SSL_shutdown - shut down a TLS/SSL connection
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.
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). As the shutdown is not specially handled in the SSLv2 protocol,
SSL_shutdown() will succeed on the first call.
The behaviour of
SSL_shutdown() additionally depends on the
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,
always succeed and return 1.
The following return values can occur:
The shutdown was successfully completed. The ``close notify'' alert was sent and the peer's ``close notify'' alert was received.
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 erroneous SSL_ERROR_SYSCALL may be flagged even
though no error occurred.
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.
SSL_get_error(3), SSL_connect(3), SSL_accept(3), SSL_set_shutdown(3), SSL_CTX_set_quiet_shutdown(3), SSL_clear(3), SSL_free(3), ssl(3), bio(3)