1SSL_SHUTDOWN(3) OpenSSL SSL_SHUTDOWN(3)
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6 SSL_shutdown - shut down a TLS/SSL connection
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9 #include <openssl/ssl.h>
10 int SSL_shutdown(SSL *ssl);
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14 SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
15 "close notify" shutdown alert to the peer.
16
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.
22 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.
33 SSL_shutdown() supports both uni- and bidirectional shutdown by its 2
35 step behaviour.
36 SSL_shutdown() only closes the write direction. It is not possible to
38 call SSL_write() after calling SSL_shutdown(). The read direction is
39 closed by the peer.
40 First to close the connection
42 When the application is the first party to send the "close notify"
43 alert, SSL_shutdown() will only send the alert and then set the
44 SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
45 be kept in the cache). SSL_shutdown() will then return with 0. If a
46 unidirectional shutdown is enough (the underlying connection shall be
47 closed anyway), this first call to SSL_shutdown() is sufficient.
48 In order to complete the bidirectional shutdown handshake, the peer
50 needs to send back a "close notify" alert. The SSL_RECEIVED_SHUTDOWN
51 flag will be set after receiving and processing it. SSL_shutdown()
52 will return 1 when it has been received.
53 The peer is still allowed to send data after receiving the "close
55 notify" event. If the peer did send data it needs to be processed by
56 calling SSL_read() before calling SSL_shutdown() a second time.
57 SSL_read() will indicate the end of the peer data by returning <= 0 and
58 SSL_get_error() returning SSL_ERROR_ZERO_RETURN. It is recommended to
59 call SSL_read() between SSL_shutdown() calls.
60 Peer closes the connection
62 If the peer already sent the "close notify" alert and it was already
63 processed implicitly inside another function (SSL_read(3)), the
64 SSL_RECEIVED_SHUTDOWN flag is set. SSL_read() will return <= 0 in that
65 case, and SSL_get_error() will return SSL_ERROR_ZERO_RETURN.
66 SSL_shutdown() will send the "close notify" alert, set the
67 SSL_SENT_SHUTDOWN flag and will immediately return with 1. Whether
68 SSL_RECEIVED_SHUTDOWN is already set can be checked using the
69 SSL_get_shutdown() (see also SSL_set_shutdown(3) call.
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72 It is recommended to do a bidirectional shutdown by checking the return
73 value of SSL_shutdown() and call it again until it returns 1 or a fatal
74 error.
75 The behaviour of SSL_shutdown() additionally depends on the underlying
77 BIO. If the underlying BIO is blocking, SSL_shutdown() will only
78 return once the handshake step has been finished or an error occurred.
79 If the underlying BIO is non-blocking, SSL_shutdown() will also return
81 when the underlying BIO could not satisfy the needs of SSL_shutdown()
82 to continue the handshake. In this case a call to SSL_get_error() with
83 the return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or
84 SSL_ERROR_WANT_WRITE. The calling process then must repeat the call
85 after taking appropriate action to satisfy the needs of SSL_shutdown().
86 The action depends on the underlying BIO. When using a non-blocking
87 socket, nothing is to be done, but select() can be used to check for
88 the required condition. When using a buffering BIO, like a BIO pair,
89 data must be written into or retrieved out of the BIO before being able
90 to continue.
91 SSL_shutdown() can be modified to only set the connection to "shutdown"
93 state but not actually send the "close notify" alert messages, see
94 SSL_CTX_set_quiet_shutdown(3). When "quiet shutdown" is enabled,
95 SSL_shutdown() will always succeed and return 1.
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98 The following return values can occur:
99 0 The shutdown is not yet finished: the "close notify" was send but
101 the peer did not send it back yet. Call SSL_shutdown() again to do
102 a bidirectional shutdown. The output of SSL_get_error(3) may be
103 misleading, as an erroneous SSL_ERROR_SYSCALL may be flagged even
104 though no error occurred.
105 1 The shutdown was successfully completed. The "close notify" alert
107 was sent and the peer's "close notify" alert was received.
108 <0 The shutdown was not successful. Call SSL_get_error(3) with the
110 return value ret to find out the reason. It can occur if an action
111 is needed to continue the operation for non-blocking BIOs.
112 It can also occur when not all data was read using SSL_read().
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116 SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3),
117 SSL_CTX_set_quiet_shutdown(3), SSL_clear(3), SSL_free(3), ssl(7),
118 bio(7)
119
121 Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
122 Licensed under the OpenSSL license (the "License"). You may not use
124 this file except in compliance with the License. You can obtain a copy
125 in the file LICENSE in the source distribution or at
126 <https://www.openssl.org/source/license.html>.
1271.1.1 2018-09-11 SSL_SHUTDOWN(3)