1SSL_CTX_SET_OPTIONS(3ossl)          OpenSSL         SSL_CTX_SET_OPTIONS(3ossl)
2
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4

NAME

6       SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options,
7       SSL_clear_options, SSL_CTX_get_options, SSL_get_options,
8       SSL_get_secure_renegotiation_support - manipulate SSL options
9

SYNOPSIS

11        #include <openssl/ssl.h>
12
13        uint64_t SSL_CTX_set_options(SSL_CTX *ctx, uint64_t options);
14        uint64_t SSL_set_options(SSL *ssl, uint64_t options);
15
16        uint64_t SSL_CTX_clear_options(SSL_CTX *ctx, uint64_t options);
17        uint64_t SSL_clear_options(SSL *ssl, uint64_t options);
18
19        uint64_t SSL_CTX_get_options(const SSL_CTX *ctx);
20        uint64_t SSL_get_options(const SSL *ssl);
21
22        long SSL_get_secure_renegotiation_support(SSL *ssl);
23

DESCRIPTION

25       SSL_CTX_set_options() adds the options set via bit-mask in options to
26       ctx.  Options already set before are not cleared!
27
28       SSL_set_options() adds the options set via bit-mask in options to ssl.
29       Options already set before are not cleared!
30
31       SSL_CTX_clear_options() clears the options set via bit-mask in options
32       to ctx.
33
34       SSL_clear_options() clears the options set via bit-mask in options to
35       ssl.
36
37       SSL_CTX_get_options() returns the options set for ctx.
38
39       SSL_get_options() returns the options set for ssl.
40
41       SSL_get_secure_renegotiation_support() indicates whether the peer
42       supports secure renegotiation.  Note, this is implemented via a macro.
43

NOTES

45       The behaviour of the SSL library can be changed by setting several
46       options.  The options are coded as bit-masks and can be combined by a
47       bitwise or operation (|).
48
49       SSL_CTX_set_options() and SSL_set_options() affect the (external)
50       protocol behaviour of the SSL library. The (internal) behaviour of the
51       API can be changed by using the similar SSL_CTX_set_mode(3) and
52       SSL_set_mode() functions.
53
54       During a handshake, the option settings of the SSL object are used.
55       When a new SSL object is created from a context using SSL_new(), the
56       current option setting is copied. Changes to ctx do not affect already
57       created SSL objects. SSL_clear() does not affect the settings.
58
59       The following bug workaround options are available:
60
61       SSL_OP_CRYPTOPRO_TLSEXT_BUG
62           Add server-hello extension from the early version of cryptopro
63           draft when GOST ciphersuite is negotiated. Required for
64           interoperability with CryptoPro CSP 3.x.
65
66       SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
67           Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
68           vulnerability affecting CBC ciphers, which cannot be handled by
69           some broken SSL implementations.  This option has no effect for
70           connections using other ciphers.
71
72       SSL_OP_SAFARI_ECDHE_ECDSA_BUG
73           Don't prefer ECDHE-ECDSA ciphers when the client appears to be
74           Safari on OS X.  OS X 10.8..10.8.3 has broken support for ECDHE-
75           ECDSA ciphers.
76
77       SSL_OP_TLSEXT_PADDING
78           Adds a padding extension to ensure the ClientHello size is never
79           between 256 and 511 bytes in length. This is needed as a workaround
80           for some implementations.
81
82       SSL_OP_ALL
83           All of the above bug workarounds.
84
85       It is usually safe to use SSL_OP_ALL to enable the bug workaround
86       options if compatibility with somewhat broken implementations is
87       desired.
88
89       The following modifying options are available:
90
91       SSL_OP_ALLOW_CLIENT_RENEGOTIATION
92           Client-initiated renegotiation is disabled by default. Use this
93           option to enable it.
94
95       SSL_OP_ALLOW_NO_DHE_KEX
96           In TLSv1.3 allow a non-(ec)dhe based key exchange mode on
97           resumption. This means that there will be no forward secrecy for
98           the resumed session.
99
100       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
101           Allow legacy insecure renegotiation between OpenSSL and unpatched
102           clients or servers. See the SECURE RENEGOTIATION section for more
103           details.
104
105       SSL_OP_CIPHER_SERVER_PREFERENCE
106           When choosing a cipher, use the server's preferences instead of the
107           client preferences. When not set, the SSL server will always follow
108           the clients preferences. When set, the SSL/TLS server will choose
109           following its own preferences.
110
111       SSL_OP_CISCO_ANYCONNECT
112           Use Cisco's version identifier of DTLS_BAD_VER when establishing a
113           DTLSv1 connection. Only available when using the deprecated
114           DTLSv1_client_method() API.
115
116       SSL_OP_CLEANSE_PLAINTEXT
117           By default TLS connections keep a copy of received plaintext
118           application data in a static buffer until it is overwritten by the
119           next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT
120           deciphered application data is cleansed by calling
121           OPENSSL_cleanse(3) after passing data to the application. Data is
122           also cleansed when releasing the connection (e.g. SSL_free(3)).
123
124           Since OpenSSL only cleanses internal buffers, the application is
125           still responsible for cleansing all other buffers. Most notably,
126           this applies to buffers passed to functions like SSL_read(3),
127           SSL_peek(3) but also like SSL_write(3).
128
129       SSL_OP_COOKIE_EXCHANGE
130           Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only
131           affects DTLS connections.
132
133       SSL_OP_DISABLE_TLSEXT_CA_NAMES
134           Disable TLS Extension CA Names. You may want to disable it for
135           security reasons or for compatibility with some Windows TLS
136           implementations crashing when this extension is larger than 1024
137           bytes.
138
139       SSL_OP_ENABLE_KTLS
140           Enable the use of kernel TLS. In order to benefit from kernel TLS
141           OpenSSL must have been compiled with support for it, and it must be
142           supported by the negotiated ciphersuites and extensions. The
143           specific ciphersuites and extensions that are supported may vary by
144           platform and kernel version.
145
146           The kernel TLS data-path implements the record layer, and the
147           encryption algorithm. The kernel will utilize the best hardware
148           available for encryption. Using the kernel data-path should reduce
149           the memory footprint of OpenSSL because no buffering is required.
150           Also, the throughput should improve because data copy is avoided
151           when user data is encrypted into kernel memory instead of the usual
152           encrypt then copy to kernel.
153
154           Kernel TLS might not support all the features of OpenSSL. For
155           instance, renegotiation, and setting the maximum fragment size is
156           not possible as of Linux 4.20.
157
158           Note that with kernel TLS enabled some cryptographic operations are
159           performed by the kernel directly and not via any available OpenSSL
160           Providers. This might be undesirable if, for example, the
161           application requires all cryptographic operations to be performed
162           by the FIPS provider.
163
164       SSL_OP_ENABLE_MIDDLEBOX_COMPAT
165           If set then dummy Change Cipher Spec (CCS) messages are sent in
166           TLSv1.3. This has the effect of making TLSv1.3 look more like
167           TLSv1.2 so that middleboxes that do not understand TLSv1.3 will not
168           drop the connection. Regardless of whether this option is set or
169           not CCS messages received from the peer will always be ignored in
170           TLSv1.3. This option is set by default. To switch it off use
171           SSL_clear_options(). A future version of OpenSSL may not set this
172           by default.
173
174       SSL_OP_IGNORE_UNEXPECTED_EOF
175           Some TLS implementations do not send the mandatory close_notify
176           alert on shutdown. If the application tries to wait for the
177           close_notify alert but the peer closes the connection without
178           sending it, an error is generated. When this option is enabled the
179           peer does not need to send the close_notify alert and a closed
180           connection will be treated as if the close_notify alert was
181           received.
182
183           You should only enable this option if the protocol running over TLS
184           can detect a truncation attack itself, and that the application is
185           checking for that truncation attack.
186
187           For more information on shutting down a connection, see
188           SSL_shutdown(3).
189
190       SSL_OP_LEGACY_SERVER_CONNECT
191           Allow legacy insecure renegotiation between OpenSSL and unpatched
192           servers only. See the SECURE RENEGOTIATION section for more
193           details.
194
195       SSL_OP_NO_ANTI_REPLAY
196           By default, when a server is configured for early data (i.e.,
197           max_early_data > 0), OpenSSL will switch on replay protection. See
198           SSL_read_early_data(3) for a description of the replay protection
199           feature. Anti-replay measures are required to comply with the
200           TLSv1.3 specification. Some applications may be able to mitigate
201           the replay risks in other ways and in such cases the built in
202           OpenSSL functionality is not required. Those applications can turn
203           this feature off by setting this option. This is a server-side
204           option only. It is ignored by clients.
205
206       SSL_OP_NO_COMPRESSION
207           Do not use compression even if it is supported. This option is set
208           by default.  To switch it off use SSL_clear_options().
209
210       SSL_OP_NO_ENCRYPT_THEN_MAC
211           Normally clients and servers will transparently attempt to
212           negotiate the RFC7366 Encrypt-then-MAC option on TLS and DTLS
213           connection.
214
215           If this option is set, Encrypt-then-MAC is disabled. Clients will
216           not propose, and servers will not accept the extension.
217
218       SSL_OP_NO_EXTENDED_MASTER_SECRET
219           Normally clients and servers will transparently attempt to
220           negotiate the RFC7627 Extended Master Secret option on TLS and DTLS
221           connection.
222
223           If this option is set, Extended Master Secret is disabled. Clients
224           will not propose, and servers will not accept the extension.
225
226       SSL_OP_NO_QUERY_MTU
227           Do not query the MTU. Only affects DTLS connections.
228
229       SSL_OP_NO_RENEGOTIATION
230           Disable all renegotiation in TLSv1.2 and earlier. Do not send
231           HelloRequest messages, and ignore renegotiation requests via
232           ClientHello.
233
234       SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
235           When performing renegotiation as a server, always start a new
236           session (i.e., session resumption requests are only accepted in the
237           initial handshake). This option is not needed for clients.
238
239       SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, SSL_OP_NO_TLSv1_2,
240       SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
241           These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or
242           TLSv1.3 protocol versions with TLS or the DTLSv1, DTLSv1.2 versions
243           with DTLS, respectively.  As of OpenSSL 1.1.0, these options are
244           deprecated, use SSL_CTX_set_min_proto_version(3) and
245           SSL_CTX_set_max_proto_version(3) instead.
246
247       SSL_OP_NO_TICKET
248           SSL/TLS supports two mechanisms for resuming sessions: session ids
249           and stateless session tickets.
250
251           When using session ids a copy of the session information is cached
252           on the server and a unique id is sent to the client. When the
253           client wishes to resume it provides the unique id so that the
254           server can retrieve the session information from its cache.
255
256           When using stateless session tickets the server uses a session
257           ticket encryption key to encrypt the session information. This
258           encrypted data is sent to the client as a "ticket". When the client
259           wishes to resume it sends the encrypted data back to the server.
260           The server uses its key to decrypt the data and resume the session.
261           In this way the server can operate statelessly - no session
262           information needs to be cached locally.
263
264           The TLSv1.3 protocol only supports tickets and does not directly
265           support session ids. However, OpenSSL allows two modes of ticket
266           operation in TLSv1.3: stateful and stateless. Stateless tickets
267           work the same way as in TLSv1.2 and below.  Stateful tickets mimic
268           the session id behaviour available in TLSv1.2 and below.  The
269           session information is cached on the server and the session id is
270           wrapped up in a ticket and sent back to the client. When the client
271           wishes to resume, it presents a ticket in the same way as for
272           stateless tickets. The server can then extract the session id from
273           the ticket and retrieve the session information from its cache.
274
275           By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET
276           option will cause stateless tickets to not be issued. In TLSv1.2
277           and below this means no ticket gets sent to the client at all. In
278           TLSv1.3 a stateful ticket will be sent. This is a server-side
279           option only.
280
281           In TLSv1.3 it is possible to suppress all tickets (stateful and
282           stateless) from being sent by calling SSL_CTX_set_num_tickets(3) or
283           SSL_set_num_tickets(3).
284
285       SSL_OP_PRIORITIZE_CHACHA
286           When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily
287           reprioritize ChaCha20-Poly1305 ciphers to the top of the server
288           cipher list if a ChaCha20-Poly1305 cipher is at the top of the
289           client cipher list. This helps those clients (e.g. mobile) use
290           ChaCha20-Poly1305 if that cipher is anywhere in the server cipher
291           list; but still allows other clients to use AES and other ciphers.
292           Requires SSL_OP_CIPHER_SERVER_PREFERENCE.
293
294       SSL_OP_TLS_ROLLBACK_BUG
295           Disable version rollback attack detection.
296
297           During the client key exchange, the client must send the same
298           information about acceptable SSL/TLS protocol levels as during the
299           first hello. Some clients violate this rule by adapting to the
300           server's answer. (Example: the client sends a SSLv2 hello and
301           accepts up to SSLv3.1=TLSv1, the server only understands up to
302           SSLv3. In this case the client must still use the same
303           SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with
304           respect to the server's answer and violate the version rollback
305           protection.)
306
307       The following options no longer have any effect but their identifiers
308       are retained for compatibility purposes:
309
310       SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
311       SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
312       SSL_OP_SSLEAY_080_CLIENT_DH_BUG
313       SSL_OP_TLS_D5_BUG
314       SSL_OP_TLS_BLOCK_PADDING_BUG
315       SSL_OP_MSIE_SSLV2_RSA_PADDING
316       SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
317       SSL_OP_MICROSOFT_SESS_ID_BUG
318       SSL_OP_NETSCAPE_CHALLENGE_BUG
319       SSL_OP_PKCS1_CHECK_1
320       SSL_OP_PKCS1_CHECK_2
321       SSL_OP_SINGLE_DH_USE
322       SSL_OP_SINGLE_ECDH_USE
323       SSL_OP_EPHEMERAL_RSA
324       SSL_OP_NETSCAPE_CA_DN_BUG
325       SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
326

SECURE RENEGOTIATION

328       OpenSSL always attempts to use secure renegotiation as described in
329       RFC5746. This counters the prefix attack described in CVE-2009-3555 and
330       elsewhere.
331
332       This attack has far reaching consequences which application writers
333       should be aware of. In the description below an implementation
334       supporting secure renegotiation is referred to as patched. A server not
335       supporting secure renegotiation is referred to as unpatched.
336
337       The following sections describe the operations permitted by OpenSSL's
338       secure renegotiation implementation.
339
340   Patched client and server
341       Connections and renegotiation are always permitted by OpenSSL
342       implementations.
343
344   Unpatched client and patched OpenSSL server
345       The initial connection succeeds but client renegotiation is denied by
346       the server with a no_renegotiation warning alert if TLS v1.0 is used or
347       a fatal handshake_failure alert in SSL v3.0.
348
349       If the patched OpenSSL server attempts to renegotiate a fatal
350       handshake_failure alert is sent. This is because the server code may be
351       unaware of the unpatched nature of the client.
352
353       If the option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then
354       renegotiation always succeeds.
355
356   Patched OpenSSL client and unpatched server
357       If the option SSL_OP_LEGACY_SERVER_CONNECT or
358       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set then initial
359       connections and renegotiation between patched OpenSSL clients and
360       unpatched servers succeeds. If neither option is set then initial
361       connections to unpatched servers will fail.
362
363       Setting the option SSL_OP_LEGACY_SERVER_CONNECT has security
364       implications; clients that are willing to connect to servers that do
365       not implement RFC 5746 secure renegotiation are subject to attacks such
366       as CVE-2009-3555.
367
368       OpenSSL client applications wishing to ensure they can connect to
369       unpatched servers should always set SSL_OP_LEGACY_SERVER_CONNECT
370
371       OpenSSL client applications that want to ensure they can not connect to
372       unpatched servers (and thus avoid any security issues) should always
373       clear SSL_OP_LEGACY_SERVER_CONNECT using SSL_CTX_clear_options() or
374       SSL_clear_options().
375
376       The difference between the SSL_OP_LEGACY_SERVER_CONNECT and
377       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION options is that
378       SSL_OP_LEGACY_SERVER_CONNECT enables initial connections and secure
379       renegotiation between OpenSSL clients and unpatched servers only, while
380       SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION allows initial connections and
381       renegotiation between OpenSSL and unpatched clients or servers.
382

RETURN VALUES

384       SSL_CTX_set_options() and SSL_set_options() return the new options bit-
385       mask after adding options.
386
387       SSL_CTX_clear_options() and SSL_clear_options() return the new options
388       bit-mask after clearing options.
389
390       SSL_CTX_get_options() and SSL_get_options() return the current bit-
391       mask.
392
393       SSL_get_secure_renegotiation_support() returns 1 is the peer supports
394       secure renegotiation and 0 if it does not.
395

SEE ALSO

397       ssl(7), SSL_new(3), SSL_clear(3), SSL_shutdown(3)
398       SSL_CTX_set_tmp_dh_callback(3), SSL_CTX_set_min_proto_version(3),
399       openssl-dhparam(1)
400

HISTORY

402       The attempt to always try to use secure renegotiation was added in
403       OpenSSL 0.9.8m.
404
405       The SSL_OP_PRIORITIZE_CHACHA and SSL_OP_NO_RENEGOTIATION options were
406       added in OpenSSL 1.1.1.
407
408       The SSL_OP_NO_EXTENDED_MASTER_SECRET and SSL_OP_IGNORE_UNEXPECTED_EOF
409       options were added in OpenSSL 3.0.
410
411       The SSL_OP_ constants and the corresponding parameter and return values
412       of the affected functions were changed to "uint64_t" type in OpenSSL
413       3.0.  For that reason it is no longer possible use the SSL_OP_ macro
414       values in preprocessor "#if" conditions. However it is still possible
415       to test whether these macros are defined or not.
416
418       Copyright 2001-2023 The OpenSSL Project Authors. All Rights Reserved.
419
420       Licensed under the Apache License 2.0 (the "License").  You may not use
421       this file except in compliance with the License.  You can obtain a copy
422       in the file LICENSE in the source distribution or at
423       <https://www.openssl.org/source/license.html>.
424
425
426
4273.1.1                             2023-08-31        SSL_CTX_SET_OPTIONS(3ossl)
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