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SSL_CTX_new, SSLv23_method, SSLv23_server_method,

 #include <openssl/ssl.h>
 SSL_CTX *SSL_CTX_new(const SSL_METHOD *method);
 const SSL_METHOD *SSLv23_method(void);
 const SSL_METHOD *SSLv23_server_method(void);
 const SSL_METHOD *SSLv23_client_method(void);
 const SSL_METHOD *TLSv1_2_method(void);
 const SSL_METHOD *TLSv1_2_server_method(void);
 const SSL_METHOD *TLSv1_2_client_method(void);
 const SSL_METHOD *TLSv1_1_method(void);
 const SSL_METHOD *TLSv1_1_server_method(void);
 const SSL_METHOD *TLSv1_1_client_method(void);
 const SSL_METHOD *TLSv1_method(void);
 const SSL_METHOD *TLSv1_server_method(void);
 const SSL_METHOD *TLSv1_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_SSL2
 const SSL_METHOD *SSLv2_method(void);
 const SSL_METHOD *SSLv2_server_method(void);
 const SSL_METHOD *SSLv2_client_method(void);
 #endif
 const SSL_METHOD *DTLS_method(void);
 const SSL_METHOD *DTLS_server_method(void);
 const SSL_METHOD *DTLS_client_method(void);
 const SSL_METHOD *DTLSv1_2_method(void);
 const SSL_METHOD *DTLSv1_2_server_method(void);
 const SSL_METHOD *DTLSv1_2_client_method(void);
 const SSL_METHOD *DTLSv1_method(void);
 const SSL_METHOD *DTLSv1_server_method(void);
 const SSL_METHOD *DTLSv1_client_method(void);

SSL_CTX_new() creates a new SSL_CTX object as framework to establish TLS/SSL enabled connections.

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:

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 SSLv2, SSLv3, TLSv1, TLSv1.1 and TLSv1.2. Most applications should use these method, and avoid the version specific methods described below.

The list of protocols available can be further limited using the SSL_OP_NO_SSLv2, SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1 and SSL_OP_NO_TLSv1_2 options of the SSL_CTX_set_options(3) or SSL_set_options(3) functions. 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 SSLv2 and SSLv3 protocols are deprecated and should generally not be used. Applications should typically use SSL_CTX_set_options(3) in combination with the SSL_OP_NO_SSLv3 flag to disable negotiation of SSLv3 via the above version-flexible SSL/TLS methods. The SSL_OP_NO_SSLv2 option is set by default, and would need to be cleared via SSL_CTX_clear_options(3) in order to enable negotiation of SSLv2.

A TLS/SSL connection established with these methods will only understand the TLSv1.2 protocol. A client will send out TLSv1.2 client hello messages and will also indicate that it only understand TLSv1.2. A server will only understand TLSv1.2 client hello messages.
A TLS/SSL connection established with these methods will only understand the TLSv1.1 protocol. A client will send out TLSv1.1 client hello messages and will also indicate that it only understand TLSv1.1. A server will only understand TLSv1.1 client hello messages.
A TLS/SSL connection established with these methods will only understand the TLSv1 protocol. A client will send out TLSv1 client hello messages and will indicate that it only understands TLSv1. A server will only understand TLSv1 client hello messages.
A TLS/SSL connection established with these methods will only understand the SSLv3 protocol. A client will send out SSLv3 client hello messages and will indicate that it only understands SSLv3. A server will only understand SSLv3 client hello messages. The SSLv3 protocol is deprecated and should not be used.
A TLS/SSL connection established with these methods will only understand the SSLv2 protocol. A client will send out SSLv2 client hello messages and will also indicate that it only understand SSLv2. A server will only understand SSLv2 client hello messages. The SSLv2 protocol offers little to no security and should not be used. As of OpenSSL 1.0.2g, EXPORT ciphers and 56-bit DES are no longer available with SSLv2.
These are the version-flexible DTLS methods.
These are the version-specific methods for DTLSv1.2.
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 its default values.

The following return values can occur:

The creation of a new SSL_CTX object failed. Check the error stack to find out the reason.
The return value points to an allocated SSL_CTX object.

SSL_CTX_set_options(3), SSL_CTX_clear_options(3), SSL_set_options(3), SSL_CTX_free(3), SSL_accept(3), ssl(3), SSL_set_connect_state(3)