NAME

       TLSv1_2_method, TLSv1_2_server_method, TLSv1_2_client_method, SSL_CTX_new, SSL_CTX_up_ref,
       SSLv3_method, SSLv3_server_method, SSLv3_client_method, TLSv1_method, TLSv1_server_method,
       TLSv1_client_method, TLSv1_1_method, TLSv1_1_server_method, TLSv1_1_client_method,
       TLS_method, TLS_server_method, TLS_client_method, SSLv23_method, SSLv23_server_method,
       SSLv23_client_method, DTLS_method, DTLS_server_method, DTLS_client_method, DTLSv1_method,
       DTLSv1_server_method, DTLSv1_client_method, DTLSv1_2_method, DTLSv1_2_server_method,
       DTLSv1_2_client_method - create a new SSL_CTX object as framework for TLS/SSL or DTLS
       enabled functions

SYNOPSIS

        #include <openssl/ssl.h>

        SSL_CTX *SSL_CTX_new(const SSL_METHOD *method);
        int SSL_CTX_up_ref(SSL_CTX *ctx);

        const SSL_METHOD *TLS_method(void);
        const SSL_METHOD *TLS_server_method(void);
        const SSL_METHOD *TLS_client_method(void);

        const SSL_METHOD *SSLv23_method(void);
        const SSL_METHOD *SSLv23_server_method(void);
        const SSL_METHOD *SSLv23_client_method(void);

        #ifndef OPENSSL_NO_SSL3_METHOD
        const SSL_METHOD *SSLv3_method(void);
        const SSL_METHOD *SSLv3_server_method(void);
        const SSL_METHOD *SSLv3_client_method(void);
        #endif

        #ifndef OPENSSL_NO_TLS1_METHOD
        const SSL_METHOD *TLSv1_method(void);
        const SSL_METHOD *TLSv1_server_method(void);
        const SSL_METHOD *TLSv1_client_method(void);
        #endif

        #ifndef OPENSSL_NO_TLS1_1_METHOD
        const SSL_METHOD *TLSv1_1_method(void);
        const SSL_METHOD *TLSv1_1_server_method(void);
        const SSL_METHOD *TLSv1_1_client_method(void);
        #endif

        #ifndef OPENSSL_NO_TLS1_2_METHOD
        const SSL_METHOD *TLSv1_2_method(void);
        const SSL_METHOD *TLSv1_2_server_method(void);
        const SSL_METHOD *TLSv1_2_client_method(void);
        #endif

        const SSL_METHOD *DTLS_method(void);
        const SSL_METHOD *DTLS_server_method(void);
        const SSL_METHOD *DTLS_client_method(void);

        #ifndef OPENSSL_NO_DTLS1_METHOD
        const SSL_METHOD *DTLSv1_method(void);
        const SSL_METHOD *DTLSv1_server_method(void);
        const SSL_METHOD *DTLSv1_client_method(void);
        #endif

        #ifndef OPENSSL_NO_DTLS1_2_METHOD
        const SSL_METHOD *DTLSv1_2_method(void);
        const SSL_METHOD *DTLSv1_2_server_method(void);
        const SSL_METHOD *DTLSv1_2_client_method(void);
        #endif

DESCRIPTION

       SSL_CTX_new() creates a new SSL_CTX object as framework to establish TLS/SSL or DTLS
       enabled connections. An SSL_CTX object is reference counted. Creating an SSL_CTX object
       for the first time increments the reference count. Freeing it (using SSL_CTX_free)
       decrements it. When the reference count drops to zero, any memory or resources allocated
       to the SSL_CTX object are freed. SSL_CTX_up_ref() increments the reference count for an
       existing SSL_CTX structure.

NOTES

       The SSL_CTX object uses method as connection method.  The methods exist in a generic type
       (for client and server use), a server only type, and a client only type.  method can be of
       the following types:

       TLS_method(), TLS_server_method(), TLS_client_method()
           These are the general-purpose version-flexible SSL/TLS methods.  The actual protocol
           version used will be negotiated to the highest version mutually supported by the
           client and the server.  The supported protocols are SSLv3, TLSv1, TLSv1.1, TLSv1.2 and
           TLSv1.3.  Applications should use these methods, and avoid the version-specific
           methods described below.

       SSLv23_method(), SSLv23_server_method(), SSLv23_client_method()
           Use of these functions is deprecated. They have been replaced with the above
           TLS_method(), TLS_server_method() and TLS_client_method() respectively. New code
           should use those functions instead.

       TLSv1_2_method(), TLSv1_2_server_method(), TLSv1_2_client_method()
           A TLS/SSL connection established with these methods will only understand the TLSv1.2
           protocol.

       TLSv1_1_method(), TLSv1_1_server_method(), TLSv1_1_client_method()
           A TLS/SSL connection established with these methods will only understand the TLSv1.1
           protocol.

       TLSv1_method(), TLSv1_server_method(), TLSv1_client_method()
           A TLS/SSL connection established with these methods will only understand the TLSv1
           protocol.

       SSLv3_method(), SSLv3_server_method(), SSLv3_client_method()
           A TLS/SSL connection established with these methods will only understand the SSLv3
           protocol.  The SSLv3 protocol is deprecated and should not be used.

       DTLS_method(), DTLS_server_method(), DTLS_client_method()
           These are the version-flexible DTLS methods.  Currently supported protocols are DTLS
           1.0 and DTLS 1.2.

       DTLSv1_2_method(), DTLSv1_2_server_method(), DTLSv1_2_client_method()
           These are the version-specific methods for DTLSv1.2.

       DTLSv1_method(), DTLSv1_server_method(), DTLSv1_client_method()
           These are the version-specific methods for DTLSv1.

       SSL_CTX_new() initializes the list of ciphers, the session cache setting, the callbacks,
       the keys and certificates and the options to their default values.

       TLS_method(), TLS_server_method(), TLS_client_method(), DTLS_method(),
       DTLS_server_method() and DTLS_client_method() are the version-flexible methods.  All other
       methods only support one specific protocol version.  Use the version-flexible methods
       instead of the version specific methods.

       If you want to limit the supported protocols for the version flexible methods you can use
       SSL_CTX_set_min_proto_version(3), SSL_set_min_proto_version(3),
       SSL_CTX_set_max_proto_version(3) and SSL_set_max_proto_version(3) functions.  Using these
       functions it is possible to choose e.g. TLS_server_method() and be able to negotiate with
       all possible clients, but to only allow newer protocols like TLS 1.0, TLS 1.1, TLS 1.2 or
       TLS 1.3.

       The list of protocols available can also be limited using the SSL_OP_NO_SSLv3,
       SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, SSL_OP_NO_TLSv1_3, SSL_OP_NO_TLSv1_2 and
       SSL_OP_NO_TLSv1_3 options of the SSL_CTX_set_options(3) or SSL_set_options(3) functions,
       but this approach is not recommended. Clients should avoid creating "holes" in the set of
       protocols they support. When disabling a protocol, make sure that you also disable either
       all previous or all subsequent protocol versions.  In clients, when a protocol version is
       disabled without disabling all previous protocol versions, the effect is to also disable
       all subsequent protocol versions.

       The SSLv3 protocol is deprecated and should generally not be used.  Applications should
       typically use SSL_CTX_set_min_proto_version(3) to set the minimum protocol to at least
       TLS1_VERSION.

RETURN VALUES

       The following return values can occur:

       NULL
           The creation of a new SSL_CTX object failed. Check the error stack to find out the
           reason.

       Pointer to an SSL_CTX object
           The return value points to an allocated SSL_CTX object.

           SSL_CTX_up_ref() returns 1 for success and 0 for failure.

HISTORY

       Support for SSLv2 and the corresponding SSLv2_method(), SSLv2_server_method() and
       SSLv2_client_method() functions where removed in OpenSSL 1.1.0.

       SSLv23_method(), SSLv23_server_method() and SSLv23_client_method() were deprecated and the
       preferred TLS_method(), TLS_server_method() and TLS_client_method() functions were
       introduced in OpenSSL 1.1.0.

       All version-specific methods were deprecated in OpenSSL 1.1.0.

SEE ALSO

       SSL_CTX_set_options(3), SSL_CTX_free(3), SSL_accept(3), SSL_CTX_set_min_proto_version(3),
       ssl(7), SSL_set_connect_state(3)

COPYRIGHT

       Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use this file except in
       compliance with the License.  You can obtain a copy in the file LICENSE in the source
       distribution or at <https://www.openssl.org/source/license.html>.