SSL_CTX_set_options.pod revision 264331
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_ALL 116 117All of the above bug workarounds. 118 119=back 120 121It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround 122options if compatibility with somewhat broken implementations is 123desired. 124 125The following B<modifying> options are available: 126 127=over 4 128 129=item SSL_OP_TLS_ROLLBACK_BUG 130 131Disable version rollback attack detection. 132 133During the client key exchange, the client must send the same information 134about acceptable SSL/TLS protocol levels as during the first hello. Some 135clients violate this rule by adapting to the server's answer. (Example: 136the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server 137only understands up to SSLv3. In this case the client must still use the 138same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect 139to the server's answer and violate the version rollback protection.) 140 141=item SSL_OP_SINGLE_DH_USE 142 143Always create a new key when using temporary/ephemeral DH parameters 144(see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>). 145This option must be used to prevent small subgroup attacks, when 146the DH parameters were not generated using "strong" primes 147(e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>). 148If "strong" primes were used, it is not strictly necessary to generate 149a new DH key during each handshake but it is also recommended. 150B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever 151temporary/ephemeral DH parameters are used. 152 153=item SSL_OP_EPHEMERAL_RSA 154 155Always use ephemeral (temporary) RSA key when doing RSA operations 156(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). 157According to the specifications this is only done, when a RSA key 158can only be used for signature operations (namely under export ciphers 159with restricted RSA keylength). By setting this option, ephemeral 160RSA keys are always used. This option breaks compatibility with the 161SSL/TLS specifications and may lead to interoperability problems with 162clients and should therefore never be used. Ciphers with EDH (ephemeral 163Diffie-Hellman) key exchange should be used instead. 164 165=item SSL_OP_CIPHER_SERVER_PREFERENCE 166 167When choosing a cipher, use the server's preferences instead of the client 168preferences. When not set, the SSL server will always follow the clients 169preferences. When set, the SSLv3/TLSv1 server will choose following its 170own preferences. Because of the different protocol, for SSLv2 the server 171will send its list of preferences to the client and the client chooses. 172 173=item SSL_OP_PKCS1_CHECK_1 174 175... 176 177=item SSL_OP_PKCS1_CHECK_2 178 179... 180 181=item SSL_OP_NETSCAPE_CA_DN_BUG 182 183If we accept a netscape connection, demand a client cert, have a 184non-self-signed CA which does not have its CA in netscape, and the 185browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta 186 187=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 188 189... 190 191=item SSL_OP_NO_SSLv2 192 193Do not use the SSLv2 protocol. 194 195=item SSL_OP_NO_SSLv3 196 197Do not use the SSLv3 protocol. 198 199=item SSL_OP_NO_TLSv1 200 201Do not use the TLSv1 protocol. 202 203=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 204 205When performing renegotiation as a server, always start a new session 206(i.e., session resumption requests are only accepted in the initial 207handshake). This option is not needed for clients. 208 209=item SSL_OP_NO_TICKET 210 211Normally clients and servers will, where possible, transparently make use 212of RFC4507bis tickets for stateless session resumption. 213 214If this option is set this functionality is disabled and tickets will 215not be used by clients or servers. 216 217=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 218 219Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 220servers. See the B<SECURE RENEGOTIATION> section for more details. 221 222=item SSL_OP_LEGACY_SERVER_CONNECT 223 224Allow legacy insecure renegotiation between OpenSSL and unpatched servers 225B<only>: this option is currently set by default. See the 226B<SECURE RENEGOTIATION> section for more details. 227 228=back 229 230=head1 SECURE RENEGOTIATION 231 232OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 233described in RFC5746. This counters the prefix attack described in 234CVE-2009-3555 and elsewhere. 235 236The deprecated and highly broken SSLv2 protocol does not support 237renegotiation at all: its use is B<strongly> discouraged. 238 239This attack has far reaching consequences which application writers should be 240aware of. In the description below an implementation supporting secure 241renegotiation is referred to as I<patched>. A server not supporting secure 242renegotiation is referred to as I<unpatched>. 243 244The following sections describe the operations permitted by OpenSSL's secure 245renegotiation implementation. 246 247=head2 Patched client and server 248 249Connections and renegotiation are always permitted by OpenSSL implementations. 250 251=head2 Unpatched client and patched OpenSSL server 252 253The initial connection suceeds but client renegotiation is denied by the 254server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 255B<handshake_failure> alert in SSL v3.0. 256 257If the patched OpenSSL server attempts to renegotiate a fatal 258B<handshake_failure> alert is sent. This is because the server code may be 259unaware of the unpatched nature of the client. 260 261If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 262renegotiation B<always> succeeds. 263 264B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 265unpatched) will result in the connection hanging if it receives a 266B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 267a B<no_renegotiation> alert as fatal and respond with a fatal 268B<handshake_failure> alert. This is because the OpenSSL API currently has 269no provision to indicate to an application that a renegotiation attempt 270was refused. 271 272=head2 Patched OpenSSL client and unpatched server. 273 274If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 275B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 276and renegotiation between patched OpenSSL clients and unpatched servers 277succeeds. If neither option is set then initial connections to unpatched 278servers will fail. 279 280The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 281though it has security implications: otherwise it would be impossible to 282connect to unpatched servers (i.e. all of them initially) and this is clearly 283not acceptable. Renegotiation is permitted because this does not add any 284additional security issues: during an attack clients do not see any 285renegotiations anyway. 286 287As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 288B<not> be set by default in a future version of OpenSSL. 289 290OpenSSL client applications wishing to ensure they can connect to unpatched 291servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 292 293OpenSSL client applications that want to ensure they can B<not> connect to 294unpatched servers (and thus avoid any security issues) should always B<clear> 295B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 296SSL_clear_options(). 297 298The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 299B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 300B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 301renegotiation between OpenSSL clients and unpatched servers B<only>, while 302B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 303and renegotiation between OpenSSL and unpatched clients or servers. 304 305=head1 RETURN VALUES 306 307SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 308after adding B<options>. 309 310SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 311after clearing B<options>. 312 313SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 314 315SSL_get_secure_renegotiation_support() returns 1 is the peer supports 316secure renegotiation and 0 if it does not. 317 318=head1 SEE ALSO 319 320L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 321L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 322L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 323L<dhparam(1)|dhparam(1)> 324 325=head1 HISTORY 326 327B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 328B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 329OpenSSL 0.9.7. 330 331B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 332enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 333and must be explicitly set. 334 335B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 336Versions up to OpenSSL 0.9.6c do not include the countermeasure that 337can be disabled with this option (in OpenSSL 0.9.6d, it was always 338enabled). 339 340SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3410.9.8m. 342 343B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 344and the function SSL_get_secure_renegotiation_support() were first added in 345OpenSSL 0.9.8m. 346 347=cut 348