xref: /freebsd-11.0-release/crypto/openssl/doc/ssl/SSL_CTX_set_options.pod

=pod

=head1 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

=head1 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);

=head1 DESCRIPTION

Note: all these functions are implemented using macros.

SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
Options already set before are not cleared!

SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
Options already set before are not cleared!

SSL_CTX_clear_options() clears the options set via bitmask in B<options>
to B<ctx>.

SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.

SSL_CTX_get_options() returns the options set for B<ctx>.

SSL_get_options() returns the options set for B<ssl>.

SSL_get_secure_renegotiation_support() indicates whether the peer supports
secure renegotiation.

=head1 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 B<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
L<SSL_CTX_set_mode(3)|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 B<ctx> do not affect already created
SSL objects. SSL_clear() does not affect the settings.

The following B<bug workaround> options are available:

=over 4

=item 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.

=item 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.

=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG

As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.

=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG

...

=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER

...

=item 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.

=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG

...

=item SSL_OP_TLS_D5_BUG

...

=item SSL_OP_TLS_BLOCK_PADDING_BUG

...

=item 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.

=item 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.

=item SSL_OP_ALL

All of the above bug workarounds.

=back

It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
options if compatibility with somewhat broken implementations is
desired.

The following B<modifying> options are available:

=over 4

=item 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.)

=item SSL_OP_SINGLE_DH_USE

Always create a new key when using temporary/ephemeral DH parameters
(see L<SSL_CTX_set_tmp_dh_callback(3)|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 L<dhparam(1)|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.
B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever
temporary/ephemeral DH parameters are used.

=item SSL_OP_EPHEMERAL_RSA

Always use ephemeral (temporary) RSA key when doing RSA operations
(see L<SSL_CTX_set_tmp_rsa_callback(3)|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.

=item 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
own preferences. Because of the different protocol, for SSLv2 the server
will send its list of preferences to the client and the client chooses.

=item SSL_OP_PKCS1_CHECK_1

...

=item SSL_OP_PKCS1_CHECK_2

...

=item 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 

=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG

...

=item SSL_OP_NO_SSLv2

Do not use the SSLv2 protocol.

=item SSL_OP_NO_SSLv3

Do not use the SSLv3 protocol.

=item SSL_OP_NO_TLSv1

Do not use the TLSv1 protocol.

=item 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.

=item 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.

=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION

Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
servers. See the B<SECURE RENEGOTIATION> section for more details.

=item SSL_OP_LEGACY_SERVER_CONNECT

Allow legacy insecure renegotiation between OpenSSL and unpatched servers
B<only>: this option is currently set by default. See the
B<SECURE RENEGOTIATION> section for more details.

=back

=head1 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 B<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 I<patched>. A server not supporting secure
renegotiation is referred to as I<unpatched>.

The following sections describe the operations permitted by OpenSSL's secure
renegotiation implementation.

=head2 Patched client and server

Connections and renegotiation are always permitted by OpenSSL implementations.

=head2 Unpatched client and patched OpenSSL server

The initial connection succeeds but client renegotiation is denied by the
server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
B<handshake_failure> alert in SSL v3.0.

If the patched OpenSSL server attempts to renegotiate a fatal
B<handshake_failure> alert is sent. This is because the server code may be
unaware of the unpatched nature of the client.

If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
renegotiation B<always> succeeds.

B<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
B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard
a B<no_renegotiation> alert as fatal and respond with a fatal
B<handshake_failure> alert. This is because the OpenSSL API currently has
no provision to indicate to an application that a renegotiation attempt
was refused.

=head2 Patched OpenSSL client and unpatched server.

If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
B<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 B<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 B<SSL_OP_LEGACY_SERVER_CONNECT> will
B<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 B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>

OpenSSL client applications that want to ensure they can B<not> connect to
unpatched servers (and thus avoid any security issues) should always B<clear>
B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
SSL_clear_options().

The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
renegotiation between OpenSSL clients and unpatched servers B<only>, while
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
and renegotiation between OpenSSL and unpatched clients or servers.

=head1 RETURN VALUES

SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
after adding B<options>.

SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
after clearing B<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.

=head1 SEE ALSO

L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>,
L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>,
L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
L<dhparam(1)|dhparam(1)>

=head1 HISTORY

B<SSL_OP_CIPHER_SERVER_PREFERENCE> and
B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in
OpenSSL 0.9.7.

B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically
enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL>
and must be explicitly set.

B<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.

B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT>
and the function SSL_get_secure_renegotiation_support() were first added in
OpenSSL 0.9.8m.

=cut