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(2,7,n) notify" shutdown(2,8) alert to the peer. NOTES SSL_shutdown() tries to send(2,n) the "close(2,7,n) notify" shutdown(2,8) alert to the peer. Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set(7,n,1 builtins) and a currently open(2,3,n) session is considered closed and good and will be kept in(1,8) the session cache for further reuse. The shutdown(2,8) procedure consists of 2 steps: the sending of the "close(2,7,n) notify" shutdown(2,8) alert and the reception of the peer's "close(2,7,n) notify" shutdown(2,8) alert. According to the TLS standard, it is acceptable for an application to only send(2,n) its shutdown(2,8) alert and then close(2,7,n) 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(2,8) procedure (bidirectional "close(2,7,n) notify" alerts) must be performed, so that the peers stay syn- chronized. SSL_shutdown() supports both uni- and bidirectional shutdown(2,8) by its 2 step behaviour. When the application is the first party to send(2,n) the "close(2,7,n) notify" alert, SSL_shutdown() will only send(2,n) the alert and the set(7,n,1 builtins) the SSL_SENT_SHUTDOWN flag (so that the session is considered good and will be kept in(1,8) cache). SSL_shutdown() will then return with 0. If a unidi- rectional shutdown(2,8) is enough (the underlying connection shall be closed anyway), this first call to SSL_shutdown() is sufficient. In order to complete the bidirectional shutdown(2,8) handshake, SSL_shutdown() must be called again. The second call will make SSL_shutdown() wait for the peer's "close(2,7,n) notify" shutdown(2,8) alert. On success, the second call to SSL_shutdown() will return with 1. If the peer already sent the "close(2,7,n) 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(2,n) the "close(2,7,n) notify" alert, set(7,n,1 builtins) the SSL_SENT_SHUTDOWN flag and will immediately return with 1. Whether SSL_RECEIVED_SHUTDOWN is already set(7,n,1 builtins) 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_shut- down() and call SSL_shutdown() again, if(3,n) the bidirectional shutdown(2,8) is not yet complete (return value of the first call is 0). As the shutdown(2,8) is not specially handled in(1,8) 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(8,n) 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(2,7,n), nothing is to be done, but select(2,7,2 select_tut)() 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(7,n,1 builtins) the connection to "shutdown(2,8)" state but not actually send(2,n) the "close(2,7,n) notify" alert messages, see SSL_CTX_set_quiet_shutdown(3). When "quiet shutdown(2,8)" is enabled, SSL_shutdown() will always succeed and return 1. RETURN VALUES The following return values can occur: 1 The shutdown(2,8) was successfully completed. The "close(2,7,n) notify" alert was sent and the peer's "close(2,7,n) notify" alert was received. 0 The shutdown(2,8) is not yet finished. Call SSL_shutdown() for a second time(1,2,n), if(3,n) a bidirectional shutdown(2,8) 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(8,n) occurred. -1 The shutdown(2,8) was not successful because a fatal error(8,n) occurred either at the protocol level or a connection failure occurred. It can also occur if(3,n) 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) 0.9.7d 2001-08-20 SSL_shutdown(3)