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       The shutdown procedure consists of 2 steps: the sending of the "close
24       notify" shutdown alert and the reception of the peer's "close notify"
25       shutdown alert. According to the TLS standard, it is acceptable for an
26       application to only send its shutdown alert and then close the
27       underlying connection without waiting for the peer's response (this way
28       resources can be saved, as the process can already terminate or serve
29       another connection).  When the underlying connection shall be used for
30       more communications, the complete shutdown procedure (bidirectional
31       "close notify" alerts) must be performed, so that the peers stay
32       synchronized.
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34       SSL_shutdown() supports both uni- and bidirectional shutdown by its 2
35       step behaviour.
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37       When the application is the first party to send the "close notify"
38       alert, SSL_shutdown() will only send the alert and then set the
39       SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
40       be kept in cache). SSL_shutdown() will then return with 0. If a
41       unidirectional shutdown is enough (the underlying connection shall be
42       closed anyway), this first call to SSL_shutdown() is sufficient. In
43       order to complete the bidirectional shutdown handshake, SSL_shutdown()
44       must be called again. The second call will make SSL_shutdown() wait for
45       the peer's "close notify" shutdown alert. On success, the second call
46       to SSL_shutdown() will return with 1.
47       If the peer already sent the "close notify" alert and it was already
48       processed implicitly inside another function (SSL_read(3)), the
49       SSL_RECEIVED_SHUTDOWN flag is set. SSL_shutdown() will send the "close
50       notify" alert, set the SSL_SENT_SHUTDOWN flag and will immediately
51       return with 1. Whether SSL_RECEIVED_SHUTDOWN is already set can be
52       checked using the SSL_get_shutdown() (see also SSL_set_shutdown(3)
53       call.
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55       It is therefore recommended, to check the return value of
56       SSL_shutdown() and call SSL_shutdown() again, if the bidirectional
57       shutdown is not yet complete (return value of the first call is 0). As
58       the shutdown is not specially handled in the SSLv2 protocol,
59       SSL_shutdown() will succeed on the first call.
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61       The behaviour of SSL_shutdown() additionally depends on the underlying
62       BIO.
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64       If the underlying BIO is blocking, SSL_shutdown() will only return once
65       the handshake step has been finished or an error occurred.
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67       If the underlying BIO is non-blocking, SSL_shutdown() will also return
68       when the underlying BIO could not satisfy the needs of SSL_shutdown()
69       to continue the handshake. In this case a call to SSL_get_error() with
70       the return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or
71       SSL_ERROR_WANT_WRITE. The calling process then must repeat the call
72       after taking appropriate action to satisfy the needs of SSL_shutdown().
73       The action depends on the underlying BIO. When using a non-blocking
74       socket, nothing is to be done, but select() can be used to check for
75       the required condition. When using a buffering BIO, like a BIO pair,
76       data must be written into or retrieved out of the BIO before being able
77       to continue.
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79       SSL_shutdown() can be modified to only set the connection to "shutdown"
80       state but not actually send the "close notify" alert messages, see
81       SSL_CTX_set_quiet_shutdown(3).  When "quiet shutdown" is enabled,
82       SSL_shutdown() will always succeed and return 1.
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RETURN VALUES

85       The following return values can occur:
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87       0   The shutdown is not yet finished. Call SSL_shutdown() for a second
88           time, if a bidirectional shutdown shall be performed.  The output
89           of SSL_get_error(3) may be misleading, as an erroneous
90           SSL_ERROR_SYSCALL may be flagged even though no error occurred.
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92       1   The shutdown was successfully completed. The "close notify" alert
93           was sent and the peer's "close notify" alert was received.
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95       <0  The shutdown was not successful because a fatal error occurred
96           either at the protocol level or a connection failure occurred. It
97           can also occur if action is need to continue the operation for non-
98           blocking BIOs.  Call SSL_get_error(3) with the return value ret to
99           find out the reason.
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SEE ALSO

102       SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3),
103       SSL_CTX_set_quiet_shutdown(3), SSL_clear(3), SSL_free(3), ssl(3),
104       bio(3)
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1081.0.2o                            2018-03-27                   SSL_shutdown(3)
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