1=pod
2
3=head1 NAME
4
5SSL_get_session, SSL_get0_session, SSL_get1_session - retrieve TLS/SSL session data
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 SSL_SESSION *SSL_get_session(const SSL *ssl);
12 SSL_SESSION *SSL_get0_session(const SSL *ssl);
13 SSL_SESSION *SSL_get1_session(SSL *ssl);
14
15=head1 DESCRIPTION
16
17SSL_get_session() returns a pointer to the B<SSL_SESSION> actually used in
18B<ssl>. The reference count of the B<SSL_SESSION> is not incremented, so
19that the pointer can become invalid by other operations.
20
21SSL_get0_session() is the same as SSL_get_session().
22
23SSL_get1_session() is the same as SSL_get_session(), but the reference
24count of the B<SSL_SESSION> is incremented by one.
25
26=head1 NOTES
27
28The ssl session contains all information required to re-establish the
29connection without a full handshake for SSL versions up to and including
30TLSv1.2. In TLSv1.3 the same is true, but sessions are established after the
31main handshake has occurred. The server will send the session information to the
32client at a time of its choosing, which may be some while after the initial
33connection is established (or never). Calling these functions on the client side
34in TLSv1.3 before the session has been established will still return an
35SSL_SESSION object but that object cannot be used for resuming the session. See
36L<SSL_SESSION_is_resumable(3)> for information on how to determine whether an
37SSL_SESSION object can be used for resumption or not.
38
39Additionally, in TLSv1.3, a server can send multiple messages that establish a
40session for a single connection. In that case the above functions will only
41return information on the last session that was received.
42
43The preferred way for applications to obtain a resumable SSL_SESSION object is
44to use a new session callback as described in L<SSL_CTX_sess_set_new_cb(3)>.
45The new session callback is only invoked when a session is actually established,
46so this avoids the problem described above where an application obtains an
47SSL_SESSION object that cannot be used for resumption in TLSv1.3. It also
48enables applications to obtain information about all sessions sent by the
49server.
50
51A session will be automatically removed from the session cache and marked as
52non-resumable if the connection is not closed down cleanly, e.g. if a fatal
53error occurs on the connection or L<SSL_shutdown(3)> is not called prior to
54L<SSL_free(3)>.
55
56In TLSv1.3 it is recommended that each SSL_SESSION object is only used for
57resumption once.
58
59SSL_get0_session() returns a pointer to the actual session. As the
60reference counter is not incremented, the pointer is only valid while
61the connection is in use. If L<SSL_clear(3)> or
62L<SSL_free(3)> is called, the session may be removed completely
63(if considered bad), and the pointer obtained will become invalid. Even
64if the session is valid, it can be removed at any time due to timeout
65during L<SSL_CTX_flush_sessions(3)>.
66
67If the data is to be kept, SSL_get1_session() will increment the reference
68count, so that the session will not be implicitly removed by other operations
69but stays in memory. In order to remove the session
70L<SSL_SESSION_free(3)> must be explicitly called once
71to decrement the reference count again.
72
73SSL_SESSION objects keep internal link information about the session cache
74list, when being inserted into one SSL_CTX object's session cache.
75One SSL_SESSION object, regardless of its reference count, must therefore
76only be used with one SSL_CTX object (and the SSL objects created
77from this SSL_CTX object).
78
79=head1 RETURN VALUES
80
81The following return values can occur:
82
83=over 4
84
85=item NULL
86
87There is no session available in B<ssl>.
88
89=item Pointer to an SSL_SESSION
90
91The return value points to the data of an SSL session.
92
93=back
94
95=head1 SEE ALSO
96
97L<ssl(7)>, L<SSL_free(3)>,
98L<SSL_clear(3)>,
99L<SSL_SESSION_free(3)>
100
101=head1 COPYRIGHT
102
103Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
104
105Licensed under the OpenSSL license (the "License").  You may not use
106this file except in compliance with the License.  You can obtain a copy
107in the file LICENSE in the source distribution or at
108L<https://www.openssl.org/source/license.html>.
109
110=cut
111