openbsd-src / lib / libssl / man / SSL_CTX_set_session_cache_mode.3

.\"	$OpenBSD: SSL_CTX_set_session_cache_mode.3,v 1.8 2025/06/08 22:52:00 schwarze Exp $
.\"	OpenSSL 67adf0a7 Dec 25 19:58:38 2016 +0100
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.Dd $Mdocdate: June 8 2025 $
.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
.Lb libssl libcrypto
.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.