SSL_CTX_set_session_cache_mode(3) OpenSSL SSL_CTX_set_session_cache_mode(3)
SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode - enable/disable session
long SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode);
long SSL_CTX_get_session_cache_mode(SSL_CTX ctx);
SSL_CTX_set_session_cache_mode() enables/disables session caching by setting the opera-
tional mode for ctx to <mode>.
SSL_CTX_get_session_cache_mode() returns the currently used cache mode.
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 either agrees to reuse the session or it starts
a full handshake (to create 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 lookups for the internal storage have been deactivated
(SSL_SESS_CACHE_NO_INTERNAL_LOOKUP), the server will try the external storage if avail-
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 connec-
tion), the application must select the session to be reused by using the SSL_set_ses-
sion(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 server looks for the corresponding session in (first) the internal session
cache (unless SSL_SESS_CACHE_NO_INTERNAL_LOOKUP is set), then (second) in the external
cache if available. 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, session-resume operations in an SSL/TLS server will not automat-
ically look up sessions in the internal cache, even if sessions are automatically
stored there. If external session caching callbacks are in use, this flag guarantees
that all lookups are directed to the external cache. As automatic lookup only applies
for SSL/TLS servers, the flag has no effect on clients.
Depending on the presence of SSL_SESS_CACHE_CLIENT and/or SSL_SESS_CACHE_SERVER, ses-
sions negotiated in an SSL/TLS handshake may be cached for possible reuse. Normally a
new session is added to the internal cache as well as any external session caching
(callback) that is configured for the SSL_CTX. This flag will prevent sessions being
stored in the internal cache (though the application can add them manually using
SSL_CTX_add_session(3)). Note: in any SSL/TLS servers where external caching is con-
figured, any successful session lookups in the external cache (ie. for session-resume
requests) would normally be copied into the local cache before processing continues -
this flag prevents these additions to the internal cache as well.
Enable both SSL_SESS_CACHE_NO_INTERNAL_LOOKUP and SSL_SESS_CACHE_NO_INTERNAL_STORE at
the same time.
The default mode is SSL_SESS_CACHE_SERVER.
SSL_CTX_set_session_cache_mode() returns the previously set cache mode.
SSL_CTX_get_session_cache_mode() returns the currently set cache mode.
ssl(3), SSL_set_session(3), SSL_session_reused(3), SSL_CTX_add_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(3), SSL_CTX_flush_sessions(3)
SSL_SESS_CACHE_NO_INTERNAL_STORE and SSL_SESS_CACHE_NO_INTERNAL were introduced in OpenSSL
0.9.7d 2003-11-20 SSL_CTX_set_session_cache_mode(3)