SSL_CTX_set_options.pod revision 277195
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 154This option is no longer implemented and is treated as no op. 155 156=item SSL_OP_CIPHER_SERVER_PREFERENCE 157 158When choosing a cipher, use the server's preferences instead of the client 159preferences. When not set, the SSL server will always follow the clients 160preferences. When set, the SSLv3/TLSv1 server will choose following its 161own preferences. Because of the different protocol, for SSLv2 the server 162will send its list of preferences to the client and the client chooses. 163 164=item SSL_OP_PKCS1_CHECK_1 165 166... 167 168=item SSL_OP_PKCS1_CHECK_2 169 170... 171 172=item SSL_OP_NETSCAPE_CA_DN_BUG 173 174If we accept a netscape connection, demand a client cert, have a 175non-self-signed CA which does not have its CA in netscape, and the 176browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta 177 178=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 179 180... 181 182=item SSL_OP_NO_SSLv2 183 184Do not use the SSLv2 protocol. 185 186=item SSL_OP_NO_SSLv3 187 188Do not use the SSLv3 protocol. 189 190=item SSL_OP_NO_TLSv1 191 192Do not use the TLSv1 protocol. 193 194=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 195 196When performing renegotiation as a server, always start a new session 197(i.e., session resumption requests are only accepted in the initial 198handshake). This option is not needed for clients. 199 200=item SSL_OP_NO_TICKET 201 202Normally clients and servers will, where possible, transparently make use 203of RFC4507bis tickets for stateless session resumption. 204 205If this option is set this functionality is disabled and tickets will 206not be used by clients or servers. 207 208=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 209 210Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 211servers. See the B<SECURE RENEGOTIATION> section for more details. 212 213=item SSL_OP_LEGACY_SERVER_CONNECT 214 215Allow legacy insecure renegotiation between OpenSSL and unpatched servers 216B<only>: this option is currently set by default. See the 217B<SECURE RENEGOTIATION> section for more details. 218 219=back 220 221=head1 SECURE RENEGOTIATION 222 223OpenSSL 0.9.8m and later always attempts to use secure renegotiation as 224described in RFC5746. This counters the prefix attack described in 225CVE-2009-3555 and elsewhere. 226 227The deprecated and highly broken SSLv2 protocol does not support 228renegotiation at all: its use is B<strongly> discouraged. 229 230This attack has far reaching consequences which application writers should be 231aware of. In the description below an implementation supporting secure 232renegotiation is referred to as I<patched>. A server not supporting secure 233renegotiation is referred to as I<unpatched>. 234 235The following sections describe the operations permitted by OpenSSL's secure 236renegotiation implementation. 237 238=head2 Patched client and server 239 240Connections and renegotiation are always permitted by OpenSSL implementations. 241 242=head2 Unpatched client and patched OpenSSL server 243 244The initial connection suceeds but client renegotiation is denied by the 245server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 246B<handshake_failure> alert in SSL v3.0. 247 248If the patched OpenSSL server attempts to renegotiate a fatal 249B<handshake_failure> alert is sent. This is because the server code may be 250unaware of the unpatched nature of the client. 251 252If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 253renegotiation B<always> succeeds. 254 255B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are 256unpatched) will result in the connection hanging if it receives a 257B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard 258a B<no_renegotiation> alert as fatal and respond with a fatal 259B<handshake_failure> alert. This is because the OpenSSL API currently has 260no provision to indicate to an application that a renegotiation attempt 261was refused. 262 263=head2 Patched OpenSSL client and unpatched server. 264 265If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 266B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 267and renegotiation between patched OpenSSL clients and unpatched servers 268succeeds. If neither option is set then initial connections to unpatched 269servers will fail. 270 271The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even 272though it has security implications: otherwise it would be impossible to 273connect to unpatched servers (i.e. all of them initially) and this is clearly 274not acceptable. Renegotiation is permitted because this does not add any 275additional security issues: during an attack clients do not see any 276renegotiations anyway. 277 278As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will 279B<not> be set by default in a future version of OpenSSL. 280 281OpenSSL client applications wishing to ensure they can connect to unpatched 282servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 283 284OpenSSL client applications that want to ensure they can B<not> connect to 285unpatched servers (and thus avoid any security issues) should always B<clear> 286B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 287SSL_clear_options(). 288 289The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 290B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 291B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 292renegotiation between OpenSSL clients and unpatched servers B<only>, while 293B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 294and renegotiation between OpenSSL and unpatched clients or servers. 295 296=head1 RETURN VALUES 297 298SSL_CTX_set_options() and SSL_set_options() return the new options bitmask 299after adding B<options>. 300 301SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask 302after clearing B<options>. 303 304SSL_CTX_get_options() and SSL_get_options() return the current bitmask. 305 306SSL_get_secure_renegotiation_support() returns 1 is the peer supports 307secure renegotiation and 0 if it does not. 308 309=head1 SEE ALSO 310 311L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>, 312L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>, 313L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>, 314L<dhparam(1)|dhparam(1)> 315 316=head1 HISTORY 317 318B<SSL_OP_CIPHER_SERVER_PREFERENCE> and 319B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in 320OpenSSL 0.9.7. 321 322B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically 323enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL> 324and must be explicitly set. 325 326B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e. 327Versions up to OpenSSL 0.9.6c do not include the countermeasure that 328can be disabled with this option (in OpenSSL 0.9.6d, it was always 329enabled). 330 331SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL 3320.9.8m. 333 334B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT> 335and the function SSL_get_secure_renegotiation_support() were first added in 336OpenSSL 0.9.8m. 337 338=cut 339