Section: OpenSSL (3SSL)
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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
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
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
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
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 go.
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
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 protection.)
Always create a new key when using temporary/ephemeral DH parameters
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.
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 1.0.1s 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 details.
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
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
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
alert in SSL
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
Patched OpenSSL client and unpatched server.
If the option SSL_OP_LEGACY_SERVER_CONNECT
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
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_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 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.
have been added in
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
and the function SSL_get_secure_renegotiation_support() were first added in
- SECURE RENEGOTIATION
- Patched client and server
- Unpatched client and patched OpenSSL server
- Patched OpenSSL client and unpatched server.
- RETURN VALUES
- SEE ALSO