09-29-2011
See RFC 4253 Section 5.1
Quote:
5.1. Old Client, New Server
Server implementations MAY support a configurable compatibility flag
that enables compatibility with old versions. When this flag is on,
the server SHOULD identify its 'protoversion' as "1.99". Clients
using protocol 2.0 MUST be able to identify this as identical to
"2.0". In this mode, the server SHOULD NOT send the Carriage Return
character (ASCII 13) after the identification string.
In the compatibility mode, the server SHOULD NOT send any further
data after sending its identification string until it has received an
identification string from the client. The server can then determine
whether the client is using an old protocol, and can revert to the
old protocol if required. In the compatibility mode, the server MUST
NOT send additional data before the identification string.
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LEARN ABOUT OSX
ssl_ctx_new
SSL_CTX_new(3) OpenSSL SSL_CTX_new(3)
NAME
SSL_CTX_new - create a new SSL_CTX object as framework for TLS/SSL enabled functions
SYNOPSIS
#include <openssl/ssl.h>
SSL_CTX *SSL_CTX_new(SSL_METHOD *method);
DESCRIPTION
SSL_CTX_new() creates a new SSL_CTX object as framework to establish TLS/SSL enabled connections.
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:
SSLv2_method(void), SSLv2_server_method(void), SSLv2_client_method(void)
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.
SSLv3_method(void), SSLv3_server_method(void), SSLv3_client_method(void)
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. This especially
means, that it will not understand SSLv2 client hello messages which are widely used for compatibility reasons, see SSLv23_*_method().
TLSv1_method(void), TLSv1_server_method(void), TLSv1_client_method(void)
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. This especially
means, that it will not understand SSLv2 client hello messages which are widely used for compatibility reasons, see SSLv23_*_method().
It will also not understand SSLv3 client hello messages.
SSLv23_method(void), SSLv23_server_method(void), SSLv23_client_method(void)
A TLS/SSL connection established with these methods will understand the SSLv2, SSLv3, and TLSv1 protocol. A client will send out SSLv2
client hello messages and will indicate that it also understands SSLv3 and TLSv1. A server will understand SSLv2, SSLv3, and TLSv1
client hello messages. This is the best choice when compatibility is a concern.
The list of protocols available can later be limited using the SSL_OP_NO_SSLv2, SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1 options of the
SSL_CTX_set_options() or SSL_set_options() functions. Using these options it is possible to choose e.g. SSLv23_server_method() and be able
to negotiate with all possible clients, but to only allow newer protocols like SSLv3 or TLSv1.
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.
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.
SEE ALSO
SSL_CTX_free(3), SSL_accept(3), ssl(3), SSL_set_connect_state(3)
50 2013-03-05 SSL_CTX_new(3)