SSL_shutdown - shut down a TLS/SSL connection
int SSL_shutdown(SSL *ssl);
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
- 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 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
- 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
If the underlying BIO is blocking
will only return
once the handshake step has been finished or an error occurred.
If the underlying BIO is non-blocking
return when the underlying BIO could not satisfy the needs of
to continue the handshake. In this case a call to
with the return value of SSL_shutdown()
. 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.
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
The following return values can occur:
- The shutdown was successfully completed. The "close
notify" alert was sent and the peer's "close notify" alert
- 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
- 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.