SSL_CTX_set_options(3) OpenSSL SSL_CTX_set_options(3) NNAAMMEE 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 SSYYNNOOPPSSIISS #include 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); DDEESSCCRRIIPPTTIIOONN Note: all these functions are implemented using macros. _S_S_L___C_T_X___s_e_t___o_p_t_i_o_n_s_(_) adds the options set via bitmask in ooppttiioonnss to ccttxx. Options already set before are not cleared! _S_S_L___s_e_t___o_p_t_i_o_n_s_(_) adds the options set via bitmask in ooppttiioonnss to ssssll. Options already set before are not cleared! _S_S_L___C_T_X___c_l_e_a_r___o_p_t_i_o_n_s_(_) clears the options set via bitmask in ooppttiioonnss to ccttxx. _S_S_L___c_l_e_a_r___o_p_t_i_o_n_s_(_) clears the options set via bitmask in ooppttiioonnss to ssssll. _S_S_L___C_T_X___g_e_t___o_p_t_i_o_n_s_(_) returns the options set for ccttxx. _S_S_L___g_e_t___o_p_t_i_o_n_s_(_) returns the options set for ssssll. _S_S_L___g_e_t___s_e_c_u_r_e___r_e_n_e_g_o_t_i_a_t_i_o_n___s_u_p_p_o_r_t_(_) indicates whether the peer sup- ports secure renegotiation. NNOOTTEESS 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 oorr operation (|). _S_S_L___C_T_X___s_e_t___o_p_t_i_o_n_s_(_) and _S_S_L___s_e_t___o_p_t_i_o_n_s_(_) affect the (external) pro- tocol behaviour of the SSL library. The (internal) behaviour of the API can be changed by using the similar _S_S_L___C_T_X___s_e_t___m_o_d_e(3) and _S_S_L___s_e_t___m_o_d_e_(_) functions. During a handshake, the option settings of the SSL object are used. When a new SSL object is created from a context using _S_S_L___n_e_w_(_), the current option setting is copied. Changes to ccttxx do not affect already created SSL objects. _S_S_L___c_l_e_a_r_(_) does not affect the settings. The following bbuugg wwoorrkkaarroouunndd options are available: SSL_OP_MICROSOFT_SESS_ID_BUG 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. SSL_OP_NETSCAPE_CHALLENGE_BUG 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 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 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. 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. SSL_OP_ALL All of the above bug workarounds. It is usually safe to use SSSSLL__OOPP__AALLLL to enable the bug workaround options if compatibility with somewhat broken implementations is desired. The following mmooddiiffyyiinngg 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 parame- ters (see _S_S_L___C_T_X___s_e_t___t_m_p___d_h___c_a_l_l_b_a_c_k(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 _d_h_p_a_r_a_m(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. SSSSLL__OOPP__SSIINNGGLLEE__DDHH__UUSSEE 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 pro- tocol, 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. As of OpenSSL 1.0.1s the SSSSLL__OOPP__NNOO__SSSSLLvv22 option is set by default. SSL_OP_NO_SSLv3 Do not use the SSLv3 protocol. It is recommended that applications should set this option. SSL_OP_NO_TLSv1 Do not use the TLSv1 protocol. SSL_OP_NO_TLSv1_1 Do not use the TLSv1.1 protocol. SSL_OP_NO_TLSv1_2 Do not use the TLSv1.2 protocol. SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 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. 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 SSEECCUURREE RREENNEEGGOOTTIIAATTIIOONN section for more details. SSL_OP_LEGACY_SERVER_CONNECT Allow legacy insecure renegotiation between OpenSSL and unpatched servers oonnllyy: this option is currently set by default. See the SSEECCUURREE RREENNEEGGOOTTIIAATTIIOONN section for more details. SSEECCUURREE RREENNEEGGOOTTIIAATTIIOONN 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 rene- gotiation at all: its use is ssttrroonnggllyy discouraged. This attack has far reaching consequences which application writers should be aware of. In the description below an implementation support- ing secure renegotiation is referred to as _p_a_t_c_h_e_d. A server not sup- porting secure renegotiation is referred to as _u_n_p_a_t_c_h_e_d. The following sections describe the operations permitted by OpenSSL's secure renegotiation implementation. PPaattcchheedd cclliieenntt aanndd sseerrvveerr Connections and renegotiation are always permitted by OpenSSL implemen- tations. UUnnppaattcchheedd cclliieenntt aanndd ppaattcchheedd OOppeennSSSSLL sseerrvveerr The initial connection succeeds but client renegotiation is denied by the server with a nnoo__rreenneeggoottiiaattiioonn warning alert if TLS v1.0 is used or a fatal hhaannddsshhaakkee__ffaaiilluurree alert in SSL v3.0. If the patched OpenSSL server attempts to renegotiate a fatal hhaanndd-- sshhaakkee__ffaaiilluurree alert is sent. This is because the server code may be unaware of the unpatched nature of the client. If the option SSSSLL__OOPP__AALLLLOOWW__UUNNSSAAFFEE__LLEEGGAACCYY__RREENNEEGGOOTTIIAATTIIOONN is set then renegotiation aallwwaayyss succeeds. NNBB:: 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 nnoo__rreenneeggoottiiaattiioonn alert. OpenSSL versions 0.9.8m and later will regard a nnoo__rreenneeggoottiiaattiioonn alert as fatal and respond with a fatal hhaanndd-- sshhaakkee__ffaaiilluurree alert. This is because the OpenSSL API currently has no provision to indicate to an application that a renegotiation attempt was refused. PPaattcchheedd OOppeennSSSSLL cclliieenntt aanndd uunnppaattcchheedd sseerrvveerr.. If the option SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT or SSSSLL__OOPP__AALLLLOOWW__UUNNSSAAFFEE__LLEEGGAACCYY__RREENNEEGGOOTTIIAATTIIOONN is set then initial connec- tions 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 SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT is currently set by default even though it has security implications: otherwise it would be impos- sible 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 SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT will nnoott be set by default in a future version of OpenSSL. OpenSSL client applications wishing to ensure they can connect to unpatched servers should always sseett SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT OpenSSL client applications that want to ensure they can nnoott connect to unpatched servers (and thus avoid any security issues) should always cclleeaarr SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT using _S_S_L___C_T_X___c_l_e_a_r___o_p_t_i_o_n_s_(_) or _S_S_L___c_l_e_a_r___o_p_t_i_o_n_s_(_). The difference between the SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT and SSSSLL__OOPP__AALLLLOOWW__UUNNSSAAFFEE__LLEEGGAACCYY__RREENNEEGGOOTTIIAATTIIOONN options is that SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT enables initial connections and secure renegotiation between OpenSSL clients and unpatched servers oonnllyy, while SSSSLL__OOPP__AALLLLOOWW__UUNNSSAAFFEE__LLEEGGAACCYY__RREENNEEGGOOTTIIAATTIIOONN allows initial connections and renegotiation between OpenSSL and unpatched clients or servers. RREETTUURRNN VVAALLUUEESS _S_S_L___C_T_X___s_e_t___o_p_t_i_o_n_s_(_) and _S_S_L___s_e_t___o_p_t_i_o_n_s_(_) return the new options bit- mask after adding ooppttiioonnss. _S_S_L___C_T_X___c_l_e_a_r___o_p_t_i_o_n_s_(_) and _S_S_L___c_l_e_a_r___o_p_t_i_o_n_s_(_) return the new options bitmask after clearing ooppttiioonnss. _S_S_L___C_T_X___g_e_t___o_p_t_i_o_n_s_(_) and _S_S_L___g_e_t___o_p_t_i_o_n_s_(_) return the current bitmask. _S_S_L___g_e_t___s_e_c_u_r_e___r_e_n_e_g_o_t_i_a_t_i_o_n___s_u_p_p_o_r_t_(_) returns 1 is the peer supports secure renegotiation and 0 if it does not. SSEEEE AALLSSOO _s_s_l(3), _S_S_L___n_e_w(3), _S_S_L___c_l_e_a_r(3), _S_S_L___C_T_X___s_e_t___t_m_p___d_h___c_a_l_l_b_a_c_k(3), _S_S_L___C_T_X___s_e_t___t_m_p___r_s_a___c_a_l_l_b_a_c_k(3), _d_h_p_a_r_a_m(1) HHIISSTTOORRYY SSSSLL__OOPP__CCIIPPHHEERR__SSEERRVVEERR__PPRREEFFEERREENNCCEE and SSSSLL__OOPP__NNOO__SSEESSSSIIOONN__RREESSUUMMPP-- TTIIOONN__OONN__RREENNEEGGOOTTIIAATTIIOONN have been added in OpenSSL 0.9.7. SSSSLL__OOPP__TTLLSS__RROOLLLLBBAACCKK__BBUUGG has been added in OpenSSL 0.9.6 and was auto- matically enabled with SSSSLL__OOPP__AALLLL. As of 0.9.7, it is no longer included in SSSSLL__OOPP__AALLLL and must be explicitly set. SSSSLL__OOPP__DDOONNTT__IINNSSEERRTT__EEMMPPTTYY__FFRRAAGGMMEENNTTSS 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). _S_S_L___C_T_X___c_l_e_a_r___o_p_t_i_o_n_s_(_) and _S_S_L___c_l_e_a_r___o_p_t_i_o_n_s_(_) were first added in OpenSSL 0.9.8m. SSSSLL__OOPP__AALLLLOOWW__UUNNSSAAFFEE__LLEEGGAACCYY__RREENNEEGGOOTTIIAATTIIOONN, SSSSLL__OOPP__LLEEGGAACCYY__SSEERRVVEERR__CCOONNNNEECCTT and the function _S_S_L___g_e_t___s_e_c_u_r_e___r_e_n_e_g_o_t_i_a_t_i_o_n___s_u_p_p_o_r_t_(_) were first added in OpenSSL 0.9.8m. 1.0.1u 2016-09-22 SSL_CTX_set_options(3)