SSL_CTX_set_session_cache_mode(3)SSL_CTX_set_session_cache_mode(3)NAME
SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode - Enable
or 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
The SSL_CTX_set_session_cache_mode() function enables or disables ses‐
sion caching by setting the operational mode for ctx to <mode>.
The SSL_CTX_get_session_cache_mode() function returns the currently
used cache mode.
NOTES
The OpenSSL library can store/retrieve SSL/TLS sessions for later re‐
use. 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 one id. The server then decides whether to
reuse the session or start the handshake for a new session.
A server will check the session in its internal session storage. If the
session is not found in internal storage or internal storage, it is
deactivated. The server will try the external storage if available.
Since a client may try to reuse a session intended for use in a differ‐
ent 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: No ses‐
sion 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() function. This option is
not activated by default. Server sessions are added to the session
cache. When a client proposes a session be reused, the session is
looked up in the internal session cache. 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. Nor‐
mally the session cache is checked for expired sessions every 255 con‐
nections using the SSL_CTX_flush_sessions() function. Since this might
lead to a delay which cannot be controlled, the automatic flushing can
be disabled and the SSL_CTX_flush_sessions() can be called explicitly
by the application. By setting this option, sessions are cached in the
internal storage but they are not looked up automatically. If an exter‐
nal session cache is enabled, sessions are looked up in the external
cache. As automatic lookup only applies for SSL/TLS servers. The
option has no effect on clients.
The default mode is SSL_SESS_CACHE_SERVER.
RETURN VALUES
The SSL_CTX_set_session_cache_mode() function returns the previously
set cache mode.
The SSL_CTX_get_session_cache_mode() function returns the currently set
cache mode.
SEE ALSO
Functions: ssl(3), SSL_set_session(3), SSL_session_reused(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)SSL_CTX_set_session_cache_mode(3)
