SSL/TLS Strong Encryption: Compatibility - Apache HTTP Server
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SSL/TLS Strong Encryption: Compatibility

All PCs are compatible. But some of them are more compatible than others.

-- Unknown

Here we talk about backward compatibility to other SSL solutions. As you perhaps know, mod_ssl is not the only existing SSL solution for Apache. Actually there are four additional major products available on the market: Ben Laurie's freely available Apache-SSL (from where mod_ssl were originally derived in 1998), Red Hat's commercial Secure Web Server (which is based on mod_ssl), Covalent's commercial Raven SSL Module (also based on mod_ssl) and finally C2Net's commercial product Stronghold (based on a different evolution branch named Sioux up to Stronghold 2.x and based on mod_ssl since Stronghold 3.x).

The idea in mod_ssl is mainly the following: because mod_ssl provides mostly a superset of the functionality of all other solutions we can easily provide backward compatibility for most of the cases. Actually there are three compatibility areas we currently address: configuration directives, environment variables and custom log functions.

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Configuration Directives

For backward compatibility to the configuration directives of other SSL solutions we do an on-the-fly mapping: directives which have a direct counterpart in mod_ssl are mapped silently while other directives lead to a warning message in the logfiles. The currently implemented directive mapping is listed in Table 1. Currently full backward compatibility is provided only for Apache-SSL 1.x and mod_ssl 2.0.x. Compatibility to Sioux 1.x and Stronghold 2.x is only partial because of special functionality in these interfaces which mod_ssl (still) doesn't provide.

Table 1: Configuration Directive Mapping

Old Directivemod_ssl DirectiveComment
Apache-SSL 1.x & mod_ssl 2.0.x compatibility:
SSLEnableSSLEngine oncompactified
SSLDisableSSLEngine offcompactified
SSLLogFile fileSSLLog filecompactified
SSLRequiredCiphers specSSLCipherSuite specrenamed
SSLRequireCipher c1 ...SSLRequire %{SSL_CIPHER} in {"c1", ...}generalized
SSLBanCipher c1 ...SSLRequire not (%{SSL_CIPHER} in {"c1", ...})generalized
SSLFakeBasicAuthSSLOptions +FakeBasicAuthmerged
SSLCacheServerPath dir-functionality removed
SSLCacheServerPort integer-functionality removed
Apache-SSL 1.x compatibility:
SSLExportClientCertificatesSSLOptions +ExportCertDatamerged
SSLCacheServerRunDir dir-functionality not supported
Sioux 1.x compatibility:
SSL_CertFile fileSSLCertificateFile filerenamed
SSL_KeyFile fileSSLCertificateKeyFile filerenamed
SSL_CipherSuite argSSLCipherSuite argrenamed
SSL_X509VerifyDir argSSLCACertificatePath argrenamed
SSL_Log fileSSLLogFile filerenamed
SSL_Connect flagSSLEngine flagrenamed
SSL_ClientAuth argSSLVerifyClient argrenamed
SSL_X509VerifyDepth argSSLVerifyDepth argrenamed
SSL_FetchKeyPhraseFrom arg-not directly mappable; use SSLPassPhraseDialog
SSL_SessionDir dir-not directly mappable; use SSLSessionCache
SSL_Require expr-not directly mappable; use SSLRequire
SSL_CertFileType arg-functionality not supported
SSL_KeyFileType arg-functionality not supported
SSL_X509VerifyPolicy arg-functionality not supported
SSL_LogX509Attributes arg-functionality not supported
Stronghold 2.x compatibility:
StrongholdAccelerator dir-functionality not supported
StrongholdKey dir-functionality not supported
StrongholdLicenseFile dir-functionality not supported
SSLFlag flagSSLEngine flagrenamed
SSLSessionLockFile fileSSLMutex filerenamed
SSLCipherList specSSLCipherSuite specrenamed
RequireSSLSSLRequireSSLrenamed
SSLErrorFile file-functionality not supported
SSLRoot dir-functionality not supported
SSL_CertificateLogDir dir-functionality not supported
AuthCertDir dir-functionality not supported
SSL_Group name-functionality not supported
SSLProxyMachineCertPath dir-functionality not supported
SSLProxyMachineCertFile file-functionality not supported
SSLProxyCACertificatePath dir-functionality not supported
SSLProxyCACertificateFile file-functionality not supported
SSLProxyVerifyDepth number-functionality not supported
SSLProxyCipherList spec-functionality not supported
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Environment Variables

When you use ``SSLOptions +CompatEnvVars'' additional environment variables are generated. They all correspond to existing official mod_ssl variables. The currently implemented variable derivation is listed in Table 2.

Table 2: Environment Variable Derivation

Old Variablemod_ssl VariableComment
SSL_PROTOCOL_VERSIONSSL_PROTOCOLrenamed
SSLEAY_VERSIONSSL_VERSION_LIBRARYrenamed
HTTPS_SECRETKEYSIZESSL_CIPHER_USEKEYSIZErenamed
HTTPS_KEYSIZESSL_CIPHER_ALGKEYSIZErenamed
HTTPS_CIPHERSSL_CIPHERrenamed
HTTPS_EXPORTSSL_CIPHER_EXPORTrenamed
SSL_SERVER_KEY_SIZESSL_CIPHER_ALGKEYSIZErenamed
SSL_SERVER_CERTIFICATESSL_SERVER_CERTrenamed
SSL_SERVER_CERT_STARTSSL_SERVER_V_STARTrenamed
SSL_SERVER_CERT_ENDSSL_SERVER_V_ENDrenamed
SSL_SERVER_CERT_SERIALSSL_SERVER_M_SERIALrenamed
SSL_SERVER_SIGNATURE_ALGORITHMSSL_SERVER_A_SIGrenamed
SSL_SERVER_DNSSL_SERVER_S_DNrenamed
SSL_SERVER_CNSSL_SERVER_S_DN_CNrenamed
SSL_SERVER_EMAILSSL_SERVER_S_DN_Emailrenamed
SSL_SERVER_OSSL_SERVER_S_DN_Orenamed
SSL_SERVER_OUSSL_SERVER_S_DN_OUrenamed
SSL_SERVER_CSSL_SERVER_S_DN_Crenamed
SSL_SERVER_SPSSL_SERVER_S_DN_SPrenamed
SSL_SERVER_LSSL_SERVER_S_DN_Lrenamed
SSL_SERVER_IDNSSL_SERVER_I_DNrenamed
SSL_SERVER_ICNSSL_SERVER_I_DN_CNrenamed
SSL_SERVER_IEMAILSSL_SERVER_I_DN_Emailrenamed
SSL_SERVER_IOSSL_SERVER_I_DN_Orenamed
SSL_SERVER_IOUSSL_SERVER_I_DN_OUrenamed
SSL_SERVER_ICSSL_SERVER_I_DN_Crenamed
SSL_SERVER_ISPSSL_SERVER_I_DN_SPrenamed
SSL_SERVER_ILSSL_SERVER_I_DN_Lrenamed
SSL_CLIENT_CERTIFICATESSL_CLIENT_CERTrenamed
SSL_CLIENT_CERT_STARTSSL_CLIENT_V_STARTrenamed
SSL_CLIENT_CERT_ENDSSL_CLIENT_V_ENDrenamed
SSL_CLIENT_CERT_SERIALSSL_CLIENT_M_SERIALrenamed
SSL_CLIENT_SIGNATURE_ALGORITHMSSL_CLIENT_A_SIGrenamed
SSL_CLIENT_DNSSL_CLIENT_S_DNrenamed
SSL_CLIENT_CNSSL_CLIENT_S_DN_CNrenamed
SSL_CLIENT_EMAILSSL_CLIENT_S_DN_Emailrenamed
SSL_CLIENT_OSSL_CLIENT_S_DN_Orenamed
SSL_CLIENT_OUSSL_CLIENT_S_DN_OUrenamed
SSL_CLIENT_CSSL_CLIENT_S_DN_Crenamed
SSL_CLIENT_SPSSL_CLIENT_S_DN_SPrenamed
SSL_CLIENT_LSSL_CLIENT_S_DN_Lrenamed
SSL_CLIENT_IDNSSL_CLIENT_I_DNrenamed
SSL_CLIENT_ICNSSL_CLIENT_I_DN_CNrenamed
SSL_CLIENT_IEMAILSSL_CLIENT_I_DN_Emailrenamed
SSL_CLIENT_IOSSL_CLIENT_I_DN_Orenamed
SSL_CLIENT_IOUSSL_CLIENT_I_DN_OUrenamed
SSL_CLIENT_ICSSL_CLIENT_I_DN_Crenamed
SSL_CLIENT_ISPSSL_CLIENT_I_DN_SPrenamed
SSL_CLIENT_ILSSL_CLIENT_I_DN_Lrenamed
SSL_EXPORTSSL_CIPHER_EXPORTrenamed
SSL_KEYSIZESSL_CIPHER_ALGKEYSIZErenamed
SSL_SECKEYSIZESSL_CIPHER_USEKEYSIZErenamed
SSL_SSLEAY_VERSIONSSL_VERSION_LIBRARYrenamed
SSL_STRONG_CRYPTO-Not supported by mod_ssl
SSL_SERVER_KEY_EXP-Not supported by mod_ssl
SSL_SERVER_KEY_ALGORITHM-Not supported by mod_ssl
SSL_SERVER_KEY_SIZE-Not supported by mod_ssl
SSL_SERVER_SESSIONDIR-Not supported by mod_ssl
SSL_SERVER_CERTIFICATELOGDIR-Not supported by mod_ssl
SSL_SERVER_CERTFILE-Not supported by mod_ssl
SSL_SERVER_KEYFILE-Not supported by mod_ssl
SSL_SERVER_KEYFILETYPE-Not supported by mod_ssl
SSL_CLIENT_KEY_EXP-Not supported by mod_ssl
SSL_CLIENT_KEY_ALGORITHM-Not supported by mod_ssl
SSL_CLIENT_KEY_SIZE-Not supported by mod_ssl
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Custom Log Functions

When mod_ssl is built into Apache or at least loaded (under DSO situation) additional functions exist for the Custom Log Format of mod_log_config as documented in the Reference Chapter. Beside the ``%{varname}x'' eXtension format function which can be used to expand any variables provided by any module, an additional Cryptography ``%{name}c'' cryptography format function exists for backward compatibility. The currently implemented function calls are listed in Table 3.

Table 3: Custom Log Cryptography Function

Function CallDescription
%...{version}c SSL protocol version
%...{cipher}c SSL cipher
%...{subjectdn}c Client Certificate Subject Distinguished Name
%...{issuerdn}c Client Certificate Issuer Distinguished Name
%...{errcode}c Certificate Verification Error (numerical)
%...{errstr}c Certificate Verification Error (string)
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