SSL_SHUTDOWN(3) OpenSSL SSL_SHUTDOWN(3)NAMESSL_shutdown - shut down a TLS/SSL connection
SYNOPSIS
#include <openssl/ssl.h>
int SSL_shutdown(SSL *ssl);
DESCRIPTIONSSL_shutdown() shuts down an active TLS/SSL connection. It
sends the "close notify" shutdown alert to the peer.
NOTESSSL_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 ses-
sion 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 pro-
cedure (bidirectional "close notify" alerts) must be per-
formed, so that the peers stay synchronized.
SSL_shutdown() supports both uni- and bidirectional shutdown
by its 2 step behaviour.
notify" alert, SSL_shutdown() will only send the alert and then
set the SSL_SENT_SHUTDOWN flag (so that the session is consideredgood and will be kept in cache). SSL_shutdown() will then return
with 0. If a unidirectional shutdown is enough (the underlyingconnection shall be closed anyway), this first call to
SSL_shutdown() is sufficient. In order to complete the bidirec-
tional 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.
When the application is the first party to send the "close
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 usingthe SSL_get_shutdown() (see also SSL_set_shutdown(3) call.
If the peer already sent the "close notify" alert and it was
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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 han-
dled 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 underly-
ing 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.
RETURN VALUES
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
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the reason.
SEE ALSOSSL_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)MirOS BSD #10-current 2005-04-29 3
