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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

     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.

1.0.2t               Last change: 2019-09-10                    1

SSL_CTX_set_options(3)       OpenSSL       SSL_CTX_set_options(3)

     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_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_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.

1.0.2t               Last change: 2019-09-10                    2

SSL_CTX_set_options(3)       OpenSSL       SSL_CTX_set_options(3)

         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.

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

     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

1.0.2t               Last change: 2019-09-10                    3

SSL_CTX_set_options(3)       OpenSSL       SSL_CTX_set_options(3)

         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.  As of OpenSSL 1.0.2g the
         SSL_OP_NO_SSLv2 option is set by default.

     SSL_OP_NO_SSLv3
         Do not use the SSLv3 protocol.  It is recommended that
         applications should set this option.

     SSL_OP_NO_TLSv1
         Do not use the TLSv1 protocol.

     SSL_OP_NO_TLSv1_1
         Do not use the TLSv1.1 protocol.

     SSL_OP_NO_TLSv1_2
         Do not use the TLSv1.2 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

1.0.2t               Last change: 2019-09-10                    4

SSL_CTX_set_options(3)       OpenSSL       SSL_CTX_set_options(3)

         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 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

1.0.2t               Last change: 2019-09-10                    5

SSL_CTX_set_options(3)       OpenSSL       SSL_CTX_set_options(3)

     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.

1.0.2t               Last change: 2019-09-10                    6

SSL_CTX_set_options(3)       OpenSSL       SSL_CTX_set_options(3)

     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.

1.0.2t               Last change: 2019-09-10                    7

See also SSL_CTX_clear_options(3)
See also SSL_CTX_get_options(3)
See also SSL_CTX_set_options(3)
See also SSL_get_options(3)
See also SSL_get_secure_renegotiation_support(3)
See also SSL_set_options(3)

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