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| 1 | =pod | ||
| 2 | |||
| 3 | =head1 NAME | ||
| 4 | |||
| 5 | SSL_shutdown - shut down a TLS/SSL connection | ||
| 6 | |||
| 7 | =head1 SYNOPSIS | ||
| 8 | |||
| 9 | #include <openssl/ssl.h> | ||
| 10 | |||
| 11 | int SSL_shutdown(SSL *ssl); | ||
| 12 | |||
| 13 | =head1 DESCRIPTION | ||
| 14 | |||
| 15 | SSL_shutdown() shuts down an active TLS/SSL connection. It sends the | ||
| 16 | "close notify" shutdown alert to the peer. | ||
| 17 | |||
| 18 | =head1 NOTES | ||
| 19 | |||
| 20 | SSL_shutdown() tries to send the "close notify" shutdown alert to the peer. | ||
| 21 | Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and | ||
| 22 | a currently open session is considered closed and good and will be kept in the | ||
| 23 | session cache for further reuse. | ||
| 24 | |||
| 25 | The shutdown procedure consists of 2 steps: the sending of the "close notify" | ||
| 26 | shutdown alert and the reception of the peer's "close notify" shutdown | ||
| 27 | alert. According to the TLS standard, it is acceptable for an application | ||
| 28 | to only send its shutdown alert and then close the underlying connection | ||
| 29 | without waiting for the peer's response (this way resources can be saved, | ||
| 30 | as the process can already terminate or serve another connection). | ||
| 31 | When the underlying connection shall be used for more communications, the | ||
| 32 | complete shutdown procedure (bidirectional "close notify" alerts) must be | ||
| 33 | performed, so that the peers stay synchronized. | ||
| 34 | |||
| 35 | SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step | ||
| 36 | behaviour. | ||
| 37 | |||
| 38 | =over 4 | ||
| 39 | |||
| 40 | =item When the application is the first party to send the "close notify" | ||
| 41 | alert, SSL_shutdown() will only send the alert and then set the | ||
| 42 | SSL_SENT_SHUTDOWN flag (so that the session is considered good and will | ||
| 43 | be kept in cache). SSL_shutdown() will then return with 0. If a unidirectional | ||
| 44 | shutdown is enough (the underlying connection shall be closed anyway), this | ||
| 45 | first call to SSL_shutdown() is sufficient. In order to complete the | ||
| 46 | bidirectional shutdown handshake, SSL_shutdown() must be called again. | ||
| 47 | The second call will make SSL_shutdown() wait for the peer's "close notify" | ||
| 48 | shutdown alert. On success, the second call to SSL_shutdown() will return | ||
| 49 | with 1. | ||
| 50 | |||
| 51 | =item If the peer already sent the "close notify" alert B<and> it was | ||
| 52 | already processed implicitly inside another function | ||
| 53 | (L<SSL_read(3)|SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set. | ||
| 54 | SSL_shutdown() will send the "close notify" alert, set the SSL_SENT_SHUTDOWN | ||
| 55 | flag and will immediately return with 1. | ||
| 56 | Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the | ||
| 57 | SSL_get_shutdown() (see also L<SSL_set_shutdown(3)|SSL_set_shutdown(3)> call. | ||
| 58 | |||
| 59 | =back | ||
| 60 | |||
| 61 | It is therefore recommended, to check the return value of SSL_shutdown() | ||
| 62 | and call SSL_shutdown() again, if the bidirectional shutdown is not yet | ||
| 63 | complete (return value of the first call is 0). As the shutdown is not | ||
| 64 | specially handled in the SSLv2 protocol, SSL_shutdown() will succeed on | ||
| 65 | the first call. | ||
| 66 | |||
| 67 | The behaviour of SSL_shutdown() additionally depends on the underlying BIO. | ||
| 68 | |||
| 69 | If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the | ||
| 70 | handshake step has been finished or an error occurred. | ||
| 71 | |||
| 72 | If the underlying BIO is B<non-blocking>, SSL_shutdown() will also return | ||
| 73 | when the underlying BIO could not satisfy the needs of SSL_shutdown() | ||
| 74 | to continue the handshake. In this case a call to SSL_get_error() with the | ||
| 75 | return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or | ||
| 76 | B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after | ||
| 77 | taking appropriate action to satisfy the needs of SSL_shutdown(). | ||
| 78 | The action depends on the underlying BIO. When using a non-blocking socket, | ||
| 79 | nothing is to be done, but select() can be used to check for the required | ||
| 80 | condition. When using a buffering BIO, like a BIO pair, data must be written | ||
| 81 | into or retrieved out of the BIO before being able to continue. | ||
| 82 | |||
| 83 | SSL_shutdown() can be modified to only set the connection to "shutdown" | ||
| 84 | state but not actually send the "close notify" alert messages, | ||
| 85 | see L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>. | ||
| 86 | When "quiet shutdown" is enabled, SSL_shutdown() will always succeed | ||
| 87 | and return 1. | ||
| 88 | |||
| 89 | =head1 RETURN VALUES | ||
| 90 | |||
| 91 | The following return values can occur: | ||
| 92 | |||
| 93 | =over 4 | ||
| 94 | |||
| 95 | =item C<0> | ||
| 96 | |||
| 97 | The shutdown is not yet finished. Call SSL_shutdown() for a second time, | ||
| 98 | if a bidirectional shutdown shall be performed. | ||
| 99 | The output of L<SSL_get_error(3)|SSL_get_error(3)> may be misleading, as an | ||
| 100 | erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred. | ||
| 101 | |||
| 102 | =item C<1> | ||
| 103 | |||
| 104 | The shutdown was successfully completed. The "close notify" alert was sent | ||
| 105 | and the peer's "close notify" alert was received. | ||
| 106 | |||
| 107 | =item C<-1> | ||
| 108 | |||
| 109 | The shutdown was not successful because a fatal error occurred either | ||
| 110 | at the protocol level or a connection failure occurred. It can also occur if | ||
| 111 | action is need to continue the operation for non-blocking BIOs. | ||
| 112 | Call L<SSL_get_error(3)|SSL_get_error(3)> with the return value B<ret> | ||
| 113 | to find out the reason. | ||
| 114 | |||
| 115 | =back | ||
| 116 | |||
| 117 | =head1 SEE ALSO | ||
| 118 | |||
| 119 | L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_connect(3)|SSL_connect(3)>, | ||
| 120 | L<SSL_accept(3)|SSL_accept(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>, | ||
| 121 | L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>, | ||
| 122 | L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)>, | ||
| 123 | L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)> | ||
| 124 | |||
| 125 | =cut | ||
