SSL_CTX_SET_SESSION_C... BSD Library Functions Manual SSL_CTX_SET_SESSION_C...NAME
SSL_CTX_set_session_cache_mode, SSL_CTX_get_session_cache_mode —
enable/disable session caching
SYNOPSIS
#include <openssl/ssl.h>
long
SSL_CTX_set_session_cache_mode(SSL_CTX ctx, long mode);
long
SSL_CTX_get_session_cache_mode(SSL_CTX ctx);
DESCRIPTIONSSL_CTX_set_session_cache_mode() enables/disables session caching by set‐
ting the operational mode for ctx to mode.
SSL_CTX_get_session_cache_mode() returns the currently used cache mode.
NOTES
The OpenSSL library can store/retrieve SSL/TLS sessions for later reuse.
The sessions can be held in memory for each ctx, if more than one SSL_CTX
object is being maintained, the sessions are unique for each SSL_CTX
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 either agrees
to reuse the session or it starts a full handshake (to create a new ses‐
sion).
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
SSL_CTX_set_session_id_context(3)).
The following session cache modes and modifiers are available:
SSL_SESS_CACHE_OFF
No session caching for client or server takes place.
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 connec‐
tion), the application must select the session to be reused by
using the SSL_set_session(3) function. This option is not acti‐
vated by default.
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 corre‐
sponding 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.
SSL_SESS_CACHE_BOTH
Enable both SSL_SESS_CACHE_CLIENT and SSL_SESS_CACHE_SERVER at
the same time.
SSL_SESS_CACHE_NO_AUTO_CLEAR
Normally the session cache is checked for expired sessions every
255 connections using the SSL_CTX_flush_sessions(3) function.
Since this may lead to a delay which cannot be controlled, the
automatic flushing may be disabled and SSL_CTX_flush_sessions(3)
can be called explicitly by the application.
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 guaran‐
tees 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.
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 hand‐
shake 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
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 nor‐
mally be copied into the local cache before processing continues
– this flag prevents these additions to the internal cache as
well.
SSL_SESS_CACHE_NO_INTERNAL
Enable both SSL_SESS_CACHE_NO_INTERNAL_LOOKUP and
SSL_SESS_CACHE_NO_INTERNAL_STORE at the same time.
The default mode is SSL_SESS_CACHE_SERVER.
RETURN VALUESSSL_CTX_set_session_cache_mode() returns the previously set cache mode.
SSL_CTX_get_session_cache_mode() returns the currently set cache mode.
SEE ALSOssl(3), SSL_CTX_add_session(3), SSL_CTX_flush_sessions(3),
SSL_CTX_sess_number(3), SSL_CTX_sess_set_cache_size(3),
SSL_CTX_sess_set_get_cb(3), SSL_CTX_set_session_id_context(3),
SSL_CTX_set_timeout(3), SSL_session_reused(3), SSL_set_session(3)HISTORY
SSL_SESS_CACHE_NO_INTERNAL_STORE and SSL_SESS_CACHE_NO_INTERNAL were
introduced in OpenSSL 0.9.6h.
BSD April 27, 2024 BSD
