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 SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless
151 When using session ids a copy of the session information is
152 cached on the server and a unique id is sent to the client. When the client
153 wishes to resume it provides the unique id so that the server can retrieve the
154 session information from its cache.
156 When using stateless session tickets the server uses a session ticket encryption
157 key to encrypt the session information. This encrypted data is sent to the
158 client as a "ticket". When the client wishes to resume it sends the encrypted
159 data back to the server. The server uses its key to decrypt the data and resume
160 the session. In this way the server can operate statelessly - no session
161 information needs to be cached locally.
163 The TLSv1.3 protocol only supports tickets and does not directly support session
164 ids. However OpenSSL allows two modes of ticket operation in TLSv1.3: stateful
165 and stateless. Stateless tickets work the same way as in TLSv1.2 and below.
166 Stateful tickets mimic the session id behaviour available in TLSv1.2 and below.
167 The session information is cached on the server and the session id is wrapped up
168 in a ticket and sent back to the client. When the client wishes to resume, it
169 presents a ticket in the same way as for stateless tickets. The server can then
170 extract the session id from the ticket and retrieve the session information from
173 By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will
174 cause stateless tickets to not be issued. In TLSv1.2 and below this means no
175 ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be
176 sent. This is a server-side option only.
178 In TLSv1.3 it is possible to suppress all tickets (stateful and stateless) from
179 being sent by calling L<SSL_CTX_set_num_tickets(3)> or
180 L<SSL_set_num_tickets(3)>.
182 =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
184 Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
185 servers. See the B<SECURE RENEGOTIATION> section for more details.
187 =item SSL_OP_LEGACY_SERVER_CONNECT
189 Allow legacy insecure renegotiation between OpenSSL and unpatched servers
190 B<only>: this option is currently set by default. See the
191 B<SECURE RENEGOTIATION> section for more details.
193 =item SSL_OP_NO_ENCRYPT_THEN_MAC
195 Normally clients and servers will transparently attempt to negotiate the
196 RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.
198 If this option is set, Encrypt-then-MAC is disabled. Clients will not
199 propose, and servers will not accept the extension.
201 =item SSL_OP_NO_RENEGOTIATION
203 Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest
204 messages, and ignore renegotiation requests via ClientHello.
206 =item SSL_OP_ALLOW_NO_DHE_KEX
208 In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means
209 that there will be no forward secrecy for the resumed session.
211 =item SSL_OP_PRIORITIZE_CHACHA
213 When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize
214 ChaCha20-Poly1305 ciphers to the top of the server cipher list if a
215 ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps
216 those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere
217 in the server cipher list; but still allows other clients to use AES and other
218 ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>.
220 =item SSL_OP_ENABLE_MIDDLEBOX_COMPAT
222 If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This
223 has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that
224 do not understand TLSv1.3 will not drop the connection. Regardless of whether
225 this option is set or not CCS messages received from the peer will always be
226 ignored in TLSv1.3. This option is set by default. To switch it off use
227 SSL_clear_options(). A future version of OpenSSL may not set this by default.
231 The following options no longer have any effect but their identifiers are
232 retained for compatibility purposes:
236 =item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
238 =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
240 =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
242 =item SSL_OP_TLS_D5_BUG
244 =item SSL_OP_TLS_BLOCK_PADDING_BUG
246 =item SSL_OP_MSIE_SSLV2_RSA_PADDING
248 =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
250 =item SSL_OP_MICROSOFT_SESS_ID_BUG
252 =item SSL_OP_NETSCAPE_CHALLENGE_BUG
254 =item SSL_OP_PKCS1_CHECK_1
256 =item SSL_OP_PKCS1_CHECK_2
258 =item SSL_OP_SINGLE_DH_USE
260 =item SSL_OP_SINGLE_ECDH_USE
262 =item SSL_OP_EPHEMERAL_RSA
266 =head1 SECURE RENEGOTIATION
268 OpenSSL always attempts to use secure renegotiation as
269 described in RFC5746. This counters the prefix attack described in
270 CVE-2009-3555 and elsewhere.
272 This attack has far reaching consequences which application writers should be
273 aware of. In the description below an implementation supporting secure
274 renegotiation is referred to as I<patched>. A server not supporting secure
275 renegotiation is referred to as I<unpatched>.
277 The following sections describe the operations permitted by OpenSSL's secure
278 renegotiation implementation.
280 =head2 Patched client and server
282 Connections and renegotiation are always permitted by OpenSSL implementations.
284 =head2 Unpatched client and patched OpenSSL server
286 The initial connection succeeds but client renegotiation is denied by the
287 server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
288 B<handshake_failure> alert in SSL v3.0.
290 If the patched OpenSSL server attempts to renegotiate a fatal
291 B<handshake_failure> alert is sent. This is because the server code may be
292 unaware of the unpatched nature of the client.
294 If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
295 renegotiation B<always> succeeds.
297 =head2 Patched OpenSSL client and unpatched server.
299 If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
300 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
301 and renegotiation between patched OpenSSL clients and unpatched servers
302 succeeds. If neither option is set then initial connections to unpatched
305 The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
306 though it has security implications: otherwise it would be impossible to
307 connect to unpatched servers (i.e. all of them initially) and this is clearly
308 not acceptable. Renegotiation is permitted because this does not add any
309 additional security issues: during an attack clients do not see any
310 renegotiations anyway.
312 As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
313 B<not> be set by default in a future version of OpenSSL.
315 OpenSSL client applications wishing to ensure they can connect to unpatched
316 servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
318 OpenSSL client applications that want to ensure they can B<not> connect to
319 unpatched servers (and thus avoid any security issues) should always B<clear>
320 B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
323 The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
324 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
325 B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
326 renegotiation between OpenSSL clients and unpatched servers B<only>, while
327 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
328 and renegotiation between OpenSSL and unpatched clients or servers.
332 SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
333 after adding B<options>.
335 SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
336 after clearing B<options>.
338 SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
340 SSL_get_secure_renegotiation_support() returns 1 is the peer supports
341 secure renegotiation and 0 if it does not.
345 L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>,
346 L<SSL_CTX_set_tmp_dh_callback(3)>,
347 L<SSL_CTX_set_min_proto_version(3)>,
352 The attempt to always try to use secure renegotiation was added in
355 B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> were added in
360 Copyright 2001-2018 The OpenSSL Project Authors. All Rights Reserved.
362 Licensed under the OpenSSL license (the "License"). You may not use
363 this file except in compliance with the License. You can obtain a copy
364 in the file LICENSE in the source distribution or at
365 L<https://www.openssl.org/source/license.html>.