SSL_CTX_SET_OPTIONS(3) BSD Library Functions Manual 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 sup‐
ports 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 bit‐
wise OR operation (|).
SSL_CTX_set_options() and SSL_set_options() affect the (external) proto‐
col 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 vul‐
nerability affecting CBC ciphers, which cannot be handled by some
broken SSL implementations. This option has no effect for con‐
nections 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 work‐
around 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 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-
parameters, see openssl(1)). If “strong” primes were used, it is
not strictly necessary to generate a new DH key during each hand‐
shake 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 opera‐
tions (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 ver‐
sions 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 renego‐
tiation 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 implementa‐
tions.
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 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 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 VALUESSSL_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.
SEE ALSOopenssl(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 automati‐
cally 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.
BSD April 18, 2024 BSD
