1=pod 2 3=head1 NAME 4 5openssl-s_client, 6s_client - SSL/TLS client program 7 8=head1 SYNOPSIS 9 10B<openssl> B<s_client> 11[B<-help>] 12[B<-connect host:port>] 13[B<-bind host:port>] 14[B<-proxy host:port>] 15[B<-unix path>] 16[B<-4>] 17[B<-6>] 18[B<-servername name>] 19[B<-noservername>] 20[B<-verify depth>] 21[B<-verify_return_error>] 22[B<-cert filename>] 23[B<-certform DER|PEM>] 24[B<-key filename>] 25[B<-keyform DER|PEM>] 26[B<-cert_chain filename>] 27[B<-build_chain>] 28[B<-xkey>] 29[B<-xcert>] 30[B<-xchain>] 31[B<-xchain_build>] 32[B<-xcertform PEM|DER>] 33[B<-xkeyform PEM|DER>] 34[B<-pass arg>] 35[B<-CApath directory>] 36[B<-CAfile filename>] 37[B<-chainCApath directory>] 38[B<-chainCAfile filename>] 39[B<-no-CAfile>] 40[B<-no-CApath>] 41[B<-requestCAfile filename>] 42[B<-dane_tlsa_domain domain>] 43[B<-dane_tlsa_rrdata rrdata>] 44[B<-dane_ee_no_namechecks>] 45[B<-attime timestamp>] 46[B<-check_ss_sig>] 47[B<-crl_check>] 48[B<-crl_check_all>] 49[B<-explicit_policy>] 50[B<-extended_crl>] 51[B<-ignore_critical>] 52[B<-inhibit_any>] 53[B<-inhibit_map>] 54[B<-no_check_time>] 55[B<-partial_chain>] 56[B<-policy arg>] 57[B<-policy_check>] 58[B<-policy_print>] 59[B<-purpose purpose>] 60[B<-suiteB_128>] 61[B<-suiteB_128_only>] 62[B<-suiteB_192>] 63[B<-trusted_first>] 64[B<-no_alt_chains>] 65[B<-use_deltas>] 66[B<-auth_level num>] 67[B<-nameopt option>] 68[B<-verify_depth num>] 69[B<-verify_email email>] 70[B<-verify_hostname hostname>] 71[B<-verify_ip ip>] 72[B<-verify_name name>] 73[B<-build_chain>] 74[B<-x509_strict>] 75[B<-reconnect>] 76[B<-showcerts>] 77[B<-debug>] 78[B<-msg>] 79[B<-nbio_test>] 80[B<-state>] 81[B<-nbio>] 82[B<-crlf>] 83[B<-ign_eof>] 84[B<-no_ign_eof>] 85[B<-psk_identity identity>] 86[B<-psk key>] 87[B<-psk_session file>] 88[B<-quiet>] 89[B<-ssl3>] 90[B<-tls1>] 91[B<-tls1_1>] 92[B<-tls1_2>] 93[B<-tls1_3>] 94[B<-no_ssl3>] 95[B<-no_tls1>] 96[B<-no_tls1_1>] 97[B<-no_tls1_2>] 98[B<-no_tls1_3>] 99[B<-dtls>] 100[B<-dtls1>] 101[B<-dtls1_2>] 102[B<-sctp>] 103[B<-sctp_label_bug>] 104[B<-fallback_scsv>] 105[B<-async>] 106[B<-max_send_frag>] 107[B<-split_send_frag>] 108[B<-max_pipelines>] 109[B<-read_buf>] 110[B<-bugs>] 111[B<-comp>] 112[B<-no_comp>] 113[B<-allow_no_dhe_kex>] 114[B<-sigalgs sigalglist>] 115[B<-curves curvelist>] 116[B<-cipher cipherlist>] 117[B<-ciphersuites val>] 118[B<-serverpref>] 119[B<-starttls protocol>] 120[B<-xmpphost hostname>] 121[B<-name hostname>] 122[B<-engine id>] 123[B<-tlsextdebug>] 124[B<-no_ticket>] 125[B<-sess_out filename>] 126[B<-sess_in filename>] 127[B<-rand file...>] 128[B<-writerand file>] 129[B<-serverinfo types>] 130[B<-status>] 131[B<-alpn protocols>] 132[B<-nextprotoneg protocols>] 133[B<-ct>] 134[B<-noct>] 135[B<-ctlogfile>] 136[B<-keylogfile file>] 137[B<-early_data file>] 138[B<-enable_pha>] 139[B<target>] 140 141=head1 DESCRIPTION 142 143The B<s_client> command implements a generic SSL/TLS client which connects 144to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for 145SSL servers. 146 147=head1 OPTIONS 148 149In addition to the options below the B<s_client> utility also supports the 150common and client only options documented 151in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)> 152manual page. 153 154=over 4 155 156=item B<-help> 157 158Print out a usage message. 159 160=item B<-connect host:port> 161 162This specifies the host and optional port to connect to. It is possible to 163select the host and port using the optional target positional argument instead. 164If neither this nor the target positional argument are specified then an attempt 165is made to connect to the local host on port 4433. 166 167=item B<-bind host:port>] 168 169This specifies the host address and or port to bind as the source for the 170connection. For Unix-domain sockets the port is ignored and the host is 171used as the source socket address. 172 173=item B<-proxy host:port> 174 175When used with the B<-connect> flag, the program uses the host and port 176specified with this flag and issues an HTTP CONNECT command to connect 177to the desired server. 178 179=item B<-unix path> 180 181Connect over the specified Unix-domain socket. 182 183=item B<-4> 184 185Use IPv4 only. 186 187=item B<-6> 188 189Use IPv6 only. 190 191=item B<-servername name> 192 193Set the TLS SNI (Server Name Indication) extension in the ClientHello message to 194the given value. 195If B<-servername> is not provided, the TLS SNI extension will be populated with 196the name given to B<-connect> if it follows a DNS name format. If B<-connect> is 197not provided either, the SNI is set to "localhost". 198This is the default since OpenSSL 1.1.1. 199 200Even though SNI should normally be a DNS name and not an IP address, if 201B<-servername> is provided then that name will be sent, regardless of whether 202it is a DNS name or not. 203 204This option cannot be used in conjunction with B<-noservername>. 205 206=item B<-noservername> 207 208Suppresses sending of the SNI (Server Name Indication) extension in the 209ClientHello message. Cannot be used in conjunction with the B<-servername> or 210<-dane_tlsa_domain> options. 211 212=item B<-cert certname> 213 214The certificate to use, if one is requested by the server. The default is 215not to use a certificate. 216 217=item B<-certform format> 218 219The certificate format to use: DER or PEM. PEM is the default. 220 221=item B<-key keyfile> 222 223The private key to use. If not specified then the certificate file will 224be used. 225 226=item B<-keyform format> 227 228The private format to use: DER or PEM. PEM is the default. 229 230=item B<-cert_chain> 231 232A file containing trusted certificates to use when attempting to build the 233client/server certificate chain related to the certificate specified via the 234B<-cert> option. 235 236=item B<-build_chain> 237 238Specify whether the application should build the certificate chain to be 239provided to the server. 240 241=item B<-xkey infile>, B<-xcert infile>, B<-xchain> 242 243Specify an extra certificate, private key and certificate chain. These behave 244in the same manner as the B<-cert>, B<-key> and B<-cert_chain> options. When 245specified, the callback returning the first valid chain will be in use by the 246client. 247 248=item B<-xchain_build> 249 250Specify whether the application should build the certificate chain to be 251provided to the server for the extra certificates provided via B<-xkey infile>, 252B<-xcert infile>, B<-xchain> options. 253 254=item B<-xcertform PEM|DER>, B<-xkeyform PEM|DER> 255 256Extra certificate and private key format respectively. 257 258=item B<-pass arg> 259 260the private key password source. For more information about the format of B<arg> 261see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)>. 262 263=item B<-verify depth> 264 265The verify depth to use. This specifies the maximum length of the 266server certificate chain and turns on server certificate verification. 267Currently the verify operation continues after errors so all the problems 268with a certificate chain can be seen. As a side effect the connection 269will never fail due to a server certificate verify failure. 270 271=item B<-verify_return_error> 272 273Return verification errors instead of continuing. This will typically 274abort the handshake with a fatal error. 275 276=item B<-nameopt option> 277 278Option which determines how the subject or issuer names are displayed. The 279B<option> argument can be a single option or multiple options separated by 280commas. Alternatively the B<-nameopt> switch may be used more than once to 281set multiple options. See the L<x509(1)> manual page for details. 282 283=item B<-CApath directory> 284 285The directory to use for server certificate verification. This directory 286must be in "hash format", see L<verify(1)> for more information. These are 287also used when building the client certificate chain. 288 289=item B<-CAfile file> 290 291A file containing trusted certificates to use during server authentication 292and to use when attempting to build the client certificate chain. 293 294=item B<-chainCApath directory> 295 296The directory to use for building the chain provided to the server. This 297directory must be in "hash format", see L<verify(1)> for more information. 298 299=item B<-chainCAfile file> 300 301A file containing trusted certificates to use when attempting to build the 302client certificate chain. 303 304=item B<-no-CAfile> 305 306Do not load the trusted CA certificates from the default file location 307 308=item B<-no-CApath> 309 310Do not load the trusted CA certificates from the default directory location 311 312=item B<-requestCAfile file> 313 314A file containing a list of certificates whose subject names will be sent 315to the server in the B<certificate_authorities> extension. Only supported 316for TLS 1.3 317 318=item B<-dane_tlsa_domain domain> 319 320Enable RFC6698/RFC7671 DANE TLSA authentication and specify the 321TLSA base domain which becomes the default SNI hint and the primary 322reference identifier for hostname checks. This must be used in 323combination with at least one instance of the B<-dane_tlsa_rrdata> 324option below. 325 326When DANE authentication succeeds, the diagnostic output will include 327the lowest (closest to 0) depth at which a TLSA record authenticated 328a chain certificate. When that TLSA record is a "2 1 0" trust 329anchor public key that signed (rather than matched) the top-most 330certificate of the chain, the result is reported as "TA public key 331verified". Otherwise, either the TLSA record "matched TA certificate" 332at a positive depth or else "matched EE certificate" at depth 0. 333 334=item B<-dane_tlsa_rrdata rrdata> 335 336Use one or more times to specify the RRDATA fields of the DANE TLSA 337RRset associated with the target service. The B<rrdata> value is 338specified in "presentation form", that is four whitespace separated 339fields that specify the usage, selector, matching type and associated 340data, with the last of these encoded in hexadecimal. Optional 341whitespace is ignored in the associated data field. For example: 342 343 $ openssl s_client -brief -starttls smtp \ 344 -connect smtp.example.com:25 \ 345 -dane_tlsa_domain smtp.example.com \ 346 -dane_tlsa_rrdata "2 1 1 347 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \ 348 -dane_tlsa_rrdata "2 1 1 349 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18" 350 ... 351 Verification: OK 352 Verified peername: smtp.example.com 353 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1 354 ... 355 356=item B<-dane_ee_no_namechecks> 357 358This disables server name checks when authenticating via DANE-EE(3) TLSA 359records. 360For some applications, primarily web browsers, it is not safe to disable name 361checks due to "unknown key share" attacks, in which a malicious server can 362convince a client that a connection to a victim server is instead a secure 363connection to the malicious server. 364The malicious server may then be able to violate cross-origin scripting 365restrictions. 366Thus, despite the text of RFC7671, name checks are by default enabled for 367DANE-EE(3) TLSA records, and can be disabled in applications where it is safe 368to do so. 369In particular, SMTP and XMPP clients should set this option as SRV and MX 370records already make it possible for a remote domain to redirect client 371connections to any server of its choice, and in any case SMTP and XMPP clients 372do not execute scripts downloaded from remote servers. 373 374=item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>, 375B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>, 376B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>, 377B<-policy_check>, B<-policy_print>, B<-purpose>, B<-suiteB_128>, 378B<-suiteB_128_only>, B<-suiteB_192>, B<-trusted_first>, B<-use_deltas>, 379B<-auth_level>, B<-verify_depth>, B<-verify_email>, B<-verify_hostname>, 380B<-verify_ip>, B<-verify_name>, B<-x509_strict> 381 382Set various certificate chain validation options. See the 383L<verify(1)> manual page for details. 384 385=item B<-reconnect> 386 387Reconnects to the same server 5 times using the same session ID, this can 388be used as a test that session caching is working. 389 390=item B<-showcerts> 391 392Displays the server certificate list as sent by the server: it only consists of 393certificates the server has sent (in the order the server has sent them). It is 394B<not> a verified chain. 395 396=item B<-prexit> 397 398Print session information when the program exits. This will always attempt 399to print out information even if the connection fails. Normally information 400will only be printed out once if the connection succeeds. This option is useful 401because the cipher in use may be renegotiated or the connection may fail 402because a client certificate is required or is requested only after an 403attempt is made to access a certain URL. Note: the output produced by this 404option is not always accurate because a connection might never have been 405established. 406 407=item B<-state> 408 409Prints out the SSL session states. 410 411=item B<-debug> 412 413Print extensive debugging information including a hex dump of all traffic. 414 415=item B<-msg> 416 417Show all protocol messages with hex dump. 418 419=item B<-trace> 420 421Show verbose trace output of protocol messages. OpenSSL needs to be compiled 422with B<enable-ssl-trace> for this option to work. 423 424=item B<-msgfile> 425 426File to send output of B<-msg> or B<-trace> to, default standard output. 427 428=item B<-nbio_test> 429 430Tests non-blocking I/O 431 432=item B<-nbio> 433 434Turns on non-blocking I/O 435 436=item B<-crlf> 437 438This option translated a line feed from the terminal into CR+LF as required 439by some servers. 440 441=item B<-ign_eof> 442 443Inhibit shutting down the connection when end of file is reached in the 444input. 445 446=item B<-quiet> 447 448Inhibit printing of session and certificate information. This implicitly 449turns on B<-ign_eof> as well. 450 451=item B<-no_ign_eof> 452 453Shut down the connection when end of file is reached in the input. 454Can be used to override the implicit B<-ign_eof> after B<-quiet>. 455 456=item B<-psk_identity identity> 457 458Use the PSK identity B<identity> when using a PSK cipher suite. 459The default value is "Client_identity" (without the quotes). 460 461=item B<-psk key> 462 463Use the PSK key B<key> when using a PSK cipher suite. The key is 464given as a hexadecimal number without leading 0x, for example -psk 4651a2b3c4d. 466This option must be provided in order to use a PSK cipher. 467 468=item B<-psk_session file> 469 470Use the pem encoded SSL_SESSION data stored in B<file> as the basis of a PSK. 471Note that this will only work if TLSv1.3 is negotiated. 472 473=item B<-ssl3>, B<-tls1>, B<-tls1_1>, B<-tls1_2>, B<-tls1_3>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>, B<-no_tls1_3> 474 475These options require or disable the use of the specified SSL or TLS protocols. 476By default B<s_client> will negotiate the highest mutually supported protocol 477version. 478When a specific TLS version is required, only that version will be offered to 479and accepted from the server. 480Note that not all protocols and flags may be available, depending on how 481OpenSSL was built. 482 483=item B<-dtls>, B<-dtls1>, B<-dtls1_2> 484 485These options make B<s_client> use DTLS protocols instead of TLS. 486With B<-dtls>, B<s_client> will negotiate any supported DTLS protocol version, 487whilst B<-dtls1> and B<-dtls1_2> will only support DTLS1.0 and DTLS1.2 488respectively. 489 490=item B<-sctp> 491 492Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in 493conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only 494available where OpenSSL has support for SCTP enabled. 495 496=item B<-sctp_label_bug> 497 498Use the incorrect behaviour of older OpenSSL implementations when computing 499endpoint-pair shared secrets for DTLS/SCTP. This allows communication with 500older broken implementations but breaks interoperability with correct 501implementations. Must be used in conjunction with B<-sctp>. This option is only 502available where OpenSSL has support for SCTP enabled. 503 504=item B<-fallback_scsv> 505 506Send TLS_FALLBACK_SCSV in the ClientHello. 507 508=item B<-async> 509 510Switch on asynchronous mode. Cryptographic operations will be performed 511asynchronously. This will only have an effect if an asynchronous capable engine 512is also used via the B<-engine> option. For test purposes the dummy async engine 513(dasync) can be used (if available). 514 515=item B<-max_send_frag int> 516 517The maximum size of data fragment to send. 518See L<SSL_CTX_set_max_send_fragment(3)> for further information. 519 520=item B<-split_send_frag int> 521 522The size used to split data for encrypt pipelines. If more data is written in 523one go than this value then it will be split into multiple pipelines, up to the 524maximum number of pipelines defined by max_pipelines. This only has an effect if 525a suitable cipher suite has been negotiated, an engine that supports pipelining 526has been loaded, and max_pipelines is greater than 1. See 527L<SSL_CTX_set_split_send_fragment(3)> for further information. 528 529=item B<-max_pipelines int> 530 531The maximum number of encrypt/decrypt pipelines to be used. This will only have 532an effect if an engine has been loaded that supports pipelining (e.g. the dasync 533engine) and a suitable cipher suite has been negotiated. The default value is 1. 534See L<SSL_CTX_set_max_pipelines(3)> for further information. 535 536=item B<-read_buf int> 537 538The default read buffer size to be used for connections. This will only have an 539effect if the buffer size is larger than the size that would otherwise be used 540and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for 541further information). 542 543=item B<-bugs> 544 545There are several known bugs in SSL and TLS implementations. Adding this 546option enables various workarounds. 547 548=item B<-comp> 549 550Enables support for SSL/TLS compression. 551This option was introduced in OpenSSL 1.1.0. 552TLS compression is not recommended and is off by default as of 553OpenSSL 1.1.0. 554 555=item B<-no_comp> 556 557Disables support for SSL/TLS compression. 558TLS compression is not recommended and is off by default as of 559OpenSSL 1.1.0. 560 561=item B<-brief> 562 563Only provide a brief summary of connection parameters instead of the 564normal verbose output. 565 566=item B<-sigalgs sigalglist> 567 568Specifies the list of signature algorithms that are sent by the client. 569The server selects one entry in the list based on its preferences. 570For example strings, see L<SSL_CTX_set1_sigalgs(3)> 571 572=item B<-curves curvelist> 573 574Specifies the list of supported curves to be sent by the client. The curve is 575ultimately selected by the server. For a list of all curves, use: 576 577 $ openssl ecparam -list_curves 578 579=item B<-cipher cipherlist> 580 581This allows the TLSv1.2 and below cipher list sent by the client to be modified. 582This list will be combined with any TLSv1.3 ciphersuites that have been 583configured. Although the server determines which ciphersuite is used it should 584take the first supported cipher in the list sent by the client. See the 585B<ciphers> command for more information. 586 587=item B<-ciphersuites val> 588 589This allows the TLSv1.3 ciphersuites sent by the client to be modified. This 590list will be combined with any TLSv1.2 and below ciphersuites that have been 591configured. Although the server determines which cipher suite is used it should 592take the first supported cipher in the list sent by the client. See the 593B<ciphers> command for more information. The format for this list is a simple 594colon (":") separated list of TLSv1.3 ciphersuite names. 595 596=item B<-starttls protocol> 597 598Send the protocol-specific message(s) to switch to TLS for communication. 599B<protocol> is a keyword for the intended protocol. Currently, the only 600supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server", 601"irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap". 602 603=item B<-xmpphost hostname> 604 605This option, when used with "-starttls xmpp" or "-starttls xmpp-server", 606specifies the host for the "to" attribute of the stream element. 607If this option is not specified, then the host specified with "-connect" 608will be used. 609 610This option is an alias of the B<-name> option for "xmpp" and "xmpp-server". 611 612=item B<-name hostname> 613 614This option is used to specify hostname information for various protocols 615used with B<-starttls> option. Currently only "xmpp", "xmpp-server", 616"smtp" and "lmtp" can utilize this B<-name> option. 617 618If this option is used with "-starttls xmpp" or "-starttls xmpp-server", 619if specifies the host for the "to" attribute of the stream element. If this 620option is not specified, then the host specified with "-connect" will be used. 621 622If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies 623the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If 624this option is not specified, then "mail.example.com" will be used. 625 626=item B<-tlsextdebug> 627 628Print out a hex dump of any TLS extensions received from the server. 629 630=item B<-no_ticket> 631 632Disable RFC4507bis session ticket support. 633 634=item B<-sess_out filename> 635 636Output SSL session to B<filename>. 637 638=item B<-sess_in sess.pem> 639 640Load SSL session from B<filename>. The client will attempt to resume a 641connection from this session. 642 643=item B<-engine id> 644 645Specifying an engine (by its unique B<id> string) will cause B<s_client> 646to attempt to obtain a functional reference to the specified engine, 647thus initialising it if needed. The engine will then be set as the default 648for all available algorithms. 649 650=item B<-rand file...> 651 652A file or files containing random data used to seed the random number 653generator. 654Multiple files can be specified separated by an OS-dependent character. 655The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for 656all others. 657 658=item [B<-writerand file>] 659 660Writes random data to the specified I<file> upon exit. 661This can be used with a subsequent B<-rand> flag. 662 663=item B<-serverinfo types> 664 665A list of comma-separated TLS Extension Types (numbers between 0 and 66665535). Each type will be sent as an empty ClientHello TLS Extension. 667The server's response (if any) will be encoded and displayed as a PEM 668file. 669 670=item B<-status> 671 672Sends a certificate status request to the server (OCSP stapling). The server 673response (if any) is printed out. 674 675=item B<-alpn protocols>, B<-nextprotoneg protocols> 676 677These flags enable the Enable the Application-Layer Protocol Negotiation 678or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the 679IETF standard and replaces NPN. 680The B<protocols> list is a comma-separated list of protocol names that 681the client should advertise support for. The list should contain the most 682desirable protocols first. Protocol names are printable ASCII strings, 683for example "http/1.1" or "spdy/3". 684An empty list of protocols is treated specially and will cause the 685client to advertise support for the TLS extension but disconnect just 686after receiving ServerHello with a list of server supported protocols. 687The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used. 688 689=item B<-ct>, B<-noct> 690 691Use one of these two options to control whether Certificate Transparency (CT) 692is enabled (B<-ct>) or disabled (B<-noct>). 693If CT is enabled, signed certificate timestamps (SCTs) will be requested from 694the server and reported at handshake completion. 695 696Enabling CT also enables OCSP stapling, as this is one possible delivery method 697for SCTs. 698 699=item B<-ctlogfile> 700 701A file containing a list of known Certificate Transparency logs. See 702L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format. 703 704=item B<-keylogfile file> 705 706Appends TLS secrets to the specified keylog file such that external programs 707(like Wireshark) can decrypt TLS connections. 708 709=item B<-early_data file> 710 711Reads the contents of the specified file and attempts to send it as early data 712to the server. This will only work with resumed sessions that support early 713data and when the server accepts the early data. 714 715=item B<-enable_pha> 716 717For TLSv1.3 only, send the Post-Handshake Authentication extension. This will 718happen whether or not a certificate has been provided via B<-cert>. 719 720=item B<[target]> 721 722Rather than providing B<-connect>, the target hostname and optional port may 723be provided as a single positional argument after all options. If neither this 724nor B<-connect> are provided, falls back to attempting to connect to localhost 725on port 4433. 726 727=back 728 729=head1 CONNECTED COMMANDS 730 731If a connection is established with an SSL server then any data received 732from the server is displayed and any key presses will be sent to the 733server. If end of file is reached then the connection will be closed down. When 734used interactively (which means neither B<-quiet> nor B<-ign_eof> have been 735given), then certain commands are also recognized which perform special 736operations. These commands are a letter which must appear at the start of a 737line. They are listed below. 738 739=over 4 740 741=item B<Q> 742 743End the current SSL connection and exit. 744 745=item B<R> 746 747Renegotiate the SSL session (TLSv1.2 and below only). 748 749=item B<B> 750 751Send a heartbeat message to the server (DTLS only) 752 753=item B<k> 754 755Send a key update message to the server (TLSv1.3 only) 756 757=item B<K> 758 759Send a key update message to the server and request one back (TLSv1.3 only) 760 761=back 762 763=head1 NOTES 764 765B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP 766server the command: 767 768 openssl s_client -connect servername:443 769 770would typically be used (https uses port 443). If the connection succeeds 771then an HTTP command can be given such as "GET /" to retrieve a web page. 772 773If the handshake fails then there are several possible causes, if it is 774nothing obvious like no client certificate then the B<-bugs>, 775B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried 776in case it is a buggy server. In particular you should play with these 777options B<before> submitting a bug report to an OpenSSL mailing list. 778 779A frequent problem when attempting to get client certificates working 780is that a web client complains it has no certificates or gives an empty 781list to choose from. This is normally because the server is not sending 782the clients certificate authority in its "acceptable CA list" when it 783requests a certificate. By using B<s_client> the CA list can be viewed 784and checked. However some servers only request client authentication 785after a specific URL is requested. To obtain the list in this case it 786is necessary to use the B<-prexit> option and send an HTTP request 787for an appropriate page. 788 789If a certificate is specified on the command line using the B<-cert> 790option it will not be used unless the server specifically requests 791a client certificate. Therefor merely including a client certificate 792on the command line is no guarantee that the certificate works. 793 794If there are problems verifying a server certificate then the 795B<-showcerts> option can be used to show all the certificates sent by the 796server. 797 798The B<s_client> utility is a test tool and is designed to continue the 799handshake after any certificate verification errors. As a result it will 800accept any certificate chain (trusted or not) sent by the peer. None test 801applications should B<not> do this as it makes them vulnerable to a MITM 802attack. This behaviour can be changed by with the B<-verify_return_error> 803option: any verify errors are then returned aborting the handshake. 804 805The B<-bind> option may be useful if the server or a firewall requires 806connections to come from some particular address and or port. 807 808=head1 BUGS 809 810Because this program has a lot of options and also because some of the 811techniques used are rather old, the C source of B<s_client> is rather hard to 812read and not a model of how things should be done. 813A typical SSL client program would be much simpler. 814 815The B<-prexit> option is a bit of a hack. We should really report 816information whenever a session is renegotiated. 817 818=head1 SEE ALSO 819 820L<SSL_CONF_cmd(3)>, L<sess_id(1)>, L<s_server(1)>, L<ciphers(1)>, 821L<SSL_CTX_set_max_send_fragment(3)>, L<SSL_CTX_set_split_send_fragment(3)>, 822L<SSL_CTX_set_max_pipelines(3)> 823 824=head1 HISTORY 825 826The B<-no_alt_chains> option was added in OpenSSL 1.1.0. 827The B<-name> option was added in OpenSSL 1.1.1. 828 829=head1 COPYRIGHT 830 831Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved. 832 833Licensed under the OpenSSL license (the "License"). You may not use 834this file except in compliance with the License. You can obtain a copy 835in the file LICENSE in the source distribution or at 836L<https://www.openssl.org/source/license.html>. 837 838=cut 839