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.

       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 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 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_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 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.

SEE ALSO

       SSL_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)

1.0.2g                                             2016-03-01                                 SSL_shutdown(3SSL)