SSL_CTX_set_options manpage

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SSL_CTX_set_options(3)		    OpenSSL		SSL_CTX_set_options(3)

       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

	#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);

       Note: all these functions are implemented using macros.

       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_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.

       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
       logical 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:

	   www.microsoft.com - when talking SSLv2, if session-id reuse is
	   performed, the session-id passed back in the server-finished
	   message is different from the one decided upon.

	   Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte
	   challenge but then appears to only use 16 bytes when generating the
	   encryption keys.  Using 16 bytes is ok but it should be ok to use
	   32.	According to the SSLv3 spec, one should use 32 bytes for the
	   challenge when operating in SSLv2/v3 compatibility mode, but as
	   mentioned above, this breaks this server so 16 bytes is the way to

	   As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.



	   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.




	   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.

	   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.

	   All of the above bug workarounds.

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

       The following modifying options are available:

	   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

	   Always create a new key when using temporary/ephemeral DH
	   parameters (see SSL_CTX_set_tmp_dh_callback(3)).  This option must
	   be used to prevent small subgroup attacks, when the DH parameters
	   were not generated using "strong" primes (e.g. when using DSA-
	   parameters, see dhparam(1)).	 If "strong" primes were used, it is
	   not strictly necessary to generate a new DH key during each
	   handshake but it is also recommended.  SSL_OP_SINGLE_DH_USE should
	   therefore be enabled whenever temporary/ephemeral DH parameters are

	   This option is no longer implemented and is treated as no op.

	   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 SSLv3/TLSv1 server will
	   choose following its own preferences. Because of the different
	   protocol, for SSLv2 the server will send its list of preferences to
	   the client and the client chooses.



	   If we accept a netscape connection, demand a client cert, have a
	   non-self-signed CA which does not have its CA in netscape, and the
	   browser has a cert, it will crash/hang.  Works for 3.x and 4.xbeta


	   Do not use the SSLv2 protocol.  As of OpenSSL 1.0.2g the
	   SSL_OP_NO_SSLv2 option is set by default.

	   Do not use the SSLv3 protocol.  It is recommended that applications
	   should set this option.

	   Do not use the TLSv1 protocol.

	   Do not use the TLSv1.1 protocol.

	   Do not use the TLSv1.2 protocol.

	   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.

	   Normally clients and servers will, where possible, transparently
	   make use of RFC4507bis tickets for stateless session resumption.

	   If this option is set this functionality is disabled and tickets
	   will not be used by clients or servers.

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

	   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.

       OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
       described in RFC5746. This counters the prefix attack described in
       CVE-2009-3555 and elsewhere.

       The deprecated and highly broken SSLv2 protocol does not support
       renegotiation at all: its use is strongly discouraged.

       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

   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.

       NB: a bug in OpenSSL clients earlier than 0.9.8m (all of which are
       unpatched) will result in the connection hanging if it receives a
       no_renegotiation alert. OpenSSL versions 0.9.8m and later will regard a
       no_renegotiation alert as fatal and respond with a fatal
       handshake_failure alert. This is because the OpenSSL API currently has
       no provision to indicate to an application that a renegotiation attempt
       was refused.

   Patched OpenSSL client and unpatched server.
       If the option SSL_OP_LEGACY_SERVER_CONNECT or
       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

       The difference between the SSL_OP_LEGACY_SERVER_CONNECT and
       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.

       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.

       ssl(3), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3),
       SSL_CTX_set_tmp_rsa_callback(3), dhparam(1)

       OpenSSL 0.9.7.

       SSL_OP_TLS_ROLLBACK_BUG has been added in OpenSSL 0.9.6 and was
       automatically enabled with SSL_OP_ALL. As of 0.9.7, it is no longer
       included in SSL_OP_ALL and must be explicitly set.

       SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS has been added in OpenSSL 0.9.6e.
       Versions up to OpenSSL 0.9.6c do not include the countermeasure that
       can be disabled with this option (in OpenSSL 0.9.6d, it was always

       SSL_CTX_clear_options() and SSL_clear_options() were first added in
       OpenSSL 0.9.8m.

       and the function SSL_get_secure_renegotiation_support() were first
       added in OpenSSL 0.9.8m.

1.0.2k				  2017-01-26		SSL_CTX_set_options(3)