NAME

       SSL_CTX_sess_set_new_cb, SSL_CTX_sess_set_remove_cb, SSL_CTX_sess_set_get_cb,
       SSL_CTX_sess_get_new_cb, SSL_CTX_sess_get_remove_cb, SSL_CTX_sess_get_get_cb - provide
       callback functions for server side external session caching

SYNOPSIS

        #include <openssl/ssl.h>

        void SSL_CTX_sess_set_new_cb(SSL_CTX *ctx,
                                     int (*new_session_cb)(SSL *, SSL_SESSION *));
        void SSL_CTX_sess_set_remove_cb(SSL_CTX *ctx,
                                        void (*remove_session_cb)(SSL_CTX *ctx,
                                                                  SSL_SESSION *));
        void SSL_CTX_sess_set_get_cb(SSL_CTX *ctx,
                                     SSL_SESSION (*get_session_cb)(SSL *,
                                                                   const unsigned char *,
                                                                   int, int *));

        int (*SSL_CTX_sess_get_new_cb(SSL_CTX *ctx))(struct ssl_st *ssl,
                                                     SSL_SESSION *sess);
        void (*SSL_CTX_sess_get_remove_cb(SSL_CTX *ctx))(struct ssl_ctx_st *ctx,
                                                         SSL_SESSION *sess);
        SSL_SESSION *(*SSL_CTX_sess_get_get_cb(SSL_CTX *ctx))(struct ssl_st *ssl,
                                                              const unsigned char *data,
                                                              int len, int *copy);

DESCRIPTION

       SSL_CTX_sess_set_new_cb() sets the callback function, which is automatically called
       whenever a new session was negotiated.

       SSL_CTX_sess_set_remove_cb() sets the callback function, which is automatically called
       whenever a session is removed by the SSL engine, because it is considered faulty or the
       session has become obsolete because of exceeding the timeout value.

       SSL_CTX_sess_set_get_cb() sets the callback function which is called, whenever a SSL/TLS
       client proposed to resume a session but the session could not be found in the internal
       session cache (see SSL_CTX_set_session_cache_mode(3)).  (SSL/TLS server only.)

       SSL_CTX_sess_get_new_cb(), SSL_CTX_sess_get_remove_cb(), and SSL_CTX_sess_get_get_cb()
       retrieve the function pointers set by the corresponding set callback functions. If a
       callback function has not been set, the NULL pointer is returned.

NOTES

       In order to allow external session caching, synchronization with the internal session
       cache is realized via callback functions. Inside these callback functions, session can be
       saved to disk or put into a database using the d2i_SSL_SESSION(3) interface.

       The new_session_cb() is called, whenever a new session has been negotiated and session
       caching is enabled (see SSL_CTX_set_session_cache_mode(3)).  The new_session_cb() is
       passed the ssl connection and the ssl session sess. If the callback returns 0, the session
       will be immediately removed again. Note that in TLSv1.3, sessions are established after
       the main handshake has completed. The server decides when to send the client the session
       information and this may occur some time after the end of the handshake (or not at all).
       This means that applications should expect the new_session_cb() function to be invoked
       during the handshake (for <= TLSv1.2) or after the handshake (for TLSv1.3). It is also
       possible in TLSv1.3 for multiple sessions to be established with a single connection. In
       these case the new_session_cb() function will be invoked multiple times.

       In TLSv1.3 it is recommended that each SSL_SESSION object is only used for resumption
       once. One way of enforcing that is for applications to call SSL_CTX_remove_session(3)
       after a session has been used.

       The remove_session_cb() is called, whenever the SSL engine removes a session from the
       internal cache. This happens when the session is removed because it is expired or when a
       connection was not shutdown cleanly. It also happens for all sessions in the internal
       session cache when SSL_CTX_free(3) is called. The remove_session_cb() is passed the ctx
       and the ssl session sess. It does not provide any feedback.

       The get_session_cb() is only called on SSL/TLS servers with the session id proposed by the
       client. The get_session_cb() is always called, also when session caching was disabled. The
       get_session_cb() is passed the ssl connection, the session id of length length at the
       memory location data. With the parameter copy the callback can require the SSL engine to
       increment the reference count of the SSL_SESSION object, Normally the reference count is
       not incremented and therefore the session must not be explicitly freed with
       SSL_SESSION_free(3).

RETURN VALUES

       SSL_CTX_sess_get_new_cb(), SSL_CTX_sess_get_remove_cb() and SSL_CTX_sess_get_get_cb()
       return different callback function pointers respectively.

SEE ALSO

       ssl(7), d2i_SSL_SESSION(3), SSL_CTX_set_session_cache_mode(3), SSL_CTX_flush_sessions(3),
       SSL_SESSION_free(3), SSL_CTX_free(3)

COPYRIGHT

       Copyright 2001-2018 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>.