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126 lines
5.0 KiB
Plaintext
126 lines
5.0 KiB
Plaintext
=pod
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=head1 NAME
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SSL_shutdown - shut down a TLS/SSL connection
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_shutdown(SSL *ssl);
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=head1 DESCRIPTION
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SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
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"close notify" shutdown alert to the peer.
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=head1 NOTES
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SSL_shutdown() tries to send the "close notify" shutdown alert to the peer.
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Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
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a currently open session is considered closed and good and will be kept in the
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session cache for further reuse.
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The shutdown procedure consists of 2 steps: the sending of the "close notify"
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shutdown alert and the reception of the peer's "close notify" shutdown
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alert. According to the TLS standard, it is acceptable for an application
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to only send its shutdown alert and then close the underlying connection
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without waiting for the peer's response (this way resources can be saved,
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as the process can already terminate or serve another connection).
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When the underlying connection shall be used for more communications, the
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complete shutdown procedure (bidirectional "close notify" alerts) must be
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performed, so that the peers stay synchronized.
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SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step
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behaviour.
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=over 4
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=item When the application is the first party to send the "close notify"
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alert, SSL_shutdown() will only send the alert and then set the
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SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
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be kept in cache). SSL_shutdown() will then return with 0. If a unidirectional
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shutdown is enough (the underlying connection shall be closed anyway), this
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first call to SSL_shutdown() is sufficient. In order to complete the
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bidirectional shutdown handshake, SSL_shutdown() must be called again.
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The second call will make SSL_shutdown() wait for the peer's "close notify"
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shutdown alert. On success, the second call to SSL_shutdown() will return
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with 1.
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=item If the peer already sent the "close notify" alert B<and> it was
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already processed implicitly inside another function
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(L<SSL_read(3)|SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
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SSL_shutdown() will send the "close notify" alert, set the SSL_SENT_SHUTDOWN
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flag and will immediately return with 1.
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Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
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SSL_get_shutdown() (see also L<SSL_set_shutdown(3)|SSL_set_shutdown(3)> call.
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=back
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It is therefore recommended, to check the return value of SSL_shutdown()
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and call SSL_shutdown() again, if the bidirectional shutdown is not yet
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complete (return value of the first call is 0). As the shutdown is not
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specially handled in the SSLv2 protocol, SSL_shutdown() will succeed on
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the first call.
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The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
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If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
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handshake step has been finished or an error occurred.
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If the underlying BIO is B<non-blocking>, SSL_shutdown() will also return
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when the underlying BIO could not satisfy the needs of SSL_shutdown()
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to continue the handshake. In this case a call to SSL_get_error() with the
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return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
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taking appropriate action to satisfy the needs of SSL_shutdown().
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The action depends on the underlying BIO. When using a non-blocking socket,
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nothing is to be done, but select() can be used to check for the required
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condition. When using a buffering BIO, like a BIO pair, data must be written
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into or retrieved out of the BIO before being able to continue.
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SSL_shutdown() can be modified to only set the connection to "shutdown"
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state but not actually send the "close notify" alert messages,
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see L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>.
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When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
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and return 1.
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=head1 RETURN VALUES
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The following return values can occur:
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=over 4
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=item 1
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The shutdown was successfully completed. The "close notify" alert was sent
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and the peer's "close notify" alert was received.
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=item 0
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The shutdown is not yet finished. Call SSL_shutdown() for a second time,
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if a bidirectional shutdown shall be performed.
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The output of L<SSL_get_error(3)|SSL_get_error(3)> may be misleading, as an
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erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.
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=item -1
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The shutdown was not successful because a fatal error occurred either
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at the protocol level or a connection failure occurred. It can also occur if
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action is need to continue the operation for non-blocking BIOs.
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Call L<SSL_get_error(3)|SSL_get_error(3)> with the return value B<ret>
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to find out the reason.
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=back
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=head1 SEE ALSO
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L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_connect(3)|SSL_connect(3)>,
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L<SSL_accept(3)|SSL_accept(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>,
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L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>,
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L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)>,
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L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>
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=cut
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