6 s_client - SSL/TLS client program
10 B<openssl> B<s_client>
12 [B<-connect host:port>]
20 [B<-verify_return_error>]
22 [B<-certform DER|PEM>]
26 [B<-CApath directory>]
30 [B<-requestCAfile filename>]
31 [B<-dane_tlsa_domain domain>]
32 [B<-dane_tlsa_rrdata rrdata>]
33 [B<-dane_ee_no_namechecks>]
34 [B<-attime timestamp>]
57 [B<-verify_depth num>]
58 [B<-verify_email email>]
59 [B<-verify_hostname hostname>]
61 [B<-verify_name name>]
97 [B<-allow_no_dhe_kex>]
98 [B<-sigalgs sigalglist>]
99 [B<-curves curvelist>]
100 [B<-cipher cipherlist>]
102 [B<-starttls protocol>]
103 [B<-xmpphost hostname>]
108 [B<-sess_out filename>]
109 [B<-sess_in filename>]
112 [B<-serverinfo types>]
115 [B<-nextprotoneg protocols>]
119 [B<-keylogfile file>]
120 [B<-early_data file>]
126 The B<s_client> command implements a generic SSL/TLS client which connects
127 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
132 In addition to the options below the B<s_client> utility also supports the
133 common and client only options documented in the
134 in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>
141 Print out a usage message.
143 =item B<-connect host:port>
145 This specifies the host and optional port to connect to. It is possible to
146 select the host and port using the optional target positional argument instead.
147 If neither this nor the target positonal argument are specified then an attempt
148 is made to connect to the local host on port 4433.
150 =item B<-proxy host:port>
152 When used with the B<-connect> flag, the program uses the host and port
153 specified with this flag and issues an HTTP CONNECT command to connect
154 to the desired server.
158 Connect over the specified Unix-domain socket.
168 =item B<-servername name>
170 Set the TLS SNI (Server Name Indication) extension in the ClientHello message to
171 the given value. If both this option and the B<-noservername> are not given, the
172 TLS SNI extension is still set to the hostname provided to the B<-connect> option,
173 or "localhost" if B<-connect> has not been supplied. This is default since OpenSSL
176 Even though SNI name should normally be a DNS name and not an IP address, this
177 option will not make the distinction when parsing B<-connect> and will send
178 IP address if one passed.
180 =item B<-noservername>
182 Suppresses sending of the SNI (Server Name Indication) extension in the
183 ClientHello message. Cannot be used in conjunction with the B<-servername> or
184 <-dane_tlsa_domain> options.
186 =item B<-cert certname>
188 The certificate to use, if one is requested by the server. The default is
189 not to use a certificate.
191 =item B<-certform format>
193 The certificate format to use: DER or PEM. PEM is the default.
195 =item B<-key keyfile>
197 The private key to use. If not specified then the certificate file will
200 =item B<-keyform format>
202 The private format to use: DER or PEM. PEM is the default.
206 the private key password source. For more information about the format of B<arg>
207 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)>.
209 =item B<-verify depth>
211 The verify depth to use. This specifies the maximum length of the
212 server certificate chain and turns on server certificate verification.
213 Currently the verify operation continues after errors so all the problems
214 with a certificate chain can be seen. As a side effect the connection
215 will never fail due to a server certificate verify failure.
217 =item B<-verify_return_error>
219 Return verification errors instead of continuing. This will typically
220 abort the handshake with a fatal error.
222 =item B<-nameopt option>
224 Option which determines how the subject or issuer names are displayed. The
225 B<option> argument can be a single option or multiple options separated by
226 commas. Alternatively the B<-nameopt> switch may be used more than once to
227 set multiple options. See the L<x509(1)> manual page for details.
229 =item B<-CApath directory>
231 The directory to use for server certificate verification. This directory
232 must be in "hash format", see B<verify> for more information. These are
233 also used when building the client certificate chain.
235 =item B<-CAfile file>
237 A file containing trusted certificates to use during server authentication
238 and to use when attempting to build the client certificate chain.
242 Do not load the trusted CA certificates from the default file location
246 Do not load the trusted CA certificates from the default directory location
248 =item B<-requestCAfile file>
250 A file containing a list of certificates whose subject names will be sent
251 to the server in the B<certificate_authorities> extension. Only supported
254 =item B<-dane_tlsa_domain domain>
256 Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
257 TLSA base domain which becomes the default SNI hint and the primary
258 reference identifier for hostname checks. This must be used in
259 combination with at least one instance of the B<-dane_tlsa_rrdata>
262 When DANE authentication succeeds, the diagnostic output will include
263 the lowest (closest to 0) depth at which a TLSA record authenticated
264 a chain certificate. When that TLSA record is a "2 1 0" trust
265 anchor public key that signed (rather than matched) the top-most
266 certificate of the chain, the result is reported as "TA public key
267 verified". Otherwise, either the TLSA record "matched TA certificate"
268 at a positive depth or else "matched EE certificate" at depth 0.
270 =item B<-dane_tlsa_rrdata rrdata>
272 Use one or more times to specify the RRDATA fields of the DANE TLSA
273 RRset associated with the target service. The B<rrdata> value is
274 specied in "presentation form", that is four whitespace separated
275 fields that specify the usage, selector, matching type and associated
276 data, with the last of these encoded in hexadecimal. Optional
277 whitespace is ignored in the associated data field. For example:
279 $ openssl s_client -brief -starttls smtp \
280 -connect smtp.example.com:25 \
281 -dane_tlsa_domain smtp.example.com \
282 -dane_tlsa_rrdata "2 1 1
283 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
284 -dane_tlsa_rrdata "2 1 1
285 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
288 Verified peername: smtp.example.com
289 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
292 =item B<-dane_ee_no_namechecks>
294 This disables server name checks when authenticating via DANE-EE(3) TLSA
296 For some applications, primarily web browsers, it is not safe to disable name
297 checks due to "unknown key share" attacks, in which a malicious server can
298 convince a client that a connection to a victim server is instead a secure
299 connection to the malicious server.
300 The malicious server may then be able to violate cross-origin scripting
302 Thus, despite the text of RFC7671, name checks are by default enabled for
303 DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
305 In particular, SMTP and XMPP clients should set this option as SRV and MX
306 records already make it possible for a remote domain to redirect client
307 connections to any server of its choice, and in any case SMTP and XMPP clients
308 do not execute scripts downloaded from remote servers.
310 =item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>,
311 B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>,
312 B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>,
313 B<-policy_check>, B<-policy_print>, B<-purpose>, B<-suiteB_128>,
314 B<-suiteB_128_only>, B<-suiteB_192>, B<-trusted_first>, B<-use_deltas>,
315 B<-auth_level>, B<-verify_depth>, B<-verify_email>, B<-verify_hostname>,
316 B<-verify_ip>, B<-verify_name>, B<-x509_strict>
318 Set various certificate chain validation options. See the
319 L<verify(1)> manual page for details.
323 Reconnects to the same server 5 times using the same session ID, this can
324 be used as a test that session caching is working.
328 Display the whole server certificate chain: normally only the server
329 certificate itself is displayed.
333 Print session information when the program exits. This will always attempt
334 to print out information even if the connection fails. Normally information
335 will only be printed out once if the connection succeeds. This option is useful
336 because the cipher in use may be renegotiated or the connection may fail
337 because a client certificate is required or is requested only after an
338 attempt is made to access a certain URL. Note: the output produced by this
339 option is not always accurate because a connection might never have been
344 Prints out the SSL session states.
348 Print extensive debugging information including a hex dump of all traffic.
352 Show all protocol messages with hex dump.
356 Show verbose trace output of protocol messages. OpenSSL needs to be compiled
357 with B<enable-ssl-trace> for this option to work.
361 File to send output of B<-msg> or B<-trace> to, default standard output.
365 Tests non-blocking I/O
369 Turns on non-blocking I/O
373 This option translated a line feed from the terminal into CR+LF as required
378 Inhibit shutting down the connection when end of file is reached in the
383 Inhibit printing of session and certificate information. This implicitly
384 turns on B<-ign_eof> as well.
388 Shut down the connection when end of file is reached in the input.
389 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
391 =item B<-psk_identity identity>
393 Use the PSK identity B<identity> when using a PSK cipher suite.
394 The default value is "Client_identity" (without the quotes).
398 Use the PSK key B<key> when using a PSK cipher suite. The key is
399 given as a hexadecimal number without leading 0x, for example -psk
401 This option must be provided in order to use a PSK cipher.
403 =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>
405 These options require or disable the use of the specified SSL or TLS protocols.
406 By default B<s_client> will negotiate the highest mutually supported protocol
408 When a specific TLS version is required, only that version will be offered to
409 and accepted from the server.
411 =item B<-dtls>, B<-dtls1>, B<-dtls1_2>
413 These options make B<s_client> use DTLS protocols instead of TLS.
414 With B<-dtls>, B<s_client> will negotiate any supported DTLS protocol version,
415 whilst B<-dtls1> and B<-dtls1_2> will only support DTLS1.0 and DTLS1.2
420 Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in
421 conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only
422 available where OpenSSL has support for SCTP enabled.
424 =item B<-fallback_scsv>
426 Send TLS_FALLBACK_SCSV in the ClientHello.
430 Switch on asynchronous mode. Cryptographic operations will be performed
431 asynchronously. This will only have an effect if an asynchronous capable engine
432 is also used via the B<-engine> option. For test purposes the dummy async engine
433 (dasync) can be used (if available).
435 =item B<-max_send_frag int>
437 The maximum size of data fragment to send.
438 See L<SSL_CTX_set_max_send_fragment(3)> for further information.
440 =item B<-split_send_frag int>
442 The size used to split data for encrypt pipelines. If more data is written in
443 one go than this value then it will be split into multiple pipelines, up to the
444 maximum number of pipelines defined by max_pipelines. This only has an effect if
445 a suitable cipher suite has been negotiated, an engine that supports pipelining
446 has been loaded, and max_pipelines is greater than 1. See
447 L<SSL_CTX_set_split_send_fragment(3)> for further information.
449 =item B<-max_pipelines int>
451 The maximum number of encrypt/decrypt pipelines to be used. This will only have
452 an effect if an engine has been loaded that supports pipelining (e.g. the dasync
453 engine) and a suitable cipher suite has been negotiated. The default value is 1.
454 See L<SSL_CTX_set_max_pipelines(3)> for further information.
456 =item B<-read_buf int>
458 The default read buffer size to be used for connections. This will only have an
459 effect if the buffer size is larger than the size that would otherwise be used
460 and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for
461 further information).
465 There are several known bug in SSL and TLS implementations. Adding this
466 option enables various workarounds.
470 Enables support for SSL/TLS compression.
471 This option was introduced in OpenSSL 1.1.0.
472 TLS compression is not recommended and is off by default as of
477 Disables support for SSL/TLS compression.
478 TLS compression is not recommended and is off by default as of
483 Only provide a brief summary of connection parameters instead of the
484 normal verbose output.
486 =item B<-sigalgs sigalglist>
488 Specifies the list of signature algorithms that are sent by the client.
489 The server selects one entry in the list based on its preferences.
490 For example strings, see L<SSL_CTX_set1_sigalgs(3)>
492 =item B<-curves curvelist>
494 Specifies the list of supported curves to be sent by the client. The curve is
495 is ultimately selected by the server. For a list of all curves, use:
497 $ openssl ecparam -list_curves
499 =item B<-cipher cipherlist>
501 This allows the cipher list sent by the client to be modified. Although
502 the server determines which cipher suite is used it should take the first
503 supported cipher in the list sent by the client. See the B<ciphers>
504 command for more information.
506 =item B<-starttls protocol>
508 Send the protocol-specific message(s) to switch to TLS for communication.
509 B<protocol> is a keyword for the intended protocol. Currently, the only
510 supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",
511 "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
513 =item B<-xmpphost hostname>
515 This option, when used with "-starttls xmpp" or "-starttls xmpp-server",
516 specifies the host for the "to" attribute of the stream element.
517 If this option is not specified, then the host specified with "-connect"
520 This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".
522 =item B<-name hostname>
524 This option is used to specify hostname information for various protocols
525 used with B<-starttls> option. Currently only "xmpp", "xmpp-server",
526 "smtp" and "lmtp" can utilize this B<-name> option.
528 If this option is used with "-starttls xmpp" or "-starttls xmpp-server",
529 if specifies the host for the "to" attribute of the stream element. If this
530 option is not specified, then the host specified with "-connect" will be used.
532 If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies
533 the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If
534 this option is not specified, then "mail.example.com" will be used.
536 =item B<-tlsextdebug>
538 Print out a hex dump of any TLS extensions received from the server.
542 Disable RFC4507bis session ticket support.
544 =item B<-sess_out filename>
546 Output SSL session to B<filename>.
548 =item B<-sess_in sess.pem>
550 Load SSL session from B<filename>. The client will attempt to resume a
551 connection from this session.
555 Specifying an engine (by its unique B<id> string) will cause B<s_client>
556 to attempt to obtain a functional reference to the specified engine,
557 thus initialising it if needed. The engine will then be set as the default
558 for all available algorithms.
560 =item B<-rand file...>
562 A file or files containing random data used to seed the random number
564 Multiple files can be specified separated by an OS-dependent character.
565 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
568 =item [B<-writerand file>]
570 Writes random data to the specified I<file> upon exit.
571 This can be used with a subsequent B<-rand> flag.
573 =item B<-serverinfo types>
575 A list of comma-separated TLS Extension Types (numbers between 0 and
576 65535). Each type will be sent as an empty ClientHello TLS Extension.
577 The server's response (if any) will be encoded and displayed as a PEM
582 Sends a certificate status request to the server (OCSP stapling). The server
583 response (if any) is printed out.
585 =item B<-alpn protocols>, B<-nextprotoneg protocols>
587 These flags enable the Enable the Application-Layer Protocol Negotiation
588 or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the
589 IETF standard and replaces NPN.
590 The B<protocols> list is a comma-separated list of protocol names that
591 the client should advertise support for. The list should contain the most
592 desirable protocols first. Protocol names are printable ASCII strings,
593 for example "http/1.1" or "spdy/3".
594 An empty list of protocols is treated specially and will cause the
595 client to advertise support for the TLS extension but disconnect just
596 after receiving ServerHello with a list of server supported protocols.
597 The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.
599 =item B<-ct>, B<-noct>
601 Use one of these two options to control whether Certificate Transparency (CT)
602 is enabled (B<-ct>) or disabled (B<-noct>).
603 If CT is enabled, signed certificate timestamps (SCTs) will be requested from
604 the server and reported at handshake completion.
606 Enabling CT also enables OCSP stapling, as this is one possible delivery method
611 A file containing a list of known Certificate Transparency logs. See
612 L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.
614 =item B<-keylogfile file>
616 Appends TLS secrets to the specified keylog file such that external programs
617 (like Wireshark) can decrypt TLS connections.
619 =item B<-early_data file>
621 Reads the contents of the specified file and attempts to send it as early data
622 to the server. This will only work with resumed sessions that support early
623 data and when the server accepts the early data.
627 For TLSv1.3 only, always send the Post-Handshake Authentication extension,
628 whether or not a certificate has been provided via B<-cert>.
632 Rather than providing B<-connect>, the target hostname and optional port may
633 be provided as a single positional argument after all options. If neither this
634 nor B<-connect> are provided, falls back to attempting to connect to localhost
639 =head1 CONNECTED COMMANDS
641 If a connection is established with an SSL server then any data received
642 from the server is displayed and any key presses will be sent to the
643 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
644 have been given), the session will be renegotiated if the line begins with an
645 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
646 connection will be closed down.
650 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
653 openssl s_client -connect servername:443
655 would typically be used (https uses port 443). If the connection succeeds
656 then an HTTP command can be given such as "GET /" to retrieve a web page.
658 If the handshake fails then there are several possible causes, if it is
659 nothing obvious like no client certificate then the B<-bugs>,
660 B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried
661 in case it is a buggy server. In particular you should play with these
662 options B<before> submitting a bug report to an OpenSSL mailing list.
664 A frequent problem when attempting to get client certificates working
665 is that a web client complains it has no certificates or gives an empty
666 list to choose from. This is normally because the server is not sending
667 the clients certificate authority in its "acceptable CA list" when it
668 requests a certificate. By using B<s_client> the CA list can be viewed
669 and checked. However some servers only request client authentication
670 after a specific URL is requested. To obtain the list in this case it
671 is necessary to use the B<-prexit> option and send an HTTP request
672 for an appropriate page.
674 If a certificate is specified on the command line using the B<-cert>
675 option it will not be used unless the server specifically requests
676 a client certificate. Therefor merely including a client certificate
677 on the command line is no guarantee that the certificate works.
679 If there are problems verifying a server certificate then the
680 B<-showcerts> option can be used to show the whole chain.
682 The B<s_client> utility is a test tool and is designed to continue the
683 handshake after any certificate verification errors. As a result it will
684 accept any certificate chain (trusted or not) sent by the peer. None test
685 applications should B<not> do this as it makes them vulnerable to a MITM
686 attack. This behaviour can be changed by with the B<-verify_return_error>
687 option: any verify errors are then returned aborting the handshake.
691 Because this program has a lot of options and also because some of the
692 techniques used are rather old, the C source of B<s_client> is rather hard to
693 read and not a model of how things should be done.
694 A typical SSL client program would be much simpler.
696 The B<-prexit> option is a bit of a hack. We should really report
697 information whenever a session is renegotiated.
701 L<SSL_CONF_cmd(3)>, L<sess_id(1)>, L<s_server(1)>, L<ciphers(1)>,
702 L<SSL_CTX_set_max_send_fragment(3)>, L<SSL_CTX_set_split_send_fragment(3)>
703 L<SSL_CTX_set_max_pipelines(3)>
707 The B<-no_alt_chains> option was first added to OpenSSL 1.1.0.
708 The B<-name> option was added in OpenSSL 1.1.1.
712 Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
714 Licensed under the OpenSSL license (the "License"). You may not use
715 this file except in compliance with the License. You can obtain a copy
716 in the file LICENSE in the source distribution or at
717 L<https://www.openssl.org/source/license.html>.