=pod

=head1 NAME

SSL_get_session, SSL_get0_session, SSL_get1_session - retrieve TLS/SSL session data

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 SSL_SESSION *SSL_get_session(const SSL *ssl);

 SSL_SESSION *SSL_get0_session(const SSL *ssl);

 SSL_SESSION *SSL_get1_session(SSL *ssl);

=head1 DESCRIPTION

SSL_get_session() returns a pointer to the B<SSL_SESSION> actually used in

B<ssl>. The reference count of the B<SSL_SESSION> is not incremented, so

that the pointer can become invalid by other operations.

SSL_get0_session() is the same as SSL_get_session().

SSL_get1_session() is the same as SSL_get_session(), but the reference

count of the B<SSL_SESSION> is incremented by one.

=head1 NOTES

The ssl session contains all information required to re-establish the

connection without a full handshake for SSL versions up to and including

TLSv1.2. In TLSv1.3 the same is true, but sessions are established after the

main handshake has occurred. The server will send the session information to the

client at a time of its choosing, which may be some while after the initial

connection is established (or never). Calling these functions on the client side

in TLSv1.3 before the session has been established will still return an

SSL_SESSION object but that object cannot be used for resuming the session. See

L<SSL_SESSION_is_resumable(3)> for information on how to determine whether an

SSL_SESSION object can be used for resumption or not.

Additionally, in TLSv1.3, a server can send multiple messages that establish a

session for a single connection. In that case the above functions will only

return information on the last session that was received.

The preferred way for applications to obtain a resumable SSL_SESSION object is

to use a new session callback as described in L<SSL_CTX_sess_set_new_cb(3)>.

The new session callback is only invoked when a session is actually established,

so this avoids the problem described above where an application obtains an

SSL_SESSION object that cannot be used for resumption in TLSv1.3. It also

enables applications to obtain information about all sessions sent by the

server.

A session will be automatically removed from the session cache and marked as

non-resumable if the connection is not closed down cleanly, e.g. if a fatal

error occurs on the connection or L<SSL_shutdown(3)> is not called prior to

L<SSL_free(3)>.

In TLSv1.3 it is recommended that each SSL_SESSION object is only used for

resumption once.

SSL_get0_session() returns a pointer to the actual session. As the

reference counter is not incremented, the pointer is only valid while

the connection is in use. If L<SSL_clear(3)> or

L<SSL_free(3)> is called, the session may be removed completely

(if considered bad), and the pointer obtained will become invalid. Even

if the session is valid, it can be removed at any time due to timeout

during L<SSL_CTX_flush_sessions(3)>.

If the data is to be kept, SSL_get1_session() will increment the reference

count, so that the session will not be implicitly removed by other operations

but stays in memory. In order to remove the session

L<SSL_SESSION_free(3)> must be explicitly called once

to decrement the reference count again.

SSL_SESSION objects keep internal link information about the session cache

list, when being inserted into one SSL_CTX object's session cache.

One SSL_SESSION object, regardless of its reference count, must therefore

only be used with one SSL_CTX object (and the SSL objects created

from this SSL_CTX object).

=head1 RETURN VALUES

The following return values can occur:

=over 4

=item NULL

There is no session available in B<ssl>.

=item Pointer to an SSL_SESSION

The return value points to the data of an SSL session.

=back

=head1 SEE ALSO

L<ssl(7)>, L<SSL_free(3)>,

L<SSL_clear(3)>,

L<SSL_SESSION_free(3)>

=head1 COPYRIGHT

Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use

this file except in compliance with the License.  You can obtain a copy

in the file LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

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