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 155This option is no longer implemented and is treated as no op. 156 157=item SSL_OP_CIPHER_SERVER_PREFERENCE 158 159When choosing a cipher, use the server's preferences instead of the client 160preferences. When not set, the SSL server will always follow the clients 161preferences. When set, the SSLv3/TLSv1 server will choose following its 162own preferences. Because of the different protocol, for SSLv2 the server 163will send its list of preferences to the client and the client chooses. 164 165=item SSL_OP_PKCS1_CHECK_1 166 167... 168 169=item SSL_OP_PKCS1_CHECK_2 170 171... 172 173=item SSL_OP_NETSCAPE_CA_DN_BUG 174 175If we accept a netscape connection, demand a client cert, have a 176non-self-signed CA which does not have its CA in netscape, and the 177browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta 178 179=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 180 181... 182 183=item SSL_OP_NO_SSLv2 184 185Do not use the SSLv2 protocol. 186 187=item SSL_OP_NO_SSLv3 188 189Do not use the SSLv3 protocol. 190 191=item SSL_OP_NO_TLSv1 192 193Do not use the TLSv1 protocol. 194 195=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 196 197When performing renegotiation as a server, always start a new session 198(i.e., session resumption requests are only accepted in the initial 199handshake). This option is not needed for clients. 200 201=item SSL_OP_NO_TICKET 202 203Normally clients and servers will, where possible, transparently make use 204of RFC4507bis tickets for stateless session resumption. 205 206If this option is set this functionality is disabled and tickets will 207not be used by clients or servers. 208 209=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 210 211Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 212servers. See the B<SECURE RENEGOTIATION> section for more details. 213 214=item SSL_OP_LEGACY_SERVER_CONNECT 215 216Allow legacy insecure renegotiation between OpenSSL and unpatched servers 217B<only>: this option is currently set by default. See the 218B<SECURE RENEGOTIATION> section for more details. 219 220=back 221 222=head1 SECURE RENEGOTIATION 223 224OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 225described in RFC5746. This counters the prefix attack described in 226CVE-2009-3555 and elsewhere. 227 228The deprecated and highly broken SSLv2 protocol does not support 229renegotiation at all: its use is B<strongly> discouraged. 230 231This attack has far reaching consequences which application writers should be 232aware of. In the description below an implementation supporting secure 233renegotiation is referred to as I<patched>. A server not supporting secure 234renegotiation is referred to as I<unpatched>. 235 236The following sections describe the operations permitted by OpenSSL's secure 237renegotiation implementation. 238 239=head2 Patched client and server 240 241Connections and renegotiation are always permitted by OpenSSL implementations. 242 243=head2 Unpatched client and patched OpenSSL server 244 245The initial connection succeeds but client renegotiation is denied by the 246server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 247B<handshake_failure> alert in SSL v3.0. 248 249If the patched OpenSSL server attempts to renegotiate a fatal 250B<handshake_failure> alert is sent. This is because the server code may be 251unaware of the unpatched nature of the client. 252 253If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 254renegotiation B<always> succeeds. 255 256B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 257unpatched) will result in the connection hanging if it receives a 258B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 259a B<no_renegotiation> alert as fatal and respond with a fatal 260B<handshake_failure> alert. This is because the OpenSSL API currently has 261no provision to indicate to an application that a renegotiation attempt 262was refused. 263 264=head2 Patched OpenSSL client and unpatched server. 265 266If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 267B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 268and renegotiation between patched OpenSSL clients and unpatched servers 269succeeds. If neither option is set then initial connections to unpatched 270servers will fail. 271 272The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 273though it has security implications: otherwise it would be impossible to 274connect to unpatched servers (i.e. all of them initially) and this is clearly 275not acceptable. Renegotiation is permitted because this does not add any 276additional security issues: during an attack clients do not see any 277renegotiations anyway. 278 279As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 280B<not> be set by default in a future version of OpenSSL. 281 282OpenSSL client applications wishing to ensure they can connect to unpatched 283servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 284 285OpenSSL client applications that want to ensure they can B<not> connect to 286unpatched servers (and thus avoid any security issues) should always B<clear> 287B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 288SSL_clear_options(). 289 290The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 291B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 292B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 293renegotiation between OpenSSL clients and unpatched servers B<only>, while 294B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 295and renegotiation between OpenSSL and unpatched clients or servers. 296 297=head1 RETURN VALUES 298 299SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 300after adding B<options>. 301 302SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 303after clearing B<options>. 304 305SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 306 307SSL_get_secure_renegotiation_support() returns 1 is the peer supports 308secure renegotiation and 0 if it does not. 309 310=head1 SEE ALSO 311 312L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 313L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 314L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 315L<dhparam(1)|dhparam(1)> 316 317=head1 HISTORY 318 319B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 320B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 321OpenSSL 0.9.7. 322 323B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 324enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 325and must be explicitly set. 326 327B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 328Versions up to OpenSSL 0.9.6c do not include the countermeasure that 329can be disabled with this option (in OpenSSL 0.9.6d, it was always 330enabled). 331 332SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3330.9.8m. 334 335B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 336and the function SSL_get_secure_renegotiation_support() were first added in 337OpenSSL 0.9.8m. 338 339=cut 340