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       SSL_CTX_set_mode, SSL_CTX_clear_mode, SSL_set_mode, SSL_clear_mode,
       SSL_CTX_get_mode, SSL_get_mode -	manipulate SSL engine mode

	#include <openssl/ssl.h>

	long SSL_CTX_set_mode(SSL_CTX *ctx, long mode);
	long SSL_CTX_clear_mode(SSL_CTX	*ctx, long mode);
	long SSL_set_mode(SSL *ssl, long mode);
	long SSL_clear_mode(SSL	*ssl, long mode);

	long SSL_CTX_get_mode(SSL_CTX *ctx);
	long SSL_get_mode(SSL *ssl);

       SSL_CTX_set_mode() adds the mode	set via	bit mask in mode to ctx.
       Options already set before are not cleared.  SSL_CTX_clear_mode()
       removes the mode	set via	bit mask in mode from ctx.

       SSL_set_mode() adds the mode set	via bit	mask in	mode to	ssl.  Options
       already set before are not cleared.  SSL_clear_mode() removes the mode
       set via bit mask	in mode	from ssl.

       SSL_CTX_get_mode() returns the mode set for ctx.

       SSL_get_mode() returns the mode set for ssl.

       The following mode changes are available:

	   Allow SSL_write_ex(..., n, &r) to return with 0 < r < n (i.e.
	   report success when just a single record has	been written). This
	   works in a similar way for SSL_write(). When	not set	(the default),
	   SSL_write_ex() or SSL_write() will only report success once the
	   complete chunk was written. Once SSL_write_ex() or SSL_write()
	   returns successful, r bytes have been written and the next call to
	   SSL_write_ex() or SSL_write() must only send	the n-r	bytes left,
	   imitating the behaviour of write().

	   Make	it possible to retry SSL_write_ex() or SSL_write() with
	   changed buffer location (the	buffer contents	must stay the same).
	   This	is not the default to avoid the	misconception that nonblocking
	   SSL_write() behaves like nonblocking	write().

	   During normal operations, non-application data records might	need
	   to be sent or received that the application is not aware of.	 If a
	   non-application data	record was processed, SSL_read_ex(3) and
	   SSL_read(3) can return with a failure and indicate the need to
	   retry with SSL_ERROR_WANT_READ.  If such a non-application data
	   record was processed, the flag SSL_MODE_AUTO_RETRY causes it	to try
	   to process the next record instead of returning.

	   In a	nonblocking environment	applications must be prepared to
	   handle incomplete read/write	operations.  Setting
	   SSL_MODE_AUTO_RETRY for a nonblocking BIO will process non-
	   application data records until either no more data is available or
	   an application data record has been processed.

	   In a	blocking environment, applications are not always prepared to
	   deal	with the functions returning intermediate reports such as
	   retry requests, and setting the SSL_MODE_AUTO_RETRY flag will cause
	   the functions to only return	after successfully processing an
	   application data record or a	failure.

	   Turning off SSL_MODE_AUTO_RETRY can be useful with blocking BIOs in
	   case	they are used in combination with something like select() or
	   poll().  Otherwise the call to SSL_read() or	SSL_read_ex() might
	   hang	when a non-application record was sent and no application data
	   was sent.

	   When	we no longer need a read buffer	or a write buffer for a	given
	   SSL,	then release the memory	we were	using to hold it.  Using this
	   flag	can save around	34k per	idle SSL connection.  This flag	has no
	   effect on SSL v2 connections, or on DTLS connections.

	   Send	TLS_FALLBACK_SCSV in the ClientHello.  To be set only by
	   applications	that reconnect with a downgraded protocol version; see
	   draft-ietf-tls-downgrade-scsv-00 for	details.

	   DO NOT ENABLE THIS if your application attempts a normal handshake.
	   Only	use this in explicit fallback retries, following the guidance
	   in draft-ietf-tls-downgrade-scsv-00.

	   Enable asynchronous processing. TLS I/O operations may indicate a
	   retry with SSL_ERROR_WANT_ASYNC with	this mode set if an
	   asynchronous	capable	engine is used to perform cryptographic
	   operations. See SSL_get_error(3).

	   Older versions of OpenSSL had a bug in the computation of the label
	   length used for computing the endpoint-pair shared secret. The bug
	   was that the	terminating zero was included in the length of the
	   label. Setting this option enables this behaviour to	allow
	   interoperability with such broken implementations. Please note that
	   setting this	option breaks interoperability with correct
	   implementations. This option	only applies to	DTLS over SCTP.

	   Disable the use of the kernel TLS egress data-path.	By default
	   kernel TLS is enabled if it is supported by the negotiated
	   ciphersuites	and extensions and OpenSSL has been compiled with
	   support for it.  The	kernel TLS data-path implements	the record
	   layer, and the crypto algorithm. The	kernel will utilize the	best
	   hardware available for crypto. Using	the kernel data-path should
	   reduce the memory footprint of OpenSSL because no buffering is
	   required. Also, the throughput should improve because data copy is
	   avoided when	user data is encrypted into kernel memory instead of
	   the usual encrypt than copy to kernel.

	   Kernel TLS might not	support	all the	features of OpenSSL. For
	   instance, renegotiation, and	setting	the maximum fragment size is
	   not possible	as of Linux 4.20.

       All modes are off by default except for SSL_MODE_AUTO_RETRY which is on
       by default since	1.1.1.

       SSL_CTX_set_mode() and SSL_set_mode() return the	new mode bit mask
       after adding mode.

       SSL_CTX_get_mode() and SSL_get_mode() return the	current	bit mask.

       ssl(7), SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or SSL_write(3),

       SSL_MODE_ASYNC was added	in OpenSSL 1.1.0.  SSL_MODE_NO_KTLS_TX was
       first added to OpenSSL 3.0.0.

       Copyright 2001-2020 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (the "License").  You	may not	use
       this file except	in compliance with the License.	 You can obtain	a copy
       in the file LICENSE in the source distribution or at

1.1.1k				  2021-03-25		   SSL_CTX_SET_MODE(3)


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