1=pod 2 3=head1 NAME 4 5SSL_shutdown - shut down a TLS/SSL connection 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 int SSL_shutdown(SSL *ssl); 12 13=head1 DESCRIPTION 14 15SSL_shutdown() shuts down an active TLS/SSL connection. It sends the 16close_notify shutdown alert to the peer. 17 18=head1 NOTES 19 20SSL_shutdown() tries to send the close_notify shutdown alert to the peer. 21Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and 22a currently open session is considered closed and good and will be kept in the 23session cache for further reuse. 24 25The shutdown procedure consists of two steps: sending of the close_notify 26shutdown alert, and reception of the peer's close_notify shutdown alert. 27The order of those two steps depends on the application. 28 29It is acceptable for an application to only send its shutdown alert and 30then close the underlying connection without waiting for the peer's response. 31This way resources can be saved, as the process can already terminate or 32serve another connection. 33This should only be done when it is known that the other side will not send more 34data, otherwise there is a risk of a truncation attack. 35 36When a client only writes and never reads from the connection, and the server 37has sent a session ticket to establish a session, the client might not be able 38to resume the session because it did not received and process the session ticket 39from the server. 40In case the application wants to be able to resume the session, it is recommended to 41do a complete shutdown procedure (bidirectional close_notify alerts). 42 43When the underlying connection shall be used for more communications, the 44complete shutdown procedure must be performed, so that the peers stay 45synchronized. 46 47SSL_shutdown() only closes the write direction. 48It is not possible to call SSL_write() after calling SSL_shutdown(). 49The read direction is closed by the peer. 50 51=head2 First to close the connection 52 53When the application is the first party to send the close_notify 54alert, SSL_shutdown() will only send the alert and then set the 55SSL_SENT_SHUTDOWN flag (so that the session is considered good and will 56be kept in the cache). 57If successful, SSL_shutdown() will return 0. 58 59If a unidirectional shutdown is enough (the underlying connection shall be 60closed anyway), this first successful call to SSL_shutdown() is sufficient. 61 62In order to complete the bidirectional shutdown handshake, the peer needs 63to send back a close_notify alert. 64The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing 65it. 66 67The peer is still allowed to send data after receiving the close_notify 68event. 69When it is done sending data, it will send the close_notify alert. 70SSL_read() should be called until all data is received. 71SSL_read() will indicate the end of the peer data by returning <= 0 72and SSL_get_error() returning SSL_ERROR_ZERO_RETURN. 73 74=head2 Peer closes the connection 75 76If the peer already sent the close_notify alert B<and> it was 77already processed implicitly inside another function 78(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set. 79SSL_read() will return <= 0 in that case, and SSL_get_error() will return 80SSL_ERROR_ZERO_RETURN. 81SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN 82flag. 83If successful, SSL_shutdown() will return 1. 84 85Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the 86SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call. 87 88=head1 NOTES 89 90The behaviour of SSL_shutdown() additionally depends on the underlying BIO. 91If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the 92handshake step has been finished or an error occurred. 93 94If the underlying BIO is B<non-blocking>, SSL_shutdown() will also return 95when the underlying BIO could not satisfy the needs of SSL_shutdown() 96to continue the handshake. In this case a call to SSL_get_error() with the 97return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or 98B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after 99taking appropriate action to satisfy the needs of SSL_shutdown(). 100The action depends on the underlying BIO. When using a non-blocking socket, 101nothing is to be done, but select() can be used to check for the required 102condition. When using a buffering BIO, like a BIO pair, data must be written 103into or retrieved out of the BIO before being able to continue. 104 105After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again 106to wait for the peer's close_notify alert. 107SSL_shutdown() will return 1 in that case. 108However, it is recommended to wait for it using SSL_read() instead. 109 110SSL_shutdown() can be modified to only set the connection to "shutdown" 111state but not actually send the close_notify alert messages, 112see L<SSL_CTX_set_quiet_shutdown(3)>. 113When "quiet shutdown" is enabled, SSL_shutdown() will always succeed 114and return 1. 115 116=head1 RETURN VALUES 117 118The following return values can occur: 119 120=over 4 121 122=item Z<>0 123 124The shutdown is not yet finished: the close_notify was sent but the peer 125did not send it back yet. 126Call SSL_read() to do a bidirectional shutdown. 127The output of L<SSL_get_error(3)> may be misleading, as an 128erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred. 129 130=item Z<>1 131 132The shutdown was successfully completed. The close_notify alert was sent 133and the peer's close_notify alert was received. 134 135=item E<lt>0 136 137The shutdown was not successful. 138Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason. 139It can occur if an action is needed to continue the operation for non-blocking 140BIOs. 141 142It can also occur when not all data was read using SSL_read(). 143 144=back 145 146=head1 SEE ALSO 147 148L<SSL_get_error(3)>, L<SSL_connect(3)>, 149L<SSL_accept(3)>, L<SSL_set_shutdown(3)>, 150L<SSL_CTX_set_quiet_shutdown(3)>, 151L<SSL_clear(3)>, L<SSL_free(3)>, 152L<ssl(7)>, L<bio(7)> 153 154=head1 COPYRIGHT 155 156Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved. 157 158Licensed under the OpenSSL license (the "License"). You may not use 159this file except in compliance with the License. You can obtain a copy 160in the file LICENSE in the source distribution or at 161L<https://www.openssl.org/source/license.html>. 162 163=cut 164