|SSL_SHUTDOWN(3)||Library Functions Manual||SSL_SHUTDOWN(3)|
shut down a TLS/SSL connection
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
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 behavior.
When the application is the first party to send the “close
SSL_shutdown() will only send
the alert and then set the
(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_RECEIVED_SHUTDOWN flag is set.
SSL_shutdown() will send the “close
notify” alert, set the
and will immediately return with 1. Whether
SSL_RECEIVED_SHUTDOWN is already set can be checked
SSL_get_shutdown() (see also the
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).
The behaviour of
additionally depends on the underlying BIO.
If the underlying BIO is
SSL_shutdown() will only
return once the handshake step has been finished or an error occurred.
If the underlying BIO is
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(3) with the return
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.
SSL_shutdown() 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.
SSL_shutdown() first appeared in SSLeay
0.8.0 and has been available since OpenBSD 2.4.
|March 27, 2018||OpenBSD-current|