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
#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, 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() allow to retrieve the function pointers of the provided 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 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).
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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