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.\" $OpenBSD: SSL_CTX_set_session_cache_mode.3,v 1.7 2019/06/12 09:36:30 schwarze Exp $ .\" OpenSSL 67adf0a7 Dec 25 19:58:38 2016 +0100 .\" .\" This file was written by Lutz Jaenicke <jaenicke@openssl.org> and .\" Geoff Thorpe <geoff@openssl.org>. .\" Copyright (c) 2001, 2002 The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. All advertising materials mentioning features or use of this .\" software must display the following acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" .\" .\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to .\" endorse or promote products derived from this software without .\" prior written permission. For written permission, please contact .\" openssl-core@openssl.org. .\" .\" 5. Products derived from this software may not be called "OpenSSL" .\" nor may "OpenSSL" appear in their names without prior written .\" permission of the OpenSSL Project. .\" .\" 6. Redistributions of any form whatsoever must retain the following .\" acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" .\" .\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY .\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: June 12 2019 $ .Dt SSL_CTX_SET_SESSION_CACHE_MODE 3 .Os .Sh NAME .Nm SSL_CTX_set_session_cache_mode , .Nm SSL_CTX_get_session_cache_mode .Nd enable/disable session caching .Sh SYNOPSIS .In openssl/ssl.h .Ft long .Fn SSL_CTX_set_session_cache_mode "SSL_CTX ctx" "long mode" .Ft long .Fn SSL_CTX_get_session_cache_mode "SSL_CTX ctx" .Sh DESCRIPTION .Fn SSL_CTX_set_session_cache_mode enables/disables session caching by setting the operational mode for .Ar ctx to .Ar mode . .Pp .Fn SSL_CTX_get_session_cache_mode returns the currently used cache mode. .Pp The OpenSSL library can store/retrieve SSL/TLS sessions for later reuse. The sessions can be held in memory for each .Fa ctx , if more than one .Vt SSL_CTX object is being maintained, the sessions are unique for each .Vt SSL_CTX object. .Pp 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). .Pp A server will look 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 .Pq Dv SSL_SESS_CACHE_NO_INTERNAL_LOOKUP , the server will try the external storage if available. .Pp 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 .Xr SSL_CTX_set_session_id_context 3 ) . .Pp The following session cache modes and modifiers are available: .Bl -tag -width Ds .It Dv SSL_SESS_CACHE_OFF No session caching for client or server takes place. .It Dv 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 .Xr SSL_set_session 3 function. This option is not activated by default. .It Dv 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 .Dv 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. .It Dv SSL_SESS_CACHE_BOTH Enable both .Dv SSL_SESS_CACHE_CLIENT and .Dv SSL_SESS_CACHE_SERVER at the same time. .It Dv SSL_SESS_CACHE_NO_AUTO_CLEAR Normally the session cache is checked for expired sessions every 255 connections using the .Xr SSL_CTX_flush_sessions 3 function. Since this may lead to a delay which cannot be controlled, the automatic flushing may be disabled and .Xr SSL_CTX_flush_sessions 3 can be called explicitly by the application. .It Dv 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. .It Dv SSL_SESS_CACHE_NO_INTERNAL_STORE Depending on the presence of .Dv SSL_SESS_CACHE_CLIENT and/or .Dv 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 .Vt SSL_CTX . This flag will prevent sessions being stored in the internal cache (though the application can add them manually using .Xr SSL_CTX_add_session 3 ) . Note: in any SSL/TLS servers where external caching is configured, any successful session lookups in the external cache (e.g., for session-resume requests) would normally be copied into the local cache before processing continues \(en this flag prevents these additions to the internal cache as well. .It Dv SSL_SESS_CACHE_NO_INTERNAL Enable both .Dv SSL_SESS_CACHE_NO_INTERNAL_LOOKUP and .Dv SSL_SESS_CACHE_NO_INTERNAL_STORE at the same time. .El .Pp The default mode is .Dv SSL_SESS_CACHE_SERVER . .Sh RETURN VALUES .Fn SSL_CTX_set_session_cache_mode returns the previously set cache mode. .Pp .Fn SSL_CTX_get_session_cache_mode returns the currently set cache mode. .Sh SEE ALSO .Xr ssl 3 , .Xr SSL_CTX_add_session 3 , .Xr SSL_CTX_ctrl 3 , .Xr SSL_CTX_flush_sessions 3 , .Xr SSL_CTX_sess_number 3 , .Xr SSL_CTX_sess_set_cache_size 3 , .Xr SSL_CTX_sess_set_get_cb 3 , .Xr SSL_CTX_set_session_id_context 3 , .Xr SSL_CTX_set_timeout 3 , .Xr SSL_session_reused 3 , .Xr SSL_set_session 3 .Sh HISTORY .Fn SSL_CTX_set_session_cache_mode and .Fn SSL_CTX_get_session_cache_mode first appeared in SSLeay 0.6.1 and have been available since .Ox 2.4 . .Pp .Dv SSL_SESS_CACHE_NO_INTERNAL_STORE and .Dv SSL_SESS_CACHE_NO_INTERNAL were introduced in OpenSSL 0.9.6h.