SSL_CTX_set_options(3) OpenSSL SSL_CTX_set_options(3)
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);
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.
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 set-
The following bug workaround options are available:
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.
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 men-
tioned above, this breaks this server so 16 bytes is the way to go.
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 220.127.116.11,
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
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.
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.
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:
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 protec-
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 rec-
ommended. SSL_OP_SINGLE_DH_USE should therefore be enabled whenever tempo-
rary/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 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.
When choosing a cipher, use the server's preferences instead of the client prefer-
ences. 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.
If we accept a netscape connection, demand a client cert, have a non-self-sighed CA
which does not have it's 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 session (i.e., session
resumption requests are only accepted in the initial handshake). This option is not
needed for clients.
SSL_CTX_set_options() and SSL_set_options() return the new options bitmask after adding
SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
ssl(3), SSL_new(3), SSL_clear(3), SSL_CTX_set_tmp_dh_callback(3),
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
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).
0.9.7a 2002-07-10 SSL_CTX_set_options(3)