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 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 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 (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.
- SSL_SESS_CACHE_NO_INTERNAL
- 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),
  SSL_CTX_flush_sessions(3)
Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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
    <https://www.openssl.org/source/license.html>.