|SSL_SHUTDOWN(3)||Library Functions Manual||SSL_SHUTDOWN(3)|
shut down a TLS/SSL connection
shuts down an active TLS/SSL connection. It sends the “close
notify” shutdown alert to the peer.
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.
supports both uni- and bidirectional shutdown by its 2 step behavior.
When the application is the first party to
send the “close notify” alert,
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
sufficient. In order to complete the bidirectional shutdown handshake,
SSL_shutdown() must be called again. The second call
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.
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
(see also the
It is therefore recommended to check the
return value of
SSL_shutdown() again, if the bidirectional
shutdown is not yet complete (return value of the first call is 0).
The behaviour of
additionally depends on the underlying BIO.
If the underlying BIO is
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
the return value of
SSL_shutdown() will yield
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.
can be modified to only set the connection to “shutdown” state
but not actually send the “close notify” alert messages; see
When “quiet shutdown” is enabled,
SSL_shutdown() will always succeed and return 1.
The following return values can occur:
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_SYSCALLmay be flagged even though no error occurred.
|December 6, 2016||OpenBSD-6.1|