S_CLIENT(1SSL)                      OpenSSL                     S_CLIENT(1SSL)

       openssl-s_client, s_client - SSL/TLS client program

       openssl s_client [-help] [-connect host:port] [-bind host:port] [-proxy
       host:port] [-unix path] [-4] [-6] [-servername name] [-noservername]
       [-verify depth] [-verify_return_error] [-cert filename] [-certform
       DER|PEM] [-key filename] [-keyform DER|PEM] [-cert_chain filename]
       [-build_chain] [-xkey] [-xcert] [-xchain] [-xchain_build] [-xcertform
       PEM|DER] [-xkeyform PEM|DER] [-pass arg] [-CApath directory] [-CAfile
       filename] [-chainCApath directory] [-chainCAfile filename] [-no-CAfile]
       [-no-CApath] [-requestCAfile filename] [-dane_tlsa_domain domain]
       [-dane_tlsa_rrdata rrdata] [-dane_ee_no_namechecks] [-attime timestamp]
       [-check_ss_sig] [-crl_check] [-crl_check_all] [-explicit_policy]
       [-extended_crl] [-ignore_critical] [-inhibit_any] [-inhibit_map]
       [-no_check_time] [-partial_chain] [-policy arg] [-policy_check]
       [-policy_print] [-purpose purpose] [-suiteB_128] [-suiteB_128_only]
       [-suiteB_192] [-trusted_first] [-no_alt_chains] [-use_deltas]
       [-auth_level num] [-nameopt option] [-verify_depth num] [-verify_email
       email] [-verify_hostname hostname] [-verify_ip ip] [-verify_name name]
       [-build_chain] [-x509_strict] [-reconnect] [-showcerts] [-debug] [-msg]
       [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof] [-no_ign_eof]
       [-psk_identity identity] [-psk key] [-psk_session file] [-quiet]
       [-ssl3] [-tls1] [-tls1_1] [-tls1_2] [-tls1_3] [-no_ssl3] [-no_tls1]
       [-no_tls1_1] [-no_tls1_2] [-no_tls1_3] [-dtls] [-dtls1] [-dtls1_2]
       [-sctp] [-fallback_scsv] [-async] [-max_send_frag] [-split_send_frag]
       [-max_pipelines] [-read_buf] [-bugs] [-comp] [-no_comp]
       [-allow_no_dhe_kex] [-sigalgs sigalglist] [-curves curvelist] [-cipher
       cipherlist] [-ciphersuites val] [-serverpref] [-starttls protocol]
       [-xmpphost hostname] [-name hostname] [-engine id] [-tlsextdebug]
       [-no_ticket] [-sess_out filename] [-sess_in filename] [-rand file...]
       [-writerand file] [-serverinfo types] [-status] [-alpn protocols]
       [-nextprotoneg protocols] [-ct] [-noct] [-ctlogfile] [-keylogfile file]
       [-early_data file] [-enable_pha] [target]

       The s_client command implements a generic SSL/TLS client which connects
       to a remote host using SSL/TLS. It is a very useful diagnostic tool for
       SSL servers.

       In addition to the options below the s_client utility also supports the
       common and client only options documented in the in the "Supported
       Command Line Commands" section of the SSL_CONF_cmd(3) manual page.

           Print out a usage message.

       -connect host:port
           This specifies the host and optional port to connect to. It is
           possible to select the host and port using the optional target
           positional argument instead.  If neither this nor the target
           positional argument are specified then an attempt is made to
           connect to the local host on port 4433.

       -bind host:port]
           This specifies the host address and or port to bind as the source
           for the connection.  For Unix-domain sockets the port is ignored
           and the host is used as the source socket address.

       -proxy host:port
           When used with the -connect flag, the program uses the host and
           port specified with this flag and issues an HTTP CONNECT command to
           connect to the desired server.

       -unix path
           Connect over the specified Unix-domain socket.

       -4  Use IPv4 only.

       -6  Use IPv6 only.

       -servername name
           Set the TLS SNI (Server Name Indication) extension in the
           ClientHello message to the given value. If both this option and the
           -noservername are not given, the TLS SNI extension is still set to
           the hostname provided to the -connect option, or "localhost" if
           -connect has not been supplied. This is default since OpenSSL

           Even though SNI name should normally be a DNS name and not an IP
           address, this option will not make the distinction when parsing
           -connect and will send IP address if one passed.

           Suppresses sending of the SNI (Server Name Indication) extension in
           the ClientHello message. Cannot be used in conjunction with the
           -servername or <-dane_tlsa_domain> options.

       -cert certname
           The certificate to use, if one is requested by the server. The
           default is not to use a certificate.

       -certform format
           The certificate format to use: DER or PEM. PEM is the default.

       -key keyfile
           The private key to use. If not specified then the certificate file
           will be used.

       -keyform format
           The private format to use: DER or PEM. PEM is the default.

           A file containing trusted certificates to use when attempting to
           build the client/server certificate chain related to the
           certificate specified via the -cert option.

           Specify whether the application should build the certificate chain
           to be provided to the server.

       -xkey infile, -xcert infile, -xchain
           Specify an extra certificate, private key and certificate chain.
           These behave in the same manner as the -cert, -key and -cert_chain
           options.  When specified, the callback returning the first valid
           chain will be in use by the client.

           Specify whether the application should build the certificate chain
           to be provided to the server for the extra certificates provided
           via -xkey infile, -xcert infile, -xchain options.

       -xcertform PEM|DER, -xkeyform PEM|DER
           Extra certificate and private key format respectively.

       -pass arg
           the private key password source. For more information about the
           format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).

       -verify depth
           The verify depth to use. This specifies the maximum length of the
           server certificate chain and turns on server certificate
           verification.  Currently the verify operation continues after
           errors so all the problems with a certificate chain can be seen. As
           a side effect the connection will never fail due to a server
           certificate verify failure.

           Return verification errors instead of continuing. This will
           typically abort the handshake with a fatal error.

       -nameopt option
           Option which determines how the subject or issuer names are
           displayed. The option argument can be a single option or multiple
           options separated by commas.  Alternatively the -nameopt switch may
           be used more than once to set multiple options. See the x509(1)
           manual page for details.

       -CApath directory
           The directory to use for server certificate verification. This
           directory must be in "hash format", see verify(1) for more
           information. These are also used when building the client
           certificate chain.

       -CAfile file
           A file containing trusted certificates to use during server
           authentication and to use when attempting to build the client
           certificate chain.

       -chainCApath directory
           The directory to use for building the chain provided to the server.
           This directory must be in "hash format", see verify(1) for more

       -chainCAfile file
           A file containing trusted certificates to use when attempting to
           build the client certificate chain.

           Do not load the trusted CA certificates from the default file

           Do not load the trusted CA certificates from the default directory

       -requestCAfile file
           A file containing a list of certificates whose subject names will
           be sent to the server in the certificate_authorities extension.
           Only supported for TLS 1.3

       -dane_tlsa_domain domain
           Enable RFC6698/RFC7671 DANE TLSA authentication and specify the
           TLSA base domain which becomes the default SNI hint and the primary
           reference identifier for hostname checks.  This must be used in
           combination with at least one instance of the -dane_tlsa_rrdata
           option below.

           When DANE authentication succeeds, the diagnostic output will
           include the lowest (closest to 0) depth at which a TLSA record
           authenticated a chain certificate.  When that TLSA record is a "2 1
           0" trust anchor public key that signed (rather than matched) the
           top-most certificate of the chain, the result is reported as "TA
           public key verified".  Otherwise, either the TLSA record "matched
           TA certificate" at a positive depth or else "matched EE
           certificate" at depth 0.

       -dane_tlsa_rrdata rrdata
           Use one or more times to specify the RRDATA fields of the DANE TLSA
           RRset associated with the target service.  The rrdata value is
           specied in "presentation form", that is four whitespace separated
           fields that specify the usage, selector, matching type and
           associated data, with the last of these encoded in hexadecimal.
           Optional whitespace is ignored in the associated data field.  For

             $ openssl s_client -brief -starttls smtp \
               -connect smtp.example.com:25 \
               -dane_tlsa_domain smtp.example.com \
               -dane_tlsa_rrdata "2 1 1
                 B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
               -dane_tlsa_rrdata "2 1 1
                 60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
             Verification: OK
             Verified peername: smtp.example.com
             DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1

           This disables server name checks when authenticating via DANE-EE(3)
           TLSA records.  For some applications, primarily web browsers, it is
           not safe to disable name checks due to "unknown key share" attacks,
           in which a malicious server can convince a client that a connection
           to a victim server is instead a secure connection to the malicious
           server.  The malicious server may then be able to violate cross-
           origin scripting restrictions.  Thus, despite the text of RFC7671,
           name checks are by default enabled for DANE-EE(3) TLSA records, and
           can be disabled in applications where it is safe to do so.  In
           particular, SMTP and XMPP clients should set this option as SRV and
           MX records already make it possible for a remote domain to redirect
           client connections to any server of its choice, and in any case
           SMTP and XMPP clients do not execute scripts downloaded from remote

       -attime, -check_ss_sig, -crl_check, -crl_check_all, -explicit_policy,
       -extended_crl, -ignore_critical, -inhibit_any, -inhibit_map,
       -no_alt_chains, -no_check_time, -partial_chain, -policy, -policy_check,
       -policy_print, -purpose, -suiteB_128, -suiteB_128_only, -suiteB_192,
       -trusted_first, -use_deltas, -auth_level, -verify_depth, -verify_email,
       -verify_hostname, -verify_ip, -verify_name, -x509_strict
           Set various certificate chain validation options. See the verify(1)
           manual page for details.

           Reconnects to the same server 5 times using the same session ID,
           this can be used as a test that session caching is working.

           Displays the server certificate list as sent by the server: it only
           consists of certificates the server has sent (in the order the
           server has sent them). It is not a verified chain.

           Print session information when the program exits. This will always
           attempt to print out information even if the connection fails.
           Normally information will only be printed out once if the
           connection succeeds. This option is useful because the cipher in
           use may be renegotiated or the connection may fail because a client
           certificate is required or is requested only after an attempt is
           made to access a certain URL. Note: the output produced by this
           option is not always accurate because a connection might never have
           been established.

           Prints out the SSL session states.

           Print extensive debugging information including a hex dump of all

           Show all protocol messages with hex dump.

           Show verbose trace output of protocol messages. OpenSSL needs to be
           compiled with enable-ssl-trace for this option to work.

           File to send output of -msg or -trace to, default standard output.

           Tests non-blocking I/O

           Turns on non-blocking I/O

           This option translated a line feed from the terminal into CR+LF as
           required by some servers.

           Inhibit shutting down the connection when end of file is reached in
           the input.

           Inhibit printing of session and certificate information.  This
           implicitly turns on -ign_eof as well.

           Shut down the connection when end of file is reached in the input.
           Can be used to override the implicit -ign_eof after -quiet.

       -psk_identity identity
           Use the PSK identity identity when using a PSK cipher suite.  The
           default value is "Client_identity" (without the quotes).

       -psk key
           Use the PSK key key when using a PSK cipher suite. The key is given
           as a hexadecimal number without leading 0x, for example -psk
           1a2b3c4d.  This option must be provided in order to use a PSK

       -psk_session file
           Use the pem encoded SSL_SESSION data stored in file as the basis of
           a PSK.  Note that this will only work if TLSv1.3 is negotiated.

       -ssl3, -tls1, -tls1_1, -tls1_2, -tls1_3, -no_ssl3, -no_tls1,
       -no_tls1_1, -no_tls1_2, -no_tls1_3
           These options require or disable the use of the specified SSL or
           TLS protocols.  By default s_client will negotiate the highest
           mutually supported protocol version.  When a specific TLS version
           is required, only that version will be offered to and accepted from
           the server.  Note that not all protocols and flags may be
           available, depending on how OpenSSL was built.

       -dtls, -dtls1, -dtls1_2
           These options make s_client use DTLS protocols instead of TLS.
           With -dtls, s_client will negotiate any supported DTLS protocol
           version, whilst -dtls1 and -dtls1_2 will only support DTLS1.0 and
           DTLS1.2 respectively.

           Use SCTP for the transport protocol instead of UDP in DTLS. Must be
           used in conjunction with -dtls, -dtls1 or -dtls1_2. This option is
           only available where OpenSSL has support for SCTP enabled.

           Send TLS_FALLBACK_SCSV in the ClientHello.

           Switch on asynchronous mode. Cryptographic operations will be
           performed asynchronously. This will only have an effect if an
           asynchronous capable engine is also used via the -engine option.
           For test purposes the dummy async engine (dasync) can be used (if

       -max_send_frag int
           The maximum size of data fragment to send.  See
           SSL_CTX_set_max_send_fragment(3) for further information.

       -split_send_frag int
           The size used to split data for encrypt pipelines. If more data is
           written in one go than this value then it will be split into
           multiple pipelines, up to the maximum number of pipelines defined
           by max_pipelines. This only has an effect if a suitable cipher
           suite has been negotiated, an engine that supports pipelining has
           been loaded, and max_pipelines is greater than 1. See
           SSL_CTX_set_split_send_fragment(3) for further information.

       -max_pipelines int
           The maximum number of encrypt/decrypt pipelines to be used. This
           will only have an effect if an engine has been loaded that supports
           pipelining (e.g. the dasync engine) and a suitable cipher suite has
           been negotiated. The default value is 1.  See
           SSL_CTX_set_max_pipelines(3) for further information.

       -read_buf int
           The default read buffer size to be used for connections. This will
           only have an effect if the buffer size is larger than the size that
           would otherwise be used and pipelining is in use (see
           SSL_CTX_set_default_read_buffer_len(3) for further information).

           There are several known bug in SSL and TLS implementations. Adding
           this option enables various workarounds.

           Enables support for SSL/TLS compression.  This option was
           introduced in OpenSSL 1.1.0.  TLS compression is not recommended
           and is off by default as of OpenSSL 1.1.0.

           Disables support for SSL/TLS compression.  TLS compression is not
           recommended and is off by default as of OpenSSL 1.1.0.

           Only provide a brief summary of connection parameters instead of
           the normal verbose output.

       -sigalgs sigalglist
           Specifies the list of signature algorithms that are sent by the
           client.  The server selects one entry in the list based on its
           preferences.  For example strings, see SSL_CTX_set1_sigalgs(3)

       -curves curvelist
           Specifies the list of supported curves to be sent by the client.
           The curve is ultimately selected by the server. For a list of all
           curves, use:

               $ openssl ecparam -list_curves

       -cipher cipherlist
           This allows the TLSv1.2 and below cipher list sent by the client to
           be modified.  This list will be combined with any TLSv1.3
           ciphersuites that have been configured. Although the server
           determines which ciphersuite is used it should take the first
           supported cipher in the list sent by the client. See the ciphers
           command for more information.

       -ciphersuites val
           This allows the TLSv1.3 ciphersuites sent by the client to be
           modified. This list will be combined with any TLSv1.2 and below
           ciphersuites that have been configured. Although the server
           determines which cipher suite is used it should take the first
           supported cipher in the list sent by the client. See the ciphers
           command for more information. The format for this list is a simple
           colon (":") separated list of TLSv1.3 ciphersuite names.

       -starttls protocol
           Send the protocol-specific message(s) to switch to TLS for
           communication.  protocol is a keyword for the intended protocol.
           Currently, the only supported keywords are "smtp", "pop3", "imap",
           "ftp", "xmpp", "xmpp-server", "irc", "postgres", "mysql", "lmtp",
           "nntp", "sieve" and "ldap".

       -xmpphost hostname
           This option, when used with "-starttls xmpp" or "-starttls xmpp-
           server", specifies the host for the "to" attribute of the stream
           element.  If this option is not specified, then the host specified
           with "-connect" will be used.

           This option is an alias of the -name option for "xmpp" and "xmpp-

       -name hostname
           This option is used to specify hostname information for various
           protocols used with -starttls option. Currently only "xmpp", "xmpp-
           server", "smtp" and "lmtp" can utilize this -name option.

           If this option is used with "-starttls xmpp" or "-starttls xmpp-
           server", if specifies the host for the "to" attribute of the stream
           element. If this option is not specified, then the host specified
           with "-connect" will be used.

           If this option is used with "-starttls lmtp" or "-starttls smtp",
           it specifies the name to use in the "LMTP LHLO" or "SMTP EHLO"
           message, respectively. If this option is not specified, then
           "mail.example.com" will be used.

           Print out a hex dump of any TLS extensions received from the

           Disable RFC4507bis session ticket support.

       -sess_out filename
           Output SSL session to filename.

       -sess_in sess.pem
           Load SSL session from filename. The client will attempt to resume a
           connection from this session.

       -engine id
           Specifying an engine (by its unique id string) will cause s_client
           to attempt to obtain a functional reference to the specified
           engine, thus initialising it if needed. The engine will then be set
           as the default for all available algorithms.

       -rand file...
           A file or files containing random data used to seed the random
           number generator.  Multiple files can be specified separated by an
           OS-dependent character.  The separator is ; for MS-Windows, , for
           OpenVMS, and : for all others.

       [-writerand file]
           Writes random data to the specified file upon exit.  This can be
           used with a subsequent -rand flag.

       -serverinfo types
           A list of comma-separated TLS Extension Types (numbers between 0
           and 65535).  Each type will be sent as an empty ClientHello TLS
           Extension.  The server's response (if any) will be encoded and
           displayed as a PEM file.

           Sends a certificate status request to the server (OCSP stapling).
           The server response (if any) is printed out.

       -alpn protocols, -nextprotoneg protocols
           These flags enable the Enable the Application-Layer Protocol
           Negotiation or Next Protocol Negotiation (NPN) extension,
           respectively. ALPN is the IETF standard and replaces NPN.  The
           protocols list is a comma-separated list of protocol names that the
           client should advertise support for. The list should contain the
           most desirable protocols first.  Protocol names are printable ASCII
           strings, for example "http/1.1" or "spdy/3".  An empty list of
           protocols is treated specially and will cause the client to
           advertise support for the TLS extension but disconnect just after
           receiving ServerHello with a list of server supported protocols.
           The flag -nextprotoneg cannot be specified if -tls1_3 is used.

       -ct, -noct
           Use one of these two options to control whether Certificate
           Transparency (CT) is enabled (-ct) or disabled (-noct).  If CT is
           enabled, signed certificate timestamps (SCTs) will be requested
           from the server and reported at handshake completion.

           Enabling CT also enables OCSP stapling, as this is one possible
           delivery method for SCTs.

           A file containing a list of known Certificate Transparency logs.
           See SSL_CTX_set_ctlog_list_file(3) for the expected file format.

       -keylogfile file
           Appends TLS secrets to the specified keylog file such that external
           programs (like Wireshark) can decrypt TLS connections.

       -early_data file
           Reads the contents of the specified file and attempts to send it as
           early data to the server. This will only work with resumed sessions
           that support early data and when the server accepts the early data.

           For TLSv1.3 only, send the Post-Handshake Authentication extension.
           This will happen whether or not a certificate has been provided via

           Rather than providing -connect, the target hostname and optional
           port may be provided as a single positional argument after all
           options. If neither this nor -connect are provided, falls back to
           attempting to connect to localhost on port 4433.

       If a connection is established with an SSL server then any data
       received from the server is displayed and any key presses will be sent
       to the server. If end of file is reached then the connection will be
       closed down. When used interactively (which means neither -quiet nor
       -ign_eof have been given), then certain commands are also recognized
       which perform special operations. These commands are a letter which
       must appear at the start of a line. They are listed below.

       Q   End the current SSL connection and exit.

       R   Renegotiate the SSL session (TLSv1.2 and below only).

       B   Send a heartbeat message to the server (DTLS only)

       k   Send a key update message to the server (TLSv1.3 only)

       K   Send a key update message to the server and request one back
           (TLSv1.3 only)

       s_client can be used to debug SSL servers. To connect to an SSL HTTP
       server the command:

        openssl s_client -connect servername:443

       would typically be used (https uses port 443). If the connection
       succeeds then an HTTP command can be given such as "GET /" to retrieve
       a web page.

       If the handshake fails then there are several possible causes, if it is
       nothing obvious like no client certificate then the -bugs, -ssl3,
       -tls1, -no_ssl3, -no_tls1 options can be tried in case it is a buggy
       server. In particular you should play with these options before
       submitting a bug report to an OpenSSL mailing list.

       A frequent problem when attempting to get client certificates working
       is that a web client complains it has no certificates or gives an empty
       list to choose from. This is normally because the server is not sending
       the clients certificate authority in its "acceptable CA list" when it
       requests a certificate. By using s_client the CA list can be viewed and
       checked. However some servers only request client authentication after
       a specific URL is requested. To obtain the list in this case it is
       necessary to use the -prexit option and send an HTTP request for an
       appropriate page.

       If a certificate is specified on the command line using the -cert
       option it will not be used unless the server specifically requests a
       client certificate. Therefor merely including a client certificate on
       the command line is no guarantee that the certificate works.

       If there are problems verifying a server certificate then the
       -showcerts option can be used to show all the certificates sent by the

       The s_client utility is a test tool and is designed to continue the
       handshake after any certificate verification errors. As a result it
       will accept any certificate chain (trusted or not) sent by the peer.
       None test applications should not do this as it makes them vulnerable
       to a MITM attack. This behaviour can be changed by with the
       -verify_return_error option: any verify errors are then returned
       aborting the handshake.

       The -bind option may be useful if the server or a firewall requires
       connections to come from some particular address and or port.

       Because this program has a lot of options and also because some of the
       techniques used are rather old, the C source of s_client is rather hard
       to read and not a model of how things should be done.  A typical SSL
       client program would be much simpler.

       The -prexit option is a bit of a hack. We should really report
       information whenever a session is renegotiated.

       SSL_CONF_cmd(3), sess_id(1), s_server(1), ciphers(1),
       SSL_CTX_set_max_send_fragment(3), SSL_CTX_set_split_send_fragment(3),

       The -no_alt_chains option was first added to OpenSSL 1.1.0.  The -name
       option was added in OpenSSL 1.1.1.

       Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1                             2023-02-06                    S_CLIENT(1SSL)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2023 Hurricane Electric. All Rights Reserved.