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_PREFER_NO_DHE_KEX
115 In TLSv1.3, on resumption let the server prefer a non-(ec)dhe based key
116 exchange mode over an (ec)dhe based one. Ignored without B<SSL_OP_ALLOW_NO_DHE_KEX>
117 being set as well. Always ignored on the client.
119 =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
121 Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
122 servers. See the B<SECURE RENEGOTIATION> section for more details.
124 =item SSL_OP_CIPHER_SERVER_PREFERENCE
126 When choosing a cipher, use the server's preferences instead of the client
127 preferences. When not set, the SSL server will always follow the clients
128 preferences. When set, the SSL/TLS server will choose following its
131 =item SSL_OP_CISCO_ANYCONNECT
133 Use Cisco's version identifier of DTLS_BAD_VER when establishing a DTLSv1
134 connection. Only available when using the deprecated DTLSv1_client_method() API.
136 =item SSL_OP_CLEANSE_PLAINTEXT
138 By default TLS and QUIC SSL objects keep a copy of received plaintext
139 application data in a static buffer until it is overwritten by the
140 next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT
141 deciphered application data is cleansed by calling OPENSSL_cleanse(3)
142 after passing data to the application. Data is also cleansed when
143 releasing the connection (e.g. L<SSL_free(3)>).
145 Since OpenSSL only cleanses internal buffers, the application is still
146 responsible for cleansing all other buffers. Most notably, this
147 applies to buffers passed to functions like L<SSL_read(3)>,
148 L<SSL_peek(3)> but also like L<SSL_write(3)>.
150 TLS connections do not buffer data to be sent in plaintext. QUIC stream
151 objects do buffer plaintext data to be sent and this option will also cause
152 that data to be cleansed when it is discarded.
154 This option can be set differently on individual QUIC stream objects and
155 has no effect on QUIC connection objects (except where a default stream is
158 =item SSL_OP_COOKIE_EXCHANGE
160 Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects
163 =item SSL_OP_DISABLE_TLSEXT_CA_NAMES
165 Disable TLS Extension CA Names. You may want to disable it for security reasons
166 or for compatibility with some Windows TLS implementations crashing when this
167 extension is larger than 1024 bytes.
169 =item SSL_OP_ENABLE_KTLS
171 Enable the use of kernel TLS. In order to benefit from kernel TLS OpenSSL must
172 have been compiled with support for it, and it must be supported by the
173 negotiated ciphersuites and extensions. The specific ciphersuites and extensions
174 that are supported may vary by platform and kernel version.
176 The kernel TLS data-path implements the record layer, and the encryption
177 algorithm. The kernel will utilize the best hardware
178 available for encryption. Using the kernel data-path should reduce the memory
179 footprint of OpenSSL because no buffering is required. Also, the throughput
180 should improve because data copy is avoided when user data is encrypted into
181 kernel memory instead of the usual encrypt then copy to kernel.
183 Kernel TLS might not support all the features of OpenSSL. For instance,
184 renegotiation, and setting the maximum fragment size is not possible as of
187 Note that with kernel TLS enabled some cryptographic operations are performed
188 by the kernel directly and not via any available OpenSSL Providers. This might
189 be undesirable if, for example, the application requires all cryptographic
190 operations to be performed by the FIPS provider.
192 =item SSL_OP_ENABLE_KTLS_TX_ZEROCOPY_SENDFILE
194 With this option, sendfile() will use the zerocopy mode, which gives a
195 performance boost when used with KTLS hardware offload. Note that invalid TLS
196 records might be transmitted if the file is changed while being sent. This
197 option has no effect if B<SSL_OP_ENABLE_KTLS> is not enabled.
199 This option only applies to Linux. KTLS sendfile on FreeBSD doesn't offer an
200 option to disable zerocopy and always runs in this mode.
202 =item SSL_OP_ENABLE_MIDDLEBOX_COMPAT
204 If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This
205 has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that
206 do not understand TLSv1.3 will not drop the connection. Regardless of whether
207 this option is set or not CCS messages received from the peer will always be
208 ignored in TLSv1.3. This option is set by default. To switch it off use
209 SSL_clear_options(). A future version of OpenSSL may not set this by default.
211 =item SSL_OP_IGNORE_UNEXPECTED_EOF
213 Some TLS implementations do not send the mandatory close_notify alert on
214 shutdown. If the application tries to wait for the close_notify alert but the
215 peer closes the connection without sending it, an error is generated. When this
216 option is enabled the peer does not need to send the close_notify alert and a
217 closed connection will be treated as if the close_notify alert was received.
219 You should only enable this option if the protocol running over TLS
220 can detect a truncation attack itself, and that the application is checking for
221 that truncation attack.
223 For more information on shutting down a connection, see L<SSL_shutdown(3)>.
225 =item SSL_OP_LEGACY_SERVER_CONNECT
227 Allow legacy insecure renegotiation between OpenSSL and unpatched servers
228 B<only>. See the B<SECURE RENEGOTIATION> section for more details.
230 =item SSL_OP_NO_ANTI_REPLAY
232 By default, when a server is configured for early data (i.e., max_early_data > 0),
233 OpenSSL will switch on replay protection. See L<SSL_read_early_data(3)> for a
234 description of the replay protection feature. Anti-replay measures are required
235 to comply with the TLSv1.3 specification. Some applications may be able to
236 mitigate the replay risks in other ways and in such cases the built in OpenSSL
237 functionality is not required. Those applications can turn this feature off by
238 setting this option. This is a server-side option only. It is ignored by
241 =item SSL_OP_NO_TX_CERTIFICATE_COMPRESSION
243 Normally clients and servers will transparently attempt to negotiate the
244 RFC8879 certificate compression option on TLSv1.3 connections.
246 If this option is set, the certificate compression extension is ignored
247 upon receipt and compressed certificates will not be sent to the peer.
249 =item SSL_OP_NO_RX_CERTIFICATE_COMPRESSION
251 Normally clients and servers will transparently attempt to negotiate the
252 RFC8879 certificate compression option on TLSv1.3 connections.
254 If this option is set, the certificate compression extension will not be sent
255 and compressed certificates will not be accepted from the peer.
257 =item SSL_OP_NO_COMPRESSION
259 Do not use TLS record compression even if it is supported. This option is set by
260 default. To switch it off use SSL_clear_options(). Note that TLS record
261 compression is not recommended and is not available at security level 2 or
262 above. From OpenSSL 3.2 the default security level is 2, so clearing this option
263 will have no effect without also changing the default security level. See
264 L<SSL_CTX_set_security_level(3)>.
266 =item SSL_OP_NO_ENCRYPT_THEN_MAC
268 Normally clients and servers will transparently attempt to negotiate the
269 RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.
271 If this option is set, Encrypt-then-MAC is disabled. Clients will not
272 propose, and servers will not accept the extension.
274 =item SSL_OP_NO_EXTENDED_MASTER_SECRET
276 Normally clients and servers will transparently attempt to negotiate the
277 RFC7627 Extended Master Secret option on TLS and DTLS connection.
279 If this option is set, Extended Master Secret is disabled. Clients will
280 not propose, and servers will not accept the extension.
282 =item SSL_OP_NO_QUERY_MTU
284 Do not query the MTU. Only affects DTLS connections.
286 =item SSL_OP_NO_RENEGOTIATION
288 Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest
289 messages, and ignore renegotiation requests via ClientHello.
291 =item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
293 When performing renegotiation as a server, always start a new session
294 (i.e., session resumption requests are only accepted in the initial
295 handshake). This option is not needed for clients.
297 =item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1,
298 SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
300 These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol
301 versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS,
303 As of OpenSSL 1.1.0, these options are deprecated, use
304 L<SSL_CTX_set_min_proto_version(3)> and
305 L<SSL_CTX_set_max_proto_version(3)> instead.
307 =item SSL_OP_NO_TICKET
309 SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless
312 When using session ids a copy of the session information is
313 cached on the server and a unique id is sent to the client. When the client
314 wishes to resume it provides the unique id so that the server can retrieve the
315 session information from its cache.
317 When using stateless session tickets the server uses a session ticket encryption
318 key to encrypt the session information. This encrypted data is sent to the
319 client as a "ticket". When the client wishes to resume it sends the encrypted
320 data back to the server. The server uses its key to decrypt the data and resume
321 the session. In this way the server can operate statelessly - no session
322 information needs to be cached locally.
324 The TLSv1.3 protocol only supports tickets and does not directly support session
325 ids. However, OpenSSL allows two modes of ticket operation in TLSv1.3: stateful
326 and stateless. Stateless tickets work the same way as in TLSv1.2 and below.
327 Stateful tickets mimic the session id behaviour available in TLSv1.2 and below.
328 The session information is cached on the server and the session id is wrapped up
329 in a ticket and sent back to the client. When the client wishes to resume, it
330 presents a ticket in the same way as for stateless tickets. The server can then
331 extract the session id from the ticket and retrieve the session information from
334 By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will
335 cause stateless tickets to not be issued. In TLSv1.2 and below this means no
336 ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be
337 sent. This is a server-side option only.
339 In TLSv1.3 it is possible to suppress all tickets (stateful and stateless) from
340 being sent by calling L<SSL_CTX_set_num_tickets(3)> or
341 L<SSL_set_num_tickets(3)>.
343 =item SSL_OP_PRIORITIZE_CHACHA
345 When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize
346 ChaCha20-Poly1305 ciphers to the top of the server cipher list if a
347 ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps
348 those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere
349 in the server cipher list; but still allows other clients to use AES and other
350 ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>.
352 =item SSL_OP_TLS_ROLLBACK_BUG
354 Disable version rollback attack detection.
356 During the client key exchange, the client must send the same information
357 about acceptable SSL/TLS protocol levels as during the first hello. Some
358 clients violate this rule by adapting to the server's answer. (Example:
359 the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
360 only understands up to SSLv3. In this case the client must still use the
361 same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
362 to the server's answer and violate the version rollback protection.)
366 The following options no longer have any effect but their identifiers are
367 retained for compatibility purposes:
371 =item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
373 =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
375 =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
377 =item SSL_OP_TLS_D5_BUG
379 =item SSL_OP_TLS_BLOCK_PADDING_BUG
381 =item SSL_OP_MSIE_SSLV2_RSA_PADDING
383 =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
385 =item SSL_OP_MICROSOFT_SESS_ID_BUG
387 =item SSL_OP_NETSCAPE_CHALLENGE_BUG
389 =item SSL_OP_PKCS1_CHECK_1
391 =item SSL_OP_PKCS1_CHECK_2
393 =item SSL_OP_SINGLE_DH_USE
395 =item SSL_OP_SINGLE_ECDH_USE
397 =item SSL_OP_EPHEMERAL_RSA
399 =item SSL_OP_NETSCAPE_CA_DN_BUG
401 =item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
405 =head1 SECURE RENEGOTIATION
407 OpenSSL always attempts to use secure renegotiation as
408 described in RFC5746. This counters the prefix attack described in
409 CVE-2009-3555 and elsewhere.
411 This attack has far reaching consequences which application writers should be
412 aware of. In the description below an implementation supporting secure
413 renegotiation is referred to as I<patched>. A server not supporting secure
414 renegotiation is referred to as I<unpatched>.
416 The following sections describe the operations permitted by OpenSSL's secure
417 renegotiation implementation.
419 =head2 Patched client and server
421 Connections and renegotiation are always permitted by OpenSSL implementations.
423 =head2 Unpatched client and patched OpenSSL server
425 The initial connection succeeds but client renegotiation is denied by the
426 server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
427 B<handshake_failure> alert in SSL v3.0.
429 If the patched OpenSSL server attempts to renegotiate a fatal
430 B<handshake_failure> alert is sent. This is because the server code may be
431 unaware of the unpatched nature of the client.
433 If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
434 renegotiation B<always> succeeds.
436 =head2 Patched OpenSSL client and unpatched server
438 If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
439 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
440 and renegotiation between patched OpenSSL clients and unpatched servers
441 succeeds. If neither option is set then initial connections to unpatched
444 Setting the option B<SSL_OP_LEGACY_SERVER_CONNECT> has security implications;
445 clients that are willing to connect to servers that do not implement
446 RFC 5746 secure renegotiation are subject to attacks such as
449 OpenSSL client applications wishing to ensure they can connect to unpatched
450 servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
452 OpenSSL client applications that want to ensure they can B<not> connect to
453 unpatched servers (and thus avoid any security issues) should always B<clear>
454 B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
457 The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
458 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
459 B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
460 renegotiation between OpenSSL clients and unpatched servers B<only>, while
461 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
462 and renegotiation between OpenSSL and unpatched clients or servers.
464 =head2 Applicability of options to QUIC connections and streams
466 These options apply to SSL objects referencing a QUIC connection:
470 =item SSL_OP_ALLOW_NO_DHE_KEX
472 =item SSL_OP_NO_TX_CERTIFICATE_COMPRESSION
474 =item SSL_OP_NO_RX_CERTIFICATE_COMPRESSION
476 =item SSL_OP_NO_TICKET
478 =item SSL_OP_PRIORITIZE_CHACHA
482 These options apply to SSL objects referencing a QUIC stream:
486 =item SSL_OP_CLEANSE_PLAINTEXT
490 Options on QUIC connections are initialized from the options set on SSL_CTX
491 before a QUIC connection SSL object is created. Options on QUIC streams are
492 initialised from the options configured on the QUIC connection SSL object
493 they are created from.
495 Setting options which relate to QUIC streams on a QUIC connection SSL object has
496 no direct effect on the QUIC connection SSL object itself, but will change the
497 options set on the default stream (if there is one) and will also determine the
498 default options set on any future streams which are created.
500 Other options not mentioned above do not have an effect and will be ignored.
502 Options which relate to QUIC streams may also be set directly on QUIC stream SSL
503 objects. Setting connection-related options on such an object has no effect.
507 SSL_CTX_set_options() and SSL_set_options() return the new options bit-mask
508 after adding B<options>.
510 SSL_CTX_clear_options() and SSL_clear_options() return the new options bit-mask
511 after clearing B<options>.
513 SSL_CTX_get_options() and SSL_get_options() return the current bit-mask.
515 SSL_get_secure_renegotiation_support() returns 1 is the peer supports
516 secure renegotiation and 0 if it does not.
520 L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>, L<SSL_shutdown(3)>
521 L<SSL_CTX_set_tmp_dh_callback(3)>,
522 L<SSL_CTX_set_min_proto_version(3)>,
523 L<openssl-dhparam(1)>
527 The attempt to always try to use secure renegotiation was added in
530 The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options
531 were added in OpenSSL 1.1.1.
533 The B<SSL_OP_NO_EXTENDED_MASTER_SECRET> and B<SSL_OP_IGNORE_UNEXPECTED_EOF>
534 options were added in OpenSSL 3.0.
536 The B<SSL_OP_> constants and the corresponding parameter and return values
537 of the affected functions were changed to C<uint64_t> type in OpenSSL 3.0.
538 For that reason it is no longer possible use the B<SSL_OP_> macro values
539 in preprocessor C<#if> conditions. However it is still possible to test
540 whether these macros are defined or not.
544 Copyright 2001-2023 The OpenSSL Project Authors. All Rights Reserved.
546 Licensed under the Apache License 2.0 (the "License"). You may not use
547 this file except in compliance with the License. You can obtain a copy
548 in the file LICENSE in the source distribution or at
549 L<https://www.openssl.org/source/license.html>.