from a PEM-encoded file:
SSLCertificateKeyFile "/usr/local/apache2/conf/ssl.key/server.key"
# To use a private key from a PKCS#11 token:
SSLCertificateKeyFile "pkcs11:token=My%20Token%20Name;id=45"

SSLCipherSuite Directive ¶

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. The optional protocol specifier can configure the Cipher Suite for a specific SSL version. Possible values include "SSL" for all SSL Protocols up to and including TLSv1.2.

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 renegotiation with the reconfigured Cipher Suite after the HTTP request was read but before the HTTP response is sent.

If the SSL library supports TLSv1.3 (OpenSSL 1.1.1 and later), the protocol specifier "TLSv1.3" can be used to configure the cipher suites for that protocol. Since TLSv1.3 does not offer renegotiations, specifying ciphers for it in a directory context is not allowed.

For a list of TLSv1.3 cipher names, see the OpenSSL documentation.

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

• Key Exchange Algorithm:
  RSA, Diffie-Hellman, Elliptic Curve Diffie-Hellman, Secure Remote Password
• Authentication Algorithm:
  RSA, Diffie-Hellman, DSS, ECDSA, or none.
• Cipher/Encryption Algorithm:
  AES, DES, Triple-DES, RC4, RC2, IDEA, etc.
• MAC Digest Algorithm:
  MD5, SHA or SHA1, SHA256, SHA384.

An SSL cipher can also be an export cipher. SSLv2 ciphers are no longer supported. 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). The actually available ciphers and aliases depends on the used openssl version. Newer openssl versions may include additional ciphers.

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 (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
• +: move matching ciphers to the 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 depends on the version of the OpenSSL libraries used. Let's suppose it is ``RC4-SHA:AES128-SHA:HIGH:MEDIUM:!aNULL:!MD5'' which means the following: Put RC4-SHA and AES128-SHA at the beginning. We do this, because these ciphers offer a good compromise between speed and security. Next, include high and medium security ciphers. Finally, remove all ciphers which do not authenticate, i.e. for SSL the Anonymous Diffie-Hellman ciphers, as well as all ciphers which use MD5 as hash algorithm, because it has been proven insufficient.

$ openssl ciphers -v 'RC4-SHA:AES128-SHA:HIGH:MEDIUM:!aNULL:!MD5'
RC4-SHA                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=SHA1
AES128-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(128)  Mac=SHA1
DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA1
...                     ...               ...     ...           ...
SEED-SHA                SSLv3 Kx=RSA      Au=RSA  Enc=SEED(128) Mac=SHA1
PSK-RC4-SHA             SSLv3 Kx=PSK      Au=PSK  Enc=RC4(128)  Mac=SHA1
KRB5-RC4-SHA            SSLv3 Kx=KRB5     Au=KRB5 Enc=RC4(128)  Mac=SHA1

The complete list of particular RSA & DH ciphers for SSL is given in Table 2.

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

SSLCompression Directive ¶

This directive allows to enable compression on the SSL level.

SSLCryptoDevice Directive ¶

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

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

# For a Broadcom accelerator:
SSLCryptoDevice ubsec

SSLEngine Directive ¶

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

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

SSLEngine can be set to optional: this enables support for RFC 2817.

SSLFIPS Directive ¶

This directive toggles the usage of the SSL library FIPS_mode flag. It must be set in the global server context and cannot be configured with conflicting settings (SSLFIPS on followed by SSLFIPS off or similar). The mode applies to all SSL library operations.

If httpd was compiled against an SSL library which did not support the FIPS_mode flag, SSLFIPS on will fail. Refer to the FIPS 140-2 Security Policy document of the SSL provider library for specific requirements to use mod_ssl in a FIPS 140-2 approved mode of operation; note that mod_ssl itself is not validated, but may be described as using FIPS 140-2 validated cryptographic module, when all components are assembled and operated under the guidelines imposed by the applicable Security Policy.

SSLHonorCipherOrder Directive ¶

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

SSLHonorCipherOrder on

SSLInsecureRenegotiation Directive ¶

As originally specified, all versions of the SSL and TLS protocols (up to and including TLS/1.2) were vulnerable to a Man-in-the-Middle attack (CVE-2009-3555) during a renegotiation. This vulnerability allowed an attacker to "prefix" a chosen plaintext to the HTTP request as seen by the web server. A protocol extension was developed which fixed this vulnerability if supported by both client and server.

If mod_ssl is linked against OpenSSL version 0.9.8m or later, by default renegotiation is only supported with clients supporting the new protocol extension. If this directive is enabled, renegotiation will be allowed with old (unpatched) clients, albeit insecurely.

SSLInsecureRenegotiation on

The SSL_SECURE_RENEG environment variable can be used from an SSI or CGI script to determine whether secure renegotiation is supported for a given SSL connection.

SSLOCSPDefaultResponder Directive ¶

This option sets the default OCSP responder to use. If SSLOCSPOverrideResponder is not enabled, the URI given will be used only if no responder URI is specified in the certificate being verified.

SSLOCSPEnable Directive ¶

This option enables OCSP validation of the client certificate chain. If this option is enabled, certificates in the client's certificate chain will be validated against an OCSP responder after normal verification (including CRL checks) have taken place. In mode 'leaf', only the client certificate itself will be validated.

The OCSP responder used is either extracted from the certificate itself, or derived by configuration; see the SSLOCSPDefaultResponder and SSLOCSPOverrideResponder directives.

SSLVerifyClient on
SSLOCSPEnable on
SSLOCSPDefaultResponder "http://responder.example.com:8888/responder"
SSLOCSPOverrideResponder on

SSLOCSPNoverify Directive ¶

Skip the OCSP responder certificates verification, mostly useful when testing an OCSP server.

SSLOCSPOverrideResponder Directive ¶

This option forces the configured default OCSP responder to be used during OCSP certificate validation, regardless of whether the certificate being validated references an OCSP responder.

SSLOCSPProxyURL Directive ¶

This option allows to set the URL of a HTTP proxy that should be used for all queries to OCSP responders.

SSLOCSPResponderCertificateFile Directive ¶

This supplies a list of trusted OCSP responder certificates to be used during OCSP responder certificate validation. The supplied certificates are implicitly trusted without any further validation. This is typically used where the OCSP responder certificate is self signed or omitted from the OCSP response.

SSLOCSPResponderTimeout Directive ¶

This option sets the timeout for queries to OCSP responders, when SSLOCSPEnable is turned on.

SSLOCSPResponseMaxAge Directive ¶

This option sets the maximum allowable age ("freshness") for OCSP responses. The default value (-1) does not enforce a maximum age, which means that OCSP responses are considered valid as long as their nextUpdate field is in the future.

SSLOCSPResponseTimeSkew Directive ¶

This option sets the maximum allowable time skew for OCSP responses (when checking their thisUpdate and nextUpdate fields).

SSLOCSPUseRequestNonce Directive ¶

This option determines whether queries to OCSP responders should contain a nonce or not. By default, a query nonce is always used and checked against the response's one. When the responder does not use nonces (e.g. Microsoft OCSP Responder), this option should be turned off.

SSLOpenSSLConfCmd Directive ¶

This directive exposes OpenSSL's SSL_CONF API to mod_ssl, allowing a flexible configuration of OpenSSL parameters without the need of implementing additional mod_ssl directives when new features are added to OpenSSL.

The set of available SSLOpenSSLConfCmd commands depends on the OpenSSL version being used for mod_ssl (at least version 1.0.2 is required). For a list of supported command names, see the section Supported configuration file commands in the SSL_CONF_cmd(3) manual page for OpenSSL.

SSLOpenSSLConfCmd Options -SessionTicket,ServerPreference
SSLOpenSSLConfCmd ECDHParameters brainpoolP256r1
SSLOpenSSLConfCmd ServerInfoFile "/usr/local/apache2/conf/server-info.pem"
SSLOpenSSLConfCmd Protocol "-ALL, TLSv1.2"
SSLOpenSSLConfCmd SignatureAlgorithms RSA+SHA384:ECDSA+SHA256

SSLOptions Directive ¶

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:

• 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.

• 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). The optional SSLUserName directive can be used to specify which part of the certificate Subject is embedded in the username. 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/''.

  Note that the AuthBasicFake directive within mod_auth_basic can be used as a more general mechanism for faking basic authentication, giving control over the structure of both the username and password.

  The usernames used for FakeBasicAuth must not include any non-ASCII characters, ASCII escape characters (such a newline), or a colon. If a colon is found, a 403 Forbidden error will be generated with httpd 2.5.1 and later.

• 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 may not be what the user expects, so enable this on a per-directory basis only, please.

• LegacyDNStringFormat

  This option influences how values of the SSL_{CLIENT,SERVER}_{I,S}_DN variables are formatted. Since version 2.3.11, Apache HTTPD uses a RFC 2253 compatible format by default. This uses commas as delimiters between the attributes, allows the use of non-ASCII characters (which are converted to UTF8), escapes various special characters with backslashes, and sorts the attributes with the "C" attribute last.

  If LegacyDNStringFormat is set, the old format will be used which sorts the "C" attribute first, uses slashes as separators, and does not handle non-ASCII and special characters in any consistent way.

SSLOptions +FakeBasicAuth -StrictRequire
<Files ~ "\.(cgi|shtml)$">
    SSLOptions +StdEnvVars -ExportCertData
</Files>

SSLPassPhraseDialog Directive ¶

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).

• |/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 one argument, a string of the form ``servername:portnumber:index'' (with index being a zero-based sequence number), which indicates for which server, TCP port and certificate number 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.

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

SSLPolicy Directive ¶

This directive applies the set of SSL* directives defined under 'name' as the base settings in the current context. Apache comes with the following pre-defined policies from Mozilla, the makers of the Firefox browser (see here for a detailed description by them.):

• modern: recommended when your server is accessible on the open Internet. Works with all modern browsers, but old devices might be unable to connect.
• intermediate: the fallback if you need to support old (but not very old) clients.
• old: when you need to give Windows XP/Internet Explorer 6 access. The last resort.

SSLPolicy applies configuration settings in place, meaning previous values are overwritten. Configuration directives following an SSLPolicy may overwrite it.

You can check the detailed description of all defined policies via the command line:

httpd -t -D DUMP_SSL_POLICIES

SSLProtocol Directive ¶

This directive can be used to control which versions of the SSL/TLS protocol will be accepted in new connections.

The available (case-insensitive) protocols are:

• 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, but is deprecated in RFC 7568.

• TLSv1

  This is the Transport Layer Security (TLS) protocol, version 1.0. It is the successor to SSLv3 and is defined in RFC 2246. It is supported by nearly every client.

• TLSv1.1 (when using OpenSSL 1.0.1 and later)

  A revision of the TLS 1.0 protocol, as defined in RFC 4346.

• TLSv1.2 (when using OpenSSL 1.0.1 and later)

  A revision of the TLS 1.1 protocol, as defined in RFC 5246.

• TLSv1.3 (when using OpenSSL 1.1.1 and later)

  A new version of the TLS protocol, as defined in RFC 8446.

• all

  This is a shortcut for ``+SSLv3 +TLSv1'' or - when using OpenSSL 1.0.1 and later - ``+SSLv3 +TLSv1 +TLSv1.1 +TLSv1.2'', respectively (except for OpenSSL versions compiled with the ``no-ssl3'' configuration option, where all does not include +SSLv3).

SSLProtocol TLSv1

SSLProxyCACertificateFile Directive ¶

This directive sets the all-in-one file where you can assemble the Certificates of Certification Authorities (CA) whose remote servers you deal with. These are used for Remote Server 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 SSLProxyCACertificatePath.

SSLProxyCACertificateFile "/usr/local/apache2/conf/ssl.crt/ca-bundle-remote-server.crt"

SSLProxyCACertificatePath Directive ¶

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

SSLProxyCACertificatePath "/usr/local/apache2/conf/ssl.crt/"

SSLProxyCARevocationCheck Directive ¶

Enables certificate revocation list (CRL) checking for the remote servers you deal with. At least one of SSLProxyCARevocationFile or SSLProxyCARevocationPath must be configured. When set to chain (recommended setting), CRL checks are applied to all certificates in the chain, while setting it to leaf limits the checks to the end-entity cert.

SSLProxyCARevocationCheck chain

SSLProxyCARevocationFile Directive ¶

This directive sets the all-in-one file where you can assemble the Certificate Revocation Lists (CRL) of Certification Authorities (CA) whose remote servers you deal with. These are used for Remote Server 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 SSLProxyCARevocationPath.

SSLProxyCARevocationFile "/usr/local/apache2/conf/ssl.crl/ca-bundle-remote-server.crl"

SSLProxyCARevocationPath Directive ¶

This directive sets the directory where you keep the Certificate Revocation Lists (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 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.

SSLProxyCARevocationPath "/usr/local/apache2/conf/ssl.crl/"

SSLProxyCheckPeerCN Directive ¶

This directive sets whether the remote server certificate's CN field is compared against the hostname of the request URL. If both are not equal a 502 status code (Bad Gateway) is sent. SSLProxyCheckPeerCN is superseded by SSLProxyCheckPeerName in release 2.4.5 and later.

In all releases 2.4.5 through 2.4.20, setting SSLProxyCheckPeerName off was sufficient to enable this behavior (as the SSLProxyCheckPeerCN default was on.) In these releases, both directives must be set to off to completely avoid remote server certificate name validation. Many users reported this to be very confusing.

As of release 2.4.21, all configurations which enable either one of the SSLProxyCheckPeerName or SSLProxyCheckPeerCN options will use the new SSLProxyCheckPeerName behavior, and all configurations which disable either one of the SSLProxyCheckPeerName or SSLProxyCheckPeerCN options will suppress all remote server certificate name validation. Only the following configuration will trigger the legacy certificate CN comparison in 2.4.21 and later releases;

SSLProxyCheckPeerCN on
SSLProxyCheckPeerName off

SSLProxyCheckPeerExpire Directive ¶

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

SSLProxyCheckPeerExpire on

SSLProxyCheckPeerName Directive ¶

This directive configures host name checking for server certificates when mod_ssl is acting as an SSL client. The check will succeed if the host name from the request URI matches one of the CN attribute(s) of the certificate's subject, or matches the subjectAltName extension. If the check fails, the SSL request is aborted and a 502 status code (Bad Gateway) is returned.

Wildcard matching is supported for specific cases: an subjectAltName entry of type dNSName, or CN attributes starting with *. will match with any host name of the same number of name elements and the same suffix. E.g. *.example.org will match foo.example.org, but will not match foo.bar.example.org, because the number of elements in the respective host names differs.

This feature was introduced in 2.4.5 and superseded the behavior of the SSLProxyCheckPeerCN directive, which only tested the exact value in the first CN attribute against the host name. However, many users were confused by the behavior of using these directives individually, so the mutual behavior of SSLProxyCheckPeerName and SSLProxyCheckPeerCN directives were improved in release 2.4.21. See the SSLProxyCheckPeerCN directive description for the original behavior and details of these improvements.

SSLProxyCipherSuite Directive ¶

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

SSLProxyEngine Directive ¶

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

Note that the SSLProxyEngine directive should not, in general, be included in a virtual host that will be acting as a forward proxy (using <Proxy> or ProxyRequests directives). SSLProxyEngine is not required to enable a forward proxy server to proxy SSL/TLS requests.

<VirtualHost _default_:443>
    SSLProxyEngine on
    #...
</VirtualHost>

SSLProxyMachineCertificateChainFile Directive ¶

This directive sets the all-in-one file where you keep the certificate chain for all of the client certs in use. This directive will be needed if the remote server presents a list of CA certificates that are not direct signers of one of the configured client certificates.

This referenced file is simply the concatenation of the various PEM-encoded certificate files. Upon startup, each client certificate configured will be examined and a chain of trust will be constructed.

SSLProxyMachineCertificateChainFile "/usr/local/apache2/conf/ssl.crt/proxyCA.pem"

SSLProxyMachineCertificateFile Directive ¶

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

This referenced file is simply the concatenation of the various PEM-encoded certificate files. Use this directive alternatively or additionally to SSLProxyMachineCertificatePath. The referenced file can contain any number of pairs of client certificate and associated private key. Each pair can be specified in either (certificate, key) or (key, certificate) order. Non-leaf (CA) certificates can also be included in the file, and are treated as if configured with SSLProxyMachineCertificateChainFile.

When challenged to provide a client certificate by a remote server, the server should provide a list of acceptable certificate authority names in the challenge. If such a list is not provided, mod_ssl will use the first configured client cert/key. If a list of CA names is provided, mod_ssl will iterate through that list, and attempt to find a configured client cert which was issued either directly by that CA, or indirectly via any number of intermediary CA certificates. The chain of intermediate CA certificates can be built from those included in the file, or configured with SSLProxyMachineCertificateChainFile. The first configured matching certificate will then be supplied in response to the challenge.

If the list of CA names is provided by the remote server, and no matching client certificate can be found, no client certificate will be provided by mod_ssl, which will likely fail the SSL/TLS handshake (depending on the remote server configuration).

SSLProxyMachineCertificateFile "/usr/local/apache2/conf/ssl.crt/proxy.pem"

SSLProxyMachineCertificatePath Directive ¶

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

mod_ssl will attempt to load every file inside the specified directory as if it was configured individually with SSLProxyMachineCertificateFile.

SSLProxyMachineCertificatePath "/usr/local/apache2/conf/proxy.crt/"

SSLProxyProtocol Directive ¶

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

Please refer to SSLProtocol for additional information.

SSLProxyVerify Directive ¶

When a proxy is configured to forward requests to a remote SSL server, this directive can be used to configure certificate verification of the remote server.

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 and require are really interesting, because level optional doesn't work with all servers and level optional_no_ca is actually against the idea of authentication (but can be used to establish SSL test pages, etc.)

SSLProxyVerify require

SSLProxyVerifyDepth Directive ¶

This directive sets how deeply mod_ssl should verify before deciding that the remote server does not have a valid certificate.

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 remote server certificate. A depth of 0 means that self-signed remote server certificates are accepted only, the default depth of 1 means the remote server 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 SSLProxyCACertificatePath), etc.

SSLProxyVerifyDepth 10

SSLRandomSeed Directive ¶

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. Its 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 a randomly chosen 128 bytes extract of the stack. 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 derivatives 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.

• 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.

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

SSLRenegBufferSize Directive ¶

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

SSLRenegBufferSize 262144

SSLRequire Directive ¶

This directive specifies a general access requirement which has to be fulfilled in order to allow access. It is 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 "in" "PeerExtList(" word ")"
           | word "=~" regex
           | word "!~" regex

wordlist ::= word
           | wordlist "," word

word     ::= digit
           | cstring
           | variable
           | function

digit    ::= [0-9]+
cstring  ::= "..."
variable ::= "%{" varname "}"
function ::= funcname "(" funcargs ")"

For varname any of the variables described in Environment Variables can be used. For funcname the available functions are listed in the ap_expr documentation.

The expression is parsed into an internal machine representation when the configuration is loaded, and then evaluated during request processing. In .htaccess context, the expression is both parsed and executed each time the .htaccess file is encountered during request processing.

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} -ge 1 and %{TIME_WDAY} -le 5          \
            and %{TIME_HOUR} -ge 8 and %{TIME_HOUR} -le 20       ) \
           or %{REMOTE_ADDR} =~ m/^192\.76\.162\.[0-9]+$/

The PeerExtList(object-ID) function expects to find zero or more instances of the X.509 certificate extension identified by the given object ID (OID) in the client certificate. The expression evaluates to true if the left-hand side string matches exactly against the value of an extension identified with this OID. (If multiple extensions with the same OID are present, at least one extension must match).

SSLRequire "foobar" in PeerExtList("1.2.3.4.5.6")

See also

• Environment Variables in Apache HTTP Server, for additional examples.
• Require expr
• Generic expression syntax in Apache HTTP Server

SSLRequireSSL Directive ¶

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.

SSLRequireSSL

SSLSessionCache Directive ¶

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 unnecessary session handshakes.

The following five 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. To use this, ensure that mod_socache_dbm is loaded.

• shmcb:/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. To use this, ensure that mod_socache_shmcb is loaded.

• 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. To use this, ensure that mod_socache_dc is loaded.

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

The ssl-cache mutex is used to serialize access to the session cache to prevent corruption. This mutex can be configured using the Mutex directive.

SSLSessionCacheTimeout Directive ¶

This directive sets the timeout in seconds for the information stored in the global/inter-process SSL Session Cache, the OpenSSL internal memory cache and for sessions resumed by TLS session resumption (RFC 5077). It can be set as low as 15 for testing, but should be set to higher values like 300 in real life.

SSLSessionCacheTimeout 600

SSLSessionTicketKeyFile Directive ¶

Optionally configures a secret key for encrypting and decrypting TLS session tickets, as defined in RFC 5077. Primarily suitable for clustered environments where TLS sessions information should be shared between multiple nodes. For single-instance httpd setups, it is recommended to not configure a ticket key file, but to rely on (random) keys generated by mod_ssl at startup, instead.

The ticket key file must contain 48 bytes of random data, preferably created from a high-entropy source. On a Unix-based system, a ticket key file can be created as follows:

Ticket keys should be rotated (replaced) on a frequent basis, as this is the only way to invalidate an existing session ticket - OpenSSL currently doesn't allow to specify a limit for ticket lifetimes. A new ticket key only gets used after restarting the web server. All existing session tickets become invalid after a restart.

SSLSessionTickets Directive ¶

This directive allows to enable or disable the use of TLS session tickets (RFC 5077).

SSLSRPUnknownUserSeed Directive ¶

This directive sets the seed used to fake SRP user parameters for unknown users, to avoid leaking whether a given user exists. Specify a secret string. If this directive is not used, then Apache will return the UNKNOWN_PSK_IDENTITY alert to clients who specify an unknown username.

SSLSRPVerifierFile Directive ¶

This directive enables TLS-SRP and sets the path to the OpenSSL SRP (Secure Remote Password) verifier file containing TLS-SRP usernames, verifiers, salts, and group parameters.

The verifier file can be created with the openssl command line utility:

The value given with the optional -userinfo parameter is available in the SSL_SRP_USERINFO request environment variable.

SSLStaplingCache Directive ¶

Configures the cache used to store OCSP responses which get included in the TLS handshake if SSLUseStapling is enabled. Configuration of a cache is mandatory for OCSP stapling. With the exception of none and nonenotnull, the same storage types are supported as with SSLSessionCache.

SSLStaplingErrorCacheTimeout Directive ¶

Sets the timeout in seconds before invalid responses in the OCSP stapling cache (configured through SSLStaplingCache) will expire. To set the cache timeout for valid responses, see SSLStaplingStandardCacheTimeout.

SSLStaplingFakeTryLater Directive ¶

When enabled and a query to an OCSP responder for stapling purposes fails, mod_ssl will synthesize a "tryLater" response for the client. Only effective if SSLStaplingReturnResponderErrors is also enabled.

SSLStaplingForceURL Directive ¶

This directive overrides the URI of an OCSP responder as obtained from the authorityInfoAccess (AIA) extension of the certificate. One potential use is when a proxy is used for retrieving OCSP queries.

SSLStaplingResponderTimeout Directive ¶

This option sets the timeout for queries to OCSP responders when SSLUseStapling is enabled and mod_ssl is querying a responder for OCSP stapling purposes.

SSLStaplingResponseMaxAge Directive ¶

This option sets the maximum allowable age ("freshness") when considering OCSP responses for stapling purposes, i.e. when SSLUseStapling is turned on. The default value (-1) does not enforce a maximum age, which means that OCSP responses are considered valid as long as their nextUpdate field is in the future.

SSLStaplingResponseTimeSkew Directive ¶

This option sets the maximum allowable time skew when mod_ssl checks the thisUpdate and nextUpdate fields of OCSP responses which get included in the TLS handshake (OCSP stapling). Only applicable if SSLUseStapling is turned on.

SSLStaplingReturnResponderErrors Directive ¶

When enabled, mod_ssl will pass responses from unsuccessful stapling related OCSP queries (such as responses with an overall status other than "successful", responses with a certificate status other than "good", expired responses etc.) on to the client. If set to off, only responses indicating a certificate status of "good" will be included in the TLS handshake.

SSLStaplingStandardCacheTimeout Directive ¶

Sets the timeout in seconds before responses in the OCSP stapling cache (configured through SSLStaplingCache) will expire. This directive applies to valid responses, while SSLStaplingErrorCacheTimeout is used for controlling the timeout for invalid/unavailable responses.

SSLStrictSNIVHostCheck Directive ¶

This directive sets whether a non-SNI client is allowed to access a name-based virtual host. If set to on in the default name-based virtual host, clients that are SNI unaware will not be allowed to access any virtual host, belonging to this particular IP / port combination. If set to on in any other virtual host, SNI unaware clients are not allowed to access this particular virtual host.

SSLStrictSNIVHostCheck on

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 character string. In particular, this may cause the environment variable REMOTE_USER to be set. The varname can be any of the SSL environment variables.

When the FakeBasicAuth option is enabled, this directive instead controls the value of the username embedded within the basic authentication header (see SSLOptions).

SSLUserName SSL_CLIENT_S_DN_CN

SSLUseStapling Directive ¶

This option enables OCSP stapling, as defined by the "Certificate Status Request" TLS extension specified in RFC 6066. If enabled (and requested by the client), mod_ssl will include an OCSP response for its own certificate in the TLS handshake. Configuring an SSLStaplingCache is a prerequisite for enabling OCSP stapling.

OCSP stapling relieves the client of querying the OCSP responder on its own, but it should be noted that with the RFC 6066 specification, the server's CertificateStatus reply may only include an OCSP response for a single cert. For server certificates with intermediate CA certificates in their chain (the typical case nowadays), stapling in its current implementation therefore only partially achieves the stated goal of "saving roundtrips and resources" - see also RFC 6961 (TLS Multiple Certificate Status Extension).

When OCSP stapling is enabled, the ssl-stapling mutex is used to control access to the OCSP stapling cache in order to prevent corruption, and the sss-stapling-refresh mutex is used to control refreshes of OCSP responses. These mutexes can be configured using the Mutex directive.

SSLVerifyClient Directive ¶

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 renegotiation 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 it need not to be (successfully) verifiable. This option cannot be relied upon for client authentication.

SSLVerifyClient require

SSLVerifyDepth Directive ¶

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 renegotiation 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.

SSLVerifyDepth 10