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>]
38 [B<-key> I<filename>|I<uri>]
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>]
60 [B<-keymatexport> I<label>]
61 [B<-keymatexportlen> I<len>]
62 [B<-msgfile> I<filename>]
69 [B<-psk_identity> I<identity>]
71 [B<-psk_session> I<file>]
77 [B<-maxfraglen> I<len>]
82 [B<-ignore_unexpected_eof>]
87 [B<-legacy_server_connect>]
88 [B<-allow_no_dhe_kex>]
89 [B<-sigalgs> I<sigalglist>]
90 [B<-curves> I<curvelist>]
91 [B<-cipher> I<cipherlist>]
92 [B<-ciphersuites> I<val>]
94 [B<-starttls> I<protocol>]
95 [B<-name> I<hostname>]
96 [B<-xmpphost> I<hostname>]
97 [B<-name> I<hostname>]
100 [B<-sess_out> I<filename>]
101 [B<-serverinfo> I<types>]
102 [B<-sess_in> I<filename>]
103 [B<-serverinfo> I<types>]
105 [B<-alpn> I<protocols>]
106 [B<-nextprotoneg> I<protocols>]
110 [B<-keylogfile> I<file>]
111 [B<-early_data> I<file>]
113 [B<-use_srtp> I<value>]
114 [B<-srpuser> I<value>]
115 [B<-srppass> I<value>]
118 [B<-srp_strength> I<number>]
119 {- $OpenSSL::safe::opt_name_synopsis -}
120 {- $OpenSSL::safe::opt_version_synopsis -}
121 {- $OpenSSL::safe::opt_x_synopsis -}
122 {- $OpenSSL::safe::opt_trust_synopsis -}
123 {- $OpenSSL::safe::opt_s_synopsis -}
124 {- $OpenSSL::safe::opt_r_synopsis -}
125 {- $OpenSSL::safe::opt_provider_synopsis -}
126 {- $OpenSSL::safe::opt_engine_synopsis -}[B<-ssl_client_engine> I<id>]
127 {- $OpenSSL::safe::opt_v_synopsis -}
132 This command implements a generic SSL/TLS client which
133 connects to a remote host using SSL/TLS. It is a I<very> useful diagnostic
134 tool for SSL servers.
138 In addition to the options below, this command also supports the
139 common and client only options documented
140 in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>
147 Print out a usage message.
149 =item B<-ssl_config> I<section>
151 Use the specified section of the configuration file to configure the B<SSL_CTX> object.
153 =item B<-connect> I<host>:I<port>
155 This specifies the host and optional port to connect to. It is possible to
156 select the host and port using the optional target positional argument instead.
157 If neither this nor the target positional argument are specified then an attempt
158 is made to connect to the local host on port 4433.
160 =item B<-host> I<hostname>
162 Host to connect to; use B<-connect> instead.
164 =item B<-port> I<port>
166 Connect to the specified port; use B<-connect> instead.
168 =item B<-bind> I<host:port>
170 This specifies the host address and or port to bind as the source for the
171 connection. For Unix-domain sockets the port is ignored and the host is
172 used as the source socket address.
174 =item B<-proxy> I<host:port>
176 When used with the B<-connect> flag, the program uses the host and port
177 specified with this flag and issues an HTTP CONNECT command to connect
178 to the desired server.
180 =item B<-proxy_user> I<userid>
182 When used with the B<-proxy> flag, the program will attempt to authenticate
183 with the specified proxy using basic (base64) authentication.
184 NB: Basic authentication is insecure; the credentials are sent to the proxy
185 in easily reversible base64 encoding before any TLS/SSL session is established.
186 Therefore, these credentials are easily recovered by anyone able to sniff/trace
187 the network. Use with caution.
189 =item B<-proxy_pass> I<arg>
191 The proxy password source, used with the B<-proxy_user> flag.
192 For more information about the format of B<arg>
193 see L<openssl-passphrase-options(1)>.
195 =item B<-unix> I<path>
197 Connect over the specified Unix-domain socket.
207 =item B<-servername> I<name>
209 Set the TLS SNI (Server Name Indication) extension in the ClientHello message to
211 If B<-servername> is not provided, the TLS SNI extension will be populated with
212 the name given to B<-connect> if it follows a DNS name format. If B<-connect> is
213 not provided either, the SNI is set to "localhost".
214 This is the default since OpenSSL 1.1.1.
216 Even though SNI should normally be a DNS name and not an IP address, if
217 B<-servername> is provided then that name will be sent, regardless of whether
218 it is a DNS name or not.
220 This option cannot be used in conjunction with B<-noservername>.
222 =item B<-noservername>
224 Suppresses sending of the SNI (Server Name Indication) extension in the
225 ClientHello message. Cannot be used in conjunction with the B<-servername> or
226 B<-dane_tlsa_domain> options.
228 =item B<-cert> I<filename>
230 The client certificate to use, if one is requested by the server.
231 The default is not to use a certificate.
233 The chain for the client certificate may be specified using B<-cert_chain>.
235 =item B<-certform> B<DER>|B<PEM>|B<P12>
237 The client certificate file format to use; unspecified by default.
238 See L<openssl-format-options(1)> for details.
242 A file or URI of untrusted certificates to use when attempting to build the
243 certificate chain related to the certificate specified via the B<-cert> option.
244 The input can be in PEM, DER, or PKCS#12 format.
246 =item B<-build_chain>
248 Specify whether the application should build the client certificate chain to be
249 provided to the server.
251 =item B<-CRL> I<filename>
253 CRL file to use to check the server's certificate.
255 =item B<-CRLform> B<DER>|B<PEM>
257 The CRL file format; unspecified by default.
258 See L<openssl-format-options(1)> for details.
260 =item B<-crl_download>
262 Download CRL from distribution points in the certificate.
264 =item B<-key> I<filename>|I<uri>
266 The client private key to use.
267 If not specified then the certificate file will be used to read also the key.
269 =item B<-keyform> B<DER>|B<PEM>|B<P12>|B<ENGINE>
271 The key format; unspecified by default.
272 See L<openssl-format-options(1)> for details.
274 =item B<-pass> I<arg>
276 the private key and certifiate file password source.
277 For more information about the format of I<arg>
278 see L<openssl-passphrase-options(1)>.
280 =item B<-verify> I<depth>
282 The verify depth to use. This specifies the maximum length of the
283 server certificate chain and turns on server certificate verification.
284 Currently the verify operation continues after errors so all the problems
285 with a certificate chain can be seen. As a side effect the connection
286 will never fail due to a server certificate verify failure.
288 =item B<-verify_return_error>
290 Return verification errors instead of continuing. This will typically
291 abort the handshake with a fatal error.
293 =item B<-verify_quiet>
295 Limit verify output to only errors.
297 =item B<-verifyCAfile> I<filename>
299 A file in PEM format containing trusted certificates to use
300 for verifying the server's certificate.
302 =item B<-verifyCApath> I<dir>
304 A directory containing trusted certificates to use
305 for verifying the server's certificate.
306 This directory must be in "hash format",
307 see L<openssl-verify(1)> for more information.
309 =item B<-verifyCAstore> I<uri>
311 The URI of a store containing trusted certificates to use
312 for verifying the server's certificate.
314 =item B<-chainCAfile> I<file>
316 A file in PEM format containing trusted certificates to use
317 when attempting to build the client certificate chain.
319 =item B<-chainCApath> I<directory>
321 A directory containing trusted certificates to use
322 for building the client certificate chain provided to the server.
323 This directory must be in "hash format",
324 see L<openssl-verify(1)> for more information.
326 =item B<-chainCAstore> I<uri>
328 The URI of a store containing trusted certificates to use
329 when attempting to build the client certificate chain.
330 The URI may indicate a single certificate, as well as a collection of them.
331 With URIs in the C<file:> scheme, this acts as B<-chainCAfile> or
332 B<-chainCApath>, depending on if the URI indicates a directory or a
334 See L<ossl_store-file(7)> for more information on the C<file:> scheme.
336 =item B<-requestCAfile> I<file>
338 A file containing a list of certificates whose subject names will be sent
339 to the server in the B<certificate_authorities> extension. Only supported
342 =item B<-dane_tlsa_domain> I<domain>
344 Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
345 TLSA base domain which becomes the default SNI hint and the primary
346 reference identifier for hostname checks. This must be used in
347 combination with at least one instance of the B<-dane_tlsa_rrdata>
350 When DANE authentication succeeds, the diagnostic output will include
351 the lowest (closest to 0) depth at which a TLSA record authenticated
352 a chain certificate. When that TLSA record is a "2 1 0" trust
353 anchor public key that signed (rather than matched) the top-most
354 certificate of the chain, the result is reported as "TA public key
355 verified". Otherwise, either the TLSA record "matched TA certificate"
356 at a positive depth or else "matched EE certificate" at depth 0.
358 =item B<-dane_tlsa_rrdata> I<rrdata>
360 Use one or more times to specify the RRDATA fields of the DANE TLSA
361 RRset associated with the target service. The I<rrdata> value is
362 specified in "presentation form", that is four whitespace separated
363 fields that specify the usage, selector, matching type and associated
364 data, with the last of these encoded in hexadecimal. Optional
365 whitespace is ignored in the associated data field. For example:
367 $ openssl s_client -brief -starttls smtp \
368 -connect smtp.example.com:25 \
369 -dane_tlsa_domain smtp.example.com \
370 -dane_tlsa_rrdata "2 1 1
371 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
372 -dane_tlsa_rrdata "2 1 1
373 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
376 Verified peername: smtp.example.com
377 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
380 =item B<-dane_ee_no_namechecks>
382 This disables server name checks when authenticating via DANE-EE(3) TLSA
384 For some applications, primarily web browsers, it is not safe to disable name
385 checks due to "unknown key share" attacks, in which a malicious server can
386 convince a client that a connection to a victim server is instead a secure
387 connection to the malicious server.
388 The malicious server may then be able to violate cross-origin scripting
390 Thus, despite the text of RFC7671, name checks are by default enabled for
391 DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
393 In particular, SMTP and XMPP clients should set this option as SRV and MX
394 records already make it possible for a remote domain to redirect client
395 connections to any server of its choice, and in any case SMTP and XMPP clients
396 do not execute scripts downloaded from remote servers.
400 Reconnects to the same server 5 times using the same session ID, this can
401 be used as a test that session caching is working.
405 Displays the server certificate list as sent by the server: it only consists of
406 certificates the server has sent (in the order the server has sent them). It is
407 B<not> a verified chain.
411 Print session information when the program exits. This will always attempt
412 to print out information even if the connection fails. Normally information
413 will only be printed out once if the connection succeeds. This option is useful
414 because the cipher in use may be renegotiated or the connection may fail
415 because a client certificate is required or is requested only after an
416 attempt is made to access a certain URL. Note: the output produced by this
417 option is not always accurate because a connection might never have been
422 Prints out the SSL session states.
426 Print extensive debugging information including a hex dump of all traffic.
430 Do not use interactive command letters.
432 =item B<-security_debug>
434 Enable security debug messages.
436 =item B<-security_debug_verbose>
438 Output more security debug output.
442 Show protocol messages.
446 Enable send/receive timeout on DTLS connections.
448 =item B<-mtu> I<size>
450 Set MTU of the link layer to the specified size.
454 Disable Encrypt-then-MAC negotiation.
456 =item B<-keymatexport> I<label>
458 Export keying material using the specified label.
460 =item B<-keymatexportlen> I<len>
462 Export the specified number of bytes of keying material; default is 20.
464 Show all protocol messages with hex dump.
468 Show verbose trace output of protocol messages.
470 =item B<-msgfile> I<filename>
472 File to send output of B<-msg> or B<-trace> to, default standard output.
476 Tests nonblocking I/O
480 Turns on nonblocking I/O
484 This option translated a line feed from the terminal into CR+LF as required
489 Inhibit shutting down the connection when end of file is reached in the
494 Inhibit printing of session and certificate information. This implicitly
495 turns on B<-ign_eof> as well.
499 Shut down the connection when end of file is reached in the input.
500 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
502 =item B<-psk_identity> I<identity>
504 Use the PSK identity I<identity> when using a PSK cipher suite.
505 The default value is "Client_identity" (without the quotes).
509 Use the PSK key I<key> when using a PSK cipher suite. The key is
510 given as a hexadecimal number without leading 0x, for example -psk
512 This option must be provided in order to use a PSK cipher.
514 =item B<-psk_session> I<file>
516 Use the pem encoded SSL_SESSION data stored in I<file> as the basis of a PSK.
517 Note that this will only work if TLSv1.3 is negotiated.
521 Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in
522 conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only
523 available where OpenSSL has support for SCTP enabled.
525 =item B<-sctp_label_bug>
527 Use the incorrect behaviour of older OpenSSL implementations when computing
528 endpoint-pair shared secrets for DTLS/SCTP. This allows communication with
529 older broken implementations but breaks interoperability with correct
530 implementations. Must be used in conjunction with B<-sctp>. This option is only
531 available where OpenSSL has support for SCTP enabled.
533 =item B<-fallback_scsv>
535 Send TLS_FALLBACK_SCSV in the ClientHello.
539 Switch on asynchronous mode. Cryptographic operations will be performed
540 asynchronously. This will only have an effect if an asynchronous capable engine
541 is also used via the B<-engine> option. For test purposes the dummy async engine
542 (dasync) can be used (if available).
544 =item B<-maxfraglen> I<len>
546 Enable Maximum Fragment Length Negotiation; allowed values are
547 C<512>, C<1024>, C<2048>, and C<4096>.
549 =item B<-max_send_frag> I<int>
551 The maximum size of data fragment to send.
552 See L<SSL_CTX_set_max_send_fragment(3)> for further information.
554 =item B<-split_send_frag> I<int>
556 The size used to split data for encrypt pipelines. If more data is written in
557 one go than this value then it will be split into multiple pipelines, up to the
558 maximum number of pipelines defined by max_pipelines. This only has an effect if
559 a suitable cipher suite has been negotiated, an engine that supports pipelining
560 has been loaded, and max_pipelines is greater than 1. See
561 L<SSL_CTX_set_split_send_fragment(3)> for further information.
563 =item B<-max_pipelines> I<int>
565 The maximum number of encrypt/decrypt pipelines to be used. This will only have
566 an effect if an engine has been loaded that supports pipelining (e.g. the dasync
567 engine) and a suitable cipher suite has been negotiated. The default value is 1.
568 See L<SSL_CTX_set_max_pipelines(3)> for further information.
570 =item B<-read_buf> I<int>
572 The default read buffer size to be used for connections. This will only have an
573 effect if the buffer size is larger than the size that would otherwise be used
574 and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for
575 further information).
577 =item B<-ignore_unexpected_eof>
579 Some TLS implementations do not send the mandatory close_notify alert on
580 shutdown. If the application tries to wait for the close_notify alert but the
581 peer closes the connection without sending it, an error is generated. When this
582 option is enabled the peer does not need to send the close_notify alert and a
583 closed connection will be treated as if the close_notify alert was received.
584 For more information on shutting down a connection, see L<SSL_shutdown(3)>.
588 There are several known bugs in SSL and TLS implementations. Adding this
589 option enables various workarounds.
593 Enables support for SSL/TLS compression.
594 This option was introduced in OpenSSL 1.1.0.
595 TLS compression is not recommended and is off by default as of
600 Disables support for SSL/TLS compression.
601 TLS compression is not recommended and is off by default as of
606 Only provide a brief summary of connection parameters instead of the
607 normal verbose output.
609 =item B<-sigalgs> I<sigalglist>
611 Specifies the list of signature algorithms that are sent by the client.
612 The server selects one entry in the list based on its preferences.
613 For example strings, see L<SSL_CTX_set1_sigalgs(3)>
615 =item B<-curves> I<curvelist>
617 Specifies the list of supported curves to be sent by the client. The curve is
618 ultimately selected by the server. For a list of all curves, use:
620 $ openssl ecparam -list_curves
622 =item B<-cipher> I<cipherlist>
624 This allows the TLSv1.2 and below cipher list sent by the client to be modified.
625 This list will be combined with any TLSv1.3 ciphersuites that have been
626 configured. Although the server determines which ciphersuite is used it should
627 take the first supported cipher in the list sent by the client. See
628 L<openssl-ciphers(1)> for more information.
630 =item B<-ciphersuites> I<val>
632 This allows the TLSv1.3 ciphersuites sent by the client to be modified. This
633 list will be combined with any TLSv1.2 and below ciphersuites that have been
634 configured. Although the server determines which cipher suite is used it should
635 take the first supported cipher in the list sent by the client. See
636 L<openssl-ciphers(1)> for more information. The format for this list is a simple
637 colon (":") separated list of TLSv1.3 ciphersuite names.
639 =item B<-starttls> I<protocol>
641 Send the protocol-specific message(s) to switch to TLS for communication.
642 I<protocol> is a keyword for the intended protocol. Currently, the only
643 supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",
644 "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
646 =item B<-xmpphost> I<hostname>
648 This option, when used with "-starttls xmpp" or "-starttls xmpp-server",
649 specifies the host for the "to" attribute of the stream element.
650 If this option is not specified, then the host specified with "-connect"
653 This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".
655 =item B<-name> I<hostname>
657 This option is used to specify hostname information for various protocols
658 used with B<-starttls> option. Currently only "xmpp", "xmpp-server",
659 "smtp" and "lmtp" can utilize this B<-name> option.
661 If this option is used with "-starttls xmpp" or "-starttls xmpp-server",
662 if specifies the host for the "to" attribute of the stream element. If this
663 option is not specified, then the host specified with "-connect" will be used.
665 If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies
666 the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If
667 this option is not specified, then "mail.example.com" will be used.
669 =item B<-tlsextdebug>
671 Print out a hex dump of any TLS extensions received from the server.
675 Disable RFC4507bis session ticket support.
677 =item B<-sess_out> I<filename>
679 Output SSL session to I<filename>.
681 =item B<-sess_in> I<filename>
683 Load SSL session from I<filename>. The client will attempt to resume a
684 connection from this session.
686 =item B<-serverinfo> I<types>
688 A list of comma-separated TLS Extension Types (numbers between 0 and
689 65535). Each type will be sent as an empty ClientHello TLS Extension.
690 The server's response (if any) will be encoded and displayed as a PEM
695 Sends a certificate status request to the server (OCSP stapling). The server
696 response (if any) is printed out.
698 =item B<-alpn> I<protocols>, B<-nextprotoneg> I<protocols>
700 These flags enable the Enable the Application-Layer Protocol Negotiation
701 or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the
702 IETF standard and replaces NPN.
703 The I<protocols> list is a comma-separated list of protocol names that
704 the client should advertise support for. The list should contain the most
705 desirable protocols first. Protocol names are printable ASCII strings,
706 for example "http/1.1" or "spdy/3".
707 An empty list of protocols is treated specially and will cause the
708 client to advertise support for the TLS extension but disconnect just
709 after receiving ServerHello with a list of server supported protocols.
710 The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.
712 =item B<-ct>, B<-noct>
714 Use one of these two options to control whether Certificate Transparency (CT)
715 is enabled (B<-ct>) or disabled (B<-noct>).
716 If CT is enabled, signed certificate timestamps (SCTs) will be requested from
717 the server and reported at handshake completion.
719 Enabling CT also enables OCSP stapling, as this is one possible delivery method
724 A file containing a list of known Certificate Transparency logs. See
725 L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.
727 =item B<-keylogfile> I<file>
729 Appends TLS secrets to the specified keylog file such that external programs
730 (like Wireshark) can decrypt TLS connections.
732 =item B<-early_data> I<file>
734 Reads the contents of the specified file and attempts to send it as early data
735 to the server. This will only work with resumed sessions that support early
736 data and when the server accepts the early data.
740 For TLSv1.3 only, send the Post-Handshake Authentication extension. This will
741 happen whether or not a certificate has been provided via B<-cert>.
743 =item B<-use_srtp> I<value>
745 Offer SRTP key management, where B<value> is a colon-separated profile list.
747 =item B<-srpuser> I<value>
749 Set the SRP username to the specified value. This option is deprecated.
751 =item B<-srppass> I<value>
753 Set the SRP password to the specified value. This option is deprecated.
755 =item B<-srp_lateuser>
757 SRP username for the second ClientHello message. This option is deprecated.
759 =item B<-srp_moregroups> This option is deprecated.
761 Tolerate other than the known B<g> and B<N> values.
763 =item B<-srp_strength> I<number>
765 Set the minimal acceptable length, in bits, for B<N>. This option is
768 {- $OpenSSL::safe::opt_version_item -}
770 {- $OpenSSL::safe::opt_name_item -}
772 {- $OpenSSL::safe::opt_x_item -}
774 {- $OpenSSL::safe::opt_trust_item -}
776 {- $OpenSSL::safe::opt_s_item -}
778 {- $OpenSSL::safe::opt_r_item -}
780 {- $OpenSSL::safe::opt_provider_item -}
782 {- $OpenSSL::safe::opt_engine_item -}
784 {- output_off() if $disabled{"deprecated-3.0"}; "" -}
785 =item B<-ssl_client_engine> I<id>
787 Specify engine to be used for client certificate operations.
788 {- output_on() if $disabled{"deprecated-3.0"}; "" -}
790 {- $OpenSSL::safe::opt_v_item -}
792 Verification errors are displayed, for debugging, but the command will
793 proceed unless the B<-verify_return_error> option is used.
795 =item I<host>:I<port>
797 Rather than providing B<-connect>, the target hostname and optional port may
798 be provided as a single positional argument after all options. If neither this
799 nor B<-connect> are provided, falls back to attempting to connect to
800 I<localhost> on port I<4433>.
804 =head1 CONNECTED COMMANDS
806 If a connection is established with an SSL server then any data received
807 from the server is displayed and any key presses will be sent to the
808 server. If end of file is reached then the connection will be closed down. When
809 used interactively (which means neither B<-quiet> nor B<-ign_eof> have been
810 given), then certain commands are also recognized which perform special
811 operations. These commands are a letter which must appear at the start of a
812 line. They are listed below.
818 End the current SSL connection and exit.
822 Renegotiate the SSL session (TLSv1.2 and below only).
826 Send a key update message to the server (TLSv1.3 only)
830 Send a key update message to the server and request one back (TLSv1.3 only)
836 This command can be used to debug SSL servers. To connect to an SSL HTTP
839 openssl s_client -connect servername:443
841 would typically be used (https uses port 443). If the connection succeeds
842 then an HTTP command can be given such as "GET /" to retrieve a web page.
844 If the handshake fails then there are several possible causes, if it is
845 nothing obvious like no client certificate then the B<-bugs>,
846 B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried
847 in case it is a buggy server. In particular you should play with these
848 options B<before> submitting a bug report to an OpenSSL mailing list.
850 A frequent problem when attempting to get client certificates working
851 is that a web client complains it has no certificates or gives an empty
852 list to choose from. This is normally because the server is not sending
853 the clients certificate authority in its "acceptable CA list" when it
854 requests a certificate. By using this command, the CA list can be viewed
855 and checked. However, some servers only request client authentication
856 after a specific URL is requested. To obtain the list in this case it
857 is necessary to use the B<-prexit> option and send an HTTP request
858 for an appropriate page.
860 If a certificate is specified on the command line using the B<-cert>
861 option it will not be used unless the server specifically requests
862 a client certificate. Therefore, merely including a client certificate
863 on the command line is no guarantee that the certificate works.
865 If there are problems verifying a server certificate then the
866 B<-showcerts> option can be used to show all the certificates sent by the
869 This command is a test tool and is designed to continue the
870 handshake after any certificate verification errors. As a result it will
871 accept any certificate chain (trusted or not) sent by the peer. Non-test
872 applications should B<not> do this as it makes them vulnerable to a MITM
873 attack. This behaviour can be changed by with the B<-verify_return_error>
874 option: any verify errors are then returned aborting the handshake.
876 The B<-bind> option may be useful if the server or a firewall requires
877 connections to come from some particular address and or port.
881 Because this program has a lot of options and also because some of the
882 techniques used are rather old, the C source for this command is rather
883 hard to read and not a model of how things should be done.
884 A typical SSL client program would be much simpler.
886 The B<-prexit> option is a bit of a hack. We should really report
887 information whenever a session is renegotiated.
892 L<openssl-sess_id(1)>,
893 L<openssl-s_server(1)>,
894 L<openssl-ciphers(1)>,
896 L<SSL_CTX_set_max_send_fragment(3)>,
897 L<SSL_CTX_set_split_send_fragment(3)>,
898 L<SSL_CTX_set_max_pipelines(3)>,
899 L<ossl_store-file(7)>
903 The B<-no_alt_chains> option was added in OpenSSL 1.1.0.
904 The B<-name> option was added in OpenSSL 1.1.1.
906 The B<-certform> option has become obsolete in OpenSSL 3.0.0 and has no effect.
908 The B<-engine> option was deprecated in OpenSSL 3.0.
912 Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.
914 Licensed under the Apache License 2.0 (the "License"). You may not use
915 this file except in compliance with the License. You can obtain a copy
916 in the file LICENSE in the source distribution or at
917 L<https://www.openssl.org/source/license.html>.