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SSL_CTX_set_options(3SSL)		     OpenSSL			SSL_CTX_set_options(3SSL)

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
       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.

       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.

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 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:

       SSL_OP_MICROSOFT_SESS_ID_BUG
	   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.

       SSL_OP_NETSCAPE_CHALLENGE_BUG
	   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 go.

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

       SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
	   ...

       SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
	   ...

       SSL_OP_MSIE_SSLV2_RSA_PADDING
	   As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.

       SSL_OP_SSLEAY_080_CLIENT_DH_BUG
	   ...

       SSL_OP_TLS_D5_BUG
	   ...

       SSL_OP_TLS_BLOCK_PADDING_BUG
	   ...

       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_ALL
	   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 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_SINGLE_DH_USE
	   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 used.

       SSL_OP_EPHEMERAL_RSA
	   Always use ephemeral (temporary) RSA key when doing RSA operations (see
	   SSL_CTX_set_tmp_rsa_callback(3)).  According to the specifications this is only done,
	   when a RSA key can only be used for signature operations (namely under export ciphers
	   with restricted RSA keylength). By setting this option, ephemeral RSA keys are always
	   used. This option breaks compatibility with the SSL/TLS specifications and may lead to
	   interoperability problems with clients and should therefore never be used. Ciphers
	   with EDH (ephemeral Diffie-Hellman) key exchange should be used instead.

       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 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.

       SSL_OP_PKCS1_CHECK_1
	   ...

       SSL_OP_PKCS1_CHECK_2
	   ...

       SSL_OP_NETSCAPE_CA_DN_BUG
	   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

       SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
	   ...

       SSL_OP_NO_SSLv2
	   Do not use the SSLv2 protocol.

       SSL_OP_NO_SSLv3
	   Do not use the SSLv3 protocol.

       SSL_OP_NO_TLSv1
	   Do not use the TLSv1 protocol.

       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_TICKET
	   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.

       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.

SECURE RENEGOTIATION
       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 implementations.

   Unpatched client and patched OpenSSL server
       The initial connection suceeds 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 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(3), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3),
       SSL_CTX_set_tmp_rsa_callback(3), dhparam(1)

HISTORY
       SSL_OP_CIPHER_SERVER_PREFERENCE and SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION have
       been added in 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 enabled).

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

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

1.0.0e					    2010-12-02			SSL_CTX_set_options(3SSL)
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