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.
+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 internal storage is
-deactivated, the server will try the external storage if available.
+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
-L<SSL_CTX_set_session_id_context(3)|SSL_CTX_set_session_id_context(3)>).
+L<SSL_CTX_set_session_id_context(3)>).
The following session cache modes and modifiers are available:
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)>
+to be reused by using the L<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.
+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.
=item SSL_SESS_CACHE_BOTH
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
+L<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
+L<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
+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.
+=item 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 L<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.
+
+=item SSL_SESS_CACHE_NO_INTERNAL
+
+Enable both SSL_SESS_CACHE_NO_INTERNAL_LOOKUP and
+SSL_SESS_CACHE_NO_INTERNAL_STORE at the same time.
+
+
=back
The default mode is SSL_SESS_CACHE_SERVER.
=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)>
+L<ssl(3)>, L<SSL_set_session(3)>,
+L<SSL_session_reused(3)>,
+L<SSL_CTX_add_session(3)>,
+L<SSL_CTX_sess_number(3)>,
+L<SSL_CTX_sess_set_cache_size(3)>,
+L<SSL_CTX_sess_set_get_cb(3)>,
+L<SSL_CTX_set_session_id_context(3)>,
+L<SSL_CTX_set_timeout(3)>,
+L<SSL_CTX_flush_sessions(3)>
=cut
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