124 lines
5 KiB
Text
124 lines
5 KiB
Text
=pod
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=head1 NAME
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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(SSL *ssl, void *buf, int num);
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=head1 DESCRIPTION
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SSL_read() tries to read B<num> bytes from the specified B<ssl> into the
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buffer B<buf>.
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=head1 NOTES
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If necessary, SSL_read() will negotiate a TLS/SSL session, if
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not already explicitly performed by L<SSL_connect(3)|SSL_connect(3)> or
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L<SSL_accept(3)|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 SSL_read() operation. The behaviour of SSL_read() 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)|SSL_set_connect_state(3)> or SSL_set_accept_state()
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before the first call to an SSL_read() or L<SSL_write(3)|SSL_write(3)>
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function.
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SSL_read() works based on the SSL/TLS records. The data are received in
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records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
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record has been completely received, it can be processed (decryption and
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check of integrity). Therefore data that was not retrieved at the last
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call of SSL_read() can still be buffered inside the SSL layer and will be
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retrieved on the next call to SSL_read(). If B<num> is higher than the
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number of bytes buffered, SSL_read() will return with the bytes buffered.
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If no more bytes are in the buffer, SSL_read() will trigger the processing
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of the next record. Only when the record has been received and processed
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completely, SSL_read() will return reporting success. At most the contents
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of the record will be returned. As the size of an SSL/TLS record may exceed
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the maximum packet size of the underlying transport (e.g. TCP), it may
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be necessary to read several packets from the transport layer before the
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record is complete and SSL_read() can succeed.
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If the underlying BIO is B<blocking>, SSL_read() 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 take place, in which case a SSL_ERROR_WANT_READ may occur.
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This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
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L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> call.
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If the underlying BIO is B<non-blocking>, SSL_read() will also return
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when the underlying BIO could not satisfy the needs of SSL_read()
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to continue the operation. In this case a call to
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L<SSL_get_error(3)|SSL_get_error(3)> with the
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return value of SSL_read() 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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call to SSL_read() can also cause write operations! The calling process
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then must repeat the call after taking appropriate action to satisfy the
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needs of SSL_read(). The action depends on the underlying BIO. When using a
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non-blocking socket, nothing is to be done, but select() can be used to check
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for the required condition. When using a buffering BIO, like a BIO pair, data
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must be written into or retrieved out of the BIO before being able to continue.
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L<SSL_pending(3)|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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SSL_read() can be called without blocking or actually receiving new
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data from the underlying socket.
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=head1 WARNING
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When an SSL_read() operation has to be repeated because of
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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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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 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)|SSL_shutdown(3)>,
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L<SSL_set_shutdown(3)|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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SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
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only be detected, whether the underlying connection was closed. It cannot
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be checked, whether the closure was initiated by the peer or by something
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else.
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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)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>,
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L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>,
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L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)>
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L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>,
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L<SSL_pending(3)|SSL_pending(3)>,
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L<SSL_shutdown(3)|SSL_shutdown(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>,
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L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>
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=cut
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