1SSL_SHUTDOWN(3)                     OpenSSL                    SSL_SHUTDOWN(3)
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NAME

6       SSL_shutdown - shut down a TLS/SSL connection
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SYNOPSIS

9        #include <openssl/ssl.h>
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11        int SSL_shutdown(SSL *ssl);
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DESCRIPTION

14       SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
15       close_notify shutdown alert to the peer.
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NOTES

18       SSL_shutdown() tries to send the close_notify shutdown alert to the
19       peer.  Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN
20       flag is set and a currently open session is considered closed and good
21       and will be kept in the session cache for further reuse.
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23       Note that SSL_shutdown() must not be called if a previous fatal error
24       has occurred on a connection i.e. if SSL_get_error() has returned
25       SSL_ERROR_SYSCALL or SSL_ERROR_SSL.
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27       The shutdown procedure consists of two steps: sending of the
28       close_notify shutdown alert, and reception of the peer's close_notify
29       shutdown alert.  The order of those two steps depends on the
30       application.
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32       It is acceptable for an application to only send its shutdown alert and
33       then close the underlying connection without waiting for the peer's
34       response.  This way resources can be saved, as the process can already
35       terminate or serve another connection.  This should only be done when
36       it is known that the other side will not send more data, otherwise
37       there is a risk of a truncation attack.
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39       When a client only writes and never reads from the connection, and the
40       server has sent a session ticket to establish a session, the client
41       might not be able to resume the session because it did not received and
42       process the session ticket from the server.  In case the application
43       wants to be able to resume the session, it is recommended to do a
44       complete shutdown procedure (bidirectional close_notify alerts).
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46       When the underlying connection shall be used for more communications,
47       the complete shutdown procedure must be performed, so that the peers
48       stay synchronized.
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50       SSL_shutdown() only closes the write direction.  It is not possible to
51       call SSL_write() after calling SSL_shutdown().  The read direction is
52       closed by the peer.
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54   First to close the connection
55       When the application is the first party to send the close_notify alert,
56       SSL_shutdown() will only send the alert and then set the
57       SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
58       be kept in the cache).  If successful, SSL_shutdown() will return 0.
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60       If a unidirectional shutdown is enough (the underlying connection shall
61       be closed anyway), this first successful call to SSL_shutdown() is
62       sufficient.
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64       In order to complete the bidirectional shutdown handshake, the peer
65       needs to send back a close_notify alert.  The SSL_RECEIVED_SHUTDOWN
66       flag will be set after receiving and processing it.
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68       The peer is still allowed to send data after receiving the close_notify
69       event.  When it is done sending data, it will send the close_notify
70       alert.  SSL_read() should be called until all data is received.
71       SSL_read() will indicate the end of the peer data by returning <= 0 and
72       SSL_get_error() returning SSL_ERROR_ZERO_RETURN.
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74   Peer closes the connection
75       If the peer already sent the close_notify alert and it was already
76       processed implicitly inside another function (SSL_read(3)), the
77       SSL_RECEIVED_SHUTDOWN flag is set.  SSL_read() will return <= 0 in that
78       case, and SSL_get_error() will return SSL_ERROR_ZERO_RETURN.
79       SSL_shutdown() will send the close_notify alert, set the
80       SSL_SENT_SHUTDOWN flag.  If successful, SSL_shutdown() will return 1.
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82       Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
83       SSL_get_shutdown() (see also SSL_set_shutdown(3) call.
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NOTES

86       The behaviour of SSL_shutdown() additionally depends on the underlying
87       BIO.  If the underlying BIO is blocking, SSL_shutdown() will only
88       return once the handshake step has been finished or an error occurred.
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90       If the underlying BIO is nonblocking, SSL_shutdown() will also return
91       when the underlying BIO could not satisfy the needs of SSL_shutdown()
92       to continue the handshake. In this case a call to SSL_get_error() with
93       the return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or
94       SSL_ERROR_WANT_WRITE. The calling process then must repeat the call
95       after taking appropriate action to satisfy the needs of SSL_shutdown().
96       The action depends on the underlying BIO. When using a nonblocking
97       socket, nothing is to be done, but select() can be used to check for
98       the required condition. When using a buffering BIO, like a BIO pair,
99       data must be written into or retrieved out of the BIO before being able
100       to continue.
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102       After SSL_shutdown() returned 0, it is possible to call SSL_shutdown()
103       again to wait for the peer's close_notify alert.  SSL_shutdown() will
104       return 1 in that case.  However, it is recommended to wait for it using
105       SSL_read() instead.
106
107       SSL_shutdown() can be modified to only set the connection to "shutdown"
108       state but not actually send the close_notify alert messages, see
109       SSL_CTX_set_quiet_shutdown(3).  When "quiet shutdown" is enabled,
110       SSL_shutdown() will always succeed and return 1.  Note that this is not
111       standard compliant behaviour.  It should only be done when the peer has
112       a way to make sure all data has been received and doesn't wait for the
113       close_notify alert message, otherwise an unexpected EOF will be
114       reported.
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116       There are implementations that do not send the required close_notify
117       alert.  If there is a need to communicate with such an implementation,
118       and it's clear that all data has been received, do not wait for the
119       peer's close_notify alert.  Waiting for the close_notify alert when the
120       peer just closes the connection will result in an error being
121       generated.
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RETURN VALUES

124       The following return values can occur:
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126       0   The shutdown is not yet finished: the close_notify was sent but the
127           peer did not send it back yet.  Call SSL_read() to do a
128           bidirectional shutdown.
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130           Unlike most other function, returning 0 does not indicate an error.
131           SSL_get_error(3) should not get called, it may misleadingly
132           indicate an error even though no error occurred.
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134       1   The shutdown was successfully completed. The close_notify alert was
135           sent and the peer's close_notify alert was received.
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137       <0  The shutdown was not successful.  Call SSL_get_error(3) with the
138           return value ret to find out the reason.  It can occur if an action
139           is needed to continue the operation for nonblocking BIOs.
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141           It can also occur when not all data was read using SSL_read().
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SEE ALSO

144       SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3),
145       SSL_CTX_set_quiet_shutdown(3), SSL_clear(3), SSL_free(3), ssl(7),
146       bio(7)
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149       Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
150
151       Licensed under the OpenSSL license (the "License").  You may not use
152       this file except in compliance with the License.  You can obtain a copy
153       in the file LICENSE in the source distribution or at
154       <https://www.openssl.org/source/license.html>.
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1581.1.1k                            2021-03-26                   SSL_SHUTDOWN(3)
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