SSL_CTX_set_options(3) OpenSSL SSL_CTX_set_options(3)NAME
SSL_CTX_set_options, SSL_set_options, SSL_CTX_get_options,
SSL_get_options - manipulate SSL engine options
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);
DESCRIPTIONSSL_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.
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
SSL_CTX_set_options() and SSL_set_options() affect the (external) pro‐
tocol behaviour of the SSL library. The (internal) behaviour of the API
can be changed by using the similar SSL_CTX_set_mode(3) and
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:
www.microsoft.com - when talking SSLv2, if session-id reuse is per‐
formed, the session-id passed back in the server-finished message
is different from the one decided upon.
Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte chal‐
lenge 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
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 18.104.22.168, '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 net‐
scape, when doing a re-connect, always takes the first cipher in
the cipher list.
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 connec‐
tions using other ciphers.
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
The following modifying options are available:
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
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-parame‐
ters, 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.
Always use ephemeral (temporary) RSA key when doing RSA operations
(see SSL_CTX_set_tmp_rsa_callback(3)). According to the specifica‐
tions this is only done, when a RSA key can only be used for signa‐
ture 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 specifica‐
tions 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.
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 pro‐
tocol, for SSLv2 the server will send his list of preferences to
the client and the client chooses.
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
Do not use the SSLv2 protocol.
Do not use the SSLv3 protocol.
Do not use the TLSv1 protocol.
When performing renegotiation as a server, always start a new ses‐
sion (i.e., session resumption requests are only accepted in the
initial handshake). This option is not needed for clients.
RETURN VALUESSSL_CTX_set_options() and SSL_set_options() return the new options bit‐
mask after adding options.
SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
SEE ALSOssl(3), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3),
SSL_OP_CIPHER_SERVER_PREFERENCE and SSL_OP_NO_SESSION_RESUMP‐
TION_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 auto‐
matically 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
0.9.7d 2003-11-20 SSL_CTX_set_options(3)