doc/man3/SSL_CTX_set_options.pod - third_party/openssl - Git at Google

 =pod

 =head1 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

 =head1 SYNOPSIS

  #include <openssl/ssl.h>

  uint64_t SSL_CTX_set_options(SSL_CTX *ctx, uint64_t options);
  uint64_t SSL_set_options(SSL *ssl, uint64_t options);

  uint64_t SSL_CTX_clear_options(SSL_CTX *ctx, uint64_t options);
  uint64_t SSL_clear_options(SSL *ssl, uint64_t options);

  uint64_t SSL_CTX_get_options(const SSL_CTX *ctx);
  uint64_t SSL_get_options(const SSL *ssl);

  long SSL_get_secure_renegotiation_support(SSL *ssl);

 =head1 DESCRIPTION

 SSL_CTX_set_options() adds the options set via bit-mask in B<options> to B<ctx>.
 Options already set before are not cleared!

 SSL_set_options() adds the options set via bit-mask in B<options> to B<ssl>.
 Options already set before are not cleared!

 SSL_CTX_clear_options() clears the options set via bit-mask in B<options>
 to B<ctx>.

 SSL_clear_options() clears the options set via bit-mask in B<options> to B<ssl>.

 SSL_CTX_get_options() returns the options set for B<ctx>.

 SSL_get_options() returns the options set for B<ssl>.

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

 =head1 NOTES

 The behaviour of the SSL library can be changed by setting several options.
 The options are coded as bit-masks and can be combined by a bitwise B<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
 L<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 B<ctx> do not affect already created
 SSL objects. SSL_clear() does not affect the settings.

 The following B<bug workaround> options are available:

 =over 4

 =item SSL_OP_CRYPTOPRO_TLSEXT_BUG

 Add server-hello extension from the early version of cryptopro draft
 when GOST ciphersuite is negotiated. Required for interoperability with CryptoPro
 CSP 3.x.

 =item 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.

 =item 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.

 =item 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.

 =item SSL_OP_ALL

 All of the above bug workarounds.

 =back

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

 The following B<modifying> options are available:

 =over 4

 =item SSL_OP_ALLOW_CLIENT_RENEGOTIATION

 Client-initiated renegotiation is disabled by default. Use
 this option to enable it.

 =item 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.

 =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION

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

 =item 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.

 =item SSL_OP_CISCO_ANYCONNECT

 Use Cisco's version identifier of DTLS_BAD_VER when establishing a DTLSv1
 connection. Only available when using the deprecated DTLSv1_client_method() API.

 =item SSL_OP_CLEANSE_PLAINTEXT

 By default TLS connections keep a copy of received plaintext
 application data in a static buffer until it is overwritten by the
 next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT
 deciphered application data is cleansed by calling OPENSSL_cleanse(3)
 after passing data to the application. Data is also cleansed when
 releasing the connection (e.g. L<SSL_free(3)>).

 Since OpenSSL only cleanses internal buffers, the application is still
 responsible for cleansing all other buffers. Most notably, this
 applies to buffers passed to functions like L<SSL_read(3)>,
 L<SSL_peek(3)> but also like L<SSL_write(3)>.

 =item SSL_OP_COOKIE_EXCHANGE

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

 =item SSL_OP_DISABLE_TLSEXT_CA_NAMES

 Disable TLS Extension CA Names. You may want to disable it for security reasons
 or for compatibility with some Windows TLS implementations crashing when this
 extension is larger than 1024 bytes.

 =item SSL_OP_ENABLE_KTLS

 Enable the use of kernel TLS. In order to benefit from kernel TLS OpenSSL must
 have been compiled with support for it, and it must be supported by the
 negotiated ciphersuites and extensions. The specific ciphersuites and extensions
 that are supported may vary by platform and kernel version.

 The kernel TLS data-path implements the record layer, and the encryption
 algorithm. The kernel will utilize the best hardware
 available for encryption. 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 then 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.

 Note that with kernel TLS enabled some cryptographic operations are performed
 by the kernel directly and not via any available OpenSSL Providers. This might
 be undesirable if, for example, the application requires all cryptographic
 operations to be performed by the FIPS provider.

 =item 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.

 =item SSL_OP_IGNORE_UNEXPECTED_EOF

 Some TLS implementations do not send the mandatory close_notify alert on
 shutdown. If the application tries to wait for the close_notify alert but the
 peer closes the connection without sending it, an error is generated. When this
 option is enabled the peer does not need to send the close_notify alert and a
 closed connection will be treated as if the close_notify alert was received.

 You should only enable this option if the protocol running over TLS
 can detect a truncation attack itself, and that the application is checking for
 that truncation attack.

 For more information on shutting down a connection, see L<SSL_shutdown(3)>.

 =item SSL_OP_LEGACY_SERVER_CONNECT

 Allow legacy insecure renegotiation between OpenSSL and unpatched servers
 B<only>. See the B<SECURE RENEGOTIATION> section for more details.

 =item 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 L<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.

 =item SSL_OP_NO_COMPRESSION

 Do not use compression even if it is supported. This option is set by default.
 To switch it off use SSL_clear_options().

 =item 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.

 =item SSL_OP_NO_EXTENDED_MASTER_SECRET

 Normally clients and servers will transparently attempt to negotiate the
 RFC7627 Extended Master Secret option on TLS and DTLS connection.

 If this option is set, Extended Master Secret is disabled. Clients will
 not propose, and servers will not accept the extension.

 =item SSL_OP_NO_QUERY_MTU

 Do not query the MTU. Only affects DTLS connections.

 =item SSL_OP_NO_RENEGOTIATION

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

 =item 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.

 =item 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
 L<SSL_CTX_set_min_proto_version(3)> and
 L<SSL_CTX_set_max_proto_version(3)> instead.

 =item 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 L<SSL_CTX_set_num_tickets(3)> or
 L<SSL_set_num_tickets(3)>.

 =item 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 B<SSL_OP_CIPHER_SERVER_PREFERENCE>.

 =item 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.)

 =back

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

 =over 4

 =item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG

 =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER

 =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG

 =item SSL_OP_TLS_D5_BUG

 =item SSL_OP_TLS_BLOCK_PADDING_BUG

 =item SSL_OP_MSIE_SSLV2_RSA_PADDING

 =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG

 =item SSL_OP_MICROSOFT_SESS_ID_BUG

 =item SSL_OP_NETSCAPE_CHALLENGE_BUG

 =item SSL_OP_PKCS1_CHECK_1

 =item SSL_OP_PKCS1_CHECK_2

 =item SSL_OP_SINGLE_DH_USE

 =item SSL_OP_SINGLE_ECDH_USE

 =item SSL_OP_EPHEMERAL_RSA

 =back

 =head1 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 I<patched>. A server not supporting secure
 renegotiation is referred to as I<unpatched>.

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

 =head2 Patched client and server

 Connections and renegotiation are always permitted by OpenSSL implementations.

 =head2 Unpatched client and patched OpenSSL server

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

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

 If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
 renegotiation B<always> succeeds.

 =head2 Patched OpenSSL client and unpatched server

 If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
 B<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.

 Setting the option B<SSL_OP_LEGACY_SERVER_CONNECT> has security implications;
 clients that are willing to connect to servers that do not implement
 RFC 5746 secure renegotiation are subject to attacks such as
 CVE-2009-3555.

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

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

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

 =head1 RETURN VALUES

 SSL_CTX_set_options() and SSL_set_options() return the new options bit-mask
 after adding B<options>.

 SSL_CTX_clear_options() and SSL_clear_options() return the new options bit-mask
 after clearing B<options>.

 SSL_CTX_get_options() and SSL_get_options() return the current bit-mask.

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

 =head1 SEE ALSO

 L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>, L<SSL_shutdown(3)>
 L<SSL_CTX_set_tmp_dh_callback(3)>,
 L<SSL_CTX_set_min_proto_version(3)>,
 L<openssl-dhparam(1)>

 =head1 HISTORY

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

 The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options
 were added in OpenSSL 1.1.1.

 The B<SSL_OP_NO_EXTENDED_MASTER_SECRET> and B<SSL_OP_IGNORE_UNEXPECTED_EOF>
 options were added in OpenSSL 3.0.

 The B<SSL_OP_> constants and the corresponding parameter and return values
 of the affected functions were changed to C<uint64_t> type in OpenSSL 3.0.
 For that reason it is no longer possible use the B<SSL_OP_> macro values
 in preprocessor C<#if> conditions. However it is still possible to test
 whether these macros are defined or not.

 =head1 COPYRIGHT

 Copyright 2001-2021 The OpenSSL Project Authors. All Rights Reserved.

 Licensed under the Apache License 2.0 (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
 L<https://www.openssl.org/source/license.html>.

 =cut
	=pod

	=head1 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

	=head1 SYNOPSIS

	#include <openssl/ssl.h>

	uint64_t SSL_CTX_set_options(SSL_CTX *ctx, uint64_t options);
	uint64_t SSL_set_options(SSL *ssl, uint64_t options);

	uint64_t SSL_CTX_clear_options(SSL_CTX *ctx, uint64_t options);
	uint64_t SSL_clear_options(SSL *ssl, uint64_t options);

	uint64_t SSL_CTX_get_options(const SSL_CTX *ctx);
	uint64_t SSL_get_options(const SSL *ssl);

	long SSL_get_secure_renegotiation_support(SSL *ssl);

	=head1 DESCRIPTION

	SSL_CTX_set_options() adds the options set via bit-mask in B<options> to B<ctx>.
	Options already set before are not cleared!

	SSL_set_options() adds the options set via bit-mask in B<options> to B<ssl>.
	Options already set before are not cleared!

	SSL_CTX_clear_options() clears the options set via bit-mask in B<options>
	to B<ctx>.

	SSL_clear_options() clears the options set via bit-mask in B<options> to B<ssl>.

	SSL_CTX_get_options() returns the options set for B<ctx>.

	SSL_get_options() returns the options set for B<ssl>.

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

	=head1 NOTES

	The behaviour of the SSL library can be changed by setting several options.
	The options are coded as bit-masks and can be combined by a bitwise B<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
	L<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 B<ctx> do not affect already created
	SSL objects. SSL_clear() does not affect the settings.

	The following B<bug workaround> options are available:

	=over 4

	=item SSL_OP_CRYPTOPRO_TLSEXT_BUG

	Add server-hello extension from the early version of cryptopro draft
	when GOST ciphersuite is negotiated. Required for interoperability with CryptoPro
	CSP 3.x.

	=item 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.

	=item 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.

	=item 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.

	=item SSL_OP_ALL

	All of the above bug workarounds.

	=back

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

	The following B<modifying> options are available:

	=over 4

	=item SSL_OP_ALLOW_CLIENT_RENEGOTIATION

	Client-initiated renegotiation is disabled by default. Use
	this option to enable it.

	=item 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.

	=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION

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

	=item 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.

	=item SSL_OP_CISCO_ANYCONNECT

	Use Cisco's version identifier of DTLS_BAD_VER when establishing a DTLSv1
	connection. Only available when using the deprecated DTLSv1_client_method() API.

	=item SSL_OP_CLEANSE_PLAINTEXT

	By default TLS connections keep a copy of received plaintext
	application data in a static buffer until it is overwritten by the
	next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT
	deciphered application data is cleansed by calling OPENSSL_cleanse(3)
	after passing data to the application. Data is also cleansed when
	releasing the connection (e.g. L<SSL_free(3)>).

	Since OpenSSL only cleanses internal buffers, the application is still
	responsible for cleansing all other buffers. Most notably, this
	applies to buffers passed to functions like L<SSL_read(3)>,
	L<SSL_peek(3)> but also like L<SSL_write(3)>.

	=item SSL_OP_COOKIE_EXCHANGE

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

	=item SSL_OP_DISABLE_TLSEXT_CA_NAMES

	Disable TLS Extension CA Names. You may want to disable it for security reasons
	or for compatibility with some Windows TLS implementations crashing when this
	extension is larger than 1024 bytes.

	=item SSL_OP_ENABLE_KTLS

	Enable the use of kernel TLS. In order to benefit from kernel TLS OpenSSL must
	have been compiled with support for it, and it must be supported by the
	negotiated ciphersuites and extensions. The specific ciphersuites and extensions
	that are supported may vary by platform and kernel version.

	The kernel TLS data-path implements the record layer, and the encryption
	algorithm. The kernel will utilize the best hardware
	available for encryption. 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 then 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.

	Note that with kernel TLS enabled some cryptographic operations are performed
	by the kernel directly and not via any available OpenSSL Providers. This might
	be undesirable if, for example, the application requires all cryptographic
	operations to be performed by the FIPS provider.

	=item 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.

	=item SSL_OP_IGNORE_UNEXPECTED_EOF

	Some TLS implementations do not send the mandatory close_notify alert on
	shutdown. If the application tries to wait for the close_notify alert but the
	peer closes the connection without sending it, an error is generated. When this
	option is enabled the peer does not need to send the close_notify alert and a
	closed connection will be treated as if the close_notify alert was received.

	You should only enable this option if the protocol running over TLS
	can detect a truncation attack itself, and that the application is checking for
	that truncation attack.

	For more information on shutting down a connection, see L<SSL_shutdown(3)>.

	=item SSL_OP_LEGACY_SERVER_CONNECT

	Allow legacy insecure renegotiation between OpenSSL and unpatched servers
	B<only>. See the B<SECURE RENEGOTIATION> section for more details.

	=item 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 L<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.

	=item SSL_OP_NO_COMPRESSION

	Do not use compression even if it is supported. This option is set by default.
	To switch it off use SSL_clear_options().

	=item 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.

	=item SSL_OP_NO_EXTENDED_MASTER_SECRET

	Normally clients and servers will transparently attempt to negotiate the
	RFC7627 Extended Master Secret option on TLS and DTLS connection.

	If this option is set, Extended Master Secret is disabled. Clients will
	not propose, and servers will not accept the extension.

	=item SSL_OP_NO_QUERY_MTU

	Do not query the MTU. Only affects DTLS connections.

	=item SSL_OP_NO_RENEGOTIATION

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

	=item 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.

	=item 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
	L<SSL_CTX_set_min_proto_version(3)> and
	L<SSL_CTX_set_max_proto_version(3)> instead.

	=item 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 L<SSL_CTX_set_num_tickets(3)> or
	L<SSL_set_num_tickets(3)>.

	=item 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 B<SSL_OP_CIPHER_SERVER_PREFERENCE>.

	=item 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.)

	=back

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

	=over 4

	=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG

	=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER

	=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG

	=item SSL_OP_TLS_D5_BUG

	=item SSL_OP_TLS_BLOCK_PADDING_BUG

	=item SSL_OP_MSIE_SSLV2_RSA_PADDING

	=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG

	=item SSL_OP_MICROSOFT_SESS_ID_BUG

	=item SSL_OP_NETSCAPE_CHALLENGE_BUG

	=item SSL_OP_PKCS1_CHECK_1

	=item SSL_OP_PKCS1_CHECK_2

	=item SSL_OP_SINGLE_DH_USE

	=item SSL_OP_SINGLE_ECDH_USE

	=item SSL_OP_EPHEMERAL_RSA

	=back

	=head1 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 I<patched>. A server not supporting secure
	renegotiation is referred to as I<unpatched>.

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

	=head2 Patched client and server

	Connections and renegotiation are always permitted by OpenSSL implementations.

	=head2 Unpatched client and patched OpenSSL server

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

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

	If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
	renegotiation B<always> succeeds.

	=head2 Patched OpenSSL client and unpatched server

	If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
	B<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.

	Setting the option B<SSL_OP_LEGACY_SERVER_CONNECT> has security implications;
	clients that are willing to connect to servers that do not implement
	RFC 5746 secure renegotiation are subject to attacks such as
	CVE-2009-3555.

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

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

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

	=head1 RETURN VALUES

	SSL_CTX_set_options() and SSL_set_options() return the new options bit-mask
	after adding B<options>.

	SSL_CTX_clear_options() and SSL_clear_options() return the new options bit-mask
	after clearing B<options>.

	SSL_CTX_get_options() and SSL_get_options() return the current bit-mask.

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

	=head1 SEE ALSO

	L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>, L<SSL_shutdown(3)>
	L<SSL_CTX_set_tmp_dh_callback(3)>,
	L<SSL_CTX_set_min_proto_version(3)>,
	L<openssl-dhparam(1)>

	=head1 HISTORY

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

	The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options
	were added in OpenSSL 1.1.1.

	The B<SSL_OP_NO_EXTENDED_MASTER_SECRET> and B<SSL_OP_IGNORE_UNEXPECTED_EOF>
	options were added in OpenSSL 3.0.

	The B<SSL_OP_> constants and the corresponding parameter and return values
	of the affected functions were changed to C<uint64_t> type in OpenSSL 3.0.
	For that reason it is no longer possible use the B<SSL_OP_> macro values
	in preprocessor C<#if> conditions. However it is still possible to test
	whether these macros are defined or not.

	=head1 COPYRIGHT

	Copyright 2001-2021 The OpenSSL Project Authors. All Rights Reserved.

	Licensed under the Apache License 2.0 (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
	L<https://www.openssl.org/source/license.html>.

	=cut