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
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
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
- 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
SSL_CTX_get_session_cache_mode() returns the currently set
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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