146 lines
6 KiB
Text
146 lines
6 KiB
Text
=pod
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=head1 NAME
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SSL_read_ex, SSL_read - read bytes from a TLS/SSL connection
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *read);
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int SSL_read(SSL *ssl, void *buf, int num);
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int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *read);
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int SSL_peek(SSL *ssl, void *buf, int num);
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=head1 DESCRIPTION
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SSL_read_ex() and SSL_read() try to read B<num> bytes from the specified B<ssl>
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into the buffer B<buf>. On success SSL_read_ex() will store the number of bytes
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actually read in B<*read>.
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SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read()
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respectively except no bytes are actually removed from the underlying BIO during
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the read, so that a subsequent call to SSL_read_ex() or SSL_read() will yield
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at least the same bytes.
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=head1 NOTES
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In the paragraphs below a "read function" is defined as one of SSL_read_ex(),
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SSL_read(), SSL_peek_ex() or SSL_peek().
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If necessary, a read function will negotiate a TLS/SSL session, if not already
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explicitly performed by L<SSL_connect(3)> or L<SSL_accept(3)>. If the
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peer requests a re-negotiation, it will be performed transparently during
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the read function operation. The behaviour of the read functions depends on the
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underlying BIO.
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For the transparent negotiation to succeed, the B<ssl> must have been
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initialized to client or server mode. This is being done by calling
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L<SSL_set_connect_state(3)> or SSL_set_accept_state() before the first
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invocation of a read function.
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The read functions work based on the SSL/TLS records. The data are received in
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records (with a maximum record size of 16kB). Only when a record has been
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completely received, can it be processed (decryption and check of integrity).
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Therefore data that was not retrieved at the last read call can still be
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buffered inside the SSL layer and will be retrieved on the next read
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call. If B<num> is higher than the number of bytes buffered then the read
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functions will return with the bytes buffered. If no more bytes are in the
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buffer, the read functions will trigger the processing of the next record.
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Only when the record has been received and processed completely will the read
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functions return reporting success. At most the contents of the record will
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be returned. As the size of an SSL/TLS record may exceed the maximum packet size
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of the underlying transport (e.g. TCP), it may be necessary to read several
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packets from the transport layer before the record is complete and the read call
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can succeed.
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If the underlying BIO is B<blocking>, a read function will only return once the
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read operation has been finished or an error occurred, except when a
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renegotiation takes place, in which case a SSL_ERROR_WANT_READ may occur. This
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behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
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L<SSL_CTX_set_mode(3)> call.
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If the underlying BIO is B<non-blocking>, a read function will also return when
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the underlying BIO could not satisfy the needs of the function to continue the
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operation. In this case a call to L<SSL_get_error(3)> with the
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return value of the read function will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
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a read function can also cause write operations! The calling process then must
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repeat the call after taking appropriate action to satisfy the needs of the read
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function. The action depends on the underlying BIO. When using a non-blocking
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socket, nothing is to be done, but select() can be used to check for the
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required condition. When using a buffering BIO, like a BIO pair, data must be
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written into or retrieved out of the BIO before being able to continue.
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L<SSL_pending(3)> can be used to find out whether there
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are buffered bytes available for immediate retrieval. In this case
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the read function can be called without blocking or actually receiving
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new data from the underlying socket.
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=head1 WARNING
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When a read function operation has to be repeated because L<SSL_get_error(3)>
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returned B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
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with the same arguments.
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=head1 RETURN VALUES
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SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure.
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Success means that 1 or more application data bytes have been read from the SSL
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connection. Failure means that no bytes could be read from the SSL connection.
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Failures can be retryable (e.g. we are waiting for more bytes to
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be delivered by the network) or non-retryable (e.g. a fatal network error). In
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the event of a failure call L<SSL_get_error(3)> to find out the reason which
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indicates whether the call is retryable or not.
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For SSL_read() and SSL_peek() the following return values can occur:
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=over 4
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=item E<gt>0
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The read operation was successful; the return value is the number of
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bytes actually read from the TLS/SSL connection.
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=item Z<>0
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The read operation was not successful. The reason may either be a clean
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shutdown due to a "close notify" alert sent by the peer (in which case
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the SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is set
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(see L<SSL_shutdown(3)>,
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L<SSL_set_shutdown(3)>). It is also possible, that
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the peer simply shut down the underlying transport and the shutdown is
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incomplete. Call SSL_get_error() with the return value B<ret> to find out,
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whether an error occurred or the connection was shut down cleanly
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(SSL_ERROR_ZERO_RETURN).
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=item E<lt>0
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The read operation was not successful, because either an error occurred
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or action must be taken by the calling process. Call SSL_get_error() with the
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return value B<ret> 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)>, L<SSL_write_ex(3)>,
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L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>,
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L<SSL_connect(3)>, L<SSL_accept(3)>
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L<SSL_set_connect_state(3)>,
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L<SSL_pending(3)>,
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L<SSL_shutdown(3)>, L<SSL_set_shutdown(3)>,
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L<ssl(3)>, L<bio(3)>
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=head1 COPYRIGHT
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Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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