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老韩Linux SRE

hanyw@hanyw:~$ man openssl-s_client |grep -Ev "^$"OPENSSL-S_CLIENT(1SSL)                                                           OpenSSL                                                          OPENSSL-S_CLIENT(1SSL)NAME       openssl-s_client - SSL/TLS client programSYNOPSIS       openssl s_client [-help] [-ssl_config section] [-connect host:port] [-host hostname] [-port port] [-bind host:port] [-proxy host:port] [-proxy_user userid]       [-proxy_pass arg] [-unix path] [-4] [-6] [-servername name] [-noservername] [-verify depth] [-verify_return_error] [-verify_quiet] [-verifyCAfile filename]       [-verifyCApath dir] [-verifyCAstore uri] [-cert filename] [-certform DER|PEM|P12] [-cert_chain filename] [-build_chain] [-CRL filename] [-CRLform DER|PEM]       [-crl_download] [-key filename|uri] [-keyform DER|PEM|P12|ENGINE] [-pass arg] [-chainCAfile filename] [-chainCApath directory] [-chainCAstore uri]       [-requestCAfile filename] [-dane_tlsa_domain domain] [-dane_tlsa_rrdata rrdata] [-dane_ee_no_namechecks] [-reconnect] [-showcerts] [-prexit] [-debug] [-trace]       [-nocommands] [-security_debug] [-security_debug_verbose] [-msg] [-timeout] [-mtu size] [-no_etm] [-keymatexport label] [-keymatexportlen len] [-msgfile       filename] [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof] [-no_ign_eof] [-psk_identity identity] [-psk key] [-psk_session file] [-quiet] [-sctp]       [-sctp_label_bug] [-fallback_scsv] [-async] [-maxfraglen len] [-max_send_frag] [-split_send_frag] [-max_pipelines] [-read_buf] [-ignore_unexpected_eof] [-bugs]       [-comp] [-no_comp] [-brief] [-legacy_server_connect] [-allow_no_dhe_kex] [-sigalgs sigalglist] [-curves curvelist] [-cipher cipherlist] [-ciphersuites val]       [-serverpref] [-starttls protocol] [-name hostname] [-xmpphost hostname] [-name hostname] [-tlsextdebug] [-no_ticket] [-sess_out filename] [-serverinfo types]       [-sess_in filename] [-serverinfo types] [-status] [-alpn protocols] [-nextprotoneg protocols] [-ct] [-noct] [-ctlogfile] [-keylogfile file] [-early_data file]       [-enable_pha] [-use_srtp value] [-srpuser value] [-srppass value] [-srp_lateuser] [-srp_moregroups] [-srp_strength number] [-nameopt option] [-no_ssl3]       [-no_tls1] [-no_tls1_1] [-no_tls1_2] [-no_tls1_3] [-ssl3] [-tls1] [-tls1_1] [-tls1_2] [-tls1_3] [-dtls] [-dtls1] [-dtls1_2] [-xkey infile] [-xcert file] [-xchain       file] [-xchain_build file] [-xcertform DER|PEM]> [-xkeyform DER|PEM]> [-CAfile file] [-no-CAfile] [-CApath dir] [-no-CApath] [-CAstore uri] [-no-CAstore] [-bugs]       [-no_comp] [-comp] [-no_ticket] [-serverpref] [-client_renegotiation] [-legacy_renegotiation] [-no_renegotiation] [-no_resumption_on_reneg]       [-legacy_server_connect] [-no_legacy_server_connect] [-no_etm] [-allow_no_dhe_kex] [-prioritize_chacha] [-strict] [-sigalgs algs] [-client_sigalgs algs] [-groups       groups] [-curves curves] [-named_curve curve] [-cipher ciphers] [-ciphersuites 1.3ciphers] [-min_protocol minprot] [-max_protocol maxprot] [-record_padding       padding] [-debug_broken_protocol] [-no_middlebox] [-rand files] [-writerand file] [-provider name] [-provider-path path] [-propquery propq] [-engine id]       [-ssl_client_engine id] [-allow_proxy_certs] [-attime timestamp] [-no_check_time] [-check_ss_sig] [-crl_check] [-crl_check_all] [-explicit_policy]       [-extended_crl] [-ignore_critical] [-inhibit_any] [-inhibit_map] [-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] [-verify_depth num] [-verify_email email] [-verify_hostname       hostname] [-verify_ip ip] [-verify_name name] [-x509_strict] [-issuer_checks] [host:port]DESCRIPTION       This 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.OPTIONS       In addition to the options below, this command also supports the common and client only options documented in the "Supported Command Line Commands" section of       the SSL_CONF_cmd(3) manual page.       -help           Print out a usage message.       -ssl_config section           Use the specified section of the configuration file to configure the SSL_CTX object.       -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.       -host hostname           Host to connect to; use -connect instead.       -port port           Connect to the specified port; use -connect instead.       -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.       -proxy_user userid           When used with the -proxy flag, the program will attempt to authenticate with the specified proxy using basic (base64) authentication.  NB: Basic           authentication is insecure; the credentials are sent to the proxy in easily reversible base64 encoding before any TLS/SSL session is established.  Therefore,           these credentials are easily recovered by anyone able to sniff/trace the network. Use with caution.       -proxy_pass arg           The proxy password source, used with the -proxy_user flag.  For more information about the format of arg see openssl-passphrase-options(1).       -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 -servername is not provided, the TLS SNI extension will           be populated with the name given to -connect if it follows a DNS name format. If -connect is not provided either, the SNI is set to "localhost".  This is the           default since OpenSSL 1.1.1.           Even though SNI should normally be a DNS name and not an IP address, if -servername is provided then that name will be sent, regardless of whether it is a           DNS name or not.           This option cannot be used in conjunction with -noservername.       -noservername           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 filename           The client certificate to use, if one is requested by the server.  The default is not to use a certificate.           The chain for the client certificate may be specified using -cert_chain.       -certform DER|PEM|P12           The client certificate file format to use; unspecified by default.  See openssl-format-options(1) for details.       -cert_chain           A file or URI of untrusted certificates to use when attempting to build the certificate chain related to the certificate specified via the -cert option.  The           input can be in PEM, DER, or PKCS#12 format.       -build_chain           Specify whether the application should build the client certificate chain to be provided to the server.       -CRL filename           CRL file to use to check the server's certificate.       -CRLform DER|PEM           The CRL file format; unspecified by default.  See openssl-format-options(1) for details.       -crl_download           Download CRL from distribution points in the certificate.       -key filename|uri           The client private key to use.  If not specified then the certificate file will be used to read also the key.       -keyform DER|PEM|P12|ENGINE           The key format; unspecified by default.  See openssl-format-options(1) for details.       -pass arg           the private key and certifiate file password source.  For more information about the format of arg see openssl-passphrase-options(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.       -verify_return_error           Return verification errors instead of continuing. This will typically abort the handshake with a fatal error.       -verify_quiet           Limit verify output to only errors.       -verifyCAfile filename           A file in PEM format containing trusted certificates to use for verifying the server's certificate.       -verifyCApath dir           A directory containing trusted certificates to use for verifying the server's certificate.  This directory must be in "hash format", see openssl-verify(1)           for more information.       -verifyCAstore uri           The URI of a store containing trusted certificates to use for verifying the server's certificate.       -chainCAfile file           A file in PEM format containing trusted certificates to use when attempting to build the client certificate chain.       -chainCApath directory           A directory containing trusted certificates to use for building the client certificate chain provided to the server.  This directory must be in "hash           format", see openssl-verify(1) for more information.       -chainCAstore uri           The URI of a store containing trusted certificates to use when attempting to build the client certificate chain.  The URI may indicate a single certificate,           as well as a collection of them.  With URIs in the "file:" scheme, this acts as -chainCAfile or -chainCApath, depending on if the URI indicates a directory           or a single file.  See ossl_store-file(7) for more information on the "file:" scheme.       -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 specified 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 example:             $ 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             ...       -dane_ee_no_namechecks           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 servers.       -reconnect           Reconnects to the same server 5 times using the same session ID, this can be used as a test that session caching is working.       -showcerts           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.       -prexit           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.       -state           Prints out the SSL session states.       -debug           Print extensive debugging information including a hex dump of all traffic.       -nocommands           Do not use interactive command letters.       -security_debug           Enable security debug messages.       -security_debug_verbose           Output more security debug output.       -msg           Show protocol messages.       -timeout           Enable send/receive timeout on DTLS connections.       -mtu size           Set MTU of the link layer to the specified size.       -no_etm           Disable Encrypt-then-MAC negotiation.       -keymatexport label           Export keying material using the specified label.       -keymatexportlen len           Export the specified number of bytes of keying material; default is 20.           Show all protocol messages with hex dump.       -trace           Show verbose trace output of protocol messages.       -msgfile filename           File to send output of -msg or -trace to, default standard output.       -nbio_test           Tests nonblocking I/O       -nbio           Turns on nonblocking I/O       -crlf           This option translated a line feed from the terminal into CR+LF as required by some servers.       -ign_eof           Inhibit shutting down the connection when end of file is reached in the input.       -quiet           Inhibit printing of session and certificate information.  This implicitly turns on -ign_eof as well.       -no_ign_eof           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 cipher.       -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.       -sctp           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.       -sctp_label_bug           Use the incorrect behaviour of older OpenSSL implementations when computing endpoint-pair shared secrets for DTLS/SCTP. This allows communication with older           broken implementations but breaks interoperability with correct implementations. Must be used in conjunction with -sctp. This option is only available where           OpenSSL has support for SCTP enabled.       -fallback_scsv           Send TLS_FALLBACK_SCSV in the ClientHello.       -async           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 available).       -maxfraglen len           Enable Maximum Fragment Length Negotiation; allowed values are 512, 1024, 2048, and 4096.       -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).       -ignore_unexpected_eof           Some TLS implementations do not send the mandatory close_notify alert on shutdown. If the application tries to wait for the close_notify alert but the peer           closes the connection without sending it, an error is generated. When this option is enabled the peer does not need to send the close_notify alert and a           closed connection will be treated as if the close_notify alert was received.  For more information on shutting down a connection, see SSL_shutdown(3).       -bugs           There are several known bugs in SSL and TLS implementations. Adding this option enables various workarounds.       -comp           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.       -no_comp           Disables support for SSL/TLS compression.  TLS compression is not recommended and is off by default as of OpenSSL 1.1.0.       -brief           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           openssl-ciphers(1) 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           openssl-ciphers(1) 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-server".       -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.       -tlsextdebug           Print out a hex dump of any TLS extensions received from the server.       -no_ticket           Disable RFC4507bis session ticket support.       -sess_out filename           Output SSL session to filename.       -sess_in filename           Load SSL session from filename. The client will attempt to resume a connection from this session.       -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.       -status           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.       -ctlogfile           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.       -enable_pha           For TLSv1.3 only, send the Post-Handshake Authentication extension. This will happen whether or not a certificate has been provided via -cert.       -use_srtp value           Offer SRTP key management, where value is a colon-separated profile list.       -srpuser value           Set the SRP username to the specified value. This option is deprecated.       -srppass value           Set the SRP password to the specified value. This option is deprecated.       -srp_lateuser           SRP username for the second ClientHello message. This option is deprecated.       -srp_moregroups  This option is deprecated.           Tolerate other than the known g and N values.       -srp_strength number           Set the minimal acceptable length, in bits, for N.  This option is deprecated.       -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2, -no_tls1_3, -ssl3, -tls1, -tls1_1, -tls1_2, -tls1_3           See "TLS Version Options" in openssl(1).       -dtls, -dtls1, -dtls1_2           These specify the use of DTLS instead of TLS.  See "TLS Version Options" in openssl(1).       -nameopt option           This specifies how the subject or issuer names are displayed.  See openssl-namedisplay-options(1) for details.       -xkey infile, -xcert file, -xchain file, -xchain_build file, -xcertform DER|PEM, -xkeyform DER|PEM           Set extended certificate verification options.  See "Extended Verification Options" in openssl-verification-options(1) for details.       -CAfile file, -no-CAfile, -CApath dir, -no-CApath, -CAstore uri, -no-CAstore           See "Trusted Certificate Options" in openssl-verification-options(1) for details.       -bugs, -comp, -no_comp, -no_ticket, -serverpref, -client_renegotiation, -legacy_renegotiation, -no_renegotiation, -no_resumption_on_reneg,       -legacy_server_connect, -no_legacy_server_connect, -no_etm -allow_no_dhe_kex, -prioritize_chacha, -strict, -sigalgs algs, -client_sigalgs algs, -groups groups,       -curves curves, -named_curve curve, -cipher ciphers, -ciphersuites 1.3ciphers, -min_protocol minprot, -max_protocol maxprot, -record_padding padding,       -debug_broken_protocol, -no_middlebox           See "SUPPORTED COMMAND LINE COMMANDS" in SSL_CONF_cmd(3) for details.       -rand files, -writerand file           See "Random State Options" in openssl(1) for details.       -provider name       -provider-path path       -propquery propq           See "Provider Options" in openssl(1), provider(7), and property(7).       -engine id           See "Engine Options" in openssl(1).  This option is deprecated.       -ssl_client_engine id           Specify engine to be used for client certificate operations.       -allow_proxy_certs, -attime, -no_check_time, -check_ss_sig, -crl_check, -crl_check_all, -explicit_policy, -extended_crl, -ignore_critical, -inhibit_any,       -inhibit_map, -no_alt_chains, -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 -issuer_checks           Set various options of certificate chain verification.  See "Verification Options" in openssl-verification-options(1) for details.           Verification errors are displayed, for debugging, but the command will proceed unless the -verify_return_error option is used.       host:port           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.CONNECTED COMMANDS       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).       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)NOTES       This command 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       this command, 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.       Therefore, 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 server.       This command 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. Non-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.BUGS       Because this program has a lot of options and also because some of the techniques used are rather old, the C source for this command 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.SEE ALSO       openssl(1), openssl-sess_id(1), openssl-s_server(1), openssl-ciphers(1), SSL_CONF_cmd(3), SSL_CTX_set_max_send_fragment(3), SSL_CTX_set_split_send_fragment(3),       SSL_CTX_set_max_pipelines(3), ossl_store-file(7)HISTORY       The -no_alt_chains option was added in OpenSSL 1.1.0.  The -name option was added in OpenSSL 1.1.1.       The -certform option has become obsolete in OpenSSL 3.0.0 and has no effect.       The -engine option was deprecated in OpenSSL 3.0.COPYRIGHT       Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.       Licensed under the Apache License 2.0 (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 <;.3.0.2                                                                          2022-07-04                                                         OPENSSL-S_CLIENT(1SSL)