249 lines
8.7 KiB
Groff
249 lines
8.7 KiB
Groff
.\" $NetBSD: SSL_read.3,v 1.3 2002/06/09 16:13:04 itojun Exp $
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.\" Mon Jun 10 00:56:34 2002
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.\" ======================================================================
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.\"
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.IX Title "SSL_read 3"
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.TH SSL_read 3 "0.9.6d" "2002-06-10" "OpenSSL"
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.UC
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.SH "NAME"
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SSL_read \- read bytes from a \s-1TLS/SSL\s0 connection.
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.SH "LIBRARY"
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libcrypto, -lcrypto
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/ssl.h>
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.Ve
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.Vb 1
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\& int SSL_read(SSL *ssl, void *buf, int num);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\fISSL_read()\fR tries to read \fBnum\fR bytes from the specified \fBssl\fR into the
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buffer \fBbuf\fR.
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.SH "NOTES"
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.IX Header "NOTES"
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If necessary, \fISSL_read()\fR will negotiate a \s-1TLS/SSL\s0 session, if
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not already explicitly performed by SSL_connect(3) or
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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 \fISSL_read()\fR operation. The behaviour of \fISSL_read()\fR depends on the
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underlying \s-1BIO\s0.
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.PP
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For the transparent negotiation to succeed, the \fBssl\fR must have been
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initialized to client or server mode. This is being done by calling
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SSL_set_connect_state(3) or \fISSL_set_accept_state()\fR
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before the first call to an \fISSL_read()\fR or SSL_write(3)
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function.
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.PP
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\&\fISSL_read()\fR works based on the \s-1SSL/TLS\s0 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 \fISSL_read()\fR can still be buffered inside the \s-1SSL\s0 layer and will be
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retrieved on the next call to \fISSL_read()\fR. If \fBnum\fR is higher than the
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number of bytes buffered, \fISSL_read()\fR will return with the bytes buffered.
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If no more bytes are in the buffer, \fISSL_read()\fR 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, \fISSL_read()\fR will return reporting success. At most the contents
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of the record will be returned. As the size of an \s-1SSL/TLS\s0 record may exceed
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the maximum packet size of the underlying transport (e.g. \s-1TCP\s0), 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 \fISSL_read()\fR can succeed.
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.PP
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If the underlying \s-1BIO\s0 is \fBblocking\fR, \fISSL_read()\fR 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 \s-1SSL_ERROR_WANT_READ\s0 may occur.
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This behaviour can be controlled with the \s-1SSL_MODE_AUTO_RETRY\s0 flag of the
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SSL_CTX_set_mode(3) call.
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.PP
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If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fISSL_read()\fR will also return
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when the underlying \s-1BIO\s0 could not satisfy the needs of \fISSL_read()\fR
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to continue the operation. In this case a call to
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SSL_get_error(3) with the
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return value of \fISSL_read()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
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\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. As at any time a re-negotiation is possible, a
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call to \fISSL_read()\fR 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 \fISSL_read()\fR. The action depends on the underlying \s-1BIO\s0. When using a
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non-blocking socket, nothing is to be done, but \fIselect()\fR can be used to check
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for the required condition. When using a buffering \s-1BIO\s0, like a \s-1BIO\s0 pair, data
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must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.
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.SH "WARNING"
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.IX Header "WARNING"
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When an \fISSL_read()\fR operation has to be repeated because of
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\&\fB\s-1SSL_ERROR_WANT_READ\s0\fR or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR, it must be repeated
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with the same arguments.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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The following return values can occur:
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.Ip ">0" 4
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.IX Item ">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 \s-1TLS/SSL\s0 connection.
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.Ip "0" 4
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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 \*(L"close notify\*(R" alert sent by the peer (in which case
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the \s-1SSL_RECEIVED_SHUTDOWN\s0 flag in the ssl shutdown state is set
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(see SSL_shutdown(3),
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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 \fISSL_get_error()\fR with the return value \fBret\fR to find out,
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whether an error occurred or the connection was shut down cleanly
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(\s-1SSL_ERROR_ZERO_RETURN\s0).
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.Sp
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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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.Ip "<0" 4
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.IX Item "<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 \fISSL_get_error()\fR with the
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return value \fBret\fR to find out the reason.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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SSL_get_error(3), SSL_write(3),
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SSL_CTX_set_mode(3), SSL_CTX_new(3),
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SSL_connect(3), SSL_accept(3)
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SSL_set_connect_state(3),
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SSL_shutdown(3), SSL_set_shutdown(3),
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ssl(3), openssl_bio(3)
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