SSL_CTX_set_session_cache_mode(3) OpenSSL SSL_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 lookup 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 (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 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.
0.9.7l 2002-10-29 SSL_CTX_set_session_cache_mode(3)
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