NAME
SSL_shutdown —
shut down a TLS/SSL
connection
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.
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 behavior.
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 cache).
SSL_shutdown() will then return 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,
SSL_shutdown() must be called again. The second call
will make SSL_shutdown() wait for the peer's
“close notify” shutdown alert. On success, the second call to
SSL_shutdown() will return 1.
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_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 the
SSL_set_shutdown(3) call).
It is therefore recommended to check the
return value of
SSL_shutdown()
and call SSL_shutdown() again, if the bidirectional
shutdown is not yet complete (return value of the first call is 0).
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(3) 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(2) 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.
RETURN VALUES
The following return values can occur:
- 0
- The shutdown is not yet finished. Call
SSL_shutdown() for a second time, if a bidirectional shutdown shall be performed. The output of SSL_get_error(3) may be misleading, as an erroneousSSL_ERROR_SYSCALLmay be flagged 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.
- −1
- The shutdown was not successful because a fatal error occurred either at the protocol level or a connection failure occurred. It can also occur if action is need to continue the operation for non-blocking BIOs. Call SSL_get_error(3) with the return value ret to find out the reason.
SEE ALSO
BIO_new(3), ssl(3), SSL_accept(3), SSL_clear(3), SSL_connect(3), SSL_CTX_set_quiet_shutdown(3), SSL_free(3), SSL_get_error(3), SSL_set_shutdown(3)
HISTORY
SSL_shutdown() first appeared in SSLeay
0.8.0 and has been available since OpenBSD 2.4.