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 bitwise 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(3) 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(3), 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
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_NETSCAPE_CHALLENGE_BUG
- As of OpenBSD 5.8, this option has no effect.
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
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_SAFARI_ECDHE_ECDSA_BUG
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_SSLEAY_080_CLIENT_DH_BUG
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_TLS_D5_BUG
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_TLS_BLOCK_PADDING_BUG
- As of OpenBSD 5.8, this option has no effect.
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_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
openssl(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 client's 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_NETSCAPE_CA_DN_BUG
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
- As of OpenBSD 5.8, this option has no effect.
SSL_OP_NO_SSLv2
- As of OpenBSD 5.6, this option has no effect as SSLv2 support has been removed. In previous versions it disabled use of the SSLv2 protocol.
SSL_OP_NO_SSLv3
- Do not use the SSLv3 protocol.
SSL_OP_NO_TLSv1
- Do not use the TLSv1.0 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
- As of OpenBSD 5.6, this option has no effect. In previous versions it allowed 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 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.
N.B.: 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
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
().
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
openssl(1), ssl(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3), SSL_CTX_set_tmp_rsa_callback(3), SSL_new(3)
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.
SSL_OP_NO_SSLv2
and
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
were
changed to have no effect in OpenBSD 5.6.