doc/man3/SSL_CONF_cmd.pod

   1 =pod
   2
   3 =head1 NAME
   4
   5 SSL_CONF_cmd_value_type,
   6 SSL_CONF_cmd - send configuration command
   7
   8 =head1 SYNOPSIS
   9
  10  #include <openssl/ssl.h>
  11
  12  int SSL_CONF_cmd(SSL_CONF_CTX *ctx, const char *option, const char *value);
  13  int SSL_CONF_cmd_value_type(SSL_CONF_CTX *ctx, const char *option);
  14
  15 =head1 DESCRIPTION
  16
  17 The function SSL_CONF_cmd() performs configuration operation B<option> with
  18 optional parameter B<value> on B<ctx>. Its purpose is to simplify application
  19 configuration of B<SSL_CTX> or B<SSL> structures by providing a common
  20 framework for command line options or configuration files.
  21
  22 SSL_CONF_cmd_value_type() returns the type of value that B<option> refers to.
  23
  24 =head1 SUPPORTED COMMAND LINE COMMANDS
  25
  26 Currently supported B<option> names for command lines (i.e. when the
  27 flag B<SSL_CONF_CMDLINE> is set) are listed below. Note: all B<option> names
  28 are case sensitive. Unless otherwise stated commands can be used by
  29 both clients and servers and the B<value> parameter is not used. The default
  30 prefix for command line commands is B<-> and that is reflected below.
  31
  32 =over 4
  33
  34 =item B<-bugs>
  35
  36 Various bug workarounds are set, same as setting B<SSL_OP_ALL>.
  37
  38 =item B<-no_comp>
  39
  40 Disables support for SSL/TLS compression, same as setting
  41 B<SSL_OP_NO_COMPRESSION>.
  42 As of OpenSSL 1.1.0, compression is off by default.
  43
  44 =item B<-comp>
  45
  46 Enables support for SSL/TLS compression, same as clearing
  47 B<SSL_OP_NO_COMPRESSION>.
  48 This command was introduced in OpenSSL 1.1.0.
  49 As of OpenSSL 1.1.0, compression is off by default.
  50
  51 =item B<-no_ticket>
  52
  53 Disables support for session tickets, same as setting B<SSL_OP_NO_TICKET>.
  54
  55 =item B<-serverpref>
  56
  57 Use server and not client preference order when determining which cipher suite,
  58 signature algorithm or elliptic curve to use for an incoming connection.
  59 Equivalent to B<SSL_OP_CIPHER_SERVER_PREFERENCE>. Only used by servers.
  60
  61 =item B<-legacyrenegotiation>
  62
  63 permits the use of unsafe legacy renegotiation. Equivalent to setting
  64 B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>.
  65
  66 =item B<-no_renegotiation>
  67
  68 Disables all attempts at renegotiation in TLSv1.2 and earlier, same as setting
  69 B<SSL_OP_NO_RENEGOTIATION>.
  70
  71 =item B<-no_resumption_on_reneg>
  72
  73 set SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION flag. Only used by servers.
  74
  75 =item B<-legacy_server_connect>, B<-no_legacy_server_connect>
  76
  77 permits or prohibits the use of unsafe legacy renegotiation for OpenSSL
  78 clients only. Equivalent to setting or clearing B<SSL_OP_LEGACY_SERVER_CONNECT>.
  79 Set by default.
  80
  81 =item B<-prioritize_chacha>
  82
  83 Prioritize ChaCha ciphers when the client has a ChaCha20 cipher at the top of
  84 its preference list. This usually indicates a client without AES hardware
  85 acceleration (e.g. mobile) is in use. Equivalent to B<SSL_OP_PRIORITIZE_CHACHA>.
  86 Only used by servers. Requires B<-serverpref>.
  87
  88 =item B<-allow_no_dhe_kex>
  89
  90 In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means
  91 that there will be no forward secrecy for the resumed session.
  92
  93 =item B<-strict>
  94
  95 enables strict mode protocol handling. Equivalent to setting
  96 B<SSL_CERT_FLAG_TLS_STRICT>.
  97
  98 =item B<-sigalgs> I<algs>
  99
 100 This sets the supported signature algorithms for TLSv1.2 and TLSv1.3.
 101 For clients this value is used directly for the supported signature
 102 algorithms extension. For servers it is used to determine which signature
 103 algorithms to support.
 104
 105 The B<algs> argument should be a colon separated list of signature
 106 algorithms in order of decreasing preference of the form B<algorithm+hash>
 107 or B<signature_scheme>. B<algorithm> is one of B<RSA>, B<DSA> or B<ECDSA> and
 108 B<hash> is a supported algorithm OID short name such as B<SHA1>, B<SHA224>,
 109 B<SHA256>, B<SHA384> of B<SHA512>.  Note: algorithm and hash names are case
 110 sensitive.  B<signature_scheme> is one of the signature schemes defined in
 111 TLSv1.3, specified using the IETF name, e.g., B<ecdsa_secp256r1_sha256>,
 112 B<ed25519>, or B<rsa_pss_pss_sha256>.
 113
 114 If this option is not set then all signature algorithms supported by the
 115 OpenSSL library are permissible.
 116
 117 Note: algorithms which specify a PKCS#1 v1.5 signature scheme (either by
 118 using B<RSA> as the B<algorithm> or by using one of the B<rsa_pkcs1_*>
 119 identifiers) are ignored in TLSv1.3 and will not be negotiated.
 120
 121 =item B<-client_sigalgs> I<algs>
 122
 123 This sets the supported signature algorithms associated with client
 124 authentication for TLSv1.2 and TLSv1.3.  For servers the B<algs> is used
 125 in the B<signature_algorithms> field of a B<CertificateRequest> message.
 126 For clients it is used to determine which signature algorithm to use with
 127 the client certificate.  If a server does not request a certificate this
 128 option has no effect.
 129
 130 The syntax of B<algs> is identical to B<-sigalgs>. If not set, then the
 131 value set for B<-sigalgs> will be used instead.
 132
 133 =item B<-groups> I<groups>
 134
 135 This sets the supported groups. For clients, the groups are sent using
 136 the supported groups extension. For servers, it is used to determine which
 137 group to use. This setting affects groups used for signatures (in TLSv1.2
 138 and earlier) and key exchange. The first group listed will also be used
 139 for the B<key_share> sent by a client in a TLSv1.3 B<ClientHello>.
 140
 141 The B<groups> argument is a colon separated list of groups. The group can
 142 be either the B<NIST> name (e.g. B<P-256>), some other commonly used name
 143 where applicable (e.g. B<X25519>, B<ffdhe2048>) or an OpenSSL OID name
 144 (e.g B<prime256v1>). Group names are case sensitive. The list should be
 145 in order of preference with the most preferred group first.
 146
 147 Currently supported groups for B<TLSv1.3> are B<P-256>, B<P-384>, B<P-521>,
 148 B<X25519>, B<X448>, B<ffdhe2048>, B<ffdhe3072>, B<ffdhe4096>, B<ffdhe6144>,
 149 B<ffdhe8192>.
 150
 151 =item B<-curves> I<groups>
 152
 153 This is a synonym for the B<-groups> command.
 154
 155 =item B<-named_curve> I<curve>
 156
 157 This sets the temporary curve used for ephemeral ECDH modes. Only used
 158 by servers.
 159
 160 The B<groups> argument is a curve name or the special value B<auto> which
 161 picks an appropriate curve based on client and server preferences. The
 162 curve can be either the B<NIST> name (e.g. B<P-256>) or an OpenSSL OID name
 163 (e.g B<prime256v1>). Curve names are case sensitive.
 164
 165 =item B<-cipher> I<ciphers>
 166
 167 Sets the TLSv1.2 and below ciphersuite list to B<ciphers>. This list will be
 168 combined with any configured TLSv1.3 ciphersuites. Note: syntax checking
 169 of B<ciphers> is currently not performed unless a B<SSL> or B<SSL_CTX>
 170 structure is associated with B<ctx>.
 171
 172 =item B<-ciphersuites> I<1.3ciphers>
 173
 174 Sets the available ciphersuites for TLSv1.3 to value. This is a
 175 colon-separated list of TLSv1.3 ciphersuite names in order of preference. This
 176 list will be combined any configured TLSv1.2 and below ciphersuites.
 177 See L<openssl-ciphers(1)> for more information.
 178
 179 =item B<-min_protocol> I<minprot>, B<-max_protocol> I<maxprot>
 180
 181 Sets the minimum and maximum supported protocol.  Currently supported
 182 protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>, B<TLSv1.2>, B<TLSv1.3>
 183 for TLS and B<DTLSv1>, B<DTLSv1.2> for DTLS, and B<None> for no limit.
 184 If either bound is not specified then only the other bound applies,
 185 if specified.  To restrict the supported protocol versions use these
 186 commands rather than the deprecated alternative commands below.
 187
 188 =item B<-record_padding> I<padding>
 189
 190 Attempts to pad TLSv1.3 records so that they are a multiple of B<padding>
 191 in length on send. A B<padding> of 0 or 1 turns off padding. Otherwise,
 192 the B<padding> must be >1 or <=16384.
 193
 194 =item B<-debug_broken_protocol>
 195
 196 =item B<-no_middlebox>
 197
 198 =back
 199
 200 =head2 Additional Options
 201
 202 The following options are accepted by SSL_CONF_cmd(), but are not
 203 processed by the OpenSSL commands.
 204
 205 =over 4
 206
 207 =item B<-cert> I<file>
 208
 209 Attempts to use B<file> as the certificate for the appropriate context. It
 210 currently uses SSL_CTX_use_certificate_chain_file() if an B<SSL_CTX>
 211 structure is set or SSL_use_certificate_file() with filetype PEM if an
 212 B<SSL> structure is set. This option is only supported if certificate
 213 operations are permitted.
 214
 215 =item B<-key> I<file>
 216
 217 Attempts to use B<file> as the private key for the appropriate context. This
 218 option is only supported if certificate operations are permitted. Note:
 219 if no B<-key> option is set then a private key is not loaded unless the
 220 flag B<SSL_CONF_FLAG_REQUIRE_PRIVATE> is set.
 221
 222 =item B<-dhparam> I<file>
 223
 224 Attempts to use B<file> as the set of temporary DH parameters for
 225 the appropriate context. This option is only supported if certificate
 226 operations are permitted.
 227
 228 =item B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>, B<-no_tls1_3>
 229
 230 Disables protocol support for SSLv3, TLSv1.0, TLSv1.1, TLSv1.2 or TLSv1.3 by
 231 setting the corresponding options B<SSL_OP_NO_SSLv3>, B<SSL_OP_NO_TLSv1>,
 232 B<SSL_OP_NO_TLSv1_1>, B<SSL_OP_NO_TLSv1_2> and B<SSL_OP_NO_TLSv1_3>
 233 respectively. These options are deprecated, use B<-min_protocol> and
 234 B<-max_protocol> instead.
 235
 236 =item B<-anti_replay>, B<-no_anti_replay>
 237
 238 Switches replay protection, on or off respectively. With replay protection on,
 239 OpenSSL will automatically detect if a session ticket has been used more than
 240 once, TLSv1.3 has been negotiated, and early data is enabled on the server. A
 241 full handshake is forced if a session ticket is used a second or subsequent
 242 time. Anti-Replay is on by default unless overridden by a configuration file and
 243 is only used by servers. Anti-replay measures are required for compliance with
 244 the TLSv1.3 specification. Some applications may be able to mitigate the replay
 245 risks in other ways and in such cases the built-in OpenSSL functionality is not
 246 required. Switching off anti-replay is equivalent to B<SSL_OP_NO_ANTI_REPLAY>.
 247
 248 =back
 249
 250 =head1 SUPPORTED CONFIGURATION FILE COMMANDS
 251
 252 Currently supported B<option> names for configuration files (i.e., when the
 253 flag B<SSL_CONF_FLAG_FILE> is set) are listed below. All configuration file
 254 B<option> names are case insensitive so B<signaturealgorithms> is recognised
 255 as well as B<SignatureAlgorithms>. Unless otherwise stated the B<value> names
 256 are also case insensitive.
 257
 258 Note: the command prefix (if set) alters the recognised B<option> values.
 259
 260 =over 4
 261
 262 =item B<CipherString>
 263
 264 Sets the ciphersuite list for TLSv1.2 and below to B<value>. This list will be
 265 combined with any configured TLSv1.3 ciphersuites. Note: syntax
 266 checking of B<value> is currently not performed unless an B<SSL> or B<SSL_CTX>
 267 structure is associated with B<ctx>.
 268
 269 =item B<Ciphersuites>
 270
 271 Sets the available ciphersuites for TLSv1.3 to B<value>. This is a
 272 colon-separated list of TLSv1.3 ciphersuite names in order of preference. This
 273 list will be combined any configured TLSv1.2 and below ciphersuites.
 274 See L<openssl-ciphers(1)> for more information.
 275
 276 =item B<Certificate>
 277
 278 Attempts to use the file B<value> as the certificate for the appropriate
 279 context. It currently uses SSL_CTX_use_certificate_chain_file() if an B<SSL_CTX>
 280 structure is set or SSL_use_certificate_file() with filetype PEM if an B<SSL>
 281 structure is set. This option is only supported if certificate operations
 282 are permitted.
 283
 284 =item B<PrivateKey>
 285
 286 Attempts to use the file B<value> as the private key for the appropriate
 287 context. This option is only supported if certificate operations
 288 are permitted. Note: if no B<PrivateKey> option is set then a private key is
 289 not loaded unless the B<SSL_CONF_FLAG_REQUIRE_PRIVATE> is set.
 290
 291 =item B<ChainCAFile>, B<ChainCAPath>, B<VerifyCAFile>, B<VerifyCAPath>
 292
 293 These options indicate a file or directory used for building certificate
 294 chains or verifying certificate chains. These options are only supported
 295 if certificate operations are permitted.
 296
 297 =item B<RequestCAFile>
 298
 299 This option indicates a file containing a set of certificates in PEM form.
 300 The subject names of the certificates are sent to the peer in the
 301 B<certificate_authorities> extension for TLS 1.3 (in ClientHello or
 302 CertificateRequest) or in a certificate request for previous versions or
 303 TLS.
 304
 305 =item B<ServerInfoFile>
 306
 307 Attempts to use the file B<value> in the "serverinfo" extension using the
 308 function SSL_CTX_use_serverinfo_file.
 309
 310 =item B<DHParameters>
 311
 312 Attempts to use the file B<value> as the set of temporary DH parameters for
 313 the appropriate context. This option is only supported if certificate
 314 operations are permitted.
 315
 316 =item B<RecordPadding>
 317
 318 Attempts to pad TLSv1.3 records so that they are a multiple of B<value> in
 319 length on send. A B<value> of 0 or 1 turns off padding. Otherwise, the
 320 B<value> must be >1 or <=16384.
 321
 322 =item B<SignatureAlgorithms>
 323
 324 This sets the supported signature algorithms for TLSv1.2 and TLSv1.3.
 325 For clients this
 326 value is used directly for the supported signature algorithms extension. For
 327 servers it is used to determine which signature algorithms to support.
 328
 329 The B<value> argument should be a colon separated list of signature algorithms
 330 in order of decreasing preference of the form B<algorithm+hash> or
 331 B<signature_scheme>. B<algorithm>
 332 is one of B<RSA>, B<DSA> or B<ECDSA> and B<hash> is a supported algorithm
 333 OID short name such as B<SHA1>, B<SHA224>, B<SHA256>, B<SHA384> of B<SHA512>.
 334 Note: algorithm and hash names are case sensitive.
 335 B<signature_scheme> is one of the signature schemes defined in TLSv1.3,
 336 specified using the IETF name, e.g., B<ecdsa_secp256r1_sha256>, B<ed25519>,
 337 or B<rsa_pss_pss_sha256>.
 338
 339 If this option is not set then all signature algorithms supported by the
 340 OpenSSL library are permissible.
 341
 342 Note: algorithms which specify a PKCS#1 v1.5 signature scheme (either by
 343 using B<RSA> as the B<algorithm> or by using one of the B<rsa_pkcs1_*>
 344 identifiers) are ignored in TLSv1.3 and will not be negotiated.
 345
 346 =item B<ClientSignatureAlgorithms>
 347
 348 This sets the supported signature algorithms associated with client
 349 authentication for TLSv1.2 and TLSv1.3.
 350 For servers the value is used in the
 351 B<signature_algorithms> field of a B<CertificateRequest> message.
 352 For clients it is
 353 used to determine which signature algorithm to use with the client certificate.
 354 If a server does not request a certificate this option has no effect.
 355
 356 The syntax of B<value> is identical to B<SignatureAlgorithms>. If not set then
 357 the value set for B<SignatureAlgorithms> will be used instead.
 358
 359 =item B<Groups>
 360
 361 This sets the supported groups. For clients, the groups are
 362 sent using the supported groups extension. For servers, it is used
 363 to determine which group to use. This setting affects groups used for
 364 signatures (in TLSv1.2 and earlier) and key exchange. The first group listed
 365 will also be used for the B<key_share> sent by a client in a TLSv1.3
 366 B<ClientHello>.
 367
 368 The B<value> argument is a colon separated list of groups. The group can be
 369 either the B<NIST> name (e.g. B<P-256>), some other commonly used name where
 370 applicable (e.g. B<X25519>, B<ffdhe2048>) or an OpenSSL OID name
 371 (e.g B<prime256v1>). Group names are case sensitive. The list should be in
 372 order of preference with the most preferred group first.
 373
 374 Currently supported groups for B<TLSv1.3> are B<P-256>, B<P-384>, B<P-521>,
 375 B<X25519>, B<X448>, B<ffdhe2048>, B<ffdhe3072>, B<ffdhe4096>, B<ffdhe6144>,
 376 B<ffdhe8192>.
 377
 378 =item B<Curves>
 379
 380 This is a synonym for the "Groups" command.
 381
 382 =item B<MinProtocol>
 383
 384 This sets the minimum supported SSL, TLS or DTLS version.
 385
 386 Currently supported protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>,
 387 B<TLSv1.2>, B<TLSv1.3>, B<DTLSv1> and B<DTLSv1.2>.
 388 The value B<None> will disable the limit.
 389
 390 =item B<MaxProtocol>
 391
 392 This sets the maximum supported SSL, TLS or DTLS version.
 393
 394 Currently supported protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>,
 395 B<TLSv1.2>, B<TLSv1.3>, B<DTLSv1> and B<DTLSv1.2>.
 396 The value B<None> will disable the limit.
 397
 398 =item B<Protocol>
 399
 400 This can be used to enable or disable certain versions of the SSL,
 401 TLS or DTLS protocol.
 402
 403 The B<value> argument is a comma separated list of supported protocols
 404 to enable or disable.
 405 If a protocol is preceded by B<-> that version is disabled.
 406
 407 All protocol versions are enabled by default.
 408 You need to disable at least one protocol version for this setting have any
 409 effect.
 410 Only enabling some protocol versions does not disable the other protocol
 411 versions.
 412
 413 Currently supported protocol values are B<SSLv3>, B<TLSv1>, B<TLSv1.1>,
 414 B<TLSv1.2>, B<TLSv1.3>, B<DTLSv1> and B<DTLSv1.2>.
 415 The special value B<ALL> refers to all supported versions.
 416
 417 This can't enable protocols that are disabled using B<MinProtocol>
 418 or B<MaxProtocol>, but can disable protocols that are still allowed
 419 by them.
 420
 421 The B<Protocol> command is fragile and deprecated; do not use it.
 422 Use B<MinProtocol> and B<MaxProtocol> instead.
 423 If you do use B<Protocol>, make sure that the resulting range of enabled
 424 protocols has no "holes", e.g. if TLS 1.0 and TLS 1.2 are both enabled, make
 425 sure to also leave TLS 1.1 enabled.
 426
 427 =item B<Options>
 428
 429 The B<value> argument is a comma separated list of various flags to set.
 430 If a flag string is preceded B<-> it is disabled.
 431 See the L<SSL_CTX_set_options(3)> function for more details of
 432 individual options.
 433
 434 Each option is listed below. Where an operation is enabled by default
 435 the B<-flag> syntax is needed to disable it.
 436
 437 B<SessionTicket>: session ticket support, enabled by default. Inverse of
 438 B<SSL_OP_NO_TICKET>: that is B<-SessionTicket> is the same as setting
 439 B<SSL_OP_NO_TICKET>.
 440
 441 B<Compression>: SSL/TLS compression support, enabled by default. Inverse
 442 of B<SSL_OP_NO_COMPRESSION>.
 443
 444 B<EmptyFragments>: use empty fragments as a countermeasure against a
 445 SSL 3.0/TLS 1.0 protocol vulnerability affecting CBC ciphers. It
 446 is set by default. Inverse of B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS>.
 447
 448 B<Bugs>: enable various bug workarounds. Same as B<SSL_OP_ALL>.
 449
 450 B<DHSingle>: enable single use DH keys, set by default. Inverse of
 451 B<SSL_OP_DH_SINGLE>. Only used by servers.
 452
 453 B<ECDHSingle>: enable single use ECDH keys, set by default. Inverse of
 454 B<SSL_OP_ECDH_SINGLE>. Only used by servers.
 455
 456 B<ServerPreference>: use server and not client preference order when
 457 determining which cipher suite, signature algorithm or elliptic curve
 458 to use for an incoming connection.  Equivalent to
 459 B<SSL_OP_CIPHER_SERVER_PREFERENCE>. Only used by servers.
 460
 461 B<PrioritizeChaCha>: prioritizes ChaCha ciphers when the client has a
 462 ChaCha20 cipher at the top of its preference list. This usually indicates
 463 a mobile client is in use. Equivalent to B<SSL_OP_PRIORITIZE_CHACHA>.
 464 Only used by servers.
 465
 466 B<NoResumptionOnRenegotiation>: set
 467 B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> flag. Only used by servers.
 468
 469 B<NoRenegotiation>: disables all attempts at renegotiation in TLSv1.2 and
 470 earlier, same as setting B<SSL_OP_NO_RENEGOTIATION>.
 471
 472 B<UnsafeLegacyRenegotiation>: permits the use of unsafe legacy renegotiation.
 473 Equivalent to B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>.
 474
 475 B<UnsafeLegacyServerConnect>: permits the use of unsafe legacy renegotiation
 476 for OpenSSL clients only. Equivalent to B<SSL_OP_LEGACY_SERVER_CONNECT>.
 477 Set by default.
 478
 479 B<EncryptThenMac>: use encrypt-then-mac extension, enabled by
 480 default. Inverse of B<SSL_OP_NO_ENCRYPT_THEN_MAC>: that is,
 481 B<-EncryptThenMac> is the same as setting B<SSL_OP_NO_ENCRYPT_THEN_MAC>.
 482
 483 B<AllowNoDHEKEX>: In TLSv1.3 allow a non-(ec)dhe based key exchange mode on
 484 resumption. This means that there will be no forward secrecy for the resumed
 485 session. Equivalent to B<SSL_OP_ALLOW_NO_DHE_KEX>.
 486
 487 B<MiddleboxCompat>: If set then dummy Change Cipher Spec (CCS) messages are sent
 488 in TLSv1.3. This has the effect of making TLSv1.3 look more like TLSv1.2 so that
 489 middleboxes that do not understand TLSv1.3 will not drop the connection. This
 490 option is set by default. A future version of OpenSSL may not set this by
 491 default. Equivalent to B<SSL_OP_ENABLE_MIDDLEBOX_COMPAT>.
 492
 493 B<AntiReplay>: If set then OpenSSL will automatically detect if a session ticket
 494 has been used more than once, TLSv1.3 has been negotiated, and early data is
 495 enabled on the server. A full handshake is forced if a session ticket is used a
 496 second or subsequent time. This option is set by default and is only used by
 497 servers. Anti-replay measures are required to comply with the TLSv1.3
 498 specification. Some applications may be able to mitigate the replay risks in
 499 other ways and in such cases the built-in OpenSSL functionality is not required.
 500 Disabling anti-replay is equivalent to setting B<SSL_OP_NO_ANTI_REPLAY>.
 501
 502 B<ExtendedMasterSecret>: use extended master secret extension, enabled by
 503 default. Inverse of B<SSL_OP_NO_EXTENDED_MASTER_SECRET>: that is,
 504 B<-ExtendedMasterSecret> is the same as setting B<SSL_OP_NO_EXTENDED_MASTER_SECRET>.
 505
 506 =item B<VerifyMode>
 507
 508 The B<value> argument is a comma separated list of flags to set.
 509
 510 B<Peer> enables peer verification: for clients only.
 511
 512 B<Request> requests but does not require a certificate from the client.
 513 Servers only.
 514
 515 B<Require> requests and requires a certificate from the client: an error
 516 occurs if the client does not present a certificate. Servers only.
 517
 518 B<Once> requests a certificate from a client only on the initial connection:
 519 not when renegotiating. Servers only.
 520
 521 B<RequestPostHandshake> configures the connection to support requests but does
 522 not require a certificate from the client post-handshake. A certificate will
 523 not be requested during the initial handshake. The server application must
 524 provide a mechanism to request a certificate post-handshake. Servers only.
 525 TLSv1.3 only.
 526
 527 B<RequiresPostHandshake> configures the connection to support requests and
 528 requires a certificate from the client post-handshake: an error occurs if the
 529 client does not present a certificate. A certificate will not be requested
 530 during the initial handshake. The server application must provide a mechanism
 531 to request a certificate post-handshake. Servers only. TLSv1.3 only.
 532
 533 =item B<ClientCAFile>, B<ClientCAPath>
 534
 535 A file or directory of certificates in PEM format whose names are used as the
 536 set of acceptable names for client CAs. Servers only. This option is only
 537 supported if certificate operations are permitted.
 538
 539 =back
 540
 541 =head1 SUPPORTED COMMAND TYPES
 542
 543 The function SSL_CONF_cmd_value_type() currently returns one of the following
 544 types:
 545
 546 =over 4
 547
 548 =item B<SSL_CONF_TYPE_UNKNOWN>
 549
 550 The B<option> string is unrecognised, this return value can be use to flag
 551 syntax errors.
 552
 553 =item B<SSL_CONF_TYPE_STRING>
 554
 555 The value is a string without any specific structure.
 556
 557 =item B<SSL_CONF_TYPE_FILE>
 558
 559 The value is a filename.
 560
 561 =item B<SSL_CONF_TYPE_DIR>
 562
 563 The value is a directory name.
 564
 565 =item B<SSL_CONF_TYPE_NONE>
 566
 567 The value string is not used e.g. a command line option which doesn't take an
 568 argument.
 569
 570 =back
 571
 572 =head1 NOTES
 573
 574 The order of operations is significant. This can be used to set either defaults
 575 or values which cannot be overridden. For example if an application calls:
 576
 577  SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
 578  SSL_CONF_cmd(ctx, userparam, uservalue);
 579
 580 it will disable SSLv3 support by default but the user can override it. If
 581 however the call sequence is:
 582
 583  SSL_CONF_cmd(ctx, userparam, uservalue);
 584  SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
 585
 586 SSLv3 is B<always> disabled and attempt to override this by the user are
 587 ignored.
 588
 589 By checking the return code of SSL_CONF_cmd() it is possible to query if a
 590 given B<option> is recognised, this is useful if SSL_CONF_cmd() values are
 591 mixed with additional application specific operations.
 592
 593 For example an application might call SSL_CONF_cmd() and if it returns
 594 -2 (unrecognised command) continue with processing of application specific
 595 commands.
 596
 597 Applications can also use SSL_CONF_cmd() to process command lines though the
 598 utility function SSL_CONF_cmd_argv() is normally used instead. One way
 599 to do this is to set the prefix to an appropriate value using
 600 SSL_CONF_CTX_set1_prefix(), pass the current argument to B<option> and the
 601 following argument to B<value> (which may be NULL).
 602
 603 In this case if the return value is positive then it is used to skip that
 604 number of arguments as they have been processed by SSL_CONF_cmd(). If -2 is
 605 returned then B<option> is not recognised and application specific arguments
 606 can be checked instead. If -3 is returned a required argument is missing
 607 and an error is indicated. If 0 is returned some other error occurred and
 608 this can be reported back to the user.
 609
 610 The function SSL_CONF_cmd_value_type() can be used by applications to
 611 check for the existence of a command or to perform additional syntax
 612 checking or translation of the command value. For example if the return
 613 value is B<SSL_CONF_TYPE_FILE> an application could translate a relative
 614 pathname to an absolute pathname.
 615
 616 =head1 RETURN VALUES
 617
 618 SSL_CONF_cmd() returns 1 if the value of B<option> is recognised and B<value> is
 619 B<NOT> used and 2 if both B<option> and B<value> are used. In other words it
 620 returns the number of arguments processed. This is useful when processing
 621 command lines.
 622
 623 A return value of -2 means B<option> is not recognised.
 624
 625 A return value of -3 means B<option> is recognised and the command requires a
 626 value but B<value> is NULL.
 627
 628 A return code of 0 indicates that both B<option> and B<value> are valid but an
 629 error occurred attempting to perform the operation: for example due to an
 630 error in the syntax of B<value> in this case the error queue may provide
 631 additional information.
 632
 633 =head1 EXAMPLES
 634
 635 Set supported signature algorithms:
 636
 637  SSL_CONF_cmd(ctx, "SignatureAlgorithms", "ECDSA+SHA256:RSA+SHA256:DSA+SHA256");
 638
 639 There are various ways to select the supported protocols.
 640
 641 This set the minimum protocol version to TLSv1, and so disables SSLv3.
 642 This is the recommended way to disable protocols.
 643
 644  SSL_CONF_cmd(ctx, "MinProtocol", "TLSv1");
 645
 646 The following also disables SSLv3:
 647
 648  SSL_CONF_cmd(ctx, "Protocol", "-SSLv3");
 649
 650 The following will first enable all protocols, and then disable
 651 SSLv3.
 652 If no protocol versions were disabled before this has the same effect as
 653 "-SSLv3", but if some versions were disables this will re-enable them before
 654 disabling SSLv3.
 655
 656  SSL_CONF_cmd(ctx, "Protocol", "ALL,-SSLv3");
 657
 658 Only enable TLSv1.2:
 659
 660  SSL_CONF_cmd(ctx, "MinProtocol", "TLSv1.2");
 661  SSL_CONF_cmd(ctx, "MaxProtocol", "TLSv1.2");
 662
 663 This also only enables TLSv1.2:
 664
 665  SSL_CONF_cmd(ctx, "Protocol", "-ALL,TLSv1.2");
 666
 667 Disable TLS session tickets:
 668
 669  SSL_CONF_cmd(ctx, "Options", "-SessionTicket");
 670
 671 Enable compression:
 672
 673  SSL_CONF_cmd(ctx, "Options", "Compression");
 674
 675 Set supported curves to P-256, P-384:
 676
 677  SSL_CONF_cmd(ctx, "Curves", "P-256:P-384");
 678
 679 =head1 SEE ALSO
 680
 681 L<ssl(7)>,
 682 L<SSL_CONF_CTX_new(3)>,
 683 L<SSL_CONF_CTX_set_flags(3)>,
 684 L<SSL_CONF_CTX_set1_prefix(3)>,
 685 L<SSL_CONF_CTX_set_ssl_ctx(3)>,
 686 L<SSL_CONF_cmd_argv(3)>,
 687 L<SSL_CTX_set_options(3)>
 688
 689 =head1 HISTORY
 690
 691 The SSL_CONF_cmd() function was added in OpenSSL 1.0.2.
 692
 693 The B<SSL_OP_NO_SSL2> option doesn't have effect since 1.1.0, but the macro
 694 is retained for backwards compatibility.
 695
 696 The B<SSL_CONF_TYPE_NONE> was added in OpenSSL 1.1.0. In earlier versions of
 697 OpenSSL passing a command which didn't take an argument would return
 698 B<SSL_CONF_TYPE_UNKNOWN>.
 699
 700 B<MinProtocol> and B<MaxProtocol> where added in OpenSSL 1.1.0.
 701
 702 B<AllowNoDHEKEX> and B<PrioritizeChaCha> were added in OpenSSL 1.1.1.
 703
 704 =head1 COPYRIGHT
 705
 706 Copyright 2012-2018 The OpenSSL Project Authors. All Rights Reserved.
 707
 708 Licensed under the Apache License 2.0 (the "License").  You may not use
 709 this file except in compliance with the License.  You can obtain a copy
 710 in the file LICENSE in the source distribution or at
 711 L<https://www.openssl.org/source/license.html>.
 712
 713 =cut