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SSL_CTX_set_session_cache_mode(3) OpenSSL SSL_CTX_set_session_cache_mode(3)
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SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode -
enable/disable session caching
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
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
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
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
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),
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
1.1.1i 2020-12-10 SSL_CTX_set_session_cache_mode(3)