6 s_client - SSL/TLS client program
10 B<openssl> B<s_client>
12 [B<-connect host:port>]
21 [B<-verify_return_error>]
23 [B<-certform DER|PEM>]
27 [B<-CApath directory>]
31 [B<-requestCAfile filename>]
32 [B<-dane_tlsa_domain domain>]
33 [B<-dane_tlsa_rrdata rrdata>]
34 [B<-dane_ee_no_namechecks>]
35 [B<-attime timestamp>]
58 [B<-verify_depth num>]
59 [B<-verify_email email>]
60 [B<-verify_hostname hostname>]
62 [B<-verify_name name>]
98 [B<-allow_no_dhe_kex>]
99 [B<-sigalgs sigalglist>]
100 [B<-curves curvelist>]
101 [B<-cipher cipherlist>]
103 [B<-starttls protocol>]
104 [B<-xmpphost hostname>]
109 [B<-sess_out filename>]
110 [B<-sess_in filename>]
113 [B<-serverinfo types>]
116 [B<-nextprotoneg protocols>]
120 [B<-keylogfile file>]
121 [B<-early_data file>]
127 The B<s_client> command implements a generic SSL/TLS client which connects
128 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
133 In addition to the options below the B<s_client> utility also supports the
134 common and client only options documented in the
135 in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>
142 Print out a usage message.
144 =item B<-connect host:port>
146 This specifies the host and optional port to connect to. It is possible to
147 select the host and port using the optional target positional argument instead.
148 If neither this nor the target positonal argument are specified then an attempt
149 is made to connect to the local host on port 4433.
151 =item B<-bind host:port>]
153 This specifies the host address and or port to bind as the source for the
154 connection. For Unix-domain sockets the port is ignored and the host is
155 used as the source socket address.
157 =item B<-proxy host:port>
159 When used with the B<-connect> flag, the program uses the host and port
160 specified with this flag and issues an HTTP CONNECT command to connect
161 to the desired server.
165 Connect over the specified Unix-domain socket.
175 =item B<-servername name>
177 Set the TLS SNI (Server Name Indication) extension in the ClientHello message to
178 the given value. If both this option and the B<-noservername> are not given, the
179 TLS SNI extension is still set to the hostname provided to the B<-connect> option,
180 or "localhost" if B<-connect> has not been supplied. This is default since OpenSSL
183 Even though SNI name should normally be a DNS name and not an IP address, this
184 option will not make the distinction when parsing B<-connect> and will send
185 IP address if one passed.
187 =item B<-noservername>
189 Suppresses sending of the SNI (Server Name Indication) extension in the
190 ClientHello message. Cannot be used in conjunction with the B<-servername> or
191 <-dane_tlsa_domain> options.
193 =item B<-cert certname>
195 The certificate to use, if one is requested by the server. The default is
196 not to use a certificate.
198 =item B<-certform format>
200 The certificate format to use: DER or PEM. PEM is the default.
202 =item B<-key keyfile>
204 The private key to use. If not specified then the certificate file will
207 =item B<-keyform format>
209 The private format to use: DER or PEM. PEM is the default.
213 the private key password source. For more information about the format of B<arg>
214 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)>.
216 =item B<-verify depth>
218 The verify depth to use. This specifies the maximum length of the
219 server certificate chain and turns on server certificate verification.
220 Currently the verify operation continues after errors so all the problems
221 with a certificate chain can be seen. As a side effect the connection
222 will never fail due to a server certificate verify failure.
224 =item B<-verify_return_error>
226 Return verification errors instead of continuing. This will typically
227 abort the handshake with a fatal error.
229 =item B<-nameopt option>
231 Option which determines how the subject or issuer names are displayed. The
232 B<option> argument can be a single option or multiple options separated by
233 commas. Alternatively the B<-nameopt> switch may be used more than once to
234 set multiple options. See the L<x509(1)> manual page for details.
236 =item B<-CApath directory>
238 The directory to use for server certificate verification. This directory
239 must be in "hash format", see B<verify> for more information. These are
240 also used when building the client certificate chain.
242 =item B<-CAfile file>
244 A file containing trusted certificates to use during server authentication
245 and to use when attempting to build the client certificate chain.
249 Do not load the trusted CA certificates from the default file location
253 Do not load the trusted CA certificates from the default directory location
255 =item B<-requestCAfile file>
257 A file containing a list of certificates whose subject names will be sent
258 to the server in the B<certificate_authorities> extension. Only supported
261 =item B<-dane_tlsa_domain domain>
263 Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
264 TLSA base domain which becomes the default SNI hint and the primary
265 reference identifier for hostname checks. This must be used in
266 combination with at least one instance of the B<-dane_tlsa_rrdata>
269 When DANE authentication succeeds, the diagnostic output will include
270 the lowest (closest to 0) depth at which a TLSA record authenticated
271 a chain certificate. When that TLSA record is a "2 1 0" trust
272 anchor public key that signed (rather than matched) the top-most
273 certificate of the chain, the result is reported as "TA public key
274 verified". Otherwise, either the TLSA record "matched TA certificate"
275 at a positive depth or else "matched EE certificate" at depth 0.
277 =item B<-dane_tlsa_rrdata rrdata>
279 Use one or more times to specify the RRDATA fields of the DANE TLSA
280 RRset associated with the target service. The B<rrdata> value is
281 specied in "presentation form", that is four whitespace separated
282 fields that specify the usage, selector, matching type and associated
283 data, with the last of these encoded in hexadecimal. Optional
284 whitespace is ignored in the associated data field. For example:
286 $ openssl s_client -brief -starttls smtp \
287 -connect smtp.example.com:25 \
288 -dane_tlsa_domain smtp.example.com \
289 -dane_tlsa_rrdata "2 1 1
290 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
291 -dane_tlsa_rrdata "2 1 1
292 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
295 Verified peername: smtp.example.com
296 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
299 =item B<-dane_ee_no_namechecks>
301 This disables server name checks when authenticating via DANE-EE(3) TLSA
303 For some applications, primarily web browsers, it is not safe to disable name
304 checks due to "unknown key share" attacks, in which a malicious server can
305 convince a client that a connection to a victim server is instead a secure
306 connection to the malicious server.
307 The malicious server may then be able to violate cross-origin scripting
309 Thus, despite the text of RFC7671, name checks are by default enabled for
310 DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
312 In particular, SMTP and XMPP clients should set this option as SRV and MX
313 records already make it possible for a remote domain to redirect client
314 connections to any server of its choice, and in any case SMTP and XMPP clients
315 do not execute scripts downloaded from remote servers.
317 =item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>,
318 B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>,
319 B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>,
320 B<-policy_check>, B<-policy_print>, B<-purpose>, B<-suiteB_128>,
321 B<-suiteB_128_only>, B<-suiteB_192>, B<-trusted_first>, B<-use_deltas>,
322 B<-auth_level>, B<-verify_depth>, B<-verify_email>, B<-verify_hostname>,
323 B<-verify_ip>, B<-verify_name>, B<-x509_strict>
325 Set various certificate chain validation options. See the
326 L<verify(1)> manual page for details.
330 Reconnects to the same server 5 times using the same session ID, this can
331 be used as a test that session caching is working.
335 Display the whole server certificate chain: normally only the server
336 certificate itself is displayed.
340 Print session information when the program exits. This will always attempt
341 to print out information even if the connection fails. Normally information
342 will only be printed out once if the connection succeeds. This option is useful
343 because the cipher in use may be renegotiated or the connection may fail
344 because a client certificate is required or is requested only after an
345 attempt is made to access a certain URL. Note: the output produced by this
346 option is not always accurate because a connection might never have been
351 Prints out the SSL session states.
355 Print extensive debugging information including a hex dump of all traffic.
359 Show all protocol messages with hex dump.
363 Show verbose trace output of protocol messages. OpenSSL needs to be compiled
364 with B<enable-ssl-trace> for this option to work.
368 File to send output of B<-msg> or B<-trace> to, default standard output.
372 Tests non-blocking I/O
376 Turns on non-blocking I/O
380 This option translated a line feed from the terminal into CR+LF as required
385 Inhibit shutting down the connection when end of file is reached in the
390 Inhibit printing of session and certificate information. This implicitly
391 turns on B<-ign_eof> as well.
395 Shut down the connection when end of file is reached in the input.
396 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
398 =item B<-psk_identity identity>
400 Use the PSK identity B<identity> when using a PSK cipher suite.
401 The default value is "Client_identity" (without the quotes).
405 Use the PSK key B<key> when using a PSK cipher suite. The key is
406 given as a hexadecimal number without leading 0x, for example -psk
408 This option must be provided in order to use a PSK cipher.
410 =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>
412 These options require or disable the use of the specified SSL or TLS protocols.
413 By default B<s_client> will negotiate the highest mutually supported protocol
415 When a specific TLS version is required, only that version will be offered to
416 and accepted from the server.
418 =item B<-dtls>, B<-dtls1>, B<-dtls1_2>
420 These options make B<s_client> use DTLS protocols instead of TLS.
421 With B<-dtls>, B<s_client> will negotiate any supported DTLS protocol version,
422 whilst B<-dtls1> and B<-dtls1_2> will only support DTLS1.0 and DTLS1.2
427 Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in
428 conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only
429 available where OpenSSL has support for SCTP enabled.
431 =item B<-fallback_scsv>
433 Send TLS_FALLBACK_SCSV in the ClientHello.
437 Switch on asynchronous mode. Cryptographic operations will be performed
438 asynchronously. This will only have an effect if an asynchronous capable engine
439 is also used via the B<-engine> option. For test purposes the dummy async engine
440 (dasync) can be used (if available).
442 =item B<-max_send_frag int>
444 The maximum size of data fragment to send.
445 See L<SSL_CTX_set_max_send_fragment(3)> for further information.
447 =item B<-split_send_frag int>
449 The size used to split data for encrypt pipelines. If more data is written in
450 one go than this value then it will be split into multiple pipelines, up to the
451 maximum number of pipelines defined by max_pipelines. This only has an effect if
452 a suitable cipher suite has been negotiated, an engine that supports pipelining
453 has been loaded, and max_pipelines is greater than 1. See
454 L<SSL_CTX_set_split_send_fragment(3)> for further information.
456 =item B<-max_pipelines int>
458 The maximum number of encrypt/decrypt pipelines to be used. This will only have
459 an effect if an engine has been loaded that supports pipelining (e.g. the dasync
460 engine) and a suitable cipher suite has been negotiated. The default value is 1.
461 See L<SSL_CTX_set_max_pipelines(3)> for further information.
463 =item B<-read_buf int>
465 The default read buffer size to be used for connections. This will only have an
466 effect if the buffer size is larger than the size that would otherwise be used
467 and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for
468 further information).
472 There are several known bug in SSL and TLS implementations. Adding this
473 option enables various workarounds.
477 Enables support for SSL/TLS compression.
478 This option was introduced in OpenSSL 1.1.0.
479 TLS compression is not recommended and is off by default as of
484 Disables support for SSL/TLS compression.
485 TLS compression is not recommended and is off by default as of
490 Only provide a brief summary of connection parameters instead of the
491 normal verbose output.
493 =item B<-sigalgs sigalglist>
495 Specifies the list of signature algorithms that are sent by the client.
496 The server selects one entry in the list based on its preferences.
497 For example strings, see L<SSL_CTX_set1_sigalgs(3)>
499 =item B<-curves curvelist>
501 Specifies the list of supported curves to be sent by the client. The curve is
502 is ultimately selected by the server. For a list of all curves, use:
504 $ openssl ecparam -list_curves
506 =item B<-cipher cipherlist>
508 This allows the cipher list sent by the client to be modified. Although
509 the server determines which cipher suite is used it should take the first
510 supported cipher in the list sent by the client. See the B<ciphers>
511 command for more information.
513 =item B<-starttls protocol>
515 Send the protocol-specific message(s) to switch to TLS for communication.
516 B<protocol> is a keyword for the intended protocol. Currently, the only
517 supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",
518 "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
520 =item B<-xmpphost hostname>
522 This option, when used with "-starttls xmpp" or "-starttls xmpp-server",
523 specifies the host for the "to" attribute of the stream element.
524 If this option is not specified, then the host specified with "-connect"
527 This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".
529 =item B<-name hostname>
531 This option is used to specify hostname information for various protocols
532 used with B<-starttls> option. Currently only "xmpp", "xmpp-server",
533 "smtp" and "lmtp" can utilize this B<-name> option.
535 If this option is used with "-starttls xmpp" or "-starttls xmpp-server",
536 if specifies the host for the "to" attribute of the stream element. If this
537 option is not specified, then the host specified with "-connect" will be used.
539 If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies
540 the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If
541 this option is not specified, then "mail.example.com" will be used.
543 =item B<-tlsextdebug>
545 Print out a hex dump of any TLS extensions received from the server.
549 Disable RFC4507bis session ticket support.
551 =item B<-sess_out filename>
553 Output SSL session to B<filename>.
555 =item B<-sess_in sess.pem>
557 Load SSL session from B<filename>. The client will attempt to resume a
558 connection from this session.
562 Specifying an engine (by its unique B<id> string) will cause B<s_client>
563 to attempt to obtain a functional reference to the specified engine,
564 thus initialising it if needed. The engine will then be set as the default
565 for all available algorithms.
567 =item B<-rand file...>
569 A file or files containing random data used to seed the random number
571 Multiple files can be specified separated by an OS-dependent character.
572 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
575 =item [B<-writerand file>]
577 Writes random data to the specified I<file> upon exit.
578 This can be used with a subsequent B<-rand> flag.
580 =item B<-serverinfo types>
582 A list of comma-separated TLS Extension Types (numbers between 0 and
583 65535). Each type will be sent as an empty ClientHello TLS Extension.
584 The server's response (if any) will be encoded and displayed as a PEM
589 Sends a certificate status request to the server (OCSP stapling). The server
590 response (if any) is printed out.
592 =item B<-alpn protocols>, B<-nextprotoneg protocols>
594 These flags enable the Enable the Application-Layer Protocol Negotiation
595 or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the
596 IETF standard and replaces NPN.
597 The B<protocols> list is a comma-separated list of protocol names that
598 the client should advertise support for. The list should contain the most
599 desirable protocols first. Protocol names are printable ASCII strings,
600 for example "http/1.1" or "spdy/3".
601 An empty list of protocols is treated specially and will cause the
602 client to advertise support for the TLS extension but disconnect just
603 after receiving ServerHello with a list of server supported protocols.
604 The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.
606 =item B<-ct>, B<-noct>
608 Use one of these two options to control whether Certificate Transparency (CT)
609 is enabled (B<-ct>) or disabled (B<-noct>).
610 If CT is enabled, signed certificate timestamps (SCTs) will be requested from
611 the server and reported at handshake completion.
613 Enabling CT also enables OCSP stapling, as this is one possible delivery method
618 A file containing a list of known Certificate Transparency logs. See
619 L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.
621 =item B<-keylogfile file>
623 Appends TLS secrets to the specified keylog file such that external programs
624 (like Wireshark) can decrypt TLS connections.
626 =item B<-early_data file>
628 Reads the contents of the specified file and attempts to send it as early data
629 to the server. This will only work with resumed sessions that support early
630 data and when the server accepts the early data.
634 For TLSv1.3 only, always send the Post-Handshake Authentication extension,
635 whether or not a certificate has been provided via B<-cert>.
639 Rather than providing B<-connect>, the target hostname and optional port may
640 be provided as a single positional argument after all options. If neither this
641 nor B<-connect> are provided, falls back to attempting to connect to localhost
646 =head1 CONNECTED COMMANDS
648 If a connection is established with an SSL server then any data received
649 from the server is displayed and any key presses will be sent to the
650 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
651 have been given), the session will be renegotiated if the line begins with an
652 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
653 connection will be closed down.
657 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
660 openssl s_client -connect servername:443
662 would typically be used (https uses port 443). If the connection succeeds
663 then an HTTP command can be given such as "GET /" to retrieve a web page.
665 If the handshake fails then there are several possible causes, if it is
666 nothing obvious like no client certificate then the B<-bugs>,
667 B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried
668 in case it is a buggy server. In particular you should play with these
669 options B<before> submitting a bug report to an OpenSSL mailing list.
671 A frequent problem when attempting to get client certificates working
672 is that a web client complains it has no certificates or gives an empty
673 list to choose from. This is normally because the server is not sending
674 the clients certificate authority in its "acceptable CA list" when it
675 requests a certificate. By using B<s_client> the CA list can be viewed
676 and checked. However some servers only request client authentication
677 after a specific URL is requested. To obtain the list in this case it
678 is necessary to use the B<-prexit> option and send an HTTP request
679 for an appropriate page.
681 If a certificate is specified on the command line using the B<-cert>
682 option it will not be used unless the server specifically requests
683 a client certificate. Therefor merely including a client certificate
684 on the command line is no guarantee that the certificate works.
686 If there are problems verifying a server certificate then the
687 B<-showcerts> option can be used to show the whole chain.
689 The B<s_client> utility is a test tool and is designed to continue the
690 handshake after any certificate verification errors. As a result it will
691 accept any certificate chain (trusted or not) sent by the peer. None test
692 applications should B<not> do this as it makes them vulnerable to a MITM
693 attack. This behaviour can be changed by with the B<-verify_return_error>
694 option: any verify errors are then returned aborting the handshake.
696 The B<-bind> option may be useful if the server or a firewall requires
697 connections to come from some particular address and or port.
701 Because this program has a lot of options and also because some of the
702 techniques used are rather old, the C source of B<s_client> is rather hard to
703 read and not a model of how things should be done.
704 A typical SSL client program would be much simpler.
706 The B<-prexit> option is a bit of a hack. We should really report
707 information whenever a session is renegotiated.
711 L<SSL_CONF_cmd(3)>, L<sess_id(1)>, L<s_server(1)>, L<ciphers(1)>,
712 L<SSL_CTX_set_max_send_fragment(3)>, L<SSL_CTX_set_split_send_fragment(3)>
713 L<SSL_CTX_set_max_pipelines(3)>
717 The B<-no_alt_chains> option was first added to OpenSSL 1.1.0.
718 The B<-name> option was added in OpenSSL 1.1.1.
722 Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
724 Licensed under the OpenSSL license (the "License"). You may not use
725 this file except in compliance with the License. You can obtain a copy
726 in the file LICENSE in the source distribution or at
727 L<https://www.openssl.org/source/license.html>.