2 {- OpenSSL::safe::output_do_not_edit_headers(); -}
6 openssl-s_client - SSL/TLS client program
10 B<openssl> B<s_client>
12 [B<-ssl_config> I<section>]
13 [B<-connect> I<host:port>]
14 [B<-host> I<hostname>]
16 [B<-bind> I<host:port>]
17 [B<-proxy> I<host:port>]
18 [B<-proxy_user> I<userid>]
19 [B<-proxy_pass> I<arg>]
23 [B<-servername> I<name>]
26 [B<-verify_return_error>]
28 [B<-verifyCAfile> I<filename>]
29 [B<-verifyCApath> I<dir>]
30 [B<-verifyCAstore> I<uri>]
31 [B<-cert> I<filename>]
32 [B<-certform> B<DER>|B<PEM>|B<P12>]
33 [B<-cert_chain> I<filename>]
36 [B<-CRLform> B<DER>|B<PEM>]
39 [B<-keyform> B<DER>|B<PEM>|B<P12>|B<ENGINE>]
41 [B<-chainCAfile> I<filename>]
42 [B<-chainCApath> I<directory>]
43 [B<-chainCAstore> I<uri>]
44 [B<-requestCAfile> I<filename>]
45 [B<-dane_tlsa_domain> I<domain>]
46 [B<-dane_tlsa_rrdata> I<rrdata>]
47 [B<-dane_ee_no_namechecks>]
55 [B<-security_debug_verbose>]
59 [B<-keymatexport> I<label>]
60 [B<-keymatexportlen> I<len>]
61 [B<-msgfile> I<filename>]
68 [B<-psk_identity> I<identity>]
70 [B<-psk_session> I<file>]
76 [B<-maxfraglen> I<len>]
85 [B<-allow_no_dhe_kex>]
86 [B<-sigalgs> I<sigalglist>]
87 [B<-curves> I<curvelist>]
88 [B<-cipher> I<cipherlist>]
89 [B<-ciphersuites> I<val>]
91 [B<-starttls> I<protocol>]
92 [B<-name> I<hostname>]
93 [B<-xmpphost> I<hostname>]
94 [B<-name> I<hostname>]
97 [B<-sess_out> I<filename>]
98 [B<-serverinfo> I<types>]
99 [B<-sess_in> I<filename>]
100 [B<-serverinfo> I<types>]
102 [B<-alpn> I<protocols>]
103 [B<-nextprotoneg> I<protocols>]
107 [B<-keylogfile> I<file>]
108 [B<-early_data> I<file>]
110 [B<-use_srtp> I<value>]
111 [B<-srpuser> I<value>]
112 [B<-srppass> I<value>]
115 [B<-srp_strength> I<number>]
116 {- $OpenSSL::safe::opt_name_synopsis -}
117 {- $OpenSSL::safe::opt_version_synopsis -}
118 {- $OpenSSL::safe::opt_x_synopsis -}
119 {- $OpenSSL::safe::opt_trust_synopsis -}
120 {- $OpenSSL::safe::opt_s_synopsis -}
121 {- $OpenSSL::safe::opt_r_synopsis -}
122 {- $OpenSSL::safe::opt_provider_synopsis -}
123 {- $OpenSSL::safe::opt_engine_synopsis -}
124 [B<-ssl_client_engine> I<id>]
125 {- $OpenSSL::safe::opt_v_synopsis -}
128 =for openssl ifdef engine ssl_client_engine ct noct ctlogfile
130 =for openssl ifdef ssl3 unix 4 6 use_srtp status trace wdebug nextprotoneg
132 =for openssl ifdef ssl3 tls1 tls1_1 tls1_2 tls1_3 dtls mtu dtls1 dtls1_2
134 =for openssl ifdef sctp_label_bug sctp
136 =for openssl ifdef srpuser srppass srp_lateuser srp_moregroups srp_strength
140 This command implements a generic SSL/TLS client which
141 connects to a remote host using SSL/TLS. It is a I<very> useful diagnostic
142 tool for SSL servers.
146 In addition to the options below, this command also supports the
147 common and client only options documented
148 in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>
155 Print out a usage message.
157 =item B<-ssl_config> I<section>
159 Use the specified section of the configuration file to configure the B<SSL_CTX> object.
161 =item B<-connect> I<host>:I<port>
163 This specifies the host and optional port to connect to. It is possible to
164 select the host and port using the optional target positional argument instead.
165 If neither this nor the target positional argument are specified then an attempt
166 is made to connect to the local host on port 4433.
168 =item B<-host> I<hostname>
170 Host to connect to; use B<-connect> instead.
172 =item B<-port> I<port>
174 Connect to the specified port; use B<-connect> instead.
176 =item B<-bind> I<host:port>
178 This specifies the host address and or port to bind as the source for the
179 connection. For Unix-domain sockets the port is ignored and the host is
180 used as the source socket address.
182 =item B<-proxy> I<host:port>
184 When used with the B<-connect> flag, the program uses the host and port
185 specified with this flag and issues an HTTP CONNECT command to connect
186 to the desired server.
188 =item B<-proxy_user> I<userid>
190 When used with the B<-proxy> flag, the program will attempt to authenticate
191 with the specified proxy using basic (base64) authentication.
192 NB: Basic authentication is insecure; the credentials are sent to the proxy
193 in easily reversible base64 encoding before any TLS/SSL session is established.
194 Therefore these credentials are easily recovered by anyone able to sniff/trace
195 the network. Use with caution.
197 =item B<-proxy_pass> I<arg>
199 The proxy password source, used with the B<-proxy_user> flag.
200 For more information about the format of B<arg>
201 see L<openssl(1)/Pass Phrase Options>.
203 =item B<-unix> I<path>
205 Connect over the specified Unix-domain socket.
215 =item B<-servername> I<name>
217 Set the TLS SNI (Server Name Indication) extension in the ClientHello message to
219 If B<-servername> is not provided, the TLS SNI extension will be populated with
220 the name given to B<-connect> if it follows a DNS name format. If B<-connect> is
221 not provided either, the SNI is set to "localhost".
222 This is the default since OpenSSL 1.1.1.
224 Even though SNI should normally be a DNS name and not an IP address, if
225 B<-servername> is provided then that name will be sent, regardless of whether
226 it is a DNS name or not.
228 This option cannot be used in conjunction with B<-noservername>.
230 =item B<-noservername>
232 Suppresses sending of the SNI (Server Name Indication) extension in the
233 ClientHello message. Cannot be used in conjunction with the B<-servername> or
234 <-dane_tlsa_domain> options.
236 =item B<-cert> I<certname>
238 The client certificate to use, if one is requested by the server.
239 The default is not to use a certificate.
241 The chain for the client certificate may be specified using B<-cert_chain>.
243 =item B<-certform> B<DER>|B<PEM>|B<P12>
245 The client certificate file format to use; the default is B<PEM>.
246 This option has no effect and is retained for backward compatibility only.
250 A file containing untrusted certificates to use when attempting to build the
251 certificate chain related to the certificate specified via the B<-cert> option.
253 =item B<-build_chain>
255 Specify whether the application should build the client certificate chain to be
256 provided to the server.
258 =item B<-CRL> I<filename>
260 CRL file to use to check the server's certificate.
262 =item B<-CRLform> B<DER>|B<PEM>
264 The CRL file format; the default is B<PEM>.
265 See L<openssl(1)/Format Options> for details.
267 =item B<-crl_download>
269 Download CRL from distribution points in the certificate.
271 =item B<-key> I<keyfile>
273 The client private key file to use.
274 If not specified then the certificate file will be used to read also the key.
276 =item B<-keyform> B<DER>|B<PEM>|B<P12>|B<ENGINE>
278 The key format; the default is B<PEM>.
279 The only value with effect is B<ENGINE>; all others have become obsolete.
280 See L<openssl(1)/Format Options> for details.
282 =item B<-pass> I<arg>
284 the private key password source. For more information about the format of I<arg>
285 see L<openssl(1)/Pass phrase options>.
287 =item B<-verify> I<depth>
289 The verify depth to use. This specifies the maximum length of the
290 server certificate chain and turns on server certificate verification.
291 Currently the verify operation continues after errors so all the problems
292 with a certificate chain can be seen. As a side effect the connection
293 will never fail due to a server certificate verify failure.
295 =item B<-verify_return_error>
297 Return verification errors instead of continuing. This will typically
298 abort the handshake with a fatal error.
300 =item B<-verify_quiet>
302 Limit verify output to only errors.
304 =item B<-verifyCAfile> I<filename>
306 A file in PEM format containing trusted certificates to use
307 for verifying the server's certificate.
309 =item B<-verifyCApath> I<dir>
311 A directory containing trusted certificates to use
312 for verifying the server's certificate.
313 This directory must be in "hash format",
314 see L<openssl-verify(1)> for more information.
316 =item B<-verifyCAstore> I<uri>
318 The URI of a store containing trusted certificates to use
319 for verifying the server's certificate.
321 =item B<-chainCAfile> I<file>
323 A file in PEM format containing trusted certificates to use
324 when attempting to build the client certificate chain.
326 =item B<-chainCApath> I<directory>
328 A directory containing trusted certificates to use
329 for building the client certificate chain provided to the server.
330 This directory must be in "hash format",
331 see L<openssl-verify(1)> for more information.
333 =item B<-chainCAstore> I<uri>
335 The URI of a store containing trusted certificates to use
336 when attempting to build the client certificate chain.
337 The URI may indicate a single certificate, as well as a collection of them.
338 With URIs in the C<file:> scheme, this acts as B<-chainCAfile> or
339 B<-chainCApath>, depending on if the URI indicates a directory or a
341 See L<ossl_store-file(7)> for more information on the C<file:> scheme.
343 =item B<-requestCAfile> I<file>
345 A file containing a list of certificates whose subject names will be sent
346 to the server in the B<certificate_authorities> extension. Only supported
349 =item B<-dane_tlsa_domain> I<domain>
351 Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
352 TLSA base domain which becomes the default SNI hint and the primary
353 reference identifier for hostname checks. This must be used in
354 combination with at least one instance of the B<-dane_tlsa_rrdata>
357 When DANE authentication succeeds, the diagnostic output will include
358 the lowest (closest to 0) depth at which a TLSA record authenticated
359 a chain certificate. When that TLSA record is a "2 1 0" trust
360 anchor public key that signed (rather than matched) the top-most
361 certificate of the chain, the result is reported as "TA public key
362 verified". Otherwise, either the TLSA record "matched TA certificate"
363 at a positive depth or else "matched EE certificate" at depth 0.
365 =item B<-dane_tlsa_rrdata> I<rrdata>
367 Use one or more times to specify the RRDATA fields of the DANE TLSA
368 RRset associated with the target service. The I<rrdata> value is
369 specified in "presentation form", that is four whitespace separated
370 fields that specify the usage, selector, matching type and associated
371 data, with the last of these encoded in hexadecimal. Optional
372 whitespace is ignored in the associated data field. For example:
374 $ openssl s_client -brief -starttls smtp \
375 -connect smtp.example.com:25 \
376 -dane_tlsa_domain smtp.example.com \
377 -dane_tlsa_rrdata "2 1 1
378 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
379 -dane_tlsa_rrdata "2 1 1
380 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
383 Verified peername: smtp.example.com
384 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
387 =item B<-dane_ee_no_namechecks>
389 This disables server name checks when authenticating via DANE-EE(3) TLSA
391 For some applications, primarily web browsers, it is not safe to disable name
392 checks due to "unknown key share" attacks, in which a malicious server can
393 convince a client that a connection to a victim server is instead a secure
394 connection to the malicious server.
395 The malicious server may then be able to violate cross-origin scripting
397 Thus, despite the text of RFC7671, name checks are by default enabled for
398 DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
400 In particular, SMTP and XMPP clients should set this option as SRV and MX
401 records already make it possible for a remote domain to redirect client
402 connections to any server of its choice, and in any case SMTP and XMPP clients
403 do not execute scripts downloaded from remote servers.
407 Reconnects to the same server 5 times using the same session ID, this can
408 be used as a test that session caching is working.
412 Displays the server certificate list as sent by the server: it only consists of
413 certificates the server has sent (in the order the server has sent them). It is
414 B<not> a verified chain.
418 Print session information when the program exits. This will always attempt
419 to print out information even if the connection fails. Normally information
420 will only be printed out once if the connection succeeds. This option is useful
421 because the cipher in use may be renegotiated or the connection may fail
422 because a client certificate is required or is requested only after an
423 attempt is made to access a certain URL. Note: the output produced by this
424 option is not always accurate because a connection might never have been
429 Prints out the SSL session states.
433 Print extensive debugging information including a hex dump of all traffic.
437 Do not use interactive command letters.
439 =item B<-security_debug>
441 Enable security debug messages.
443 =item B<-security_debug_verbose>
445 Output more security debug output.
449 Show protocol messages.
453 Enable send/receive timeout on DTLS connections.
455 =item B<-mtu> I<size>
457 Set MTU of the link layer to the specified size.
459 =item B<-keymatexport> I<label>
461 Export keying material using the specified label.
463 =item B<-keymatexportlen> I<len>
465 Export the specified number of bytes of keying material; default is 20.
467 Show all protocol messages with hex dump.
471 Show verbose trace output of protocol messages. OpenSSL needs to be compiled
472 with B<enable-ssl-trace> for this option to work.
474 =item B<-msgfile> I<filename>
476 File to send output of B<-msg> or B<-trace> to, default standard output.
480 Tests non-blocking I/O
484 Turns on non-blocking I/O
488 This option translated a line feed from the terminal into CR+LF as required
493 Inhibit shutting down the connection when end of file is reached in the
498 Inhibit printing of session and certificate information. This implicitly
499 turns on B<-ign_eof> as well.
503 Shut down the connection when end of file is reached in the input.
504 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
506 =item B<-psk_identity> I<identity>
508 Use the PSK identity I<identity> when using a PSK cipher suite.
509 The default value is "Client_identity" (without the quotes).
513 Use the PSK key I<key> when using a PSK cipher suite. The key is
514 given as a hexadecimal number without leading 0x, for example -psk
516 This option must be provided in order to use a PSK cipher.
518 =item B<-psk_session> I<file>
520 Use the pem encoded SSL_SESSION data stored in I<file> as the basis of a PSK.
521 Note that this will only work if TLSv1.3 is negotiated.
525 Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in
526 conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only
527 available where OpenSSL has support for SCTP enabled.
529 =item B<-sctp_label_bug>
531 Use the incorrect behaviour of older OpenSSL implementations when computing
532 endpoint-pair shared secrets for DTLS/SCTP. This allows communication with
533 older broken implementations but breaks interoperability with correct
534 implementations. Must be used in conjunction with B<-sctp>. This option is only
535 available where OpenSSL has support for SCTP enabled.
537 =item B<-fallback_scsv>
539 Send TLS_FALLBACK_SCSV in the ClientHello.
543 Switch on asynchronous mode. Cryptographic operations will be performed
544 asynchronously. This will only have an effect if an asynchronous capable engine
545 is also used via the B<-engine> option. For test purposes the dummy async engine
546 (dasync) can be used (if available).
548 =item B<-maxfraglen> I<len>
550 Enable Maximum Fragment Length Negotiation; allowed values are
551 C<512>, C<1024>, C<2048>, and C<4096>.
553 =item B<-max_send_frag> I<int>
555 The maximum size of data fragment to send.
556 See L<SSL_CTX_set_max_send_fragment(3)> for further information.
558 =item B<-split_send_frag> I<int>
560 The size used to split data for encrypt pipelines. If more data is written in
561 one go than this value then it will be split into multiple pipelines, up to the
562 maximum number of pipelines defined by max_pipelines. This only has an effect if
563 a suitable cipher suite has been negotiated, an engine that supports pipelining
564 has been loaded, and max_pipelines is greater than 1. See
565 L<SSL_CTX_set_split_send_fragment(3)> for further information.
567 =item B<-max_pipelines> I<int>
569 The maximum number of encrypt/decrypt pipelines to be used. This will only have
570 an effect if an engine has been loaded that supports pipelining (e.g. the dasync
571 engine) and a suitable cipher suite has been negotiated. The default value is 1.
572 See L<SSL_CTX_set_max_pipelines(3)> for further information.
574 =item B<-read_buf> I<int>
576 The default read buffer size to be used for connections. This will only have an
577 effect if the buffer size is larger than the size that would otherwise be used
578 and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for
579 further information).
583 There are several known bugs in SSL and TLS implementations. Adding this
584 option enables various workarounds.
588 Enables support for SSL/TLS compression.
589 This option was introduced in OpenSSL 1.1.0.
590 TLS compression is not recommended and is off by default as of
595 Disables support for SSL/TLS compression.
596 TLS compression is not recommended and is off by default as of
601 Only provide a brief summary of connection parameters instead of the
602 normal verbose output.
604 =item B<-sigalgs> I<sigalglist>
606 Specifies the list of signature algorithms that are sent by the client.
607 The server selects one entry in the list based on its preferences.
608 For example strings, see L<SSL_CTX_set1_sigalgs(3)>
610 =item B<-curves> I<curvelist>
612 Specifies the list of supported curves to be sent by the client. The curve is
613 ultimately selected by the server. For a list of all curves, use:
615 $ openssl ecparam -list_curves
617 =item B<-cipher> I<cipherlist>
619 This allows the TLSv1.2 and below cipher list sent by the client to be modified.
620 This list will be combined with any TLSv1.3 ciphersuites that have been
621 configured. Although the server determines which ciphersuite is used it should
622 take the first supported cipher in the list sent by the client. See
623 L<openssl-ciphers(1)> for more information.
625 =item B<-ciphersuites> I<val>
627 This allows the TLSv1.3 ciphersuites sent by the client to be modified. This
628 list will be combined with any TLSv1.2 and below ciphersuites that have been
629 configured. Although the server determines which cipher suite is used it should
630 take the first supported cipher in the list sent by the client. See
631 L<openssl-ciphers(1)> for more information. The format for this list is a simple
632 colon (":") separated list of TLSv1.3 ciphersuite names.
634 =item B<-starttls> I<protocol>
636 Send the protocol-specific message(s) to switch to TLS for communication.
637 I<protocol> is a keyword for the intended protocol. Currently, the only
638 supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",
639 "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
641 =item B<-xmpphost> I<hostname>
643 This option, when used with "-starttls xmpp" or "-starttls xmpp-server",
644 specifies the host for the "to" attribute of the stream element.
645 If this option is not specified, then the host specified with "-connect"
648 This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".
650 =item B<-name> I<hostname>
652 This option is used to specify hostname information for various protocols
653 used with B<-starttls> option. Currently only "xmpp", "xmpp-server",
654 "smtp" and "lmtp" can utilize this B<-name> option.
656 If this option is used with "-starttls xmpp" or "-starttls xmpp-server",
657 if specifies the host for the "to" attribute of the stream element. If this
658 option is not specified, then the host specified with "-connect" will be used.
660 If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies
661 the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If
662 this option is not specified, then "mail.example.com" will be used.
664 =item B<-tlsextdebug>
666 Print out a hex dump of any TLS extensions received from the server.
670 Disable RFC4507bis session ticket support.
672 =item B<-sess_out> I<filename>
674 Output SSL session to I<filename>.
676 =item B<-sess_in> I<filename>
678 Load SSL session from I<filename>. The client will attempt to resume a
679 connection from this session.
681 =item B<-serverinfo> I<types>
683 A list of comma-separated TLS Extension Types (numbers between 0 and
684 65535). Each type will be sent as an empty ClientHello TLS Extension.
685 The server's response (if any) will be encoded and displayed as a PEM
690 Sends a certificate status request to the server (OCSP stapling). The server
691 response (if any) is printed out.
693 =item B<-alpn> I<protocols>, B<-nextprotoneg> I<protocols>
695 These flags enable the Enable the Application-Layer Protocol Negotiation
696 or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the
697 IETF standard and replaces NPN.
698 The I<protocols> list is a comma-separated list of protocol names that
699 the client should advertise support for. The list should contain the most
700 desirable protocols first. Protocol names are printable ASCII strings,
701 for example "http/1.1" or "spdy/3".
702 An empty list of protocols is treated specially and will cause the
703 client to advertise support for the TLS extension but disconnect just
704 after receiving ServerHello with a list of server supported protocols.
705 The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.
707 =item B<-ct>, B<-noct>
709 Use one of these two options to control whether Certificate Transparency (CT)
710 is enabled (B<-ct>) or disabled (B<-noct>).
711 If CT is enabled, signed certificate timestamps (SCTs) will be requested from
712 the server and reported at handshake completion.
714 Enabling CT also enables OCSP stapling, as this is one possible delivery method
719 A file containing a list of known Certificate Transparency logs. See
720 L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.
722 =item B<-keylogfile> I<file>
724 Appends TLS secrets to the specified keylog file such that external programs
725 (like Wireshark) can decrypt TLS connections.
727 =item B<-early_data> I<file>
729 Reads the contents of the specified file and attempts to send it as early data
730 to the server. This will only work with resumed sessions that support early
731 data and when the server accepts the early data.
735 For TLSv1.3 only, send the Post-Handshake Authentication extension. This will
736 happen whether or not a certificate has been provided via B<-cert>.
738 =item B<-use_srtp> I<value>
740 Offer SRTP key management, where B<value> is a colon-separated profile list.
742 =item B<-srpuser> I<value>
744 Set the SRP username to the specified value.
746 =item B<-srppass> I<value>
748 Set the SRP password to the specified value.
750 =item B<-srp_lateuser>
752 SRP username for the second ClientHello message.
754 =item B<-srp_moregroups>
756 Tolerate other than the known B<g> and B<N> values.
758 =item B<-srp_strength> I<number>
760 Set the minimal acceptable length, in bits, for B<N>.
762 {- $OpenSSL::safe::opt_version_item -}
764 {- $OpenSSL::safe::opt_name_item -}
766 {- $OpenSSL::safe::opt_x_item -}
768 {- $OpenSSL::safe::opt_trust_item -}
770 {- $OpenSSL::safe::opt_s_item -}
772 {- $OpenSSL::safe::opt_r_item -}
774 {- $OpenSSL::safe::opt_provider_item -}
776 {- $OpenSSL::safe::opt_engine_item -}
778 =item B<-ssl_client_engine> I<id>
780 Specify engine to be used for client certificate operations.
782 {- $OpenSSL::safe::opt_v_item -}
784 Verification errors are displayed, for debugging, but the command will
785 proceed unless the B<-verify_return_error> option is used.
787 =item I<host>:I<port>
789 Rather than providing B<-connect>, the target hostname and optional port may
790 be provided as a single positional argument after all options. If neither this
791 nor B<-connect> are provided, falls back to attempting to connect to
792 I<localhost> on port I<4433>.
796 =head1 CONNECTED COMMANDS
798 If a connection is established with an SSL server then any data received
799 from the server is displayed and any key presses will be sent to the
800 server. If end of file is reached then the connection will be closed down. When
801 used interactively (which means neither B<-quiet> nor B<-ign_eof> have been
802 given), then certain commands are also recognized which perform special
803 operations. These commands are a letter which must appear at the start of a
804 line. They are listed below.
810 End the current SSL connection and exit.
814 Renegotiate the SSL session (TLSv1.2 and below only).
818 Send a key update message to the server (TLSv1.3 only)
822 Send a key update message to the server and request one back (TLSv1.3 only)
828 This command can be used to debug SSL servers. To connect to an SSL HTTP
831 openssl s_client -connect servername:443
833 would typically be used (https uses port 443). If the connection succeeds
834 then an HTTP command can be given such as "GET /" to retrieve a web page.
836 If the handshake fails then there are several possible causes, if it is
837 nothing obvious like no client certificate then the B<-bugs>,
838 B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried
839 in case it is a buggy server. In particular you should play with these
840 options B<before> submitting a bug report to an OpenSSL mailing list.
842 A frequent problem when attempting to get client certificates working
843 is that a web client complains it has no certificates or gives an empty
844 list to choose from. This is normally because the server is not sending
845 the clients certificate authority in its "acceptable CA list" when it
846 requests a certificate. By using this command, the CA list can be viewed
847 and checked. However some servers only request client authentication
848 after a specific URL is requested. To obtain the list in this case it
849 is necessary to use the B<-prexit> option and send an HTTP request
850 for an appropriate page.
852 If a certificate is specified on the command line using the B<-cert>
853 option it will not be used unless the server specifically requests
854 a client certificate. Therefore merely including a client certificate
855 on the command line is no guarantee that the certificate works.
857 If there are problems verifying a server certificate then the
858 B<-showcerts> option can be used to show all the certificates sent by the
861 This command is a test tool and is designed to continue the
862 handshake after any certificate verification errors. As a result it will
863 accept any certificate chain (trusted or not) sent by the peer. None test
864 applications should B<not> do this as it makes them vulnerable to a MITM
865 attack. This behaviour can be changed by with the B<-verify_return_error>
866 option: any verify errors are then returned aborting the handshake.
868 The B<-bind> option may be useful if the server or a firewall requires
869 connections to come from some particular address and or port.
873 Because this program has a lot of options and also because some of the
874 techniques used are rather old, the C source for this command is rather
875 hard to read and not a model of how things should be done.
876 A typical SSL client program would be much simpler.
878 The B<-prexit> option is a bit of a hack. We should really report
879 information whenever a session is renegotiated.
884 L<openssl-sess_id(1)>,
885 L<openssl-s_server(1)>,
886 L<openssl-ciphers(1)>,
888 L<SSL_CTX_set_max_send_fragment(3)>,
889 L<SSL_CTX_set_split_send_fragment(3)>,
890 L<SSL_CTX_set_max_pipelines(3)>,
891 L<ossl_store-file(7)>
895 The B<-no_alt_chains> option was added in OpenSSL 1.1.0.
896 The B<-name> option was added in OpenSSL 1.1.1.
898 The B<-certform> option has become obsolete in OpenSSL 3.0.0 and has no effect.
900 All B<-keyform> values except B<ENGINE> have become obsolete in OpenSSL 3.0.0
905 Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
907 Licensed under the Apache License 2.0 (the "License"). You may not use
908 this file except in compliance with the License. You can obtain a copy
909 in the file LICENSE in the source distribution or at
910 L<https://www.openssl.org/source/license.html>.