5 openssl-s_client - SSL/TLS client program
11 [B<-connect host:port>]
14 [B<-proxy_user userid>]
22 [B<-verify_return_error>]
24 [B<-certform DER|PEM>]
27 [B<-cert_chain filename>]
33 [B<-xcertform PEM|DER>]
34 [B<-xkeyform PEM|DER>]
36 [B<-CApath directory>]
38 [B<-chainCApath directory>]
39 [B<-chainCAfile filename>]
42 [B<-requestCAfile filename>]
43 [B<-dane_tlsa_domain domain>]
44 [B<-dane_tlsa_rrdata rrdata>]
45 [B<-dane_ee_no_namechecks>]
46 [B<-attime timestamp>]
69 [B<-verify_depth num>]
70 [B<-verify_email email>]
71 [B<-verify_hostname hostname>]
73 [B<-verify_name name>]
86 [B<-psk_identity identity>]
88 [B<-psk_session file>]
108 [B<-split_send_frag>]
114 [B<-allow_no_dhe_kex>]
115 [B<-sigalgs sigalglist>]
116 [B<-curves curvelist>]
117 [B<-cipher cipherlist>]
118 [B<-ciphersuites val>]
120 [B<-starttls protocol>]
121 [B<-xmpphost hostname>]
126 [B<-sess_out filename>]
127 [B<-sess_in filename>]
130 [B<-serverinfo types>]
133 [B<-nextprotoneg protocols>]
137 [B<-keylogfile file>]
138 [B<-early_data file>]
144 The B<s_client> command implements a generic SSL/TLS client which connects
145 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
150 In addition to the options below the B<s_client> utility also supports the
151 common and client only options documented
152 in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>
159 Print out a usage message.
161 =item B<-connect host:port>
163 This specifies the host and optional port to connect to. It is possible to
164 select the host and port using the optional target positional argument instead.
165 If neither this nor the target positional argument are specified then an attempt
166 is made to connect to the local host on port 4433.
168 =item B<-bind 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 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 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 arg>
191 The proxy password source, used with the B<-proxy_user> flag.
192 For more information about the format of B<arg> see the B<PASS PHRASE ARGUMENTS>
193 section in L<openssl(1)>.
197 Connect over the specified Unix-domain socket.
207 =item B<-servername 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 <-dane_tlsa_domain> options.
228 =item B<-cert certname>
230 The certificate to use, if one is requested by the server. The default is
231 not to use a certificate.
233 =item B<-certform format>
235 The certificate format to use: DER or PEM. PEM is the default.
237 =item B<-key keyfile>
239 The private key to use. If not specified then the certificate file will
242 =item B<-keyform format>
244 The private format to use: DER or PEM. PEM is the default.
248 A file containing trusted certificates to use when attempting to build the
249 client/server certificate chain related to the certificate specified via the
252 =item B<-build_chain>
254 Specify whether the application should build the certificate chain to be
255 provided to the server.
257 =item B<-xkey infile>, B<-xcert infile>, B<-xchain>
259 Specify an extra certificate, private key and certificate chain. These behave
260 in the same manner as the B<-cert>, B<-key> and B<-cert_chain> options. When
261 specified, the callback returning the first valid chain will be in use by the
264 =item B<-xchain_build>
266 Specify whether the application should build the certificate chain to be
267 provided to the server for the extra certificates provided via B<-xkey infile>,
268 B<-xcert infile>, B<-xchain> options.
270 =item B<-xcertform PEM|DER>, B<-xkeyform PEM|DER>
272 Extra certificate and private key format respectively.
276 the private key password source. For more information about the format of B<arg>
277 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)>.
279 =item B<-verify depth>
281 The verify depth to use. This specifies the maximum length of the
282 server certificate chain and turns on server certificate verification.
283 Currently the verify operation continues after errors so all the problems
284 with a certificate chain can be seen. As a side effect the connection
285 will never fail due to a server certificate verify failure.
287 =item B<-verify_return_error>
289 Return verification errors instead of continuing. This will typically
290 abort the handshake with a fatal error.
292 =item B<-nameopt option>
294 Option which determines how the subject or issuer names are displayed. The
295 B<option> argument can be a single option or multiple options separated by
296 commas. Alternatively the B<-nameopt> switch may be used more than once to
297 set multiple options. See the L<x509(1)> manual page for details.
299 =item B<-CApath directory>
301 The directory to use for server certificate verification. This directory
302 must be in "hash format", see L<verify(1)> for more information. These are
303 also used when building the client certificate chain.
305 =item B<-CAfile file>
307 A file containing trusted certificates to use during server authentication
308 and to use when attempting to build the client certificate chain.
310 =item B<-chainCApath directory>
312 The directory to use for building the chain provided to the server. This
313 directory must be in "hash format", see L<verify(1)> for more information.
315 =item B<-chainCAfile file>
317 A file containing trusted certificates to use when attempting to build the
318 client certificate chain.
322 Do not load the trusted CA certificates from the default file location
326 Do not load the trusted CA certificates from the default directory location
328 =item B<-requestCAfile file>
330 A file containing a list of certificates whose subject names will be sent
331 to the server in the B<certificate_authorities> extension. Only supported
334 =item B<-dane_tlsa_domain domain>
336 Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
337 TLSA base domain which becomes the default SNI hint and the primary
338 reference identifier for hostname checks. This must be used in
339 combination with at least one instance of the B<-dane_tlsa_rrdata>
342 When DANE authentication succeeds, the diagnostic output will include
343 the lowest (closest to 0) depth at which a TLSA record authenticated
344 a chain certificate. When that TLSA record is a "2 1 0" trust
345 anchor public key that signed (rather than matched) the top-most
346 certificate of the chain, the result is reported as "TA public key
347 verified". Otherwise, either the TLSA record "matched TA certificate"
348 at a positive depth or else "matched EE certificate" at depth 0.
350 =item B<-dane_tlsa_rrdata rrdata>
352 Use one or more times to specify the RRDATA fields of the DANE TLSA
353 RRset associated with the target service. The B<rrdata> value is
354 specied in "presentation form", that is four whitespace separated
355 fields that specify the usage, selector, matching type and associated
356 data, with the last of these encoded in hexadecimal. Optional
357 whitespace is ignored in the associated data field. For example:
359 $ openssl s_client -brief -starttls smtp \
360 -connect smtp.example.com:25 \
361 -dane_tlsa_domain smtp.example.com \
362 -dane_tlsa_rrdata "2 1 1
363 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
364 -dane_tlsa_rrdata "2 1 1
365 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
368 Verified peername: smtp.example.com
369 DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
372 =item B<-dane_ee_no_namechecks>
374 This disables server name checks when authenticating via DANE-EE(3) TLSA
376 For some applications, primarily web browsers, it is not safe to disable name
377 checks due to "unknown key share" attacks, in which a malicious server can
378 convince a client that a connection to a victim server is instead a secure
379 connection to the malicious server.
380 The malicious server may then be able to violate cross-origin scripting
382 Thus, despite the text of RFC7671, name checks are by default enabled for
383 DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
385 In particular, SMTP and XMPP clients should set this option as SRV and MX
386 records already make it possible for a remote domain to redirect client
387 connections to any server of its choice, and in any case SMTP and XMPP clients
388 do not execute scripts downloaded from remote servers.
390 =item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>,
391 B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>,
392 B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>,
393 B<-policy_check>, B<-policy_print>, B<-purpose>, B<-suiteB_128>,
394 B<-suiteB_128_only>, B<-suiteB_192>, B<-trusted_first>, B<-use_deltas>,
395 B<-auth_level>, B<-verify_depth>, B<-verify_email>, B<-verify_hostname>,
396 B<-verify_ip>, B<-verify_name>, B<-x509_strict>
398 Set various certificate chain validation options. See the
399 L<verify(1)> manual page for details.
403 Reconnects to the same server 5 times using the same session ID, this can
404 be used as a test that session caching is working.
408 Displays the server certificate list as sent by the server: it only consists of
409 certificates the server has sent (in the order the server has sent them). It is
410 B<not> a verified chain.
414 Print session information when the program exits. This will always attempt
415 to print out information even if the connection fails. Normally information
416 will only be printed out once if the connection succeeds. This option is useful
417 because the cipher in use may be renegotiated or the connection may fail
418 because a client certificate is required or is requested only after an
419 attempt is made to access a certain URL. Note: the output produced by this
420 option is not always accurate because a connection might never have been
425 Prints out the SSL session states.
429 Print extensive debugging information including a hex dump of all traffic.
433 Show all protocol messages with hex dump.
437 Show verbose trace output of protocol messages. OpenSSL needs to be compiled
438 with B<enable-ssl-trace> for this option to work.
442 File to send output of B<-msg> or B<-trace> to, default standard output.
446 Tests non-blocking I/O
450 Turns on non-blocking I/O
454 This option translated a line feed from the terminal into CR+LF as required
459 Inhibit shutting down the connection when end of file is reached in the
464 Inhibit printing of session and certificate information. This implicitly
465 turns on B<-ign_eof> as well.
469 Shut down the connection when end of file is reached in the input.
470 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
472 =item B<-psk_identity identity>
474 Use the PSK identity B<identity> when using a PSK cipher suite.
475 The default value is "Client_identity" (without the quotes).
479 Use the PSK key B<key> when using a PSK cipher suite. The key is
480 given as a hexadecimal number without leading 0x, for example -psk
482 This option must be provided in order to use a PSK cipher.
484 =item B<-psk_session file>
486 Use the pem encoded SSL_SESSION data stored in B<file> as the basis of a PSK.
487 Note that this will only work if TLSv1.3 is negotiated.
489 =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>
491 These options require or disable the use of the specified SSL or TLS protocols.
492 By default B<s_client> will negotiate the highest mutually supported protocol
494 When a specific TLS version is required, only that version will be offered to
495 and accepted from the server.
496 Note that not all protocols and flags may be available, depending on how
499 =item B<-dtls>, B<-dtls1>, B<-dtls1_2>
501 These options make B<s_client> use DTLS protocols instead of TLS.
502 With B<-dtls>, B<s_client> will negotiate any supported DTLS protocol version,
503 whilst B<-dtls1> and B<-dtls1_2> will only support DTLS1.0 and DTLS1.2
508 Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in
509 conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only
510 available where OpenSSL has support for SCTP enabled.
512 =item B<-sctp_label_bug>
514 Use the incorrect behaviour of older OpenSSL implementations when computing
515 endpoint-pair shared secrets for DTLS/SCTP. This allows communication with
516 older broken implementations but breaks interoperability with correct
517 implementations. Must be used in conjunction with B<-sctp>. This option is only
518 available where OpenSSL has support for SCTP enabled.
520 =item B<-fallback_scsv>
522 Send TLS_FALLBACK_SCSV in the ClientHello.
526 Switch on asynchronous mode. Cryptographic operations will be performed
527 asynchronously. This will only have an effect if an asynchronous capable engine
528 is also used via the B<-engine> option. For test purposes the dummy async engine
529 (dasync) can be used (if available).
531 =item B<-max_send_frag int>
533 The maximum size of data fragment to send.
534 See L<SSL_CTX_set_max_send_fragment(3)> for further information.
536 =item B<-split_send_frag int>
538 The size used to split data for encrypt pipelines. If more data is written in
539 one go than this value then it will be split into multiple pipelines, up to the
540 maximum number of pipelines defined by max_pipelines. This only has an effect if
541 a suitable cipher suite has been negotiated, an engine that supports pipelining
542 has been loaded, and max_pipelines is greater than 1. See
543 L<SSL_CTX_set_split_send_fragment(3)> for further information.
545 =item B<-max_pipelines int>
547 The maximum number of encrypt/decrypt pipelines to be used. This will only have
548 an effect if an engine has been loaded that supports pipelining (e.g. the dasync
549 engine) and a suitable cipher suite has been negotiated. The default value is 1.
550 See L<SSL_CTX_set_max_pipelines(3)> for further information.
552 =item B<-read_buf int>
554 The default read buffer size to be used for connections. This will only have an
555 effect if the buffer size is larger than the size that would otherwise be used
556 and pipelining is in use (see L<SSL_CTX_set_default_read_buffer_len(3)> for
557 further information).
561 There are several known bugs in SSL and TLS implementations. Adding this
562 option enables various workarounds.
566 Enables support for SSL/TLS compression.
567 This option was introduced in OpenSSL 1.1.0.
568 TLS compression is not recommended and is off by default as of
573 Disables support for SSL/TLS compression.
574 TLS compression is not recommended and is off by default as of
579 Only provide a brief summary of connection parameters instead of the
580 normal verbose output.
582 =item B<-sigalgs sigalglist>
584 Specifies the list of signature algorithms that are sent by the client.
585 The server selects one entry in the list based on its preferences.
586 For example strings, see L<SSL_CTX_set1_sigalgs(3)>
588 =item B<-curves curvelist>
590 Specifies the list of supported curves to be sent by the client. The curve is
591 ultimately selected by the server. For a list of all curves, use:
593 $ openssl ecparam -list_curves
595 =item B<-cipher cipherlist>
597 This allows the TLSv1.2 and below cipher list sent by the client to be modified.
598 This list will be combined with any TLSv1.3 ciphersuites that have been
599 configured. Although the server determines which ciphersuite is used it should
600 take the first supported cipher in the list sent by the client. See the
601 B<ciphers> command for more information.
603 =item B<-ciphersuites val>
605 This allows the TLSv1.3 ciphersuites sent by the client to be modified. This
606 list will be combined with any TLSv1.2 and below ciphersuites that have been
607 configured. Although the server determines which cipher suite is used it should
608 take the first supported cipher in the list sent by the client. See the
609 B<ciphers> command for more information. The format for this list is a simple
610 colon (":") separated list of TLSv1.3 ciphersuite names.
612 =item B<-starttls protocol>
614 Send the protocol-specific message(s) to switch to TLS for communication.
615 B<protocol> is a keyword for the intended protocol. Currently, the only
616 supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",
617 "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
619 =item B<-xmpphost hostname>
621 This option, when used with "-starttls xmpp" or "-starttls xmpp-server",
622 specifies the host for the "to" attribute of the stream element.
623 If this option is not specified, then the host specified with "-connect"
626 This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".
628 =item B<-name hostname>
630 This option is used to specify hostname information for various protocols
631 used with B<-starttls> option. Currently only "xmpp", "xmpp-server",
632 "smtp" and "lmtp" can utilize this B<-name> option.
634 If this option is used with "-starttls xmpp" or "-starttls xmpp-server",
635 if specifies the host for the "to" attribute of the stream element. If this
636 option is not specified, then the host specified with "-connect" will be used.
638 If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies
639 the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If
640 this option is not specified, then "mail.example.com" will be used.
642 =item B<-tlsextdebug>
644 Print out a hex dump of any TLS extensions received from the server.
648 Disable RFC4507bis session ticket support.
650 =item B<-sess_out filename>
652 Output SSL session to B<filename>.
654 =item B<-sess_in sess.pem>
656 Load SSL session from B<filename>. The client will attempt to resume a
657 connection from this session.
661 Specifying an engine (by its unique B<id> string) will cause B<s_client>
662 to attempt to obtain a functional reference to the specified engine,
663 thus initialising it if needed. The engine will then be set as the default
664 for all available algorithms.
666 =item B<-rand file...>
668 A file or files containing random data used to seed the random number
670 Multiple files can be specified separated by an OS-dependent character.
671 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
674 =item [B<-writerand file>]
676 Writes random data to the specified I<file> upon exit.
677 This can be used with a subsequent B<-rand> flag.
679 =item B<-serverinfo types>
681 A list of comma-separated TLS Extension Types (numbers between 0 and
682 65535). Each type will be sent as an empty ClientHello TLS Extension.
683 The server's response (if any) will be encoded and displayed as a PEM
688 Sends a certificate status request to the server (OCSP stapling). The server
689 response (if any) is printed out.
691 =item B<-alpn protocols>, B<-nextprotoneg protocols>
693 These flags enable the Enable the Application-Layer Protocol Negotiation
694 or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the
695 IETF standard and replaces NPN.
696 The B<protocols> list is a comma-separated list of protocol names that
697 the client should advertise support for. The list should contain the most
698 desirable protocols first. Protocol names are printable ASCII strings,
699 for example "http/1.1" or "spdy/3".
700 An empty list of protocols is treated specially and will cause the
701 client to advertise support for the TLS extension but disconnect just
702 after receiving ServerHello with a list of server supported protocols.
703 The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.
705 =item B<-ct>, B<-noct>
707 Use one of these two options to control whether Certificate Transparency (CT)
708 is enabled (B<-ct>) or disabled (B<-noct>).
709 If CT is enabled, signed certificate timestamps (SCTs) will be requested from
710 the server and reported at handshake completion.
712 Enabling CT also enables OCSP stapling, as this is one possible delivery method
717 A file containing a list of known Certificate Transparency logs. See
718 L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.
720 =item B<-keylogfile file>
722 Appends TLS secrets to the specified keylog file such that external programs
723 (like Wireshark) can decrypt TLS connections.
725 =item B<-early_data file>
727 Reads the contents of the specified file and attempts to send it as early data
728 to the server. This will only work with resumed sessions that support early
729 data and when the server accepts the early data.
733 For TLSv1.3 only, send the Post-Handshake Authentication extension. This will
734 happen whether or not a certificate has been provided via B<-cert>.
738 Rather than providing B<-connect>, the target hostname and optional port may
739 be provided as a single positional argument after all options. If neither this
740 nor B<-connect> are provided, falls back to attempting to connect to localhost
745 =head1 CONNECTED COMMANDS
747 If a connection is established with an SSL server then any data received
748 from the server is displayed and any key presses will be sent to the
749 server. If end of file is reached then the connection will be closed down. When
750 used interactively (which means neither B<-quiet> nor B<-ign_eof> have been
751 given), then certain commands are also recognized which perform special
752 operations. These commands are a letter which must appear at the start of a
753 line. They are listed below.
759 End the current SSL connection and exit.
763 Renegotiate the SSL session (TLSv1.2 and below only).
767 Send a key update message to the server (TLSv1.3 only)
771 Send a key update message to the server and request one back (TLSv1.3 only)
777 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
780 openssl s_client -connect servername:443
782 would typically be used (https uses port 443). If the connection succeeds
783 then an HTTP command can be given such as "GET /" to retrieve a web page.
785 If the handshake fails then there are several possible causes, if it is
786 nothing obvious like no client certificate then the B<-bugs>,
787 B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried
788 in case it is a buggy server. In particular you should play with these
789 options B<before> submitting a bug report to an OpenSSL mailing list.
791 A frequent problem when attempting to get client certificates working
792 is that a web client complains it has no certificates or gives an empty
793 list to choose from. This is normally because the server is not sending
794 the clients certificate authority in its "acceptable CA list" when it
795 requests a certificate. By using B<s_client> the CA list can be viewed
796 and checked. However some servers only request client authentication
797 after a specific URL is requested. To obtain the list in this case it
798 is necessary to use the B<-prexit> option and send an HTTP request
799 for an appropriate page.
801 If a certificate is specified on the command line using the B<-cert>
802 option it will not be used unless the server specifically requests
803 a client certificate. Therefor merely including a client certificate
804 on the command line is no guarantee that the certificate works.
806 If there are problems verifying a server certificate then the
807 B<-showcerts> option can be used to show all the certificates sent by the
810 The B<s_client> utility is a test tool and is designed to continue the
811 handshake after any certificate verification errors. As a result it will
812 accept any certificate chain (trusted or not) sent by the peer. None test
813 applications should B<not> do this as it makes them vulnerable to a MITM
814 attack. This behaviour can be changed by with the B<-verify_return_error>
815 option: any verify errors are then returned aborting the handshake.
817 The B<-bind> option may be useful if the server or a firewall requires
818 connections to come from some particular address and or port.
822 Because this program has a lot of options and also because some of the
823 techniques used are rather old, the C source of B<s_client> is rather hard to
824 read and not a model of how things should be done.
825 A typical SSL client program would be much simpler.
827 The B<-prexit> option is a bit of a hack. We should really report
828 information whenever a session is renegotiated.
833 L<openssl-sess_id(1)>,
834 L<openssl-s_server(1)>,
835 L<openssl-ciphers(1)>,
837 L<SSL_CTX_set_max_send_fragment(3)>,
838 L<SSL_CTX_set_split_send_fragment(3)>,
839 L<SSL_CTX_set_max_pipelines(3)>
843 The B<-no_alt_chains> option was added in OpenSSL 1.1.0.
844 The B<-name> option was added in OpenSSL 1.1.1.
848 Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.
850 Licensed under the Apache License 2.0 (the "License"). You may not use
851 this file except in compliance with the License. You can obtain a copy
852 in the file LICENSE in the source distribution or at
853 L<https://www.openssl.org/source/license.html>.