180 lines
6.8 KiB
Plaintext
180 lines
6.8 KiB
Plaintext
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=pod
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=head1 NAME
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SSL_shutdown - shut down a TLS/SSL connection
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_shutdown(SSL *ssl);
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=head1 DESCRIPTION
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SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
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close_notify shutdown alert to the peer.
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=head1 NOTES
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SSL_shutdown() tries to send the close_notify shutdown alert to the peer.
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Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
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a currently open session is considered closed and good and will be kept in the
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session cache for further reuse.
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Note that SSL_shutdown() must not be called if a previous fatal error has
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occurred on a connection i.e. if SSL_get_error() has returned SSL_ERROR_SYSCALL
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or SSL_ERROR_SSL.
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The shutdown procedure consists of two steps: sending of the close_notify
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shutdown alert, and reception of the peer's close_notify shutdown alert.
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The order of those two steps depends on the application.
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It is acceptable for an application to only send its shutdown alert and
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then close the underlying connection without waiting for the peer's response.
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This way resources can be saved, as the process can already terminate or
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serve another connection.
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This should only be done when it is known that the other side will not send more
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data, otherwise there is a risk of a truncation attack.
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When a client only writes and never reads from the connection, and the server
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has sent a session ticket to establish a session, the client might not be able
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to resume the session because it did not received and process the session ticket
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from the server.
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In case the application wants to be able to resume the session, it is recommended to
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do a complete shutdown procedure (bidirectional close_notify alerts).
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When the underlying connection shall be used for more communications, the
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complete shutdown procedure must be performed, so that the peers stay
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synchronized.
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SSL_shutdown() only closes the write direction.
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It is not possible to call SSL_write() after calling SSL_shutdown().
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The read direction is closed by the peer.
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=head2 First to close the connection
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When the application is the first party to send the close_notify
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alert, SSL_shutdown() will only send the alert and then set the
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SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
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be kept in the cache).
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If successful, SSL_shutdown() will return 0.
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If a unidirectional shutdown is enough (the underlying connection shall be
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closed anyway), this first successful call to SSL_shutdown() is sufficient.
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In order to complete the bidirectional shutdown handshake, the peer needs
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to send back a close_notify alert.
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The SSL_RECEIVED_SHUTDOWN flag will be set after receiving and processing
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it.
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The peer is still allowed to send data after receiving the close_notify
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event.
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When it is done sending data, it will send the close_notify alert.
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SSL_read() should be called until all data is received.
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SSL_read() will indicate the end of the peer data by returning <= 0
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and SSL_get_error() returning SSL_ERROR_ZERO_RETURN.
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=head2 Peer closes the connection
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If the peer already sent the close_notify alert B<and> it was
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already processed implicitly inside another function
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(L<SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
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SSL_read() will return <= 0 in that case, and SSL_get_error() will return
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SSL_ERROR_ZERO_RETURN.
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SSL_shutdown() will send the close_notify alert, set the SSL_SENT_SHUTDOWN
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flag.
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If successful, SSL_shutdown() will return 1.
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Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
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SSL_get_shutdown() (see also L<SSL_set_shutdown(3)> call.
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=head1 NOTES
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The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
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If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
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handshake step has been finished or an error occurred.
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If the underlying BIO is B<nonblocking>, SSL_shutdown() will also return
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when the underlying BIO could not satisfy the needs of SSL_shutdown()
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to continue the handshake. In this case a call to SSL_get_error() with the
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return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
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taking appropriate action to satisfy the needs of SSL_shutdown().
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The action depends on the underlying BIO. When using a nonblocking socket,
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nothing is to be done, but select() can be used to check for the required
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condition. When using a buffering BIO, like a BIO pair, data must be written
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into or retrieved out of the BIO before being able to continue.
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After SSL_shutdown() returned 0, it is possible to call SSL_shutdown() again
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to wait for the peer's close_notify alert.
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SSL_shutdown() will return 1 in that case.
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However, it is recommended to wait for it using SSL_read() instead.
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SSL_shutdown() can be modified to only set the connection to "shutdown"
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state but not actually send the close_notify alert messages,
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see L<SSL_CTX_set_quiet_shutdown(3)>.
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When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
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and return 1.
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Note that this is not standard compliant behaviour.
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It should only be done when the peer has a way to make sure all
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data has been received and doesn't wait for the close_notify alert
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message, otherwise an unexpected EOF will be reported.
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There are implementations that do not send the required close_notify alert.
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If there is a need to communicate with such an implementation, and it's clear
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that all data has been received, do not wait for the peer's close_notify alert.
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Waiting for the close_notify alert when the peer just closes the connection will
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result in an error being generated.
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=head1 RETURN VALUES
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The following return values can occur:
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=over 4
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=item Z<>0
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The shutdown is not yet finished: the close_notify was sent but the peer
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did not send it back yet.
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Call SSL_read() to do a bidirectional shutdown.
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Unlike most other function, returning 0 does not indicate an error.
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L<SSL_get_error(3)> should not get called, it may misleadingly
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indicate an error even though no error occurred.
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=item Z<>1
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The shutdown was successfully completed. The close_notify alert was sent
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and the peer's close_notify alert was received.
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=item E<lt>0
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The shutdown was not successful.
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Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
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It can occur if an action is needed to continue the operation for nonblocking
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BIOs.
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It can also occur when not all data was read using SSL_read().
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=back
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=head1 SEE ALSO
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L<SSL_get_error(3)>, L<SSL_connect(3)>,
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L<SSL_accept(3)>, L<SSL_set_shutdown(3)>,
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L<SSL_CTX_set_quiet_shutdown(3)>,
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L<SSL_clear(3)>, L<SSL_free(3)>,
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L<ssl(7)>, L<bio(7)>
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=head1 COPYRIGHT
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Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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