Ubuntu Manpage: SSL_CTX_sess_set_new_cb, SSL_CTX_sess_set_remove_cb, SSL_CTX_sess_set_get_cb, SSL_CTX_sess_get_new

Provided by: libssl-doc_3.0.13-0ubuntu3.5_all

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 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.

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
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).

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.

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