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SSL_shutdown(3)




SSL_shutdown(3)              OpenSSL              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 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

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SSL_shutdown(3)              OpenSSL              SSL_shutdown(3)

          SSL_shutdown() will return with 1." 4 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." 4

     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.

1.0.2t               Last change: 2019-09-10                    2

SSL_shutdown(3)              OpenSSL              SSL_shutdown(3)


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)

1.0.2t               Last change: 2019-09-10                    3


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