SSL_shutdown(3)

NAME

       SSL_shutdown - shut down a TLS/SSL connection

SYNOPSIS

        #include <openssl/ssl.h>

        int SSL_shutdown(SSL *ssl);

DESCRIPTION

       SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
       "close notify" shutdown alert to the peer.

NOTES

       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.

       Note that SSL_shutdown() must not be called if a previous fatal error
       has occurred on a connection i.e. if SSL_get_error() has returned
       SSL_ERROR_SYSCALL or SSL_ERROR_SSL.

       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 underly-
       ing 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 syn-
       chronized.

       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 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 bidi-
              rectional 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." 4 If a unidirec-
              tional 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_shut-
              down(3) call." 4

       It is therefore recommended, to check the return value of SSL_shut-
       down() 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.

RETURN VALUES

       The following return values can occur:

       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 successfully completed. The "close notify" alert
           was sent and the peer's "close notify" alert was received.

       <0  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_shutdown(3)

NAME

SYNOPSIS

DESCRIPTION

NOTES

RETURN VALUES

SEE ALSO