MirOS Manual: 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 ses-
     sion 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 pro-
     cedure (bidirectional "close notify" alerts) must be per-
     formed, 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 bidirec-
      tional 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.

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SSL_SHUTDOWN(3)              OpenSSL              SSL_SHUTDOWN(3)

     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 han-
     dled 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 underly-
     ing 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:

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

     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 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

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SSL_SHUTDOWN(3)              OpenSSL              SSL_SHUTDOWN(3)

         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)

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