SSL_CTX_SESS_SET_GET_CB(3) | Library Functions Manual | SSL_CTX_SESS_SET_GET_CB(3)

# 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

library “libssl” libcrypto
`#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))`(SSL *ssl, SSL_SESSION *sess);

void
`(*SSL_CTX_sess_get_remove_cb(SSL_CTX *ctx))`(SSL_CTX *ctx, SSL_SESSION *sess);

SSL_SESSION *
`(*SSL_CTX_sess_get_get_cb(SSL_CTX *ctx))`(SSL *ssl, const unsigned char *data, int len, int *copy);

int
`(*new_session_cb)`(SSL *ssl, SSL_SESSION *sess);

void
`(*remove_session_cb)`(SSL_CTX *ctx, SSL_SESSION *sess);

SSL_SESSION *
`(*get_session_cb)`(SSL *ssl, 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 proposes to resume a session but the session cannot 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 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`() function 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`() function is passed the ssl connection and the ssl session sess. If the callback returns 0, the session will be immediately removed again.

The `remove_session_cb`() function 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 shut down cleanly. It also happens for all sessions in the internal session cache when SSL_CTX_free(3) is called. The `remove_session_cb`() function is passed the ctx and the ssl session sess. It does not provide any feedback.

The `get_session_cb`() function is only called on SSL/TLS servers with the session id proposed by the client. The `get_session_cb`() function is always called, also when session caching was disabled. The `get_session_cb`() function 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).

# SEE ALSO

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

# HISTORY

`SSL_CTX_sess_set_new_cb`(), `SSL_CTX_sess_set_get_cb`(), `SSL_CTX_sess_get_new_cb`(), and `SSL_CTX_sess_get_get_cb`() first appeared in SSLeay 0.6.0. `SSL_CTX_sess_set_remove_cb`() and `SSL_CTX_sess_get_remove_cb`() first appeared in SSLeay 0.8.0. These functions have been available since OpenBSD 2.4.

June 8, 2025 | Debian