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135 .IX Title "SSL_CTX_set_session_cache_mode 3"
136 .TH SSL_CTX_set_session_cache_mode 3 "2016-09-26" "1.0.2j" "OpenSSL"
137 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
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142 SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode \- enable/disable session caching
144 .IX Header "SYNOPSIS"
146 \& #include <openssl/ssl.h>
148 \& long SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode);
149 \& long SSL_CTX_get_session_cache_mode(SSL_CTX ctx);
152 .IX Header "DESCRIPTION"
153 \&\fISSL_CTX_set_session_cache_mode()\fR enables/disables session caching
154 by setting the operational mode for \fBctx\fR to <mode>.
156 \&\fISSL_CTX_get_session_cache_mode()\fR returns the currently used cache mode.
159 The OpenSSL library can store/retrieve \s-1SSL/TLS\s0 sessions for later reuse.
160 The sessions can be held in memory for each \fBctx\fR, if more than one
161 \&\s-1SSL_CTX\s0 object is being maintained, the sessions are unique for each \s-1SSL_CTX\s0
164 In order to reuse a session, a client must send the session's id to the
165 server. It can only send exactly one id. The server then either
166 agrees to reuse the session or it starts a full handshake (to create a new
169 A server will lookup up the session in its internal session storage. If the
170 session is not found in internal storage or lookups for the internal storage
171 have been deactivated (\s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0), the server will try
172 the external storage if available.
174 Since a client may try to reuse a session intended for use in a different
175 context, the session id context must be set by the server (see
176 \&\fISSL_CTX_set_session_id_context\fR\|(3)).
178 The following session cache modes and modifiers are available:
179 .IP "\s-1SSL_SESS_CACHE_OFF\s0" 4
180 .IX Item "SSL_SESS_CACHE_OFF"
181 No session caching for client or server takes place.
182 .IP "\s-1SSL_SESS_CACHE_CLIENT\s0" 4
183 .IX Item "SSL_SESS_CACHE_CLIENT"
184 Client sessions are added to the session cache. As there is no reliable way
185 for the OpenSSL library to know whether a session should be reused or which
186 session to choose (due to the abstract \s-1BIO\s0 layer the \s-1SSL\s0 engine does not
187 have details about the connection), the application must select the session
188 to be reused by using the \fISSL_set_session\fR\|(3)
189 function. This option is not activated by default.
190 .IP "\s-1SSL_SESS_CACHE_SERVER\s0" 4
191 .IX Item "SSL_SESS_CACHE_SERVER"
192 Server sessions are added to the session cache. When a client proposes a
193 session to be reused, the server looks for the corresponding session in (first)
194 the internal session cache (unless \s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0 is set),
195 then (second) in the external cache if available. If the session is found, the
196 server will try to reuse the session. This is the default.
197 .IP "\s-1SSL_SESS_CACHE_BOTH\s0" 4
198 .IX Item "SSL_SESS_CACHE_BOTH"
199 Enable both \s-1SSL_SESS_CACHE_CLIENT\s0 and \s-1SSL_SESS_CACHE_SERVER\s0 at the same time.
200 .IP "\s-1SSL_SESS_CACHE_NO_AUTO_CLEAR\s0" 4
201 .IX Item "SSL_SESS_CACHE_NO_AUTO_CLEAR"
202 Normally the session cache is checked for expired sessions every
203 255 connections using the
204 \&\fISSL_CTX_flush_sessions\fR\|(3) function. Since
205 this may lead to a delay which cannot be controlled, the automatic
206 flushing may be disabled and
207 \&\fISSL_CTX_flush_sessions\fR\|(3) can be called
208 explicitly by the application.
209 .IP "\s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0" 4
210 .IX Item "SSL_SESS_CACHE_NO_INTERNAL_LOOKUP"
211 By setting this flag, session-resume operations in an \s-1SSL/TLS\s0 server will not
212 automatically look up sessions in the internal cache, even if sessions are
213 automatically stored there. If external session caching callbacks are in use,
214 this flag guarantees that all lookups are directed to the external cache.
215 As automatic lookup only applies for \s-1SSL/TLS\s0 servers, the flag has no effect on
217 .IP "\s-1SSL_SESS_CACHE_NO_INTERNAL_STORE\s0" 4
218 .IX Item "SSL_SESS_CACHE_NO_INTERNAL_STORE"
219 Depending on the presence of \s-1SSL_SESS_CACHE_CLIENT\s0 and/or \s-1SSL_SESS_CACHE_SERVER,\s0
220 sessions negotiated in an \s-1SSL/TLS\s0 handshake may be cached for possible reuse.
221 Normally a new session is added to the internal cache as well as any external
222 session caching (callback) that is configured for the \s-1SSL_CTX.\s0 This flag will
223 prevent sessions being stored in the internal cache (though the application can
224 add them manually using \fISSL_CTX_add_session\fR\|(3)). Note:
225 in any \s-1SSL/TLS\s0 servers where external caching is configured, any successful
226 session lookups in the external cache (ie. for session-resume requests) would
227 normally be copied into the local cache before processing continues \- this flag
228 prevents these additions to the internal cache as well.
229 .IP "\s-1SSL_SESS_CACHE_NO_INTERNAL\s0" 4
230 .IX Item "SSL_SESS_CACHE_NO_INTERNAL"
231 Enable both \s-1SSL_SESS_CACHE_NO_INTERNAL_LOOKUP\s0 and
232 \&\s-1SSL_SESS_CACHE_NO_INTERNAL_STORE\s0 at the same time.
234 The default mode is \s-1SSL_SESS_CACHE_SERVER.\s0
236 .IX Header "RETURN VALUES"
237 \&\fISSL_CTX_set_session_cache_mode()\fR returns the previously set cache mode.
239 \&\fISSL_CTX_get_session_cache_mode()\fR returns the currently set cache mode.
241 .IX Header "SEE ALSO"
242 \&\fIssl\fR\|(3), \fISSL_set_session\fR\|(3),
243 \&\fISSL_session_reused\fR\|(3),
244 \&\fISSL_CTX_add_session\fR\|(3),
245 \&\fISSL_CTX_sess_number\fR\|(3),
246 \&\fISSL_CTX_sess_set_cache_size\fR\|(3),
247 \&\fISSL_CTX_sess_set_get_cb\fR\|(3),
248 \&\fISSL_CTX_set_session_id_context\fR\|(3),
249 \&\fISSL_CTX_set_timeout\fR\|(3),
250 \&\fISSL_CTX_flush_sessions\fR\|(3)
253 \&\s-1SSL_SESS_CACHE_NO_INTERNAL_STORE\s0 and \s-1SSL_SESS_CACHE_NO_INTERNAL\s0
254 were introduced in OpenSSL 0.9.6h.