5 SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options,
6 SSL_clear_options, SSL_CTX_get_options, SSL_get_options,
7 SSL_get_secure_renegotiation_support - manipulate SSL options
11 #include <openssl/ssl.h>
13 long SSL_CTX_set_options(SSL_CTX *ctx, long options);
14 long SSL_set_options(SSL *ssl, long options);
16 long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
17 long SSL_clear_options(SSL *ssl, long options);
19 long SSL_CTX_get_options(SSL_CTX *ctx);
20 long SSL_get_options(SSL *ssl);
22 long SSL_get_secure_renegotiation_support(SSL *ssl);
26 SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
27 Options already set before are not cleared!
29 SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
30 Options already set before are not cleared!
32 SSL_CTX_clear_options() clears the options set via bitmask in B<options>
35 SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.
37 SSL_CTX_get_options() returns the options set for B<ctx>.
39 SSL_get_options() returns the options set for B<ssl>.
41 SSL_get_secure_renegotiation_support() indicates whether the peer supports
43 Note, this is implemented via a macro.
47 The behaviour of the SSL library can be changed by setting several options.
48 The options are coded as bitmasks and can be combined by a bitwise B<or>
51 SSL_CTX_set_options() and SSL_set_options() affect the (external)
52 protocol behaviour of the SSL library. The (internal) behaviour of
53 the API can be changed by using the similar
54 L<SSL_CTX_set_mode(3)> and SSL_set_mode() functions.
56 During a handshake, the option settings of the SSL object are used. When
57 a new SSL object is created from a context using SSL_new(), the current
58 option setting is copied. Changes to B<ctx> do not affect already created
59 SSL objects. SSL_clear() does not affect the settings.
61 The following B<bug workaround> options are available:
65 =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
69 =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
73 =item SSL_OP_SAFARI_ECDHE_ECDSA_BUG
75 Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
76 OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
78 =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
82 =item SSL_OP_TLS_D5_BUG
86 =item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
88 Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
89 vulnerability affecting CBC ciphers, which cannot be handled by some
90 broken SSL implementations. This option has no effect for connections
93 =item SSL_OP_TLSEXT_PADDING
95 Adds a padding extension to ensure the ClientHello size is never between
96 256 and 511 bytes in length. This is needed as a workaround for some
101 All of the above bug workarounds.
105 It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
106 options if compatibility with somewhat broken implementations is
109 The following B<modifying> options are available:
113 =item SSL_OP_TLS_ROLLBACK_BUG
115 Disable version rollback attack detection.
117 During the client key exchange, the client must send the same information
118 about acceptable SSL/TLS protocol levels as during the first hello. Some
119 clients violate this rule by adapting to the server's answer. (Example:
120 the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
121 only understands up to SSLv3. In this case the client must still use the
122 same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
123 to the server's answer and violate the version rollback protection.)
125 =item SSL_OP_SINGLE_DH_USE
127 Always create a new key when using temporary/ephemeral DH parameters
128 (see L<SSL_CTX_set_tmp_dh_callback(3)>).
129 This option must be used to prevent small subgroup attacks, when
130 the DH parameters were not generated using "strong" primes
131 (e.g. when using DSA-parameters, see L<dhparam(1)>).
132 If "strong" primes were used, it is not strictly necessary to generate
133 a new DH key during each handshake but it is also recommended.
134 B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever
135 temporary/ephemeral DH parameters are used.
137 =item SSL_OP_EPHEMERAL_RSA
139 This option is no longer implemented and is treated as no op.
141 =item SSL_OP_CIPHER_SERVER_PREFERENCE
143 When choosing a cipher, use the server's preferences instead of the client
144 preferences. When not set, the SSL server will always follow the clients
145 preferences. When set, the SSL/TLS server will choose following its
148 =item SSL_OP_PKCS1_CHECK_1
152 =item SSL_OP_PKCS1_CHECK_2
157 =item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1,
158 SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
160 These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol
161 versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS,
163 As of OpenSSL 1.1.0, these options are deprecated, use
164 L<SSL_CTX_set_min_proto_version(3)> and
165 L<SSL_CTX_set_max_proto_version(3)> instead.
167 =item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
169 When performing renegotiation as a server, always start a new session
170 (i.e., session resumption requests are only accepted in the initial
171 handshake). This option is not needed for clients.
173 =item SSL_OP_NO_TICKET
175 Normally clients and servers will, where possible, transparently make use
176 of RFC4507bis tickets for stateless session resumption.
178 If this option is set this functionality is disabled and tickets will
179 not be used by clients or servers.
181 =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
183 Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
184 servers. See the B<SECURE RENEGOTIATION> section for more details.
186 =item SSL_OP_LEGACY_SERVER_CONNECT
188 Allow legacy insecure renegotiation between OpenSSL and unpatched servers
189 B<only>: this option is currently set by default. See the
190 B<SECURE RENEGOTIATION> section for more details.
192 =item SSL_OP_NO_ENCRYPT_THEN_MAC
194 Normally clients and servers will transparently attempt to negotiate the
195 RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.
197 If this option is set, Encrypt-then-MAC is disabled. Clients will not
198 propose, and servers will not accept the extension.
202 =head1 SECURE RENEGOTIATION
204 OpenSSL always attempts to use secure renegotiation as
205 described in RFC5746. This counters the prefix attack described in
206 CVE-2009-3555 and elsewhere.
208 This attack has far reaching consequences which application writers should be
209 aware of. In the description below an implementation supporting secure
210 renegotiation is referred to as I<patched>. A server not supporting secure
211 renegotiation is referred to as I<unpatched>.
213 The following sections describe the operations permitted by OpenSSL's secure
214 renegotiation implementation.
216 =head2 Patched client and server
218 Connections and renegotiation are always permitted by OpenSSL implementations.
220 =head2 Unpatched client and patched OpenSSL server
222 The initial connection succeeds but client renegotiation is denied by the
223 server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
224 B<handshake_failure> alert in SSL v3.0.
226 If the patched OpenSSL server attempts to renegotiate a fatal
227 B<handshake_failure> alert is sent. This is because the server code may be
228 unaware of the unpatched nature of the client.
230 If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
231 renegotiation B<always> succeeds.
233 =head2 Patched OpenSSL client and unpatched server.
235 If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
236 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
237 and renegotiation between patched OpenSSL clients and unpatched servers
238 succeeds. If neither option is set then initial connections to unpatched
241 The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
242 though it has security implications: otherwise it would be impossible to
243 connect to unpatched servers (i.e. all of them initially) and this is clearly
244 not acceptable. Renegotiation is permitted because this does not add any
245 additional security issues: during an attack clients do not see any
246 renegotiations anyway.
248 As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
249 B<not> be set by default in a future version of OpenSSL.
251 OpenSSL client applications wishing to ensure they can connect to unpatched
252 servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
254 OpenSSL client applications that want to ensure they can B<not> connect to
255 unpatched servers (and thus avoid any security issues) should always B<clear>
256 B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
259 The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
260 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
261 B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
262 renegotiation between OpenSSL clients and unpatched servers B<only>, while
263 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
264 and renegotiation between OpenSSL and unpatched clients or servers.
268 SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
269 after adding B<options>.
271 SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
272 after clearing B<options>.
274 SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
276 SSL_get_secure_renegotiation_support() returns 1 is the peer supports
277 secure renegotiation and 0 if it does not.
281 L<ssl(3)>, L<SSL_new(3)>, L<SSL_clear(3)>,
282 L<SSL_CTX_set_tmp_dh_callback(3)>,
283 L<SSL_CTX_set_min_proto_version(3)>,
288 The attempt to always try to use secure renegotiation was added in
293 Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
295 Licensed under the OpenSSL license (the "License"). You may not use
296 this file except in compliance with the License. You can obtain a copy
297 in the file LICENSE in the source distribution or at
298 L<https://www.openssl.org/source/license.html>.