NAME

       SSL_shutdown - shut down a TLS/SSL connection

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>.