SSL_shutdown - shut down a TLS/SSL connection
#include <openssl/ssl.h>
int SSL_shutdown(SSL *ssl);
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).
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.
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.
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().
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 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>.