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>]
102 [B<-ciphersuites val>]
104 [B<-starttls protocol>]
105 [B<-xmpphost hostname>]
110 [B<-sess_out filename>]
111 [B<-sess_in filename>]
114 [B<-serverinfo types>]
117 [B<-nextprotoneg protocols>]
121 [B<-keylogfile file>]
122 [B<-early_data file>]
128 The B<s_client> command implements a generic SSL/TLS client which connects
129 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
134 In addition to the options below the B<s_client> utility also supports the
135 common and client only options documented in the
136 in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>
143 Print out a usage message.
145 =item B<-connect host:port>
147 This specifies the host and optional port to connect to. It is possible to
148 select the host and port using the optional target positional argument instead.
149 If neither this nor the target positional argument are specified then an attempt
150 is made to connect to the local host on port 4433.
152 =item B<-bind host:port>]
154 This specifies the host address and or port to bind as the source for the
155 connection. For Unix-domain sockets the port is ignored and the host is
156 used as the source socket address.
158 =item B<-proxy host:port>
160 When used with the B<-connect> flag, the program uses the host and port
161 specified with this flag and issues an HTTP CONNECT command to connect
162 to the desired server.
166 Connect over the specified Unix-domain socket.
176 =item B<-servername name>
178 Set the TLS SNI (Server Name Indication) extension in the ClientHello message to
179 the given value. If both this option and the B<-noservername> are not given, the
180 TLS SNI extension is still set to the hostname provided to the B<-connect> option,
181 or "localhost" if B<-connect> has not been supplied. This is default since OpenSSL
184 Even though SNI name should normally be a DNS name and not an IP address, this
185 option will not make the distinction when parsing B<-connect> and will send
186 IP address if one passed.
188 =item B<-noservername>
190 Suppresses sending of the SNI (Server Name Indication) extension in the
191 ClientHello message. Cannot be used in conjunction with the B<-servername> or
192 <-dane_tlsa_domain> options.
194 =item B<-cert certname>
196 The certificate to use, if one is requested by the server. The default is
197 not to use a certificate.
199 =item B<-certform format>
201 The certificate format to use: DER or PEM. PEM is the default.
203 =item B<-key keyfile>
205 The private key to use. If not specified then the certificate file will
208 =item B<-keyform format>
210 The private format to use: DER or PEM. PEM is the default.
214 the private key password source. For more information about the format of B<arg>
215 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)>.
217 =item B<-verify depth>
219 The verify depth to use. This specifies the maximum length of the
220 server certificate chain and turns on server certificate verification.
221 Currently the verify operation continues after errors so all the problems
222 with a certificate chain can be seen. As a side effect the connection
223 will never fail due to a server certificate verify failure.
225 =item B<-verify_return_error>
227 Return verification errors instead of continuing. This will typically
228 abort the handshake with a fatal error.
230 =item B<-nameopt option>
232 Option which determines how the subject or issuer names are displayed. The
233 B<option> argument can be a single option or multiple options separated by
234 commas. Alternatively the B<-nameopt> switch may be used more than once to
235 set multiple options. See the L<x509(1)> manual page for details.
237 =item B<-CApath directory>
239 The directory to use for server certificate verification. This directory
240 must be in "hash format", see B<verify> for more information. These are
241 also used when building the client certificate chain.
243 =item B<-CAfile file>
245 A file containing trusted certificates to use during server authentication
246 and to use when attempting to build the client certificate chain.
250 Do not load the trusted CA certificates from the default file location
254 Do not load the trusted CA certificates from the default directory location
256 =item B<-requestCAfile file>
258 A file containing a list of certificates whose subject names will be sent
259 to the server in the B<certificate_authorities> extension. Only supported
262 =item B<-dane_tlsa_domain domain>
264 Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
265 TLSA base domain which becomes the default SNI hint and the primary
266 reference identifier for hostname checks. This must be used in
267 combination with at least one instance of the B<-dane_tlsa_rrdata>
270 When DANE authentication succeeds, the diagnostic output will include
271 the lowest (closest to 0) depth at which a TLSA record authenticated
272 a chain certificate. When that TLSA record is a "2 1 0" trust
273 anchor public key that signed (rather than matched) the top-most
274 certificate of the chain, the result is reported as "TA public key
275 verified". Otherwise, either the TLSA record "matched TA certificate"
276 at a positive depth or else "matched EE certificate" at depth 0.
278 =item B<-dane_tlsa_rrdata rrdata>
280 Use one or more times to specify the RRDATA fields of the DANE TLSA
281 RRset associated with the target service. The B<rrdata> value is
282 specied in "presentation form", that is four whitespace separated
283 fields that specify the usage, selector, matching type and associated
284 data, with the last of these encoded in hexadecimal. Optional
285 whitespace is ignored in the associated data field. For example:
287 $ openssl s_client -brief -starttls smtp \
288 -connect smtp.example.com:25 \
289 -dane_tlsa_domain smtp.example.com \
290 -dane_tlsa_rrdata "2 1 1
291 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
292 -dane_tlsa_rrdata "2 1 1
293 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
296 Verified peername: smtp.example.com
297 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
300 =item B<-dane_ee_no_namechecks>
302 This disables server name checks when authenticating via DANE-EE(3) TLSA
304 For some applications, primarily web browsers, it is not safe to disable name
305 checks due to "unknown key share" attacks, in which a malicious server can
306 convince a client that a connection to a victim server is instead a secure
307 connection to the malicious server.
308 The malicious server may then be able to violate cross-origin scripting
310 Thus, despite the text of RFC7671, name checks are by default enabled for
311 DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
313 In particular, SMTP and XMPP clients should set this option as SRV and MX
314 records already make it possible for a remote domain to redirect client
315 connections to any server of its choice, and in any case SMTP and XMPP clients
316 do not execute scripts downloaded from remote servers.
318 =item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>,
319 B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>,
320 B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>,
321 B<-policy_check>, B<-policy_print>, B<-purpose>, B<-suiteB_128>,
322 B<-suiteB_128_only>, B<-suiteB_192>, B<-trusted_first>, B<-use_deltas>,
323 B<-auth_level>, B<-verify_depth>, B<-verify_email>, B<-verify_hostname>,
324 B<-verify_ip>, B<-verify_name>, B<-x509_strict>
326 Set various certificate chain validation options. See the
327 L<verify(1)> manual page for details.
331 Reconnects to the same server 5 times using the same session ID, this can
332 be used as a test that session caching is working.
336 Displays the server certificate list as sent by the server: it only consists of
337 certificates the server has sent (in the order the server has sent them). It is
338 B<not> a verified chain.
342 Print session information when the program exits. This will always attempt
343 to print out information even if the connection fails. Normally information
344 will only be printed out once if the connection succeeds. This option is useful
345 because the cipher in use may be renegotiated or the connection may fail
346 because a client certificate is required or is requested only after an
347 attempt is made to access a certain URL. Note: the output produced by this
348 option is not always accurate because a connection might never have been
353 Prints out the SSL session states.
357 Print extensive debugging information including a hex dump of all traffic.
361 Show all protocol messages with hex dump.
365 Show verbose trace output of protocol messages. OpenSSL needs to be compiled
366 with B<enable-ssl-trace> for this option to work.
370 File to send output of B<-msg> or B<-trace> to, default standard output.
374 Tests non-blocking I/O
378 Turns on non-blocking I/O
382 This option translated a line feed from the terminal into CR+LF as required
387 Inhibit shutting down the connection when end of file is reached in the
392 Inhibit printing of session and certificate information. This implicitly
393 turns on B<-ign_eof> as well.
397 Shut down the connection when end of file is reached in the input.
398 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
400 =item B<-psk_identity identity>
402 Use the PSK identity B<identity> when using a PSK cipher suite.
403 The default value is "Client_identity" (without the quotes).
407 Use the PSK key B<key> when using a PSK cipher suite. The key is
408 given as a hexadecimal number without leading 0x, for example -psk
410 This option must be provided in order to use a PSK cipher.
412 =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>
414 These options require or disable the use of the specified SSL or TLS protocols.
415 By default B<s_client> will negotiate the highest mutually supported protocol
417 When a specific TLS version is required, only that version will be offered to
418 and accepted from the server.
420 =item B<-dtls>, B<-dtls1>, B<-dtls1_2>
422 These options make B<s_client> use DTLS protocols instead of TLS.
423 With B<-dtls>, B<s_client> will negotiate any supported DTLS protocol version,
424 whilst B<-dtls1> and B<-dtls1_2> will only support DTLS1.0 and DTLS1.2
429 Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in
430 conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only
431 available where OpenSSL has support for SCTP enabled.
433 =item B<-fallback_scsv>
435 Send TLS_FALLBACK_SCSV in the ClientHello.
439 Switch on asynchronous mode. Cryptographic operations will be performed
440 asynchronously. This will only have an effect if an asynchronous capable engine
441 is also used via the B<-engine> option. For test purposes the dummy async engine
442 (dasync) can be used (if available).
444 =item B<-max_send_frag int>
446 The maximum size of data fragment to send.
447 See L<SSL_CTX_set_max_send_fragment(3)> for further information.
449 =item B<-split_send_frag int>
451 The size used to split data for encrypt pipelines. If more data is written in
452 one go than this value then it will be split into multiple pipelines, up to the
453 maximum number of pipelines defined by max_pipelines. This only has an effect if
454 a suitable cipher suite has been negotiated, an engine that supports pipelining
455 has been loaded, and max_pipelines is greater than 1. See
456 L<SSL_CTX_set_split_send_fragment(3)> for further information.
458 =item B<-max_pipelines int>
460 The maximum number of encrypt/decrypt pipelines to be used. This will only have
461 an effect if an engine has been loaded that supports pipelining (e.g. the dasync
462 engine) and a suitable cipher suite has been negotiated. The default value is 1.
463 See L<SSL_CTX_set_max_pipelines(3)> for further information.
465 =item B<-read_buf int>
467 The default read buffer size to be used for connections. This will only have an
468 effect if the buffer size is larger than the size that would otherwise be used
469 and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for
470 further information).
474 There are several known bug in SSL and TLS implementations. Adding this
475 option enables various workarounds.
479 Enables support for SSL/TLS compression.
480 This option was introduced in OpenSSL 1.1.0.
481 TLS compression is not recommended and is off by default as of
486 Disables support for SSL/TLS compression.
487 TLS compression is not recommended and is off by default as of
492 Only provide a brief summary of connection parameters instead of the
493 normal verbose output.
495 =item B<-sigalgs sigalglist>
497 Specifies the list of signature algorithms that are sent by the client.
498 The server selects one entry in the list based on its preferences.
499 For example strings, see L<SSL_CTX_set1_sigalgs(3)>
501 =item B<-curves curvelist>
503 Specifies the list of supported curves to be sent by the client. The curve is
504 ultimately selected by the server. For a list of all curves, use:
506 $ openssl ecparam -list_curves
508 =item B<-cipher cipherlist>
510 This allows the TLSv1.2 and below cipher list sent by the client to be modified.
511 This list will be combined with any TLSv1.3 ciphersuites that have been
512 configured. Although the server determines which ciphersuite is used it should
513 take the first supported cipher in the list sent by the client. See the
514 B<ciphers> command for more information.
516 =item B<-ciphersuites val>
518 This allows the TLSv1.3 ciphersuites sent by the client to be modified. This
519 list will be combined with any TLSv1.2 and below ciphersuites that have been
520 configured. Although the server determines which cipher suite is used it should
521 take the first supported cipher in the list sent by the client. See the
522 B<ciphers> command for more information. The format for this list is a simple
523 colon (":") separated list of TLSv1.3 ciphersuite names.
525 =item B<-starttls protocol>
527 Send the protocol-specific message(s) to switch to TLS for communication.
528 B<protocol> is a keyword for the intended protocol. Currently, the only
529 supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",
530 "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
532 =item B<-xmpphost hostname>
534 This option, when used with "-starttls xmpp" or "-starttls xmpp-server",
535 specifies the host for the "to" attribute of the stream element.
536 If this option is not specified, then the host specified with "-connect"
539 This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".
541 =item B<-name hostname>
543 This option is used to specify hostname information for various protocols
544 used with B<-starttls> option. Currently only "xmpp", "xmpp-server",
545 "smtp" and "lmtp" can utilize this B<-name> option.
547 If this option is used with "-starttls xmpp" or "-starttls xmpp-server",
548 if specifies the host for the "to" attribute of the stream element. If this
549 option is not specified, then the host specified with "-connect" will be used.
551 If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies
552 the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If
553 this option is not specified, then "mail.example.com" will be used.
555 =item B<-tlsextdebug>
557 Print out a hex dump of any TLS extensions received from the server.
561 Disable RFC4507bis session ticket support.
563 =item B<-sess_out filename>
565 Output SSL session to B<filename>.
567 =item B<-sess_in sess.pem>
569 Load SSL session from B<filename>. The client will attempt to resume a
570 connection from this session.
574 Specifying an engine (by its unique B<id> string) will cause B<s_client>
575 to attempt to obtain a functional reference to the specified engine,
576 thus initialising it if needed. The engine will then be set as the default
577 for all available algorithms.
579 =item B<-rand file...>
581 A file or files containing random data used to seed the random number
583 Multiple files can be specified separated by an OS-dependent character.
584 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
587 =item [B<-writerand file>]
589 Writes random data to the specified I<file> upon exit.
590 This can be used with a subsequent B<-rand> flag.
592 =item B<-serverinfo types>
594 A list of comma-separated TLS Extension Types (numbers between 0 and
595 65535). Each type will be sent as an empty ClientHello TLS Extension.
596 The server's response (if any) will be encoded and displayed as a PEM
601 Sends a certificate status request to the server (OCSP stapling). The server
602 response (if any) is printed out.
604 =item B<-alpn protocols>, B<-nextprotoneg protocols>
606 These flags enable the Enable the Application-Layer Protocol Negotiation
607 or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the
608 IETF standard and replaces NPN.
609 The B<protocols> list is a comma-separated list of protocol names that
610 the client should advertise support for. The list should contain the most
611 desirable protocols first. Protocol names are printable ASCII strings,
612 for example "http/1.1" or "spdy/3".
613 An empty list of protocols is treated specially and will cause the
614 client to advertise support for the TLS extension but disconnect just
615 after receiving ServerHello with a list of server supported protocols.
616 The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.
618 =item B<-ct>, B<-noct>
620 Use one of these two options to control whether Certificate Transparency (CT)
621 is enabled (B<-ct>) or disabled (B<-noct>).
622 If CT is enabled, signed certificate timestamps (SCTs) will be requested from
623 the server and reported at handshake completion.
625 Enabling CT also enables OCSP stapling, as this is one possible delivery method
630 A file containing a list of known Certificate Transparency logs. See
631 L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.
633 =item B<-keylogfile file>
635 Appends TLS secrets to the specified keylog file such that external programs
636 (like Wireshark) can decrypt TLS connections.
638 =item B<-early_data file>
640 Reads the contents of the specified file and attempts to send it as early data
641 to the server. This will only work with resumed sessions that support early
642 data and when the server accepts the early data.
646 For TLSv1.3 only, always send the Post-Handshake Authentication extension,
647 whether or not a certificate has been provided via B<-cert>.
651 Rather than providing B<-connect>, the target hostname and optional port may
652 be provided as a single positional argument after all options. If neither this
653 nor B<-connect> are provided, falls back to attempting to connect to localhost
658 =head1 CONNECTED COMMANDS
660 If a connection is established with an SSL server then any data received
661 from the server is displayed and any key presses will be sent to the
662 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
663 have been given), the session will be renegotiated if the line begins with an
664 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
665 connection will be closed down.
669 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
672 openssl s_client -connect servername:443
674 would typically be used (https uses port 443). If the connection succeeds
675 then an HTTP command can be given such as "GET /" to retrieve a web page.
677 If the handshake fails then there are several possible causes, if it is
678 nothing obvious like no client certificate then the B<-bugs>,
679 B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried
680 in case it is a buggy server. In particular you should play with these
681 options B<before> submitting a bug report to an OpenSSL mailing list.
683 A frequent problem when attempting to get client certificates working
684 is that a web client complains it has no certificates or gives an empty
685 list to choose from. This is normally because the server is not sending
686 the clients certificate authority in its "acceptable CA list" when it
687 requests a certificate. By using B<s_client> the CA list can be viewed
688 and checked. However some servers only request client authentication
689 after a specific URL is requested. To obtain the list in this case it
690 is necessary to use the B<-prexit> option and send an HTTP request
691 for an appropriate page.
693 If a certificate is specified on the command line using the B<-cert>
694 option it will not be used unless the server specifically requests
695 a client certificate. Therefor merely including a client certificate
696 on the command line is no guarantee that the certificate works.
698 If there are problems verifying a server certificate then the
699 B<-showcerts> option can be used to show all the certificates sent by the
702 The B<s_client> utility is a test tool and is designed to continue the
703 handshake after any certificate verification errors. As a result it will
704 accept any certificate chain (trusted or not) sent by the peer. None test
705 applications should B<not> do this as it makes them vulnerable to a MITM
706 attack. This behaviour can be changed by with the B<-verify_return_error>
707 option: any verify errors are then returned aborting the handshake.
709 The B<-bind> option may be useful if the server or a firewall requires
710 connections to come from some particular address and or port.
714 Because this program has a lot of options and also because some of the
715 techniques used are rather old, the C source of B<s_client> is rather hard to
716 read and not a model of how things should be done.
717 A typical SSL client program would be much simpler.
719 The B<-prexit> option is a bit of a hack. We should really report
720 information whenever a session is renegotiated.
724 L<SSL_CONF_cmd(3)>, L<sess_id(1)>, L<s_server(1)>, L<ciphers(1)>,
725 L<SSL_CTX_set_max_send_fragment(3)>, L<SSL_CTX_set_split_send_fragment(3)>
726 L<SSL_CTX_set_max_pipelines(3)>
730 The B<-no_alt_chains> option was first added to OpenSSL 1.1.0.
731 The B<-name> option was added in OpenSSL 1.1.1.
735 Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
737 Licensed under the OpenSSL license (the "License"). You may not use
738 this file except in compliance with the License. You can obtain a copy
739 in the file LICENSE in the source distribution or at
740 L<https://www.openssl.org/source/license.html>.