Updated: 2021/Apr/14

SSL_CTX_sess_set_get_cb(3)          OpenSSL         SSL_CTX_sess_set_get_cb(3)

       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

       libcrypto, -lcrypto

        #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);

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

       SSL_CTX_sess_set_remove_cb() sets the callback function that is called
       whenever a session is removed by the SSL engine.  For example, this can
       occur because a session is considered faulty or has become obsolete
       because of exceeding the timeout value.

       SSL_CTX_sess_set_get_cb() sets the callback function that is called
       whenever a 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)).  (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.

       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 nascent ssl session sess.  Since sessions are
       reference-counted objects, the reference count on the session is
       incremented before the callback, on behalf of the application.  If the
       callback returns 0, the session will be immediately removed from the
       internal cache and the reference count released. If the callback
       returns 1, the application retains the reference (for an entry in the
       application-maintained "external session cache"), and is responsible
       for calling SSL_SESSION_free() when the session reference is no longer
       in use.

       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 can happen 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, and is given
       the session id proposed by the client. The get_session_cb() is always
       called, even when session caching was disabled. The get_session_cb() is
       passed the ssl connection and the session id of length length at the
       memory location data. By setting the parameter copy to 1, the callback
       can require the SSL engine to increment the reference count of the
       SSL_SESSION object; setting copy to 0 causes the reference count to
       remain unchanged.  If the get_session_cb() does not write to copy, the
       reference count is incremented and the session must be explicitly freed
       with SSL_SESSION_free(3).

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

       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 2001-2020 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

1.1.1i                            2020-03-22        SSL_CTX_sess_set_get_cb(3)