SSL_CTX_set_options.pod revision 267256
1=pod 2 3=head1 NAME 4 5SSL_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 6 7=head1 SYNOPSIS 8 9 #include <openssl/ssl.h> 10 11 long SSL_CTX_set_options(SSL_CTX *ctx, long options); 12 long SSL_set_options(SSL *ssl, long options); 13 14 long SSL_CTX_clear_options(SSL_CTX *ctx, long options); 15 long SSL_clear_options(SSL *ssl, long options); 16 17 long SSL_CTX_get_options(SSL_CTX *ctx); 18 long SSL_get_options(SSL *ssl); 19 20 long SSL_get_secure_renegotiation_support(SSL *ssl); 21 22=head1 DESCRIPTION 23 24Note: all these functions are implemented using macros. 25 26SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>. 27Options already set before are not cleared! 28 29SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>. 30Options already set before are not cleared! 31 32SSL_CTX_clear_options() clears the options set via bitmask in B<options> 33to B<ctx>. 34 35SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>. 36 37SSL_CTX_get_options() returns the options set for B<ctx>. 38 39SSL_get_options() returns the options set for B<ssl>. 40 41SSL_get_secure_renegotiation_support() indicates whether the peer supports 42secure renegotiation. 43 44=head1 NOTES 45 46The behaviour of the SSL library can be changed by setting several options. 47The options are coded as bitmasks and can be combined by a logical B<or> 48operation (|). 49 50SSL_CTX_set_options() and SSL_set_options() affect the (external) 51protocol behaviour of the SSL library. The (internal) behaviour of 52the API can be changed by using the similar 53L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> and SSL_set_mode() functions. 54 55During a handshake, the option settings of the SSL object are used. When 56a new SSL object is created from a context using SSL_new(), the current 57option setting is copied. Changes to B<ctx> do not affect already created 58SSL objects. SSL_clear() does not affect the settings. 59 60The following B<bug workaround> options are available: 61 62=over 4 63 64=item SSL_OP_MICROSOFT_SESS_ID_BUG 65 66www.microsoft.com - when talking SSLv2, if session-id reuse is 67performed, the session-id passed back in the server-finished message 68is different from the one decided upon. 69 70=item SSL_OP_NETSCAPE_CHALLENGE_BUG 71 72Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte 73challenge but then appears to only use 16 bytes when generating the 74encryption keys. Using 16 bytes is ok but it should be ok to use 32. 75According to the SSLv3 spec, one should use 32 bytes for the challenge 76when operating in SSLv2/v3 compatibility mode, but as mentioned above, 77this breaks this server so 16 bytes is the way to go. 78 79=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG 80 81As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect. 82 83=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG 84 85... 86 87=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER 88 89... 90 91=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG 92 93Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X. 94OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers. 95 96=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG 97 98... 99 100=item SSL_OP_TLS_D5_BUG 101 102... 103 104=item SSL_OP_TLS_BLOCK_PADDING_BUG 105 106... 107 108=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS 109 110Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol 111vulnerability affecting CBC ciphers, which cannot be handled by some 112broken SSL implementations. This option has no effect for connections 113using other ciphers. 114 115=item SSL_OP_TLSEXT_PADDING 116 117Adds a padding extension to ensure the ClientHello size is never between 118256 and 511 bytes in length. This is needed as a workaround for some 119implementations. 120 121=item SSL_OP_ALL 122 123All of the above bug workarounds. 124 125=back 126 127It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround 128options if compatibility with somewhat broken implementations is 129desired. 130 131The following B<modifying> options are available: 132 133=over 4 134 135=item SSL_OP_TLS_ROLLBACK_BUG 136 137Disable version rollback attack detection. 138 139During the client key exchange, the client must send the same information 140about acceptable SSL/TLS protocol levels as during the first hello. Some 141clients violate this rule by adapting to the server's answer. (Example: 142the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server 143only understands up to SSLv3. In this case the client must still use the 144same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect 145to the server's answer and violate the version rollback protection.) 146 147=item SSL_OP_SINGLE_DH_USE 148 149Always create a new key when using temporary/ephemeral DH parameters 150(see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>). 151This option must be used to prevent small subgroup attacks, when 152the DH parameters were not generated using "strong" primes 153(e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>). 154If "strong" primes were used, it is not strictly necessary to generate 155a new DH key during each handshake but it is also recommended. 156B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever 157temporary/ephemeral DH parameters are used. 158 159=item SSL_OP_EPHEMERAL_RSA 160 161Always use ephemeral (temporary) RSA key when doing RSA operations 162(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). 163According to the specifications this is only done, when a RSA key 164can only be used for signature operations (namely under export ciphers 165with restricted RSA keylength). By setting this option, ephemeral 166RSA keys are always used. This option breaks compatibility with the 167SSL/TLS specifications and may lead to interoperability problems with 168clients and should therefore never be used. Ciphers with EDH (ephemeral 169Diffie-Hellman) key exchange should be used instead. 170 171=item SSL_OP_CIPHER_SERVER_PREFERENCE 172 173When choosing a cipher, use the server's preferences instead of the client 174preferences. When not set, the SSL server will always follow the clients 175preferences. When set, the SSLv3/TLSv1 server will choose following its 176own preferences. Because of the different protocol, for SSLv2 the server 177will send its list of preferences to the client and the client chooses. 178 179=item SSL_OP_PKCS1_CHECK_1 180 181... 182 183=item SSL_OP_PKCS1_CHECK_2 184 185... 186 187=item SSL_OP_NETSCAPE_CA_DN_BUG 188 189If we accept a netscape connection, demand a client cert, have a 190non-self-signed CA which does not have its CA in netscape, and the 191browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta 192 193=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 194 195... 196 197=item SSL_OP_NO_SSLv2 198 199Do not use the SSLv2 protocol. 200 201=item SSL_OP_NO_SSLv3 202 203Do not use the SSLv3 protocol. 204 205=item SSL_OP_NO_TLSv1 206 207Do not use the TLSv1 protocol. 208 209=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 210 211When performing renegotiation as a server, always start a new session 212(i.e., session resumption requests are only accepted in the initial 213handshake). This option is not needed for clients. 214 215=item SSL_OP_NO_TICKET 216 217Normally clients and servers will, where possible, transparently make use 218of RFC4507bis tickets for stateless session resumption. 219 220If this option is set this functionality is disabled and tickets will 221not be used by clients or servers. 222 223=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 224 225Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 226servers. See the B<SECURE RENEGOTIATION> section for more details. 227 228=item SSL_OP_LEGACY_SERVER_CONNECT 229 230Allow legacy insecure renegotiation between OpenSSL and unpatched servers 231B<only>: this option is currently set by default. See the 232B<SECURE RENEGOTIATION> section for more details. 233 234=back 235 236=head1 SECURE RENEGOTIATION 237 238OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 239described in RFC5746. This counters the prefix attack described in 240CVE-2009-3555 and elsewhere. 241 242The deprecated and highly broken SSLv2 protocol does not support 243renegotiation at all: its use is B<strongly> discouraged. 244 245This attack has far reaching consequences which application writers should be 246aware of. In the description below an implementation supporting secure 247renegotiation is referred to as I<patched>. A server not supporting secure 248renegotiation is referred to as I<unpatched>. 249 250The following sections describe the operations permitted by OpenSSL's secure 251renegotiation implementation. 252 253=head2 Patched client and server 254 255Connections and renegotiation are always permitted by OpenSSL implementations. 256 257=head2 Unpatched client and patched OpenSSL server 258 259The initial connection suceeds but client renegotiation is denied by the 260server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 261B<handshake_failure> alert in SSL v3.0. 262 263If the patched OpenSSL server attempts to renegotiate a fatal 264B<handshake_failure> alert is sent. This is because the server code may be 265unaware of the unpatched nature of the client. 266 267If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 268renegotiation B<always> succeeds. 269 270B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 271unpatched) will result in the connection hanging if it receives a 272B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 273a B<no_renegotiation> alert as fatal and respond with a fatal 274B<handshake_failure> alert. This is because the OpenSSL API currently has 275no provision to indicate to an application that a renegotiation attempt 276was refused. 277 278=head2 Patched OpenSSL client and unpatched server. 279 280If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 281B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 282and renegotiation between patched OpenSSL clients and unpatched servers 283succeeds. If neither option is set then initial connections to unpatched 284servers will fail. 285 286The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 287though it has security implications: otherwise it would be impossible to 288connect to unpatched servers (i.e. all of them initially) and this is clearly 289not acceptable. Renegotiation is permitted because this does not add any 290additional security issues: during an attack clients do not see any 291renegotiations anyway. 292 293As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 294B<not> be set by default in a future version of OpenSSL. 295 296OpenSSL client applications wishing to ensure they can connect to unpatched 297servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 298 299OpenSSL client applications that want to ensure they can B<not> connect to 300unpatched servers (and thus avoid any security issues) should always B<clear> 301B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 302SSL_clear_options(). 303 304The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 305B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 306B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 307renegotiation between OpenSSL clients and unpatched servers B<only>, while 308B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 309and renegotiation between OpenSSL and unpatched clients or servers. 310 311=head1 RETURN VALUES 312 313SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 314after adding B<options>. 315 316SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 317after clearing B<options>. 318 319SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 320 321SSL_get_secure_renegotiation_support() returns 1 is the peer supports 322secure renegotiation and 0 if it does not. 323 324=head1 SEE ALSO 325 326L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 327L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 328L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 329L<dhparam(1)|dhparam(1)> 330 331=head1 HISTORY 332 333B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 334B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 335OpenSSL 0.9.7. 336 337B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 338enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 339and must be explicitly set. 340 341B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 342Versions up to OpenSSL 0.9.6c do not include the countermeasure that 343can be disabled with this option (in OpenSSL 0.9.6d, it was always 344enabled). 345 346SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3470.9.8m. 348 349B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 350and the function SSL_get_secure_renegotiation_support() were first added in 351OpenSSL 0.9.8m. 352 353=cut 354