SSL_CTX_set_session_cache_mode(3)

NAME
SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode - enable/disable session caching

SYNOPSIS

 #include <openssl/ssl.h>
 long SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode);
 long SSL_CTX_get_session_cache_mode(SSL_CTX ctx);

DESCRIPTION
SSL_CTX_set_session_cache_mode() enables/disables session caching by setting the operational mode for ctx to <mode>.

SSL_CTX_get_session_cache_mode() returns the currently used cache mode.

NOTES
The OpenSSL library can store/retrieve SSL/TLS sessions for later reuse. The sessions can be held in memory for each ctx, if more than one SSL_CTX object is being maintained, the sessions are unique for each SSL_CTX object.

In order to reuse a session, a client must send the session's id to the server. It can only send exactly one id. The server then decides whether it agrees in reusing the session or starts the handshake for a new session.

A server will lookup up the session in its internal session storage. If the session is not found in internal storage or internal storage is deactivated, the server will try the external storage if available.

Since a client may try to reuse a session intended for use in a different context, the session id context must be set by the server (see SSL_CTX_set_session_id_context(3)).

The following session cache modes and modifiers are available: No session caching for client or server takes place. Client sessions are added to the session cache. As there is no reliable way for the OpenSSL library to know whether a session should be reused or which session to choose (due to the abstract BIO layer the SSL engine does not have details about the connection), the application must select the session to be reused by using the SSL_set_session(3) function. This option is not activated by default. Server sessions are added to the session cache. When a client proposes a session to be reused, the session is looked up in the internal session cache. If the session is found, the server will try to reuse the session. This is the default. Enable both SSL_SESS_CACHE_CLIENT and SSL_SESS_CACHE_SERVER at the same time. Normally the session cache is checked for expired sessions every 255 connections using the SSL_CTX_flush_sessions(3) function. Since this may lead to a delay which cannot be controlled, the automatic flushing may be disabled and SSL_CTX_flush_sessions(3) can be called explicitly by the application. By setting this flag sessions are cached in the internal storage but they are not looked up automatically. If an external session cache is enabled, sessions are looked up in the external cache. As automatic lookup only applies for SSL/TLS servers, the flag has no effect on clients.

The default mode is SSL_SESS_CACHE_SERVER.

RETURN VALUES
SSL_CTX_set_session_cache_mode() returns the previously set cache mode.

SSL_CTX_get_session_cache_mode() returns the currently set cache mode.

SEE ALSO
ssl(3), SSL_set_session(3), SSL_CTX_sess_number(3), SSL_CTX_sess_set_cache_size(3), SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3), SSL_CTX_set_timeout.pod(3), SSL_CTX_flush_sessions(3)