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