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138.\" ======================================================================
139.\"
140.IX Title "SSL_CTX_set_session_cache_mode 3"
141.TH SSL_CTX_set_session_cache_mode 3 "0.9.7a" "2003-02-19" "OpenSSL"
142.UC
143.SH "NAME"
144SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode \- enable/disable session caching
145.SH "SYNOPSIS"
146.IX Header "SYNOPSIS"
147.Vb 1
148\& #include <openssl/ssl.h>
149.Ve
150.Vb 2
151\& long SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode);
152\& long SSL_CTX_get_session_cache_mode(SSL_CTX ctx);
153.Ve
154.SH "DESCRIPTION"
155.IX Header "DESCRIPTION"
156\&\fISSL_CTX_set_session_cache_mode()\fR enables/disables session caching
157by setting the operational mode for \fBctx\fR to <mode>.
158.PP
159\&\fISSL_CTX_get_session_cache_mode()\fR returns the currently used cache mode.
160.SH "NOTES"
161.IX Header "NOTES"
162The OpenSSL library can store/retrieve \s-1SSL/TLS\s0 sessions for later reuse.
163The sessions can be held in memory for each \fBctx\fR, if more than one
164\&\s-1SSL_CTX\s0 object is being maintained, the sessions are unique for each \s-1SSL_CTX\s0
165object.
166.PP
167In order to reuse a session, a client must send the session's id to the
168server. It can only send exactly one id. The server then either
169agrees to reuse the session or it starts a full handshake (to create a new
170session).
171.PP
172A server will lookup up the session in its internal session storage. If the
173session is not found in internal storage or lookups for the internal storage
174have been deactivated (\s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0), the server will try
175the external storage if available.
176.PP
177Since a client may try to reuse a session intended for use in a different
178context, the session id context must be set by the server (see
179SSL_CTX_set_session_id_context(3)).
180.PP
181The following session cache modes and modifiers are available:
182.Ip "\s-1SSL_SESS_CACHE_OFF\s0" 4
183.IX Item "SSL_SESS_CACHE_OFF"
184No session caching for client or server takes place.
185.Ip "\s-1SSL_SESS_CACHE_CLIENT\s0" 4
186.IX Item "SSL_SESS_CACHE_CLIENT"
187Client sessions are added to the session cache. As there is no reliable way
188for the OpenSSL library to know whether a session should be reused or which
189session to choose (due to the abstract \s-1BIO\s0 layer the \s-1SSL\s0 engine does not
190have details about the connection), the application must select the session
191to be reused by using the SSL_set_session(3)
192function. This option is not activated by default.
193.Ip "\s-1SSL_SESS_CACHE_SERVER\s0" 4
194.IX Item "SSL_SESS_CACHE_SERVER"
195Server sessions are added to the session cache. When a client proposes a
196session to be reused, the server looks for the corresponding session in (first)
197the internal session cache (unless \s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0 is set),
198then (second) in the external cache if available. If the session is found, the
199server will try to reuse the session. This is the default.
200.Ip "\s-1SSL_SESS_CACHE_BOTH\s0" 4
201.IX Item "SSL_SESS_CACHE_BOTH"
202Enable both \s-1SSL_SESS_CACHE_CLIENT\s0 and \s-1SSL_SESS_CACHE_SERVER\s0 at the same time.
203.Ip "\s-1SSL_SESS_CACHE_NO_AUTO_CLEAR\s0" 4
204.IX Item "SSL_SESS_CACHE_NO_AUTO_CLEAR"
205Normally the session cache is checked for expired sessions every
206255 connections using the
207SSL_CTX_flush_sessions(3) function. Since
208this may lead to a delay which cannot be controlled, the automatic
209flushing may be disabled and
210SSL_CTX_flush_sessions(3) can be called
211explicitly by the application.
212.Ip "\s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0" 4
213.IX Item "SSL_SESS_CACHE_NO_INTERNAL_LOOKUP"
214By setting this flag, session-resume operations in an \s-1SSL/TLS\s0 server will not
215automatically look up sessions in the internal cache, even if sessions are
216automatically stored there. If external session caching callbacks are in use,
217this flag guarantees that all lookups are directed to the external cache.
218As automatic lookup only applies for \s-1SSL/TLS\s0 servers, the flag has no effect on
219clients.
220.Ip "\s-1SSL_SESS_CACHE_NO_INTERNAL_STORE\s0" 4
221.IX Item "SSL_SESS_CACHE_NO_INTERNAL_STORE"
222Depending on the presence of \s-1SSL_SESS_CACHE_CLIENT\s0 and/or \s-1SSL_SESS_CACHE_SERVER\s0,
223sessions negotiated in an \s-1SSL/TLS\s0 handshake may be cached for possible reuse.
224Normally a new session is added to the internal cache as well as any external
225session caching (callback) that is configured for the \s-1SSL_CTX\s0. This flag will
226prevent sessions being stored in the internal cache (though the application can
227add them manually using SSL_CTX_add_session(3)). Note:
228in any \s-1SSL/TLS\s0 servers where external caching is configured, any successful
229session lookups in the external cache (ie. for session-resume requests) would
230normally be copied into the local cache before processing continues \- this flag
231prevents these additions to the internal cache as well.
232.Ip "\s-1SSL_SESS_CACHE_NO_INTERNAL\s0" 4
233.IX Item "SSL_SESS_CACHE_NO_INTERNAL"
234Enable both \s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0 and
235\&\s-1SSL_SESS_CACHE_NO_INTERNAL_STORE\s0 at the same time.
236.PP
237The default mode is \s-1SSL_SESS_CACHE_SERVER\s0.
238.SH "RETURN VALUES"
239.IX Header "RETURN VALUES"
240\&\fISSL_CTX_set_session_cache_mode()\fR returns the previously set cache mode.
241.PP
242\&\fISSL_CTX_get_session_cache_mode()\fR returns the currently set cache mode.
243.SH "SEE ALSO"
244.IX Header "SEE ALSO"
245ssl(3), SSL_set_session(3),
246SSL_session_reused(3),
247SSL_CTX_add_session(3),
248SSL_CTX_sess_number(3),
249SSL_CTX_sess_set_cache_size(3),
250SSL_CTX_sess_set_get_cb(3),
251SSL_CTX_set_session_id_context(3),
252SSL_CTX_set_timeout(3),
253SSL_CTX_flush_sessions(3)
254.SH "HISTORY"
255.IX Header "HISTORY"
256\&\s-1SSL_SESS_CACHE_NO_INTERNAL_STORE\s0 and \s-1SSL_SESS_CACHE_NO_INTERNAL\s0
257were introduced in OpenSSL 0.9.6h.