1=pod 2 3=head1 NAME 4 5SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode - enable/disable session caching 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 long SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode); 12 long SSL_CTX_get_session_cache_mode(SSL_CTX ctx); 13 14=head1 DESCRIPTION 15 16SSL_CTX_set_session_cache_mode() enables/disables session caching 17by setting the operational mode for B<ctx> to <mode>. 18 19SSL_CTX_get_session_cache_mode() returns the currently used cache mode. 20 21=head1 NOTES 22 23The OpenSSL library can store/retrieve SSL/TLS sessions for later reuse. 24The sessions can be held in memory for each B<ctx>, if more than one 25SSL_CTX object is being maintained, the sessions are unique for each SSL_CTX 26object. 27 28In order to reuse a session, a client must send the session's id to the 29server. It can only send exactly one id. The server then either 30agrees to reuse the session or it starts a full handshake (to create a new 31session). 32 33A server will look up the session in its internal session storage. If the 34session is not found in internal storage or lookups for the internal storage 35have been deactivated (SSL_SESS_CACHE_NO_INTERNAL_LOOKUP), the server will try 36the external storage if available. 37 38Since a client may try to reuse a session intended for use in a different 39context, the session id context must be set by the server (see 40L<SSL_CTX_set_session_id_context(3)>). 41 42The following session cache modes and modifiers are available: 43 44=over 4 45 46=item SSL_SESS_CACHE_OFF 47 48No session caching for client or server takes place. 49 50=item SSL_SESS_CACHE_CLIENT 51 52Client sessions are added to the session cache. As there is no reliable way 53for the OpenSSL library to know whether a session should be reused or which 54session to choose (due to the abstract BIO layer the SSL engine does not 55have details about the connection), the application must select the session 56to be reused by using the L<SSL_set_session(3)> 57function. This option is not activated by default. 58 59=item SSL_SESS_CACHE_SERVER 60 61Server sessions are added to the session cache. When a client proposes a 62session to be reused, the server looks for the corresponding session in (first) 63the internal session cache (unless SSL_SESS_CACHE_NO_INTERNAL_LOOKUP is set), 64then (second) in the external cache if available. If the session is found, the 65server will try to reuse the session. This is the default. 66 67=item SSL_SESS_CACHE_BOTH 68 69Enable both SSL_SESS_CACHE_CLIENT and SSL_SESS_CACHE_SERVER at the same time. 70 71=item SSL_SESS_CACHE_NO_AUTO_CLEAR 72 73Normally the session cache is checked for expired sessions every 74255 connections using the 75L<SSL_CTX_flush_sessions(3)> function. Since 76this may lead to a delay which cannot be controlled, the automatic 77flushing may be disabled and 78L<SSL_CTX_flush_sessions(3)> can be called 79explicitly by the application. 80 81=item SSL_SESS_CACHE_NO_INTERNAL_LOOKUP 82 83By setting this flag, session-resume operations in an SSL/TLS server will not 84automatically look up sessions in the internal cache, even if sessions are 85automatically stored there. If external session caching callbacks are in use, 86this flag guarantees that all lookups are directed to the external cache. 87As automatic lookup only applies for SSL/TLS servers, the flag has no effect on 88clients. 89 90=item SSL_SESS_CACHE_NO_INTERNAL_STORE 91 92Depending on the presence of SSL_SESS_CACHE_CLIENT and/or SSL_SESS_CACHE_SERVER, 93sessions negotiated in an SSL/TLS handshake may be cached for possible reuse. 94Normally a new session is added to the internal cache as well as any external 95session caching (callback) that is configured for the SSL_CTX. This flag will 96prevent sessions being stored in the internal cache (though the application can 97add them manually using L<SSL_CTX_add_session(3)>). Note: 98in any SSL/TLS servers where external caching is configured, any successful 99session lookups in the external cache (i.e. for session-resume requests) would 100normally be copied into the local cache before processing continues - this flag 101prevents these additions to the internal cache as well. 102 103=item SSL_SESS_CACHE_NO_INTERNAL 104 105Enable both SSL_SESS_CACHE_NO_INTERNAL_LOOKUP and 106SSL_SESS_CACHE_NO_INTERNAL_STORE at the same time. 107 108 109=back 110 111The default mode is SSL_SESS_CACHE_SERVER. 112 113=head1 RETURN VALUES 114 115SSL_CTX_set_session_cache_mode() returns the previously set cache mode. 116 117SSL_CTX_get_session_cache_mode() returns the currently set cache mode. 118 119 120=head1 SEE ALSO 121 122L<ssl(7)>, L<SSL_set_session(3)>, 123L<SSL_session_reused(3)>, 124L<SSL_CTX_add_session(3)>, 125L<SSL_CTX_sess_number(3)>, 126L<SSL_CTX_sess_set_cache_size(3)>, 127L<SSL_CTX_sess_set_get_cb(3)>, 128L<SSL_CTX_set_session_id_context(3)>, 129L<SSL_CTX_set_timeout(3)>, 130L<SSL_CTX_flush_sessions(3)> 131 132=head1 COPYRIGHT 133 134Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved. 135 136Licensed under the OpenSSL license (the "License"). You may not use 137this file except in compliance with the License. You can obtain a copy 138in the file LICENSE in the source distribution or at 139L<https://www.openssl.org/source/license.html>. 140 141=cut 142