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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
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