forked from cory/tildefriends
276 lines
11 KiB
Groff
276 lines
11 KiB
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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 "2020-04-21" "1.1.1g" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek \&\- read bytes from a TLS/SSL connection
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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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\&
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\& int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
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\& int SSL_read(SSL *ssl, void *buf, int num);
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\&
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\& int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
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\& int SSL_peek(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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\&\fBSSL_read_ex()\fR and \fBSSL_read()\fR try to read \fBnum\fR bytes from the specified \fBssl\fR
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into the buffer \fBbuf\fR. On success \fBSSL_read_ex()\fR will store the number of bytes
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actually read in \fB*readbytes\fR.
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.PP
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\&\fBSSL_peek_ex()\fR and \fBSSL_peek()\fR are identical to \fBSSL_read_ex()\fR and \fBSSL_read()\fR
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respectively except no bytes are actually removed from the underlying \s-1BIO\s0 during
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the read, so that a subsequent call to \fBSSL_read_ex()\fR or \fBSSL_read()\fR will yield
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at least the same bytes.
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.SH "NOTES"
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.IX Header "NOTES"
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In the paragraphs below a \*(L"read function\*(R" is defined as one of \fBSSL_read_ex()\fR,
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\&\fBSSL_read()\fR, \fBSSL_peek_ex()\fR or \fBSSL_peek()\fR.
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.PP
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If necessary, a read function will negotiate a \s-1TLS/SSL\s0 session, if not already
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explicitly performed by \fBSSL_connect\fR\|(3) or \fBSSL_accept\fR\|(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 \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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\&\fBSSL_set_connect_state\fR\|(3) or \fBSSL_set_accept_state()\fR before the first
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invocation of a read function.
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.PP
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The read functions work 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). 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 \s-1SSL\s0 layer and will be retrieved on the next read
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call. If \fBnum\fR 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 one record will
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be returned. As the size of an \s-1SSL/TLS\s0 record may exceed the maximum packet size
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of the underlying transport (e.g. \s-1TCP\s0), 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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.PP
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If \fB\s-1SSL_MODE_AUTO_RETRY\s0\fR has been switched off and a non-application data
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record has been processed, the read function can return and set the error to
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\&\fB\s-1SSL_ERROR_WANT_READ\s0\fR.
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In this case there might still be unprocessed data available in the \fB\s-1BIO\s0\fR.
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If read ahead was set using \fBSSL_CTX_set_read_ahead\fR\|(3), there might also still
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be unprocessed data available in the \fB\s-1SSL\s0\fR.
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This behaviour can be controlled using the \fBSSL_CTX_set_mode\fR\|(3) call.
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.PP
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If the underlying \s-1BIO\s0 is \fBblocking\fR, 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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non-application data record has been processed and \fB\s-1SSL_MODE_AUTO_RETRY\s0\fR is
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not set.
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Note that if \fB\s-1SSL_MODE_AUTO_RETRY\s0\fR is set and only non-application data is
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available the call will hang.
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.PP
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If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, a read function will also return when
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the underlying \s-1BIO\s0 could not satisfy the needs of the function to continue the
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operation.
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In this case a call to \fBSSL_get_error\fR\|(3) with the
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return value of the read function will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
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\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR.
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As at any time it's possible that non-application data needs to be sent,
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a read function can also cause write operations.
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The calling process then must repeat the call after taking appropriate action
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to satisfy the needs of the read function.
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The action depends on the underlying \s-1BIO.\s0
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When using a non-blocking socket, nothing is to be done, but \fBselect()\fR can be
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used to check for the required condition.
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When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data must be written into or
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retrieved out of the \s-1BIO\s0 before being able to continue.
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.PP
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\&\fBSSL_pending\fR\|(3) can be used to find out whether there
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are buffered bytes available for immediate retrieval.
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In this case the read function can be called without blocking or actually
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receiving new data from the underlying socket.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fBSSL_read_ex()\fR and \fBSSL_peek_ex()\fR 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 \s-1SSL\s0
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connection.
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Failure means that no bytes could be read from the \s-1SSL\s0 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).
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In the event of a failure call \fBSSL_get_error\fR\|(3) to find out the reason which
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indicates whether the call is retryable or not.
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.PP
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For \fBSSL_read()\fR and \fBSSL_peek()\fR 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.
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The return value is the number of bytes actually read from the \s-1TLS/SSL\s0
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connection.
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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 the connection was closed,
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an error occurred or action must be taken by the calling process.
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Call \fBSSL_get_error\fR\|(3) with the return value \fBret\fR to find out the reason.
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.Sp
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Old documentation indicated a difference between 0 and \-1, and that \-1 was
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retryable.
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You should instead call \fBSSL_get_error()\fR to find out if it's retryable.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fBSSL_get_error\fR\|(3), \fBSSL_write_ex\fR\|(3),
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\&\fBSSL_CTX_set_mode\fR\|(3), \fBSSL_CTX_new\fR\|(3),
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\&\fBSSL_connect\fR\|(3), \fBSSL_accept\fR\|(3)
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\&\fBSSL_set_connect_state\fR\|(3),
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\&\fBSSL_pending\fR\|(3),
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\&\fBSSL_shutdown\fR\|(3), \fBSSL_set_shutdown\fR\|(3),
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\&\fBssl\fR\|(7), \fBbio\fR\|(7)
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.SH "HISTORY"
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.IX Header "HISTORY"
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The \fBSSL_read_ex()\fR and \fBSSL_peek_ex()\fR functions were added in OpenSSL 1.1.1.
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2000\-2019 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the OpenSSL license (the \*(L"License\*(R"). 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 \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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