xref: /freebsd-10.3-release/crypto/openssl/doc/ssl/SSL_CTX_set_session_cache_mode.pod

=pod

=head1 NAME

SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode - enable/disable session caching

=head1 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);

=head1 DESCRIPTION

SSL_CTX_set_session_cache_mode() enables/disables session caching
by setting the operational mode for B<ctx> to <mode>.

SSL_CTX_get_session_cache_mode() returns the currently used cache mode.

=head1 NOTES

The OpenSSL library can store/retrieve SSL/TLS sessions for later reuse.
The sessions can be held in memory for each B<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 decides whether it
agrees in reusing the session or starts the handshake for 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 internal storage 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 different
context, the session id context must be set by the server (see
L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>).

The following session cache modes and modifiers are available:

=over 4

=item SSL_SESS_CACHE_OFF

No session caching for client or server takes place.

=item 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 L<SSL_set_session(3)|SSL_set_session(3)>
function. This option is not activated by default.

=item SSL_SESS_CACHE_SERVER

Server sessions are added to the session cache. When a client proposes a
session to 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.

=item SSL_SESS_CACHE_BOTH

Enable both SSL_SESS_CACHE_CLIENT and SSL_SESS_CACHE_SERVER at the same time.

=item SSL_SESS_CACHE_NO_AUTO_CLEAR

Normally the session cache is checked for expired sessions every
255 connections using the
L<SSL_CTX_flush_sessions(3)|SSL_CTX_flush_sessions(3)> function. Since
this may lead to a delay which cannot be controlled, the automatic
flushing may be disabled and
L<SSL_CTX_flush_sessions(3)|SSL_CTX_flush_sessions(3)> can be called
explicitly by the application.

=item SSL_SESS_CACHE_NO_INTERNAL_LOOKUP

By setting this flag sessions are cached in the internal storage but
they are not looked up automatically. If an external session cache
is enabled, sessions are looked up in the external cache. As automatic
lookup only applies for SSL/TLS servers, the flag has no effect on
clients.

=back

The default mode is SSL_SESS_CACHE_SERVER.

=head1 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.


=head1 SEE ALSO

L<ssl(3)|ssl(3)>, L<SSL_set_session(3)|SSL_set_session(3)>,
L<SSL_CTX_sess_number(3)|SSL_CTX_sess_number(3)>,
L<SSL_CTX_sess_set_cache_size(3)|SSL_CTX_sess_set_cache_size(3)>,
L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>,
L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>,
L<SSL_CTX_set_timeout(3)|SSL_CTX_set_timeout(3)>,
L<SSL_CTX_flush_sessions(3)|SSL_CTX_flush_sessions(3)>

=cut