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 161This option is no longer implemented and is treated as no op. 162 163=item SSL_OP_CIPHER_SERVER_PREFERENCE 164 165When choosing a cipher, use the server's preferences instead of the client 166preferences. When not set, the SSL server will always follow the clients 167preferences. When set, the SSLv3/TLSv1 server will choose following its 168own preferences. Because of the different protocol, for SSLv2 the server 169will send its list of preferences to the client and the client chooses. 170 171=item SSL_OP_PKCS1_CHECK_1 172 173... 174 175=item SSL_OP_PKCS1_CHECK_2 176 177... 178 179=item SSL_OP_NETSCAPE_CA_DN_BUG 180 181If we accept a netscape connection, demand a client cert, have a 182non-self-signed CA which does not have its CA in netscape, and the 183browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta 184 185=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 186 187... 188 189=item SSL_OP_NO_SSLv2 190 191Do not use the SSLv2 protocol. 192 193=item SSL_OP_NO_SSLv3 194 195Do not use the SSLv3 protocol. 196 197=item SSL_OP_NO_TLSv1 198 199Do not use the TLSv1 protocol. 200 201=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 202 203When performing renegotiation as a server, always start a new session 204(i.e., session resumption requests are only accepted in the initial 205handshake). This option is not needed for clients. 206 207=item SSL_OP_NO_TICKET 208 209Normally clients and servers will, where possible, transparently make use 210of RFC4507bis tickets for stateless session resumption. 211 212If this option is set this functionality is disabled and tickets will 213not be used by clients or servers. 214 215=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 216 217Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 218servers. See the B<SECURE RENEGOTIATION> section for more details. 219 220=item SSL_OP_LEGACY_SERVER_CONNECT 221 222Allow legacy insecure renegotiation between OpenSSL and unpatched servers 223B<only>: this option is currently set by default. See the 224B<SECURE RENEGOTIATION> section for more details. 225 226=back 227 228=head1 SECURE RENEGOTIATION 229 230OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 231described in RFC5746. This counters the prefix attack described in 232CVE-2009-3555 and elsewhere. 233 234The deprecated and highly broken SSLv2 protocol does not support 235renegotiation at all: its use is B<strongly> discouraged. 236 237This attack has far reaching consequences which application writers should be 238aware of. In the description below an implementation supporting secure 239renegotiation is referred to as I<patched>. A server not supporting secure 240renegotiation is referred to as I<unpatched>. 241 242The following sections describe the operations permitted by OpenSSL's secure 243renegotiation implementation. 244 245=head2 Patched client and server 246 247Connections and renegotiation are always permitted by OpenSSL implementations. 248 249=head2 Unpatched client and patched OpenSSL server 250 251The initial connection succeeds but client renegotiation is denied by the 252server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 253B<handshake_failure> alert in SSL v3.0. 254 255If the patched OpenSSL server attempts to renegotiate a fatal 256B<handshake_failure> alert is sent. This is because the server code may be 257unaware of the unpatched nature of the client. 258 259If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 260renegotiation B<always> succeeds. 261 262B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 263unpatched) will result in the connection hanging if it receives a 264B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 265a B<no_renegotiation> alert as fatal and respond with a fatal 266B<handshake_failure> alert. This is because the OpenSSL API currently has 267no provision to indicate to an application that a renegotiation attempt 268was refused. 269 270=head2 Patched OpenSSL client and unpatched server. 271 272If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 273B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 274and renegotiation between patched OpenSSL clients and unpatched servers 275succeeds. If neither option is set then initial connections to unpatched 276servers will fail. 277 278The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 279though it has security implications: otherwise it would be impossible to 280connect to unpatched servers (i.e. all of them initially) and this is clearly 281not acceptable. Renegotiation is permitted because this does not add any 282additional security issues: during an attack clients do not see any 283renegotiations anyway. 284 285As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 286B<not> be set by default in a future version of OpenSSL. 287 288OpenSSL client applications wishing to ensure they can connect to unpatched 289servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 290 291OpenSSL client applications that want to ensure they can B<not> connect to 292unpatched servers (and thus avoid any security issues) should always B<clear> 293B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 294SSL_clear_options(). 295 296The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 297B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 298B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 299renegotiation between OpenSSL clients and unpatched servers B<only>, while 300B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 301and renegotiation between OpenSSL and unpatched clients or servers. 302 303=head1 RETURN VALUES 304 305SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 306after adding B<options>. 307 308SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 309after clearing B<options>. 310 311SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 312 313SSL_get_secure_renegotiation_support() returns 1 is the peer supports 314secure renegotiation and 0 if it does not. 315 316=head1 SEE ALSO 317 318L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 319L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 320L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 321L<dhparam(1)|dhparam(1)> 322 323=head1 HISTORY 324 325B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 326B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 327OpenSSL 0.9.7. 328 329B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 330enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 331and must be explicitly set. 332 333B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 334Versions up to OpenSSL 0.9.6c do not include the countermeasure that 335can be disabled with this option (in OpenSSL 0.9.6d, it was always 336enabled). 337 338SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3390.9.8m. 340 341B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 342and the function SSL_get_secure_renegotiation_support() were first added in 343OpenSSL 0.9.8m. 344 345=cut 346