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.
The shutdown procedure consists of 2 steps: the sending of the "close notify" shutdown alert and the reception of the peer's "close notify" shutdown alert. According to the TLS standard, 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). When the underlying connection shall be used for more communications, the complete shutdown procedure (bidirectional "close notify" alerts) must be performed, so that the peers stay synchronized.
SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step behaviour.
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.
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). SSL_shutdown() will then return with 0. If a unidirectional shutdown is enough (the underlying connection shall be closed anyway), this first 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. SSL_shutdown() will return 1 when it has been received.
The peer is still allowed to send data after receiving the "close notify" event. If the peer did send data it needs to be processed by calling SSL_read() before calling SSL_shutdown() a second time. SSL_read() will indicate the end of the peer data by returning <= 0 and SSL_get_error() returning SSL_ERROR_ZERO_RETURN. It is recommended to call SSL_read() between SSL_shutdown() calls.
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 and will immediately return with 1. Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the SSL_get_shutdown() (see also SSL_set_shutdown(3) call.
It is recommended to do a bidirectional shutdown by checking the return value of SSL_shutdown() and call it again until it returns 1 or a fatal error.
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 non-blocking, 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 non-blocking 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.
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.
The following return values can occur:
The shutdown is not yet finished: the "close notify" was send but the peer did not send it back yet. Call SSL_shutdown() again to do a bidirectional shutdown. The output of SSL_get_error(3) may be misleading, as an erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.
The shutdown was successfully completed. The "close notify" alert was sent and the peer's "close notify" alert was received.
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 non-blocking BIOs.
It can also occur when not all data was read using SSL_read().
Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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.