NAME
SSL_CTX_set_options, SSL_set_options, SSL_CTX_get_options, SSL_get_options — manipulate SSL engine 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_get_options(SSL_CTX
*ctx); long SSL_get_options(SSL *ssl);
DESCRIPTION
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_get_options() returns the options set for ctx.
SSL_get_options() returns the options set for ssl.
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 (|). Options can only be added
but can never be reset.
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
ssl3.netscape.com:443, first a connection is established with
RC4-MD5. If it is then resumed, we end up using DES-CBC3-SHA. It
should be RC4-MD5 according to 7.6.1.3, 'cipher_suite'.
Netscape-Enterprise/2.01 (https://merchant.netscape.com) has
this bug. It only really shows up when connecting via SSLv2/v3 then
reconnecting via SSLv3. The cipher list changes....
NEW INFORMATION. Try connecting with a cipher list of just
DES-CBC-SHA:RC4-MD5. For some weird reason, each new connection
uses RC4-MD5, but a re-connect tries to use DES-CBC-SHA. So netscape,
when doing a re-connect, always takes the first cipher in the cipher
list.
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
...
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
...
SSL_OP_MSIE_SSLV2_RSA_PADDING
...
SSL_OP_SSLEAY_080_CLIENT_DH_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_AL
All of the above bug workarounds.
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:
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 L<ssl_ctx_set_tmp_dh_callback>).
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
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 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 his list of preferences
to the client and the client chooses.
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.
RETURN VALUES
SSL_CTX_set_options() and SSL_set_options() return the new
options bitmask after adding options.
SSL_CTX_get_options() and SSL_get_options() return the current
bitmask.
SEE ALSO
ssl(3), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3), SSL_CTX_set_tmp_rsa_callback(3), dhparam(1)
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).
