/usr/man/cat.3/SSL_CTX_set_session_cache_mode.3(/usr/man/cat.3/SSL_CTX_set_session_cache_mode.3)

SSL_CTX_set_session_cache_mode(3OpenSSSSL_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 either agrees to reuse the session or it
     starts a full handshake (to create a new session).

     A server will look 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 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:

     SSL_SESS_CACHE_OFF
         No session caching for client or server takes place.

     SSL_SESS_CACHE_CLIENT
         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

1.0.2t               Last change: 2019-09-10                    1

SSL_CTX_set_session_cache_mode(3OpenSSSSL_CTX_set_session_cache_mode(3)

         using the SSL_set_session(3) function. This option is
         not activated by default.

     SSL_SESS_CACHE_SERVER
         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.

     SSL_SESS_CACHE_BOTH
         Enable both SSL_SESS_CACHE_CLIENT and
         SSL_SESS_CACHE_SERVER at the same time.

     SSL_SESS_CACHE_NO_AUTO_CLEAR
         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.

     SSL_SESS_CACHE_NO_INTERNAL_LOOKUP
         By setting this flag, session-resume operations in an
         SSL/TLS server will not automatically 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.

     SSL_SESS_CACHE_NO_INTERNAL_STORE
         Depending on the presence of SSL_SESS_CACHE_CLIENT
         and/or SSL_SESS_CACHE_SERVER, sessions 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 configured, 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.

     SSL_SESS_CACHE_NO_INTERNAL
         Enable both SSL_SESS_CACHE_NO_INTERNAL_LOOKUP and

1.0.2t               Last change: 2019-09-10                    2

SSL_CTX_set_session_cache_mode(3OpenSSSSL_CTX_set_session_cache_mode(3)

         SSL_SESS_CACHE_NO_INTERNAL_STORE at the same time.

     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_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)

HISTORY

     SSL_SESS_CACHE_NO_INTERNAL_STORE and
     SSL_SESS_CACHE_NO_INTERNAL were introduced in OpenSSL
     0.9.6h.

1.0.2t               Last change: 2019-09-10                    3

See also SSL_CTX_get_session_cache_mode(3)