1=pod 2 3=head1 NAME 4 5SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options, 6SSL_clear_options, SSL_CTX_get_options, SSL_get_options, 7SSL_get_secure_renegotiation_support - manipulate SSL options 8 9=head1 SYNOPSIS 10 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 24=head1 DESCRIPTION 25 26SSL_CTX_set_options() adds the options set via bit-mask in B<options> to B<ctx>. 27B<ctx> B<MUST NOT> be NULL. 28Options already set before are not cleared! 29 30SSL_set_options() adds the options set via bit-mask in B<options> to B<ssl>. 31Options already set before are not cleared! 32 33SSL_CTX_clear_options() clears the options set via bit-mask in B<options> 34to B<ctx>. 35 36SSL_clear_options() clears the options set via bit-mask in B<options> to B<ssl>. 37 38SSL_CTX_get_options() returns the options set for B<ctx>. 39 40SSL_get_options() returns the options set for B<ssl>. 41 42SSL_get_secure_renegotiation_support() indicates whether the peer supports 43secure renegotiation. 44Note, this is implemented via a macro. 45 46=head1 NOTES 47 48The behaviour of the SSL library can be changed by setting several options. 49The options are coded as bit-masks and can be combined by a bitwise B<or> 50operation (|). 51 52SSL_CTX_set_options() and SSL_set_options() affect the (external) 53protocol behaviour of the SSL library. The (internal) behaviour of 54the API can be changed by using the similar 55L<SSL_CTX_set_mode(3)> and SSL_set_mode() functions. 56 57During a handshake, the option settings of the SSL object are used. When 58a new SSL object is created from a context using SSL_new(), the current 59option setting is copied. Changes to B<ctx> do not affect already created 60SSL objects. SSL_clear() does not affect the settings. 61 62The following B<bug workaround> options are available: 63 64=over 4 65 66=item SSL_OP_CRYPTOPRO_TLSEXT_BUG 67 68Add server-hello extension from the early version of cryptopro draft 69when GOST ciphersuite is negotiated. Required for interoperability with CryptoPro 70CSP 3.x. 71 72=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS 73 74Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol 75vulnerability affecting CBC ciphers, which cannot be handled by some 76broken SSL implementations. This option has no effect for connections 77using other ciphers. 78 79=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG 80 81Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X. 82OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers. 83 84=item SSL_OP_TLSEXT_PADDING 85 86Adds a padding extension to ensure the ClientHello size is never between 87256 and 511 bytes in length. This is needed as a workaround for some 88implementations. 89 90=item SSL_OP_ALL 91 92All of the above bug workarounds. 93 94=back 95 96It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround 97options if compatibility with somewhat broken implementations is 98desired. 99 100The following B<modifying> options are available: 101 102=over 4 103 104=item SSL_OP_ALLOW_CLIENT_RENEGOTIATION 105 106Client-initiated renegotiation is disabled by default. Use 107this option to enable it. 108 109=item SSL_OP_ALLOW_NO_DHE_KEX 110 111In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means 112that there will be no forward secrecy for the resumed session. 113 114=item SSL_OP_PREFER_NO_DHE_KEX 115 116In TLSv1.3, on resumption let the server prefer a non-(ec)dhe based key 117exchange mode over an (ec)dhe based one. Ignored without B<SSL_OP_ALLOW_NO_DHE_KEX> 118being set as well. Always ignored on the client. 119 120=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION 121 122Allow legacy insecure renegotiation between OpenSSL and unpatched clients or 123servers. See the B<SECURE RENEGOTIATION> section for more details. 124 125=item SSL_OP_CIPHER_SERVER_PREFERENCE 126 127When choosing a cipher, use the server's preferences instead of the client 128preferences. When not set, the SSL server will always follow the clients 129preferences. When set, the SSL/TLS server will choose following its 130own preferences. 131 132=item SSL_OP_CISCO_ANYCONNECT 133 134Use Cisco's version identifier of DTLS_BAD_VER when establishing a DTLSv1 135connection. Only available when using the deprecated DTLSv1_client_method() API. 136 137=item SSL_OP_CLEANSE_PLAINTEXT 138 139By default TLS and QUIC SSL objects keep a copy of received plaintext 140application data in a static buffer until it is overwritten by the 141next portion of data. When enabling SSL_OP_CLEANSE_PLAINTEXT 142deciphered application data is cleansed by calling OPENSSL_cleanse(3) 143after passing data to the application. Data is also cleansed when 144releasing the connection (e.g. L<SSL_free(3)>). 145 146Since OpenSSL only cleanses internal buffers, the application is still 147responsible for cleansing all other buffers. Most notably, this 148applies to buffers passed to functions like L<SSL_read(3)>, 149L<SSL_peek(3)> but also like L<SSL_write(3)>. 150 151TLS connections do not buffer data to be sent in plaintext. QUIC stream 152objects do buffer plaintext data to be sent and this option will also cause 153that data to be cleansed when it is discarded. 154 155This option can be set differently on individual QUIC stream objects and 156has no effect on QUIC connection objects (except where a default stream is 157being used). 158 159=item SSL_OP_COOKIE_EXCHANGE 160 161Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects 162DTLS connections. 163 164=item SSL_OP_DISABLE_TLSEXT_CA_NAMES 165 166Disable TLS Extension CA Names. You may want to disable it for security reasons 167or for compatibility with some Windows TLS implementations crashing when this 168extension is larger than 1024 bytes. 169 170=item SSL_OP_ENABLE_KTLS 171 172Enable the use of kernel TLS. In order to benefit from kernel TLS OpenSSL must 173have been compiled with support for it, and it must be supported by the 174negotiated ciphersuites and extensions. The specific ciphersuites and extensions 175that are supported may vary by platform and kernel version. 176 177The kernel TLS data-path implements the record layer, and the encryption 178algorithm. The kernel will utilize the best hardware 179available for encryption. Using the kernel data-path should reduce the memory 180footprint of OpenSSL because no buffering is required. Also, the throughput 181should improve because data copy is avoided when user data is encrypted into 182kernel memory instead of the usual encrypt then copy to kernel. 183 184Kernel TLS might not support all the features of OpenSSL. For instance, 185renegotiation, and setting the maximum fragment size is not possible as of 186Linux 4.20. 187 188Note that with kernel TLS enabled some cryptographic operations are performed 189by the kernel directly and not via any available OpenSSL Providers. This might 190be undesirable if, for example, the application requires all cryptographic 191operations to be performed by the FIPS provider. 192 193=item SSL_OP_ENABLE_KTLS_TX_ZEROCOPY_SENDFILE 194 195With this option, sendfile() will use the zerocopy mode, which gives a 196performance boost when used with KTLS hardware offload. Note that invalid TLS 197records might be transmitted if the file is changed while being sent. This 198option has no effect if B<SSL_OP_ENABLE_KTLS> is not enabled. 199 200This option only applies to Linux. KTLS sendfile on FreeBSD doesn't offer an 201option to disable zerocopy and always runs in this mode. 202 203=item SSL_OP_ENABLE_MIDDLEBOX_COMPAT 204 205If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This 206has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that 207do not understand TLSv1.3 will not drop the connection. Regardless of whether 208this option is set or not CCS messages received from the peer will always be 209ignored in TLSv1.3. This option is set by default. To switch it off use 210SSL_clear_options(). A future version of OpenSSL may not set this by default. 211 212=item SSL_OP_IGNORE_UNEXPECTED_EOF 213 214Some TLS implementations do not send the mandatory close_notify alert on 215shutdown. If the application tries to wait for the close_notify alert but the 216peer closes the connection without sending it, an error is generated. When this 217option is enabled the peer does not need to send the close_notify alert and a 218closed connection will be treated as if the close_notify alert was received. 219 220You should only enable this option if the protocol running over TLS 221can detect a truncation attack itself, and that the application is checking for 222that truncation attack. 223 224For more information on shutting down a connection, see L<SSL_shutdown(3)>. 225 226=item SSL_OP_LEGACY_SERVER_CONNECT 227 228Allow legacy insecure renegotiation between OpenSSL and unpatched servers 229B<only>. See the B<SECURE RENEGOTIATION> section for more details. 230 231=item SSL_OP_NO_ANTI_REPLAY 232 233By default, when a server is configured for early data (i.e., max_early_data > 0), 234OpenSSL will switch on replay protection. See L<SSL_read_early_data(3)> for a 235description of the replay protection feature. Anti-replay measures are required 236to comply with the TLSv1.3 specification. Some applications may be able to 237mitigate the replay risks in other ways and in such cases the built in OpenSSL 238functionality is not required. Those applications can turn this feature off by 239setting this option. This is a server-side option only. It is ignored by 240clients. 241 242=item SSL_OP_NO_TX_CERTIFICATE_COMPRESSION 243 244Normally clients and servers will transparently attempt to negotiate the 245RFC8879 certificate compression option on TLSv1.3 connections. 246 247If this option is set, the certificate compression extension is ignored 248upon receipt and compressed certificates will not be sent to the peer. 249 250=item SSL_OP_NO_RX_CERTIFICATE_COMPRESSION 251 252Normally clients and servers will transparently attempt to negotiate the 253RFC8879 certificate compression option on TLSv1.3 connections. 254 255If this option is set, the certificate compression extension will not be sent 256and compressed certificates will not be accepted from the peer. 257 258=item SSL_OP_NO_COMPRESSION 259 260Do not use TLS record compression even if it is supported. This option is set by 261default. To switch it off use SSL_clear_options(). Note that TLS record 262compression is not recommended and is not available at security level 2 or 263above. From OpenSSL 3.2 the default security level is 2, so clearing this option 264will have no effect without also changing the default security level. See 265L<SSL_CTX_set_security_level(3)>. 266 267=item SSL_OP_NO_ENCRYPT_THEN_MAC 268 269Normally clients and servers will transparently attempt to negotiate the 270RFC7366 Encrypt-then-MAC option on TLS and DTLS connection. 271 272If this option is set, Encrypt-then-MAC is disabled. Clients will not 273propose, and servers will not accept the extension. 274 275=item SSL_OP_NO_EXTENDED_MASTER_SECRET 276 277Normally clients and servers will transparently attempt to negotiate the 278RFC7627 Extended Master Secret option on TLS and DTLS connection. 279 280If this option is set, Extended Master Secret is disabled. Clients will 281not propose, and servers will not accept the extension. 282 283=item SSL_OP_NO_QUERY_MTU 284 285Do not query the MTU. Only affects DTLS connections. 286 287=item SSL_OP_NO_RENEGOTIATION 288 289Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest 290messages, and ignore renegotiation requests via ClientHello. 291 292=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION 293 294When performing renegotiation as a server, always start a new session 295(i.e., session resumption requests are only accepted in the initial 296handshake). This option is not needed for clients. 297 298=item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, 299SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2 300 301These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol 302versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS, 303respectively. 304As of OpenSSL 1.1.0, these options are deprecated, use 305L<SSL_CTX_set_min_proto_version(3)> and 306L<SSL_CTX_set_max_proto_version(3)> instead. 307 308=item SSL_OP_NO_TICKET 309 310SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless 311session tickets. 312 313When using session ids a copy of the session information is 314cached on the server and a unique id is sent to the client. When the client 315wishes to resume it provides the unique id so that the server can retrieve the 316session information from its cache. 317 318When using stateless session tickets the server uses a session ticket encryption 319key to encrypt the session information. This encrypted data is sent to the 320client as a "ticket". When the client wishes to resume it sends the encrypted 321data back to the server. The server uses its key to decrypt the data and resume 322the session. In this way the server can operate statelessly - no session 323information needs to be cached locally. 324 325The TLSv1.3 protocol only supports tickets and does not directly support session 326ids. However, OpenSSL allows two modes of ticket operation in TLSv1.3: stateful 327and stateless. Stateless tickets work the same way as in TLSv1.2 and below. 328Stateful tickets mimic the session id behaviour available in TLSv1.2 and below. 329The session information is cached on the server and the session id is wrapped up 330in a ticket and sent back to the client. When the client wishes to resume, it 331presents a ticket in the same way as for stateless tickets. The server can then 332extract the session id from the ticket and retrieve the session information from 333its cache. 334 335By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will 336cause stateless tickets to not be issued. In TLSv1.2 and below this means no 337ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be 338sent. This is a server-side option only. 339 340In TLSv1.3 it is possible to suppress all tickets (stateful and stateless) from 341being sent by calling L<SSL_CTX_set_num_tickets(3)> or 342L<SSL_set_num_tickets(3)>. 343 344=item SSL_OP_PRIORITIZE_CHACHA 345 346When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize 347ChaCha20-Poly1305 ciphers to the top of the server cipher list if a 348ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps 349those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere 350in the server cipher list; but still allows other clients to use AES and other 351ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>. 352 353=item SSL_OP_TLS_ROLLBACK_BUG 354 355Disable version rollback attack detection. 356 357During the client key exchange, the client must send the same information 358about acceptable SSL/TLS protocol levels as during the first hello. Some 359clients violate this rule by adapting to the server's answer. (Example: 360the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server 361only understands up to SSLv3. In this case the client must still use the 362same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect 363to the server's answer and violate the version rollback protection.) 364 365=back 366 367The following options no longer have any effect but their identifiers are 368retained for compatibility purposes: 369 370=over 4 371 372=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG 373 374=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER 375 376=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG 377 378=item SSL_OP_TLS_D5_BUG 379 380=item SSL_OP_TLS_BLOCK_PADDING_BUG 381 382=item SSL_OP_MSIE_SSLV2_RSA_PADDING 383 384=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG 385 386=item SSL_OP_MICROSOFT_SESS_ID_BUG 387 388=item SSL_OP_NETSCAPE_CHALLENGE_BUG 389 390=item SSL_OP_PKCS1_CHECK_1 391 392=item SSL_OP_PKCS1_CHECK_2 393 394=item SSL_OP_SINGLE_DH_USE 395 396=item SSL_OP_SINGLE_ECDH_USE 397 398=item SSL_OP_EPHEMERAL_RSA 399 400=item SSL_OP_NETSCAPE_CA_DN_BUG 401 402=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG 403 404=back 405 406=head1 SECURE RENEGOTIATION 407 408OpenSSL always attempts to use secure renegotiation as 409described in RFC5746. This counters the prefix attack described in 410CVE-2009-3555 and elsewhere. 411 412This attack has far reaching consequences which application writers should be 413aware of. In the description below an implementation supporting secure 414renegotiation is referred to as I<patched>. A server not supporting secure 415renegotiation is referred to as I<unpatched>. 416 417The following sections describe the operations permitted by OpenSSL's secure 418renegotiation implementation. 419 420=head2 Patched client and server 421 422Connections and renegotiation are always permitted by OpenSSL implementations. 423 424=head2 Unpatched client and patched OpenSSL server 425 426The initial connection succeeds but client renegotiation is denied by the 427server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal 428B<handshake_failure> alert in SSL v3.0. 429 430If the patched OpenSSL server attempts to renegotiate a fatal 431B<handshake_failure> alert is sent. This is because the server code may be 432unaware of the unpatched nature of the client. 433 434If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then 435renegotiation B<always> succeeds. 436 437=head2 Patched OpenSSL client and unpatched server 438 439If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or 440B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections 441and renegotiation between patched OpenSSL clients and unpatched servers 442succeeds. If neither option is set then initial connections to unpatched 443servers will fail. 444 445Setting the option B<SSL_OP_LEGACY_SERVER_CONNECT> has security implications; 446clients that are willing to connect to servers that do not implement 447RFC 5746 secure renegotiation are subject to attacks such as 448CVE-2009-3555. 449 450OpenSSL client applications wishing to ensure they can connect to unpatched 451servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT> 452 453OpenSSL client applications that want to ensure they can B<not> connect to 454unpatched servers (and thus avoid any security issues) should always B<clear> 455B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or 456SSL_clear_options(). 457 458The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and 459B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that 460B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure 461renegotiation between OpenSSL clients and unpatched servers B<only>, while 462B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections 463and renegotiation between OpenSSL and unpatched clients or servers. 464 465=head2 Applicability of options to QUIC connections and streams 466 467These options apply to SSL objects referencing a QUIC connection: 468 469=over 4 470 471=item SSL_OP_ALLOW_NO_DHE_KEX 472 473=item SSL_OP_NO_TX_CERTIFICATE_COMPRESSION 474 475=item SSL_OP_NO_RX_CERTIFICATE_COMPRESSION 476 477=item SSL_OP_NO_TICKET 478 479=item SSL_OP_PRIORITIZE_CHACHA 480 481=back 482 483These options apply to SSL objects referencing a QUIC stream: 484 485=over 4 486 487=item SSL_OP_CLEANSE_PLAINTEXT 488 489=back 490 491Options on QUIC connections are initialized from the options set on SSL_CTX 492before a QUIC connection SSL object is created. Options on QUIC streams are 493initialised from the options configured on the QUIC connection SSL object 494they are created from. 495 496Setting options which relate to QUIC streams on a QUIC connection SSL object has 497no direct effect on the QUIC connection SSL object itself, but will change the 498options set on the default stream (if there is one) and will also determine the 499default options set on any future streams which are created. 500 501Other options not mentioned above do not have an effect and will be ignored. 502 503Options which relate to QUIC streams may also be set directly on QUIC stream SSL 504objects. Setting connection-related options on such an object has no effect. 505 506=head1 RETURN VALUES 507 508SSL_CTX_set_options() and SSL_set_options() return the new options bit-mask 509after adding B<options>. 510 511SSL_CTX_clear_options() and SSL_clear_options() return the new options bit-mask 512after clearing B<options>. 513 514SSL_CTX_get_options() and SSL_get_options() return the current bit-mask. 515 516SSL_get_secure_renegotiation_support() returns 1 is the peer supports 517secure renegotiation and 0 if it does not. 518 519=head1 SEE ALSO 520 521L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>, L<SSL_shutdown(3)> 522L<SSL_CTX_set_tmp_dh_callback(3)>, 523L<SSL_CTX_set_min_proto_version(3)>, 524L<openssl-dhparam(1)> 525 526=head1 HISTORY 527 528The attempt to always try to use secure renegotiation was added in 529OpenSSL 0.9.8m. 530 531The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options 532were added in OpenSSL 1.1.1. 533 534The B<SSL_OP_NO_EXTENDED_MASTER_SECRET> and B<SSL_OP_IGNORE_UNEXPECTED_EOF> 535options were added in OpenSSL 3.0. 536 537The B<SSL_OP_> constants and the corresponding parameter and return values 538of the affected functions were changed to C<uint64_t> type in OpenSSL 3.0. 539For that reason it is no longer possible use the B<SSL_OP_> macro values 540in preprocessor C<#if> conditions. However it is still possible to test 541whether these macros are defined or not. 542 543=head1 COPYRIGHT 544 545Copyright 2001-2023 The OpenSSL Project Authors. All Rights Reserved. 546 547Licensed under the Apache License 2.0 (the "License"). You may not use 548this file except in compliance with the License. You can obtain a copy 549in the file LICENSE in the source distribution or at 550L<https://www.openssl.org/source/license.html>. 551 552=cut 553
