SSL_CTX_set_session_cache_mode (3)

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NAME

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);

DESCRIPTION

SSL_CTX_get_session_cache_mode() returns the currently used cache mode.

NOTES

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).

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 (

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 connection), the application must select the session to be reused by using the SSL_set_session(3) function. This option is not activated 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 corresponding 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 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.

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

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

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 (ie. for session-resume requests) would normally 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

RETURN VALUES

SSL_CTX_get_session_cache_mode() returns the currently set cache mode.

SEE ALSO

HISTORY