SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode -
       enable/disable session caching

        #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);

       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.

       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

       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

       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

       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 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 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.

           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 (i.e. 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(7), 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),

       Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

3.0.0-alpha12                      2021-02-18  SSL_CTX_SET_SESSION_CACHE_MODE(3)