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 uint64_t SSL_CTX_set_options(SSL_CTX *ctx, uint64_t options);
14 uint64_t SSL_set_options(SSL *ssl, uint64_t options);
16 uint64_t SSL_CTX_clear_options(SSL_CTX *ctx, uint64_t options);
17 uint64_t SSL_clear_options(SSL *ssl, uint64_t options);
19 uint64_t SSL_CTX_get_options(const SSL_CTX *ctx);
20 uint64_t SSL_get_options(const SSL *ssl);
22 long SSL_get_secure_renegotiation_support(SSL *ssl);
26 SSL_CTX_set_options() adds the options set via bit-mask in B<options> to B<ctx>.
27 Options already set before are not cleared!
29 SSL_set_options() adds the options set via bit-mask in B<options> to B<ssl>.
30 Options already set before are not cleared!
32 SSL_CTX_clear_options() clears the options set via bit-mask in B<options>
35 SSL_clear_options() clears the options set via bit-mask 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 bit-masks 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_CRYPTOPRO_TLSEXT_BUG
67 Add server-hello extension from the early version of cryptopro draft
68 when GOST ciphersuite is negotiated. Required for interoperability with CryptoPro
71 =item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
73 Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
74 vulnerability affecting CBC ciphers, which cannot be handled by some
75 broken SSL implementations. This option has no effect for connections
78 =item SSL_OP_SAFARI_ECDHE_ECDSA_BUG
80 Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
81 OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
83 =item SSL_OP_TLSEXT_PADDING
85 Adds a padding extension to ensure the ClientHello size is never between
86 256 and 511 bytes in length. This is needed as a workaround for some
91 All of the above bug workarounds.
95 It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
96 options if compatibility with somewhat broken implementations is
99 The following B<modifying> options are available:
103 =item SSL_OP_ALLOW_CLIENT_RENEGOTIATION
105 Client-initiated renegotiation is disabled by default. Use
106 this option to enable it.
108 =item SSL_OP_ALLOW_NO_DHE_KEX
110 In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means
111 that there will be no forward secrecy for the resumed session.
113 =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
115 Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
116 servers. See the B<SECURE RENEGOTIATION> section for more details.
118 =item SSL_OP_CIPHER_SERVER_PREFERENCE
120 When choosing a cipher, use the server's preferences instead of the client
121 preferences. When not set, the SSL server will always follow the clients
122 preferences. When set, the SSL/TLS server will choose following its
125 =item SSL_OP_CISCO_ANYCONNECT
127 Use Cisco's version identifier of DTLS_BAD_VER when establishing a DTLSv1
128 connection. Only available when using the deprecated DTLSv1_client_method() API.
130 =item SSL_OP_CLEANSE_PLAINTEXT
132 By default TLS connections keep a copy of received plaintext
133 application data in a static buffer until it is overwritten by the
134 next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT
135 deciphered application data is cleansed by calling OPENSSL_cleanse(3)
136 after passing data to the application. Data is also cleansed when
137 releasing the connection (e.g. L<SSL_free(3)>).
139 Since OpenSSL only cleanses internal buffers, the application is still
140 responsible for cleansing all other buffers. Most notably, this
141 applies to buffers passed to functions like L<SSL_read(3)>,
142 L<SSL_peek(3)> but also like L<SSL_write(3)>.
144 =item SSL_OP_COOKIE_EXCHANGE
146 Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects
149 =item SSL_OP_DISABLE_TLSEXT_CA_NAMES
151 Disable TLS Extension CA Names. You may want to disable it for security reasons
152 or for compatibility with some Windows TLS implementations crashing when this
153 extension is larger than 1024 bytes.
155 =item SSL_OP_ENABLE_KTLS
157 Enable the use of kernel TLS. In order to benefit from kernel TLS OpenSSL must
158 have been compiled with support for it, and it must be supported by the
159 negotiated ciphersuites and extensions. The specific ciphersuites and extensions
160 that are supported may vary by platform and kernel version.
162 The kernel TLS data-path implements the record layer, and the encryption
163 algorithm. The kernel will utilize the best hardware
164 available for encryption. Using the kernel data-path should reduce the memory
165 footprint of OpenSSL because no buffering is required. Also, the throughput
166 should improve because data copy is avoided when user data is encrypted into
167 kernel memory instead of the usual encrypt then copy to kernel.
169 Kernel TLS might not support all the features of OpenSSL. For instance,
170 renegotiation, and setting the maximum fragment size is not possible as of
173 Note that with kernel TLS enabled some cryptographic operations are performed
174 by the kernel directly and not via any available OpenSSL Providers. This might
175 be undesirable if, for example, the application requires all cryptographic
176 operations to be performed by the FIPS provider.
178 =item SSL_OP_ENABLE_MIDDLEBOX_COMPAT
180 If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This
181 has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that
182 do not understand TLSv1.3 will not drop the connection. Regardless of whether
183 this option is set or not CCS messages received from the peer will always be
184 ignored in TLSv1.3. This option is set by default. To switch it off use
185 SSL_clear_options(). A future version of OpenSSL may not set this by default.
187 =item SSL_OP_IGNORE_UNEXPECTED_EOF
189 Some TLS implementations do not send the mandatory close_notify alert on
190 shutdown. If the application tries to wait for the close_notify alert but the
191 peer closes the connection without sending it, an error is generated. When this
192 option is enabled the peer does not need to send the close_notify alert and a
193 closed connection will be treated as if the close_notify alert was received.
195 You should only enable this option if the protocol running over TLS
196 can detect a truncation attack itself, and that the application is checking for
197 that truncation attack.
199 For more information on shutting down a connection, see L<SSL_shutdown(3)>.
201 =item SSL_OP_LEGACY_SERVER_CONNECT
203 Allow legacy insecure renegotiation between OpenSSL and unpatched servers
204 B<only>. See the B<SECURE RENEGOTIATION> section for more details.
206 =item SSL_OP_NO_ANTI_REPLAY
208 By default, when a server is configured for early data (i.e., max_early_data > 0),
209 OpenSSL will switch on replay protection. See L<SSL_read_early_data(3)> for a
210 description of the replay protection feature. Anti-replay measures are required
211 to comply with the TLSv1.3 specification. Some applications may be able to
212 mitigate the replay risks in other ways and in such cases the built in OpenSSL
213 functionality is not required. Those applications can turn this feature off by
214 setting this option. This is a server-side opton only. It is ignored by
217 =item SSL_OP_NO_COMPRESSION
219 Do not use compression even if it is supported. This option is set by default.
220 To switch it off use SSL_clear_options().
222 =item SSL_OP_NO_ENCRYPT_THEN_MAC
224 Normally clients and servers will transparently attempt to negotiate the
225 RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.
227 If this option is set, Encrypt-then-MAC is disabled. Clients will not
228 propose, and servers will not accept the extension.
230 =item SSL_OP_NO_EXTENDED_MASTER_SECRET
232 Normally clients and servers will transparently attempt to negotiate the
233 RFC7627 Extended Master Secret option on TLS and DTLS connection.
235 If this option is set, Extended Master Secret is disabled. Clients will
236 not propose, and servers will not accept the extension.
238 =item SSL_OP_NO_QUERY_MTU
240 Do not query the MTU. Only affects DTLS connections.
242 =item SSL_OP_NO_RENEGOTIATION
244 Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest
245 messages, and ignore renegotiation requests via ClientHello.
247 =item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
249 When performing renegotiation as a server, always start a new session
250 (i.e., session resumption requests are only accepted in the initial
251 handshake). This option is not needed for clients.
253 =item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1,
254 SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
256 These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol
257 versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS,
259 As of OpenSSL 1.1.0, these options are deprecated, use
260 L<SSL_CTX_set_min_proto_version(3)> and
261 L<SSL_CTX_set_max_proto_version(3)> instead.
263 =item SSL_OP_NO_TICKET
265 SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless
268 When using session ids a copy of the session information is
269 cached on the server and a unique id is sent to the client. When the client
270 wishes to resume it provides the unique id so that the server can retrieve the
271 session information from its cache.
273 When using stateless session tickets the server uses a session ticket encryption
274 key to encrypt the session information. This encrypted data is sent to the
275 client as a "ticket". When the client wishes to resume it sends the encrypted
276 data back to the server. The server uses its key to decrypt the data and resume
277 the session. In this way the server can operate statelessly - no session
278 information needs to be cached locally.
280 The TLSv1.3 protocol only supports tickets and does not directly support session
281 ids. However, OpenSSL allows two modes of ticket operation in TLSv1.3: stateful
282 and stateless. Stateless tickets work the same way as in TLSv1.2 and below.
283 Stateful tickets mimic the session id behaviour available in TLSv1.2 and below.
284 The session information is cached on the server and the session id is wrapped up
285 in a ticket and sent back to the client. When the client wishes to resume, it
286 presents a ticket in the same way as for stateless tickets. The server can then
287 extract the session id from the ticket and retrieve the session information from
290 By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will
291 cause stateless tickets to not be issued. In TLSv1.2 and below this means no
292 ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be
293 sent. This is a server-side option only.
295 In TLSv1.3 it is possible to suppress all tickets (stateful and stateless) from
296 being sent by calling L<SSL_CTX_set_num_tickets(3)> or
297 L<SSL_set_num_tickets(3)>.
299 =item SSL_OP_PRIORITIZE_CHACHA
301 When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize
302 ChaCha20-Poly1305 ciphers to the top of the server cipher list if a
303 ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps
304 those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere
305 in the server cipher list; but still allows other clients to use AES and other
306 ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>.
308 =item SSL_OP_TLS_ROLLBACK_BUG
310 Disable version rollback attack detection.
312 During the client key exchange, the client must send the same information
313 about acceptable SSL/TLS protocol levels as during the first hello. Some
314 clients violate this rule by adapting to the server's answer. (Example:
315 the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
316 only understands up to SSLv3. In this case the client must still use the
317 same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
318 to the server's answer and violate the version rollback protection.)
322 The following options no longer have any effect but their identifiers are
323 retained for compatibility purposes:
327 =item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
329 =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
331 =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
333 =item SSL_OP_TLS_D5_BUG
335 =item SSL_OP_TLS_BLOCK_PADDING_BUG
337 =item SSL_OP_MSIE_SSLV2_RSA_PADDING
339 =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
341 =item SSL_OP_MICROSOFT_SESS_ID_BUG
343 =item SSL_OP_NETSCAPE_CHALLENGE_BUG
345 =item SSL_OP_PKCS1_CHECK_1
347 =item SSL_OP_PKCS1_CHECK_2
349 =item SSL_OP_SINGLE_DH_USE
351 =item SSL_OP_SINGLE_ECDH_USE
353 =item SSL_OP_EPHEMERAL_RSA
357 =head1 SECURE RENEGOTIATION
359 OpenSSL always attempts to use secure renegotiation as
360 described in RFC5746. This counters the prefix attack described in
361 CVE-2009-3555 and elsewhere.
363 This attack has far reaching consequences which application writers should be
364 aware of. In the description below an implementation supporting secure
365 renegotiation is referred to as I<patched>. A server not supporting secure
366 renegotiation is referred to as I<unpatched>.
368 The following sections describe the operations permitted by OpenSSL's secure
369 renegotiation implementation.
371 =head2 Patched client and server
373 Connections and renegotiation are always permitted by OpenSSL implementations.
375 =head2 Unpatched client and patched OpenSSL server
377 The initial connection succeeds but client renegotiation is denied by the
378 server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
379 B<handshake_failure> alert in SSL v3.0.
381 If the patched OpenSSL server attempts to renegotiate a fatal
382 B<handshake_failure> alert is sent. This is because the server code may be
383 unaware of the unpatched nature of the client.
385 If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
386 renegotiation B<always> succeeds.
388 =head2 Patched OpenSSL client and unpatched server
390 If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
391 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
392 and renegotiation between patched OpenSSL clients and unpatched servers
393 succeeds. If neither option is set then initial connections to unpatched
396 Setting the option B<SSL_OP_LEGACY_SERVER_CONNECT> has security implications;
397 clients that are willing to connect to servers that do not implement
398 RFC 5746 secure renegotiation are subject to attacks such as
401 OpenSSL client applications wishing to ensure they can connect to unpatched
402 servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
404 OpenSSL client applications that want to ensure they can B<not> connect to
405 unpatched servers (and thus avoid any security issues) should always B<clear>
406 B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
409 The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
410 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
411 B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
412 renegotiation between OpenSSL clients and unpatched servers B<only>, while
413 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
414 and renegotiation between OpenSSL and unpatched clients or servers.
418 SSL_CTX_set_options() and SSL_set_options() return the new options bit-mask
419 after adding B<options>.
421 SSL_CTX_clear_options() and SSL_clear_options() return the new options bit-mask
422 after clearing B<options>.
424 SSL_CTX_get_options() and SSL_get_options() return the current bit-mask.
426 SSL_get_secure_renegotiation_support() returns 1 is the peer supports
427 secure renegotiation and 0 if it does not.
431 L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>, L<SSL_shutdown(3)>
432 L<SSL_CTX_set_tmp_dh_callback(3)>,
433 L<SSL_CTX_set_min_proto_version(3)>,
434 L<openssl-dhparam(1)>
438 The attempt to always try to use secure renegotiation was added in
441 The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options
442 were added in OpenSSL 1.1.1.
444 The B<SSL_OP_NO_EXTENDED_MASTER_SECRET> and B<SSL_OP_IGNORE_UNEXPECTED_EOF>
445 options were added in OpenSSL 3.0.
447 The B<SSL_OP_> constants and the corresponding parameter and return values
448 of the affected functions were changed to C<uint64_t> type in OpenSSL 3.0.
449 For that reason it is no longer possible use the B<SSL_OP_> macro values
450 in preprocessor C<#if> conditions. However it is still possible to test
451 whether these macros are defined or not.
455 Copyright 2001-2021 The OpenSSL Project Authors. All Rights Reserved.
457 Licensed under the Apache License 2.0 (the "License"). You may not use
458 this file except in compliance with the License. You can obtain a copy
459 in the file LICENSE in the source distribution or at
460 L<https://www.openssl.org/source/license.html>.