SSL_shutdown - shut down a TLS/SSL connection
libcrypto, -lcrypto
#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.
When the application is the first party to send the
""close notify"" alert, SSL_shutdown() will only send the
alert and the set the SSL_SENT_SHUTDOWN flag (so that the
session is considered good and will be kept in 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, 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 with 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
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). 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 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:
1 The shutdown was successfully completed. The "close
notify" alert was sent and the peer's "close notify"
alert was received.
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 erroneous SSL_ERROR_SYSCALL may be
flagged even though no error occurred.
-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.
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), openssl_bio(3)
2002-06-10 0.9.6g SSL_shutdown(3)
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