151 lines
5.6 KiB
Text
151 lines
5.6 KiB
Text
=pod
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=head1 NAME
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SSL_shutdown - shut down a TLS/SSL connection
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_shutdown(SSL *ssl);
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=head1 DESCRIPTION
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SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
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"close notify" shutdown alert to the peer.
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=head1 NOTES
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SSL_shutdown() tries to send the "close notify" shutdown alert to the peer.
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Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
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a currently open session is considered closed and good and will be kept in the
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session cache for further reuse.
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The shutdown procedure consists of 2 steps: the sending of the "close notify"
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shutdown alert and the reception of the peer's "close notify" shutdown
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alert. According to the TLS standard, it is acceptable for an application
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to only send its shutdown alert and then close the underlying connection
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without waiting for the peer's response (this way resources can be saved,
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as the process can already terminate or serve another connection).
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When the underlying connection shall be used for more communications, the
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complete shutdown procedure (bidirectional "close notify" alerts) must be
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performed, so that the peers stay synchronized.
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SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step
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behaviour.
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SSL_shutdown() only closes the write direction.
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It is not possible to call SSL_write() after calling SSL_shutdown().
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The read direction is closed by the peer.
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=head2 First to close the connection
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When the application is the first party to send the "close notify"
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alert, SSL_shutdown() will only send the alert and then set the
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SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
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be kept in the cache).
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SSL_shutdown() will then return with 0.
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If a unidirectional shutdown is enough (the underlying connection shall be
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closed anyway), this first call to SSL_shutdown() is sufficient.
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In order to complete the bidirectional shutdown handshake, the peer needs
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to send back a "close notify" alert.
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The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
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it.
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SSL_shutdown() will return 1 when it has been received.
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The peer is still allowed to send data after receiving the "close notify"
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event.
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If the peer did send data it needs to be processed by calling SSL_read()
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before calling SSL_shutdown() a second time.
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SSL_read() will indicate the end of the peer data by returning <= 0
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and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.
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It is recommended to call SSL_read() between SSL_shutdown() calls.
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=head2 Peer closes the connection
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If the peer already sent the "close notify" alert B<and> it was
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already processed implicitly inside another function
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(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
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SSL_read() will return <= 0 in that case, and SSL_get_error() will return
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SSL_ERROR_ZERO_RETURN.
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SSL_shutdown() will send the "close notify" alert, set the SSL_SENT_SHUTDOWN
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flag and will immediately return with 1.
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Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
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SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.
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=head1 NOTES
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It is recommended to do a bidirectional shutdown by checking the return value
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of SSL_shutdown() and call it again until it returns 1 or a fatal error.
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The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
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If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
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handshake step has been finished or an error occurred.
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If the underlying BIO is B<non-blocking>, SSL_shutdown() will also return
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when the underlying BIO could not satisfy the needs of SSL_shutdown()
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to continue the handshake. In this case a call to SSL_get_error() with the
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return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
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taking appropriate action to satisfy the needs of SSL_shutdown().
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The action depends on the underlying BIO. When using a non-blocking socket,
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nothing is to be done, but select() can be used to check for the required
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condition. When using a buffering BIO, like a BIO pair, data must be written
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into or retrieved out of the BIO before being able to continue.
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SSL_shutdown() can be modified to only set the connection to "shutdown"
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state but not actually send the "close notify" alert messages,
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see L<SSL_CTX_set_quiet_shutdown(3)>.
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When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
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and return 1.
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=head1 RETURN VALUES
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The following return values can occur:
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=over 4
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=item Z<>0
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The shutdown is not yet finished: the "close notify" was send but the peer
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did not send it back yet.
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Call SSL_shutdown() again to do a bidirectional shutdown.
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The output of L<SSL_get_error(3)> may be misleading, as an
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erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.
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=item Z<>1
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The shutdown was successfully completed. The "close notify" alert was sent
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and the peer's "close notify" alert was received.
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=item E<lt>0
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The shutdown was not successful.
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Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
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It can occur if an action is needed to continue the operation for non-blocking
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BIOs.
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It can also occur when not all data was read using SSL_read().
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=back
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=head1 SEE ALSO
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L<SSL_get_error(3)>, L<SSL_connect(3)>,
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L<SSL_accept(3)>, L<SSL_set_shutdown(3)>,
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L<SSL_CTX_set_quiet_shutdown(3)>,
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L<SSL_clear(3)>, L<SSL_free(3)>,
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L<ssl(7)>, L<bio(7)>
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=head1 COPYRIGHT
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Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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