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SSL_shutdown(3) OpenSSL SSL_shutdown(3)
NAME
SSL_shutdown - shut down a TLS/SSL connection
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#include <openssl/ssl.h>
int SSL_shutdown(SSL *ssl);
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 blocking, SSL_shutdown() will only return
once the handshake step has been finished or an error occurred.
If the underlying BIO is 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 SSL_ERROR_WANT_READ or
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
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
SSL_OP_IGNORE_UNEXPECTED_EOF. For more information see
SSL_CTX_set_options(3).
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.
Peer closes the connection
If the peer already sent the close_notify alert and it was already
processed implicitly inside another function (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 SSL_set_shutdown(3) call.
RETURN VALUES
The following return values can occur:
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.
SSL_get_error(3) should not get called, it may misleadingly
indicate an error even though no error occurred.
1 The shutdown was successfully completed. The close_notify alert was
sent and the peer's close_notify alert was received.
<0 The shutdown was not successful. Call SSL_get_error(3) with the
return value 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().
SEE ALSO
SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3),
SSL_CTX_set_quiet_shutdown(3), SSL_CTX_set_options(3) SSL_clear(3),
SSL_free(3), ssl(7), bio(7)
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
<https://www.openssl.org/source/license.html>.
3.0.12 2023-05-07 SSL_shutdown(3)