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 25Note that SSL_shutdown() must not be called if a previous fatal error has 26occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL 27or SSL_ERROR_SSL. 28 29The shutdown procedure consists of two steps: sending of the close_notify 30shutdown alert, and reception of the peer's close_notify shutdown alert. 31The order of those two steps depends on the application. 32 33It is acceptable for an application to only send its shutdown alert and 34then close the underlying connection without waiting for the peer's response. 35This way resources can be saved, as the process can already terminate or 36serve another connection. 37This should only be done when it is known that the other side will not send more 38data, otherwise there is a risk of a truncation attack. 39 40When a client only writes and never reads from the connection, and the server 41has sent a session ticket to establish a session, the client might not be able 42to resume the session because it did not received and process the session ticket 43from the server. 44In case the application wants to be able to resume the session, it is recommended to 45do a complete shutdown procedure (bidirectional close_notify alerts). 46 47When the underlying connection shall be used for more communications, the 48complete shutdown procedure must be performed, so that the peers stay 49synchronized. 50 51SSL_shutdown() only closes the write direction. 52It is not possible to call SSL_write() after calling SSL_shutdown(). 53The read direction is closed by the peer. 54 55=head2 First to close the connection 56 57When the application is the first party to send the close_notify 58alert, SSL_shutdown() will only send the alert and then set the 59SSL_SENT_SHUTDOWN flag (so that the session is considered good and will 60be kept in the cache). 61If successful, SSL_shutdown() will return 0. 62 63If a unidirectional shutdown is enough (the underlying connection shall be 64closed anyway), this first successful call to SSL_shutdown() is sufficient. 65 66In order to complete the bidirectional shutdown handshake, the peer needs 67to send back a close_notify alert. 68The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing 69it. 70 71The peer is still allowed to send data after receiving the close_notify 72event. 73When it is done sending data, it will send the close_notify alert. 74SSL_read() should be called until all data is received. 75SSL_read() will indicate the end of the peer data by returning <= 0 76and SSL_get_error() returning SSL_ERROR_ZERO_RETURN. 77 78=head2 Peer closes the connection 79 80If the peer already sent the close_notify alert B<and> it was 81already processed implicitly inside another function 82(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set. 83SSL_read() will return <= 0 in that case, and SSL_get_error() will return 84SSL_ERROR_ZERO_RETURN. 85SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN 86flag. 87If successful, SSL_shutdown() will return 1. 88 89Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the 90SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call. 91 92=head1 NOTES 93 94The behaviour of SSL_shutdown() additionally depends on the underlying BIO. 95If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the 96handshake step has been finished or an error occurred. 97 98If the underlying BIO is B<nonblocking>, SSL_shutdown() will also return 99when the underlying BIO could not satisfy the needs of SSL_shutdown() 100to continue the handshake. In this case a call to SSL_get_error() with the 101return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or 102B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after 103taking appropriate action to satisfy the needs of SSL_shutdown(). 104The action depends on the underlying BIO. When using a nonblocking socket, 105nothing is to be done, but select() can be used to check for the required 106condition. When using a buffering BIO, like a BIO pair, data must be written 107into or retrieved out of the BIO before being able to continue. 108 109After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again 110to wait for the peer's close_notify alert. 111SSL_shutdown() will return 1 in that case. 112However, it is recommended to wait for it using SSL_read() instead. 113 114SSL_shutdown() can be modified to only set the connection to "shutdown" 115state but not actually send the close_notify alert messages, 116see L<SSL_CTX_set_quiet_shutdown(3)>. 117When "quiet shutdown" is enabled, SSL_shutdown() will always succeed 118and return 1. 119Note that this is not standard compliant behaviour. 120It should only be done when the peer has a way to make sure all 121data has been received and doesn't wait for the close_notify alert 122message, otherwise an unexpected EOF will be reported. 123 124There are implementations that do not send the required close_notify alert. 125If there is a need to communicate with such an implementation, and it's clear 126that all data has been received, do not wait for the peer's close_notify alert. 127Waiting for the close_notify alert when the peer just closes the connection will 128result in an error being generated. 129 130=head1 RETURN VALUES 131 132The following return values can occur: 133 134=over 4 135 136=item Z<>0 137 138The shutdown is not yet finished: the close_notify was sent but the peer 139did not send it back yet. 140Call SSL_read() to do a bidirectional shutdown. 141 142Unlike most other function, returning 0 does not indicate an error. 143L<SSL_get_error(3)> should not get called, it may misleadingly 144indicate an error even though no error occurred. 145 146=item Z<>1 147 148The shutdown was successfully completed. The close_notify alert was sent 149and the peer's close_notify alert was received. 150 151=item E<lt>0 152 153The shutdown was not successful. 154Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason. 155It can occur if an action is needed to continue the operation for nonblocking 156BIOs. 157 158It can also occur when not all data was read using SSL_read(). 159 160=back 161 162=head1 SEE ALSO 163 164L<SSL_get_error(3)>, L<SSL_connect(3)>, 165L<SSL_accept(3)>, L<SSL_set_shutdown(3)>, 166L<SSL_CTX_set_quiet_shutdown(3)>, 167L<SSL_clear(3)>, L<SSL_free(3)>, 168L<ssl(7)>, L<bio(7)> 169 170=head1 COPYRIGHT 171 172Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved. 173 174Licensed under the OpenSSL license (the "License"). You may not use 175this file except in compliance with the License. You can obtain a copy 176in the file LICENSE in the source distribution or at 177L<https://www.openssl.org/source/license.html>. 178 179=cut 180