SSL_CTX_set_session_cache_modOpeSSL_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);
DESCRIPTIONSSL_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 VALUESSSL_CTX_set_session_cache_mode() returns the previously
set cache mode.
SSL_CTX_get_session_cache_mode() returns the currently set
cache mode.
SEE ALSOssl(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.
29/Oct/2002 0.SSL_CTX_set_session_cache_mode(3)
