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DESCRIPTION

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 behavior.

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 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 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 the 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).

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(3) 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(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 SSL_CTX_set_quiet_shutdown(3). When “quiet shutdown” is enabled, SSL_shutdown() will always succeed and return 1.