SSL_CTX_set_options.pod revision 238405
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_MSIE_SSLV2_RSA_PADDING 92 93As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect. 94 95=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG 96 97... 98 99=item SSL_OP_TLS_D5_BUG 100 101... 102 103=item SSL_OP_TLS_BLOCK_PADDING_BUG 104 105... 106 107=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS 108 109Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol 110vulnerability affecting CBC ciphers, which cannot be handled by some 111broken SSL implementations. This option has no effect for connections 112using other ciphers. 113 114=item SSL_OP_ALL 115 116All of the above bug workarounds. 117 118=back 119 120It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround 121options if compatibility with somewhat broken implementations is 122desired. 123 124The following B<modifying> options are available: 125 126=over 4 127 128=item SSL_OP_TLS_ROLLBACK_BUG 129 130Disable version rollback attack detection. 131 132During the client key exchange, the client must send the same information 133about acceptable SSL/TLS protocol levels as during the first hello. Some 134clients violate this rule by adapting to the server's answer. (Example: 135the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server 136only understands up to SSLv3. In this case the client must still use the 137same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect 138to the server's answer and violate the version rollback protection.) 139 140=item SSL_OP_SINGLE_DH_USE 141 142Always create a new key when using temporary/ephemeral DH parameters 143(see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>). 144This option must be used to prevent small subgroup attacks, when 145the DH parameters were not generated using "strong" primes 146(e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>). 147If "strong" primes were used, it is not strictly necessary to generate 148a new DH key during each handshake but it is also recommended. 149B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever 150temporary/ephemeral DH parameters are used. 151 152=item SSL_OP_EPHEMERAL_RSA 153 154Always use ephemeral (temporary) RSA key when doing RSA operations 155(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>). 156According to the specifications this is only done, when a RSA key 157can only be used for signature operations (namely under export ciphers 158with restricted RSA keylength). By setting this option, ephemeral 159RSA keys are always used. This option breaks compatibility with the 160SSL/TLS specifications and may lead to interoperability problems with 161clients and should therefore never be used. Ciphers with EDH (ephemeral 162Diffie-Hellman) key exchange should be used instead. 163 164=item SSL_OP_CIPHER_SERVER_PREFERENCE 165 166When choosing a cipher, use the server's preferences instead of the client 167preferences. When not set, the SSL server will always follow the clients 168preferences. When set, the SSLv3/TLSv1 server will choose following its 169own preferences. Because of the different protocol, for SSLv2 the server 170will send its list of preferences to the client and the client chooses. 171 172=item SSL_OP_PKCS1_CHECK_1 173 174... 175 176=item SSL_OP_PKCS1_CHECK_2 177 178... 179 180=item SSL_OP_NETSCAPE_CA_DN_BUG 181 182If we accept a netscape connection, demand a client cert, have a 183non-self-signed CA which does not have its CA in netscape, and the 184browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta 185 186=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 187 188... 189 190=item SSL_OP_NO_SSLv2 191 192Do not use the SSLv2 protocol. 193 194=item SSL_OP_NO_SSLv3 195 196Do not use the SSLv3 protocol. 197 198=item SSL_OP_NO_TLSv1 199 200Do not use the TLSv1 protocol. 201 202=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 203 204When performing renegotiation as a server, always start a new session 205(i.e., session resumption requests are only accepted in the initial 206handshake). This option is not needed for clients. 207 208=item SSL_OP_NO_TICKET 209 210Normally clients and servers will, where possible, transparently make use 211of RFC4507bis tickets for stateless session resumption. 212 213If this option is set this functionality is disabled and tickets will 214not be used by clients or servers. 215 216=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 217 218Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 219servers. See the B<SECURE RENEGOTIATION> section for more details. 220 221=item SSL_OP_LEGACY_SERVER_CONNECT 222 223Allow legacy insecure renegotiation between OpenSSL and unpatched servers 224B<only>: this option is currently set by default. See the 225B<SECURE RENEGOTIATION> section for more details. 226 227=back 228 229=head1 SECURE RENEGOTIATION 230 231OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 232described in RFC5746. This counters the prefix attack described in 233CVE-2009-3555 and elsewhere. 234 235The deprecated and highly broken SSLv2 protocol does not support 236renegotiation at all: its use is B<strongly> discouraged. 237 238This attack has far reaching consequences which application writers should be 239aware of. In the description below an implementation supporting secure 240renegotiation is referred to as I<patched>. A server not supporting secure 241renegotiation is referred to as I<unpatched>. 242 243The following sections describe the operations permitted by OpenSSL's secure 244renegotiation implementation. 245 246=head2 Patched client and server 247 248Connections and renegotiation are always permitted by OpenSSL implementations. 249 250=head2 Unpatched client and patched OpenSSL server 251 252The initial connection suceeds but client renegotiation is denied by the 253server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 254B<handshake_failure> alert in SSL v3.0. 255 256If the patched OpenSSL server attempts to renegotiate a fatal 257B<handshake_failure> alert is sent. This is because the server code may be 258unaware of the unpatched nature of the client. 259 260If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 261renegotiation B<always> succeeds. 262 263B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 264unpatched) will result in the connection hanging if it receives a 265B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 266a B<no_renegotiation> alert as fatal and respond with a fatal 267B<handshake_failure> alert. This is because the OpenSSL API currently has 268no provision to indicate to an application that a renegotiation attempt 269was refused. 270 271=head2 Patched OpenSSL client and unpatched server. 272 273If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 274B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 275and renegotiation between patched OpenSSL clients and unpatched servers 276succeeds. If neither option is set then initial connections to unpatched 277servers will fail. 278 279The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 280though it has security implications: otherwise it would be impossible to 281connect to unpatched servers (i.e. all of them initially) and this is clearly 282not acceptable. Renegotiation is permitted because this does not add any 283additional security issues: during an attack clients do not see any 284renegotiations anyway. 285 286As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 287B<not> be set by default in a future version of OpenSSL. 288 289OpenSSL client applications wishing to ensure they can connect to unpatched 290servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 291 292OpenSSL client applications that want to ensure they can B<not> connect to 293unpatched servers (and thus avoid any security issues) should always B<clear> 294B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 295SSL_clear_options(). 296 297The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 298B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 299B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 300renegotiation between OpenSSL clients and unpatched servers B<only>, while 301B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 302and renegotiation between OpenSSL and unpatched clients or servers. 303 304=head1 RETURN VALUES 305 306SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 307after adding B<options>. 308 309SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 310after clearing B<options>. 311 312SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 313 314SSL_get_secure_renegotiation_support() returns 1 is the peer supports 315secure renegotiation and 0 if it does not. 316 317=head1 SEE ALSO 318 319L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 320L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 321L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 322L<dhparam(1)|dhparam(1)> 323 324=head1 HISTORY 325 326B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 327B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 328OpenSSL 0.9.7. 329 330B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 331enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 332and must be explicitly set. 333 334B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 335Versions up to OpenSSL 0.9.6c do not include the countermeasure that 336can be disabled with this option (in OpenSSL 0.9.6d, it was always 337enabled). 338 339SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3400.9.8m. 341 342B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 343and the function SSL_get_secure_renegotiation_support() were first added in 344OpenSSL 0.9.8m. 345 346=cut 347