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SSL_shutdown(3)                     OpenSSL                    SSL_shutdown(3)



NAME
       SSL_shutdown - shut down a TLS/SSL connection

LIBRARY
       libcrypto, -lcrypto

SYNOPSIS
        #include <openssl/ssl.h>

        int SSL_shutdown(SSL *ssl);

DESCRIPTION
       SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
       "close notify" shutdown alert to the peer.

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.

   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.

   Peer closes the connection
       If the peer already sent the "close notify" alert and it was already
       processed implicitly inside another function (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 SSL_set_shutdown(3) call.

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 blocking, SSL_shutdown() will only return
       once the handshake step has been finished or an error occurred.

       If the underlying BIO is 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 SSL_ERROR_WANT_READ or
       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
       SSL_CTX_set_quiet_shutdown(3).  When "quiet shutdown" is enabled,
       SSL_shutdown() will always succeed and return 1.

RETURN VALUES
       The following return values can occur:

       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 SSL_get_error(3) may be
           misleading, as an erroneous SSL_ERROR_SYSCALL may be flagged even
           though no error occurred.

       1   The shutdown was successfully completed. The "close notify" alert
           was sent and the peer's "close notify" alert was received.

       <0  The shutdown was not successful.  Call SSL_get_error(3) with the
           return value 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().

SEE ALSO
       SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3),
       SSL_CTX_set_quiet_shutdown(3), SSL_clear(3), SSL_free(3), ssl(7),
       bio(7)

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



1.1.1                             2018-09-17                   SSL_shutdown(3)