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_SAFARI_ECDHE_ECDSA_BUG
67 Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
68 OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
70 =item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
72 Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
73 vulnerability affecting CBC ciphers, which cannot be handled by some
74 broken SSL implementations. This option has no effect for connections
77 =item SSL_OP_TLSEXT_PADDING
79 Adds a padding extension to ensure the ClientHello size is never between
80 256 and 511 bytes in length. This is needed as a workaround for some
85 All of the above bug workarounds plus B<SSL_OP_LEGACY_SERVER_CONNECT> as
90 It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
91 options if compatibility with somewhat broken implementations is
94 The following B<modifying> options are available:
98 =item SSL_OP_TLS_ROLLBACK_BUG
100 Disable version rollback attack detection.
102 During the client key exchange, the client must send the same information
103 about acceptable SSL/TLS protocol levels as during the first hello. Some
104 clients violate this rule by adapting to the server's answer. (Example:
105 the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
106 only understands up to SSLv3. In this case the client must still use the
107 same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
108 to the server's answer and violate the version rollback protection.)
110 =item SSL_OP_CIPHER_SERVER_PREFERENCE
112 When choosing a cipher, use the server's preferences instead of the client
113 preferences. When not set, the SSL server will always follow the clients
114 preferences. When set, the SSL/TLS server will choose following its
117 =item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1,
118 SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
120 These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol
121 versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS,
123 As of OpenSSL 1.1.0, these options are deprecated, use
124 L<SSL_CTX_set_min_proto_version(3)> and
125 L<SSL_CTX_set_max_proto_version(3)> instead.
127 =item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
129 When performing renegotiation as a server, always start a new session
130 (i.e., session resumption requests are only accepted in the initial
131 handshake). This option is not needed for clients.
133 =item SSL_OP_NO_COMPRESSION
135 Do not use compression even if it is supported.
137 =item SSL_OP_NO_QUERY_MTU
139 Do not query the MTU. Only affects DTLS connections.
141 =item SSL_OP_COOKIE_EXCHANGE
143 Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects
146 =item SSL_OP_NO_TICKET
148 Normally clients and servers will, where possible, transparently make use
149 of RFC4507bis tickets for stateless session resumption.
151 If this option is set this functionality is disabled and tickets will
152 not be used by clients or servers.
154 This option only applies to TLSv1.2 and below. It is ignored for TLSv1.3.
156 =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
158 Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
159 servers. See the B<SECURE RENEGOTIATION> section for more details.
161 =item SSL_OP_LEGACY_SERVER_CONNECT
163 Allow legacy insecure renegotiation between OpenSSL and unpatched servers
164 B<only>: this option is currently set by default. See the
165 B<SECURE RENEGOTIATION> section for more details.
167 =item SSL_OP_NO_ENCRYPT_THEN_MAC
169 Normally clients and servers will transparently attempt to negotiate the
170 RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.
172 If this option is set, Encrypt-then-MAC is disabled. Clients will not
173 propose, and servers will not accept the extension.
175 =item SSL_OP_NO_RENEGOTIATION
177 Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest
178 messages, and ignore renegotiation requests via ClientHello.
180 =item SSL_OP_ALLOW_NO_DHE_KEX
182 In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means
183 that there will be no forward secrecy for the resumed session.
185 =item SSL_OP_PRIORITIZE_CHACHA
187 When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize
188 ChaCha20-Poly1305 ciphers to the top of the server cipher list if a
189 ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps
190 those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere
191 in the server cipher list; but still allows other clients to use AES and other
192 ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>.
194 =item SSL_OP_ENABLE_MIDDLEBOX_COMPAT
196 If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This
197 has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that
198 do not understand TLSv1.3 will not drop the connection. Regardless of whether
199 this option is set or not CCS messages received from the peer will always be
200 ignored in TLSv1.3. This option is set by default. To switch it off use
201 SSL_clear_options(). A future version of OpenSSL may not set this by default.
205 The following options no longer have any effect but their identifiers are
206 retained for compatibility purposes:
210 =item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
212 =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
214 =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
216 =item SSL_OP_TLS_D5_BUG
218 =item SSL_OP_TLS_BLOCK_PADDING_BUG
220 =item SSL_OP_MSIE_SSLV2_RSA_PADDING
222 =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
224 =item SSL_OP_MICROSOFT_SESS_ID_BUG
226 =item SSL_OP_NETSCAPE_CHALLENGE_BUG
228 =item SSL_OP_PKCS1_CHECK_1
230 =item SSL_OP_PKCS1_CHECK_2
232 =item SSL_OP_SINGLE_DH_USE
234 =item SSL_OP_SINGLE_ECDH_USE
236 =item SSL_OP_EPHEMERAL_RSA
240 =head1 SECURE RENEGOTIATION
242 OpenSSL always attempts to use secure renegotiation as
243 described in RFC5746. This counters the prefix attack described in
244 CVE-2009-3555 and elsewhere.
246 This attack has far reaching consequences which application writers should be
247 aware of. In the description below an implementation supporting secure
248 renegotiation is referred to as I<patched>. A server not supporting secure
249 renegotiation is referred to as I<unpatched>.
251 The following sections describe the operations permitted by OpenSSL's secure
252 renegotiation implementation.
254 =head2 Patched client and server
256 Connections and renegotiation are always permitted by OpenSSL implementations.
258 =head2 Unpatched client and patched OpenSSL server
260 The initial connection succeeds but client renegotiation is denied by the
261 server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
262 B<handshake_failure> alert in SSL v3.0.
264 If the patched OpenSSL server attempts to renegotiate a fatal
265 B<handshake_failure> alert is sent. This is because the server code may be
266 unaware of the unpatched nature of the client.
268 If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
269 renegotiation B<always> succeeds.
271 =head2 Patched OpenSSL client and unpatched server.
273 If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
274 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
275 and renegotiation between patched OpenSSL clients and unpatched servers
276 succeeds. If neither option is set then initial connections to unpatched
279 The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
280 though it has security implications: otherwise it would be impossible to
281 connect to unpatched servers (i.e. all of them initially) and this is clearly
282 not acceptable. Renegotiation is permitted because this does not add any
283 additional security issues: during an attack clients do not see any
284 renegotiations anyway.
286 As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
287 B<not> be set by default in a future version of OpenSSL.
289 OpenSSL client applications wishing to ensure they can connect to unpatched
290 servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
292 OpenSSL client applications that want to ensure they can B<not> connect to
293 unpatched servers (and thus avoid any security issues) should always B<clear>
294 B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
297 The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
298 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
299 B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
300 renegotiation between OpenSSL clients and unpatched servers B<only>, while
301 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
302 and renegotiation between OpenSSL and unpatched clients or servers.
306 SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
307 after adding B<options>.
309 SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
310 after clearing B<options>.
312 SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
314 SSL_get_secure_renegotiation_support() returns 1 is the peer supports
315 secure renegotiation and 0 if it does not.
319 L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>,
320 L<SSL_CTX_set_tmp_dh_callback(3)>,
321 L<SSL_CTX_set_min_proto_version(3)>,
326 The attempt to always try to use secure renegotiation was added in
329 B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> were added in
334 Copyright 2001-2018 The OpenSSL Project Authors. All Rights Reserved.
336 Licensed under the OpenSSL license (the "License"). You may not use
337 this file except in compliance with the License. You can obtain a copy
338 in the file LICENSE in the source distribution or at
339 L<https://www.openssl.org/source/license.html>.