SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode - enable/disable
long SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode);
long SSL_CTX_get_session_cache_mode(SSL_CTX ctx);
enables/disables session caching by
setting the operational mode for ctx
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 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
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
- 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 time.
- 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 (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.
- 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.
returns the previously set cache mode.
returns the currently set cache mode.
SSL_SESS_CACHE_NO_INTERNAL_STORE and SSL_SESS_CACHE_NO_INTERNAL were introduced
in OpenSSL 0.9.6h.