doc/man3/SSL_shutdown.pod - third_party/openssl - Git at Google

 =pod

 =head1 NAME

 SSL_shutdown - shut down a TLS/SSL connection

 =head1 SYNOPSIS

  #include <openssl/ssl.h>

  int SSL_shutdown(SSL *ssl);

 =head1 DESCRIPTION

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

 SSL_shutdown() tries to send the close_notify shutdown alert to the peer.
 Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
 a currently open session is considered closed and good and will be kept in the
 session cache for further reuse.

 Note that SSL_shutdown() must not be called if a previous fatal error has
 occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL
 or SSL_ERROR_SSL.

 The shutdown procedure consists of two steps: sending of the close_notify
 shutdown alert, and reception of the peer's close_notify shutdown alert.
 The order of those two steps depends on the application.

 It is acceptable for an application to only send its shutdown alert and
 then close the underlying connection without waiting for the peer's response.
 This way resources can be saved, as the process can already terminate or
 serve another connection.
 This should only be done when it is known that the other side will not send more
 data, otherwise there is a risk of a truncation attack.

 When a client only writes and never reads from the connection, and the server
 has sent a session ticket to establish a session, the client might not be able
 to resume the session because it did not received and process the session ticket
 from the server.
 In case the application wants to be able to resume the session, it is recommended to
 do a complete shutdown procedure (bidirectional close_notify alerts).

 When the underlying connection shall be used for more communications, the
 complete shutdown procedure must be performed, so that the peers stay
 synchronized.

 SSL_shutdown() only closes the write direction.
 It is not possible to call SSL_write() after calling SSL_shutdown().
 The read direction is closed by the peer.

 The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
 If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
 handshake step has been finished or an error occurred.

 If the underlying BIO is B<nonblocking>, SSL_shutdown() will also return
 when the underlying BIO could not satisfy the needs of SSL_shutdown()
 to continue the handshake. In this case a call to SSL_get_error() with the
 return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
 B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
 taking appropriate action to satisfy the needs of SSL_shutdown().
 The action depends on the underlying BIO. When using a nonblocking socket,
 nothing is to be done, but select() can be used to check for the required
 condition. When using a buffering BIO, like a BIO pair, data must be written
 into or retrieved out of the BIO before being able to continue.

 After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again
 to wait for the peer's close_notify alert.
 SSL_shutdown() will return 1 in that case.
 However, it is recommended to wait for it using SSL_read() instead.

 SSL_shutdown() can be modified to only set the connection to "shutdown"
 state but not actually send the close_notify alert messages,
 see L<SSL_CTX_set_quiet_shutdown(3)>.
 When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
 and return 1.
 Note that this is not standard compliant behaviour.
 It should only be done when the peer has a way to make sure all
 data has been received and doesn't wait for the close_notify alert
 message, otherwise an unexpected EOF will be reported.

 There are implementations that do not send the required close_notify alert.
 If there is a need to communicate with such an implementation, and it's clear
 that all data has been received, do not wait for the peer's close_notify alert.
 Waiting for the close_notify alert when the peer just closes the connection
 will result in an error being generated.
 The error can be ignored using the B<SSL_OP_IGNORE_UNEXPECTED_EOF>.
 For more information see L<SSL_CTX_set_options(3)>.

 =head2 First to close the connection

 When the application is the first party to send the close_notify
 alert, SSL_shutdown() will only send the alert and then set the
 SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
 be kept in the cache).
 If successful, SSL_shutdown() will return 0.

 If a unidirectional shutdown is enough (the underlying connection shall be
 closed anyway), this first successful call to SSL_shutdown() is sufficient.

 In order to complete the bidirectional shutdown handshake, the peer needs
 to send back a close_notify alert.
 The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
 it.

 The peer is still allowed to send data after receiving the close_notify
 event.
 When it is done sending data, it will send the close_notify alert.
 SSL_read() should be called until all data is received.
 SSL_read() will indicate the end of the peer data by returning <= 0
 and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.

 =head2 Peer closes the connection

 If the peer already sent the close_notify alert B<and> it was
 already processed implicitly inside another function
 (L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
 SSL_read() will return <= 0 in that case, and SSL_get_error() will return
 SSL_ERROR_ZERO_RETURN.
 SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN
 flag.
 If successful, SSL_shutdown() will return 1.

 Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
 SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.

 =head1 RETURN VALUES

 The following return values can occur:

 =over 4

 =item Z<>0

 The shutdown is not yet finished: the close_notify was sent but the peer
 did not send it back yet.
 Call SSL_read() to do a bidirectional shutdown.

 Unlike most other function, returning 0 does not indicate an error.
 L<SSL_get_error(3)> should not get called, it may misleadingly
 indicate an error even though no error occurred.

 =item Z<>1

 The shutdown was successfully completed. The close_notify alert was sent
 and the peer's close_notify alert was received.

 =item E<lt>0

 The shutdown was not successful.
 Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
 It can occur if an action is needed to continue the operation for nonblocking
 BIOs.

 It can also occur when not all data was read using SSL_read().

 =back

 =head1 SEE ALSO

 L<SSL_get_error(3)>, L<SSL_connect(3)>,
 L<SSL_accept(3)>, L<SSL_set_shutdown(3)>,
 L<SSL_CTX_set_quiet_shutdown(3)>, L<SSL_CTX_set_options(3)>
 L<SSL_clear(3)>, L<SSL_free(3)>,
 L<ssl(7)>, L<bio(7)>

 =head1 COPYRIGHT

 Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

 Licensed under the Apache License 2.0 (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy
 in the file LICENSE in the source distribution or at
 L<https://www.openssl.org/source/license.html>.

 =cut
	=pod

	=head1 NAME

	SSL_shutdown - shut down a TLS/SSL connection

	=head1 SYNOPSIS

	#include <openssl/ssl.h>

	int SSL_shutdown(SSL *ssl);

	=head1 DESCRIPTION

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

	SSL_shutdown() tries to send the close_notify shutdown alert to the peer.
	Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
	a currently open session is considered closed and good and will be kept in the
	session cache for further reuse.

	Note that SSL_shutdown() must not be called if a previous fatal error has
	occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL
	or SSL_ERROR_SSL.

	The shutdown procedure consists of two steps: sending of the close_notify
	shutdown alert, and reception of the peer's close_notify shutdown alert.
	The order of those two steps depends on the application.

	It is acceptable for an application to only send its shutdown alert and
	then close the underlying connection without waiting for the peer's response.
	This way resources can be saved, as the process can already terminate or
	serve another connection.
	This should only be done when it is known that the other side will not send more
	data, otherwise there is a risk of a truncation attack.

	When a client only writes and never reads from the connection, and the server
	has sent a session ticket to establish a session, the client might not be able
	to resume the session because it did not received and process the session ticket
	from the server.
	In case the application wants to be able to resume the session, it is recommended to
	do a complete shutdown procedure (bidirectional close_notify alerts).

	When the underlying connection shall be used for more communications, the
	complete shutdown procedure must be performed, so that the peers stay
	synchronized.

	SSL_shutdown() only closes the write direction.
	It is not possible to call SSL_write() after calling SSL_shutdown().
	The read direction is closed by the peer.

	The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
	If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
	handshake step has been finished or an error occurred.

	If the underlying BIO is B<nonblocking>, SSL_shutdown() will also return
	when the underlying BIO could not satisfy the needs of SSL_shutdown()
	to continue the handshake. In this case a call to SSL_get_error() with the
	return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
	B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
	taking appropriate action to satisfy the needs of SSL_shutdown().
	The action depends on the underlying BIO. When using a nonblocking socket,
	nothing is to be done, but select() can be used to check for the required
	condition. When using a buffering BIO, like a BIO pair, data must be written
	into or retrieved out of the BIO before being able to continue.

	After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again
	to wait for the peer's close_notify alert.
	SSL_shutdown() will return 1 in that case.
	However, it is recommended to wait for it using SSL_read() instead.

	SSL_shutdown() can be modified to only set the connection to "shutdown"
	state but not actually send the close_notify alert messages,
	see L<SSL_CTX_set_quiet_shutdown(3)>.
	When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
	and return 1.
	Note that this is not standard compliant behaviour.
	It should only be done when the peer has a way to make sure all
	data has been received and doesn't wait for the close_notify alert
	message, otherwise an unexpected EOF will be reported.

	There are implementations that do not send the required close_notify alert.
	If there is a need to communicate with such an implementation, and it's clear
	that all data has been received, do not wait for the peer's close_notify alert.
	Waiting for the close_notify alert when the peer just closes the connection
	will result in an error being generated.
	The error can be ignored using the B<SSL_OP_IGNORE_UNEXPECTED_EOF>.
	For more information see L<SSL_CTX_set_options(3)>.

	=head2 First to close the connection

	When the application is the first party to send the close_notify
	alert, SSL_shutdown() will only send the alert and then set the
	SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
	be kept in the cache).
	If successful, SSL_shutdown() will return 0.

	If a unidirectional shutdown is enough (the underlying connection shall be
	closed anyway), this first successful call to SSL_shutdown() is sufficient.

	In order to complete the bidirectional shutdown handshake, the peer needs
	to send back a close_notify alert.
	The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
	it.

	The peer is still allowed to send data after receiving the close_notify
	event.
	When it is done sending data, it will send the close_notify alert.
	SSL_read() should be called until all data is received.
	SSL_read() will indicate the end of the peer data by returning <= 0
	and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.

	=head2 Peer closes the connection

	If the peer already sent the close_notify alert B<and> it was
	already processed implicitly inside another function
	(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
	SSL_read() will return <= 0 in that case, and SSL_get_error() will return
	SSL_ERROR_ZERO_RETURN.
	SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN
	flag.
	If successful, SSL_shutdown() will return 1.

	Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
	SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.

	=head1 RETURN VALUES

	The following return values can occur:

	=over 4

	=item Z<>0

	The shutdown is not yet finished: the close_notify was sent but the peer
	did not send it back yet.
	Call SSL_read() to do a bidirectional shutdown.

	Unlike most other function, returning 0 does not indicate an error.
	L<SSL_get_error(3)> should not get called, it may misleadingly
	indicate an error even though no error occurred.

	=item Z<>1

	The shutdown was successfully completed. The close_notify alert was sent
	and the peer's close_notify alert was received.

	=item E<lt>0

	The shutdown was not successful.
	Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
	It can occur if an action is needed to continue the operation for nonblocking
	BIOs.

	It can also occur when not all data was read using SSL_read().

	=back

	=head1 SEE ALSO

	L<SSL_get_error(3)>, L<SSL_connect(3)>,
	L<SSL_accept(3)>, L<SSL_set_shutdown(3)>,
	L<SSL_CTX_set_quiet_shutdown(3)>, L<SSL_CTX_set_options(3)>
	L<SSL_clear(3)>, L<SSL_free(3)>,
	L<ssl(7)>, L<bio(7)>

	=head1 COPYRIGHT

	Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

	Licensed under the Apache License 2.0 (the "License"). You may not use
	this file except in compliance with the License. You can obtain a copy
	in the file LICENSE in the source distribution or at
	L<https://www.openssl.org/source/license.html>.

	=cut