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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
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_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 sup-
ports 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) pro-
tocol behaviour of the SSL library. The (internal) behaviour of the API
can be changed by using the similar SSL_CTX_set_mode(3) and
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 per-
formed, 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 chal-
lenge 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
This option has no effect anymore.
As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.
Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol vul-
nerability affecting CBC ciphers, which cannot be handled by some
broken SSL implementations. This option has no effect for connec-
tions using other ciphers.
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 parame-
ters (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-parame-
ters, 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.
Always use ephemeral (temporary) RSA key when doing RSA operations
(see SSL_CTX_set_tmp_rsa_callback(3)). According to the specifica-
tions this is only done, when a RSA key can only be used for signa-
ture 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 specifica-
tions 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.
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 pro-
tocol, 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.
Do not use the SSLv3 protocol.
Do not use the TLSv1 protocol.
When performing renegotiation as a server, always start a new ses-
sion (i.e., session resumption requests are only accepted in the
initial handshake). This option is not needed for clients.
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 support-
ing secure renegotiation is referred to as patched. A server not sup-
porting 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 implemen-
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 hand-
shake_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 hand-
shake_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 or
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then initial connec-
tions 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 impos-
sible 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_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.
SSL_CTX_set_options() and SSL_set_options() return the new options bit-
mask 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_OP_CIPHER_SERVER_PREFERENCE and SSL_OP_NO_SESSION_RESUMP-
TION_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 auto-
matically 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
and the function SSL_get_secure_renegotiation_support() were first
added in OpenSSL 0.9.8m.
0.9.8e-rhel5 2014-12-17 SSL_CTX_set_options(3)
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