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SSL_CTX_SET_OPTIONS(3)		    OpenSSL		SSL_CTX_SET_OPTIONS(3)

NAME
       SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options,
       SSL_clear_options, SSL_CTX_get_options, SSL_get_options,
       SSL_get_secure_renegotiation_support - manipulate SSL options

SYNOPSIS
	#include <openssl/ssl.h>

	long SSL_CTX_set_options(SSL_CTX *ctx, long options);
	long SSL_set_options(SSL *ssl, long options);

	long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
	long SSL_clear_options(SSL *ssl, long options);

	long SSL_CTX_get_options(SSL_CTX *ctx);
	long SSL_get_options(SSL *ssl);

	long SSL_get_secure_renegotiation_support(SSL *ssl);

DESCRIPTION
       SSL_CTX_set_options() adds the options set via bitmask in options to
       ctx.  Options already set before	are not	cleared!

       SSL_set_options() adds the options set via bitmask in options to	ssl.
       Options already set before are not cleared!

       SSL_CTX_clear_options() clears the options set via bitmask in options
       to ctx.

       SSL_clear_options() clears the options set via bitmask in options to
       ssl.

       SSL_CTX_get_options() returns the options set for ctx.

       SSL_get_options() returns the options set for ssl.

       SSL_get_secure_renegotiation_support() indicates	whether	the peer
       supports	secure renegotiation.  Note, this is implemented via a macro.

NOTES
       The behaviour of	the SSL	library	can be changed by setting several
       options.	 The options are coded as bitmasks and can be combined by a
       bitwise or operation (|).

       SSL_CTX_set_options() and SSL_set_options() affect the (external)
       protocol	behaviour of the SSL library. The (internal) behaviour of the
       API can be changed by using the similar SSL_CTX_set_mode(3) and
       SSL_set_mode() functions.

       During a	handshake, the option settings of the SSL object are used.
       When a new SSL object is	created	from a context using SSL_new(),	the
       current option setting is copied. Changes to ctx	do not affect already
       created SSL objects. SSL_clear()	does not affect	the settings.

       The following bug workaround options are	available:

       SSL_OP_SAFARI_ECDHE_ECDSA_BUG
	   Don't prefer	ECDHE-ECDSA ciphers when the client appears to be
	   Safari on OS	X.  OS X 10.8..10.8.3 has broken support for ECDHE-
	   ECDSA ciphers.

       SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
	   Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
	   vulnerability affecting CBC ciphers,	which cannot be	handled	by
	   some	broken SSL implementations.  This option has no	effect for
	   connections using other ciphers.

       SSL_OP_TLSEXT_PADDING
	   Adds	a padding extension to ensure the ClientHello size is never
	   between 256 and 511 bytes in	length.	This is	needed as a workaround
	   for some implementations.

       SSL_OP_ALL
	   All of the above bug	workarounds plus SSL_OP_LEGACY_SERVER_CONNECT
	   as mentioned	below.

       It is usually safe to use SSL_OP_ALL to enable the bug workaround
       options if compatibility	with somewhat broken implementations is
       desired.

       The following modifying options are available:

       SSL_OP_TLS_ROLLBACK_BUG
	   Disable version rollback attack detection.

	   During the client key exchange, the client must send	the same
	   information about acceptable	SSL/TLS	protocol levels	as during the
	   first hello.	Some clients violate this rule by adapting to the
	   server's answer. (Example: the client sends a SSLv2 hello and
	   accepts up to SSLv3.1=TLSv1,	the server only	understands up to
	   SSLv3. In this case the client must still use the same
	   SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with
	   respect to the server's answer and violate the version rollback
	   protection.)

       SSL_OP_CIPHER_SERVER_PREFERENCE
	   When	choosing a cipher, use the server's preferences	instead	of the
	   client preferences. When not	set, the SSL server will always	follow
	   the clients preferences. When set, the SSL/TLS server will choose
	   following its own preferences.

       SSL_OP_NO_SSLv3,	SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, SSL_OP_NO_TLSv1_2,
       SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
	   These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or
	   TLSv1.3 protocol versions with TLS or the DTLSv1, DTLSv1.2 versions
	   with	DTLS, respectively.  As	of OpenSSL 1.1.0, these	options	are
	   deprecated, use SSL_CTX_set_min_proto_version(3) and
	   SSL_CTX_set_max_proto_version(3) instead.

       SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
	   When	performing renegotiation as a server, always start a new
	   session (i.e., session resumption requests are only accepted	in the
	   initial handshake). This option is not needed for clients.

       SSL_OP_NO_COMPRESSION
	   Do not use compression even if it is	supported.

       SSL_OP_NO_QUERY_MTU
	   Do not query	the MTU. Only affects DTLS connections.

       SSL_OP_COOKIE_EXCHANGE
	   Turn	on Cookie Exchange as described	in RFC4347 Section 4.2.1. Only
	   affects DTLS	connections.

       SSL_OP_NO_TICKET
	   SSL/TLS supports two	mechanisms for resuming	sessions: session ids
	   and stateless session tickets.

	   When	using session ids a copy of the	session	information is cached
	   on the server and a unique id is sent to the	client.	When the
	   client wishes to resume it provides the unique id so	that the
	   server can retrieve the session information from its	cache.

	   When	using stateless	session	tickets	the server uses	a session
	   ticket encryption key to encrypt the	session	information. This
	   encrypted data is sent to the client	as a "ticket". When the	client
	   wishes to resume it sends the encrypted data	back to	the server.
	   The server uses its key to decrypt the data and resume the session.
	   In this way the server can operate statelessly - no session
	   information needs to	be cached locally.

	   The TLSv1.3 protocol	only supports tickets and does not directly
	   support session ids.	However	OpenSSL	allows two modes of ticket
	   operation in	TLSv1.3: stateful and stateless. Stateless tickets
	   work	the same way as	in TLSv1.2 and below.  Stateful	tickets	mimic
	   the session id behaviour available in TLSv1.2 and below.  The
	   session information is cached on the	server and the session id is
	   wrapped up in a ticket and sent back	to the client. When the	client
	   wishes to resume, it	presents a ticket in the same way as for
	   stateless tickets. The server can then extract the session id from
	   the ticket and retrieve the session information from	its cache.

	   By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET
	   option will cause stateless tickets to not be issued. In TLSv1.2
	   and below this means	no ticket gets sent to the client at all. In
	   TLSv1.3 a stateful ticket will be sent. This	is a server-side
	   option only.

	   In TLSv1.3 it is possible to	suppress all tickets (stateful and
	   stateless) from being sent by calling SSL_CTX_set_num_tickets(3) or
	   SSL_set_num_tickets(3).

       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
	   Allow legacy	insecure renegotiation between OpenSSL and unpatched
	   clients or servers. See the SECURE RENEGOTIATION section for	more
	   details.

       SSL_OP_LEGACY_SERVER_CONNECT
	   Allow legacy	insecure renegotiation between OpenSSL and unpatched
	   servers only: this option is	currently set by default. See the
	   SECURE RENEGOTIATION	section	for more details.

       SSL_OP_NO_ENCRYPT_THEN_MAC
	   Normally clients and	servers	will transparently attempt to
	   negotiate the RFC7366 Encrypt-then-MAC option on TLS	and DTLS
	   connection.

	   If this option is set, Encrypt-then-MAC is disabled.	Clients	will
	   not propose,	and servers will not accept the	extension.

       SSL_OP_NO_RENEGOTIATION
	   Disable all renegotiation in	TLSv1.2	and earlier. Do	not send
	   HelloRequest	messages, and ignore renegotiation requests via
	   ClientHello.

       SSL_OP_ALLOW_NO_DHE_KEX
	   In TLSv1.3 allow a non-(ec)dhe based	key exchange mode on
	   resumption. This means that there will be no	forward	secrecy	for
	   the resumed session.

       SSL_OP_PRIORITIZE_CHACHA
	   When	SSL_OP_CIPHER_SERVER_PREFERENCE	is set,	temporarily
	   reprioritize	ChaCha20-Poly1305 ciphers to the top of	the server
	   cipher list if a ChaCha20-Poly1305 cipher is	at the top of the
	   client cipher list. This helps those	clients	(e.g. mobile) use
	   ChaCha20-Poly1305 if	that cipher is anywhere	in the server cipher
	   list; but still allows other	clients	to use AES and other ciphers.
	   Requires SSL_OP_CIPHER_SERVER_PREFERENCE.

       SSL_OP_ENABLE_MIDDLEBOX_COMPAT
	   If set then dummy Change Cipher Spec	(CCS) messages are sent	in
	   TLSv1.3. This has the effect	of making TLSv1.3 look more like
	   TLSv1.2 so that middleboxes that do not understand TLSv1.3 will not
	   drop	the connection.	Regardless of whether this option is set or
	   not CCS messages received from the peer will	always be ignored in
	   TLSv1.3. This option	is set by default. To switch it	off use
	   SSL_clear_options().	A future version of OpenSSL may	not set	this
	   by default.

       SSL_OP_NO_ANTI_REPLAY
	   By default, when a server is	configured for early data (i.e.,
	   max_early_data > 0),	OpenSSL	will switch on replay protection. See
	   SSL_read_early_data(3) for a	description of the replay protection
	   feature. Anti-replay	measures are required to comply	with the
	   TLSv1.3 specification. Some applications may	be able	to mitigate
	   the replay risks in other ways and in such cases the	built in
	   OpenSSL functionality is not	required. Those	applications can turn
	   this	feature	off by setting this option. This is a server-side
	   opton only. It is ignored by	clients.

       The following options no	longer have any	effect but their identifiers
       are retained for	compatibility purposes:

       SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
       SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
       SSL_OP_SSLEAY_080_CLIENT_DH_BUG
       SSL_OP_TLS_D5_BUG
       SSL_OP_TLS_BLOCK_PADDING_BUG
       SSL_OP_MSIE_SSLV2_RSA_PADDING
       SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
       SSL_OP_MICROSOFT_SESS_ID_BUG
       SSL_OP_NETSCAPE_CHALLENGE_BUG
       SSL_OP_PKCS1_CHECK_1
       SSL_OP_PKCS1_CHECK_2
       SSL_OP_SINGLE_DH_USE
       SSL_OP_SINGLE_ECDH_USE
       SSL_OP_EPHEMERAL_RSA

SECURE RENEGOTIATION
       OpenSSL always attempts to use secure renegotiation as described	in
       RFC5746.	This counters the prefix attack	described in CVE-2009-3555 and
       elsewhere.

       This attack has far reaching consequences which application writers
       should be aware of. In the description below an implementation
       supporting secure renegotiation is referred to as patched. A server not
       supporting secure renegotiation is referred to as unpatched.

       The following sections describe the operations permitted	by OpenSSL's
       secure renegotiation implementation.

   Patched client and server
       Connections and renegotiation are always	permitted by OpenSSL
       implementations.

   Unpatched client and	patched	OpenSSL	server
       The initial connection succeeds but client renegotiation	is denied by
       the server with a no_renegotiation warning alert	if TLS v1.0 is used or
       a fatal handshake_failure alert in SSL v3.0.

       If the patched OpenSSL server attempts to renegotiate a fatal
       handshake_failure alert is sent.	This is	because	the server code	may be
       unaware of the unpatched	nature of the client.

       If the option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then
       renegotiation always succeeds.

   Patched OpenSSL client and unpatched	server.
       If the option SSL_OP_LEGACY_SERVER_CONNECT or
       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION	is set then initial
       connections and renegotiation between patched OpenSSL clients and
       unpatched servers succeeds. If neither option is	set then initial
       connections to unpatched	servers	will fail.

       The option SSL_OP_LEGACY_SERVER_CONNECT is currently set	by default
       even though it has security implications: otherwise it would be
       impossible to connect to	unpatched servers (i.e.	all of them initially)
       and this	is clearly not acceptable. Renegotiation is permitted because
       this does not add any additional	security issues: during	an attack
       clients do not see any renegotiations anyway.

       As more servers become patched the option SSL_OP_LEGACY_SERVER_CONNECT
       will not	be set by default in a future version of OpenSSL.

       OpenSSL client applications wishing to ensure they can connect to
       unpatched servers should	always set SSL_OP_LEGACY_SERVER_CONNECT

       OpenSSL client applications that	want to	ensure they can	not connect to
       unpatched servers (and thus avoid any security issues) should always
       clear SSL_OP_LEGACY_SERVER_CONNECT using	SSL_CTX_clear_options()	or
       SSL_clear_options().

       The difference between the SSL_OP_LEGACY_SERVER_CONNECT and
       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION	options	is that
       SSL_OP_LEGACY_SERVER_CONNECT enables initial connections	and secure
       renegotiation between OpenSSL clients and unpatched servers only, while
       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION	allows initial connections and
       renegotiation between OpenSSL and unpatched clients or servers.

RETURN VALUES
       SSL_CTX_set_options() and SSL_set_options() return the new options
       bitmask after adding options.

       SSL_CTX_clear_options() and SSL_clear_options() return the new options
       bitmask after clearing options.

       SSL_CTX_get_options() and SSL_get_options() return the current bitmask.

       SSL_get_secure_renegotiation_support() returns 1	is the peer supports
       secure renegotiation and	0 if it	does not.

SEE ALSO
       ssl(7), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3),
       SSL_CTX_set_min_proto_version(3), dhparam(1)

HISTORY
       The attempt to always try to use	secure renegotiation was added in
       Openssl 0.9.8m.

       SSL_OP_PRIORITIZE_CHACHA	and SSL_OP_NO_RENEGOTIATION were added in
       OpenSSL 1.1.1.

COPYRIGHT
       Copyright 2001-2018 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
       <https://www.openssl.org/source/license.html>.

1.1.1a				  2018-11-20		SSL_CTX_SET_OPTIONS(3)

NAME | SYNOPSIS | DESCRIPTION | NOTES | SECURE RENEGOTIATION | RETURN VALUES | SEE ALSO | HISTORY | COPYRIGHT

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