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 if extension support 213is explicitly set when OpenSSL is compiled. 214 215If this option is set this functionality is disabled and tickets will 216not be used by clients or servers. 217 218=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 219 220Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 221servers. See the B<SECURE RENEGOTIATION> section for more details. 222 223=item SSL_OP_LEGACY_SERVER_CONNECT 224 225Allow legacy insecure renegotiation between OpenSSL and unpatched servers 226B<only>: this option is currently set by default. See the 227B<SECURE RENEGOTIATION> section for more details. 228 229=back 230 231=head1 SECURE RENEGOTIATION 232 233OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 234described in RFC5746. This counters the prefix attack described in 235CVE-2009-3555 and elsewhere. 236 237The deprecated and highly broken SSLv2 protocol does not support 238renegotiation at all: its use is B<strongly> discouraged. 239 240This attack has far reaching consequences which application writers should be 241aware of. In the description below an implementation supporting secure 242renegotiation is referred to as I<patched>. A server not supporting secure 243renegotiation is referred to as I<unpatched>. 244 245The following sections describe the operations permitted by OpenSSL's secure 246renegotiation implementation. 247 248=head2 Patched client and server 249 250Connections and renegotiation are always permitted by OpenSSL implementations. 251 252=head2 Unpatched client and patched OpenSSL server 253 254The initial connection suceeds but client renegotiation is denied by the 255server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 256B<handshake_failure> alert in SSL v3.0. 257 258If the patched OpenSSL server attempts to renegotiate a fatal 259B<handshake_failure> alert is sent. This is because the server code may be 260unaware of the unpatched nature of the client. 261 262If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 263renegotiation B<always> succeeds. 264 265B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 266unpatched) will result in the connection hanging if it receives a 267B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 268a B<no_renegotiation> alert as fatal and respond with a fatal 269B<handshake_failure> alert. This is because the OpenSSL API currently has 270no provision to indicate to an application that a renegotiation attempt 271was refused. 272 273=head2 Patched OpenSSL client and unpatched server. 274 275If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 276B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 277and renegotiation between patched OpenSSL clients and unpatched servers 278succeeds. If neither option is set then initial connections to unpatched 279servers will fail. 280 281The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 282though it has security implications: otherwise it would be impossible to 283connect to unpatched servers (i.e. all of them initially) and this is clearly 284not acceptable. Renegotiation is permitted because this does not add any 285additional security issues: during an attack clients do not see any 286renegotiations anyway. 287 288As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 289B<not> be set by default in a future version of OpenSSL. 290 291OpenSSL client applications wishing to ensure they can connect to unpatched 292servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 293 294OpenSSL client applications that want to ensure they can B<not> connect to 295unpatched servers (and thus avoid any security issues) should always B<clear> 296B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 297SSL_clear_options(). 298 299The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 300B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 301B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 302renegotiation between OpenSSL clients and unpatched servers B<only>, while 303B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 304and renegotiation between OpenSSL and unpatched clients or servers. 305 306=head1 RETURN VALUES 307 308SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 309after adding B<options>. 310 311SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 312after clearing B<options>. 313 314SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 315 316SSL_get_secure_renegotiation_support() returns 1 is the peer supports 317secure renegotiation and 0 if it does not. 318 319=head1 SEE ALSO 320 321L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 322L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 323L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 324L<dhparam(1)|dhparam(1)> 325 326=head1 HISTORY 327 328B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 329B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 330OpenSSL 0.9.7. 331 332B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 333enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 334and must be explicitly set. 335 336B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 337Versions up to OpenSSL 0.9.6c do not include the countermeasure that 338can be disabled with this option (in OpenSSL 0.9.6d, it was always 339enabled). 340 341SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3420.9.8m. 343 344B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 345and the function SSL_get_secure_renegotiation_support() were first added in 346OpenSSL 0.9.8m. 347 348=cut 349