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>.