Ubuntu Manpage: SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options, SSL_clear

Provided by: libssl-doc_1.0.1f-1ubuntu2.27_all

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 logical 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() 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:

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.

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.

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
...

SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
...

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.

SSL_OP_SSLEAY_080_CLIENT_DH_BUG
...

SSL_OP_TLS_D5_BUG
...

SSL_OP_TLS_BLOCK_PADDING_BUG
...

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_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 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.

SSL_OP_EPHEMERAL_RSA
This option is no longer implemented and is treated as no op.

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.

SSL_OP_PKCS1_CHECK_1
...

SSL_OP_PKCS1_CHECK_2
...

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

SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
...

SSL_OP_NO_SSLv2
Do not use the SSLv2 protocol.

SSL_OP_NO_SSLv3
Do not use the SSLv3 protocol.

SSL_OP_NO_TLSv1
Do not use the TLSv1 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
Allow 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 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 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 was refused.

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 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().

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 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.

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.