openssl s_client -help
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This implements a generic SSL/TLS client which can establish a transparent connection to a remote server speaking SSL/TLS. It's intended for testing purposes only and provides only rudimentary interface functionality but internally uses mostly all the functionality of the OpenSSL ssl library.
$ openssl s_client -help Usage: s_client [options] Valid options are: -help Display this summary -host val Use -connect instead -port +int Use -connect instead -connect val TCP/IP where to connect (default is :4433) -bind val bind local address for connection -proxy val Connect to via specified proxy to the real server -unix val Connect over the specified Unix-domain socket -4 Use IPv4 only -6 Use IPv6 only -verify +int Turn on peer certificate verification -cert infile Certificate file to use, PEM format assumed -certform PEM|DER Certificate format (PEM or DER) PEM default -nameopt val Various certificate name options -key val Private key file to use, if not in -cert file -keyform PEM|DER|ENGINE Key format (PEM, DER or engine) PEM default -pass val Private key file pass phrase source -CApath dir PEM format directory of CA's -CAfile infile PEM format file of CA's -no-CAfile Do not load the default certificates file -no-CApath Do not load certificates from the default certificates directory -requestCAfile infile PEM format file of CA names to send to the server -dane_tlsa_domain val DANE TLSA base domain -dane_tlsa_rrdata val DANE TLSA rrdata presentation form -dane_ee_no_namechecks Disable name checks when matching DANE-EE(3) TLSA records -reconnect Drop and re-make the connection with the same Session-ID -showcerts Show all certificates sent by the server -debug Extra output -msg Show protocol messages -msgfile outfile File to send output of -msg or -trace, instead of stdout -nbio_test More ssl protocol testing -state Print the ssl states -crlf Convert LF from terminal into CRLF -quiet No s_client output -ign_eof Ignore input eof (default when -quiet) -no_ign_eof Don't ignore input eof -starttls val Use the appropriate STARTTLS command before starting TLS -xmpphost val Alias of -name option for "-starttls xmpp[-server]" -rand val Load the file(s) into the random number generator -writerand outfile Write random data to the specified file -sess_out outfile File to write SSL session to -sess_in infile File to read SSL session from -use_srtp val Offer SRTP key management with a colon-separated profile list -keymatexport val Export keying material using label -keymatexportlen +int Export len bytes of keying material (default 20) -maxfraglen +int Enable Maximum Fragment Length Negotiation (len values: 512, 1024, 2048 and 4096) -fallback_scsv Send the fallback SCSV -name val Hostname to use for "-starttls lmtp", "-starttls smtp" or "-starttls xmpp[-server]" -CRL infile CRL file to use -crl_download Download CRL from distribution points -CRLform PEM|DER CRL format (PEM or DER) PEM is default -verify_return_error Close connection on verification error -verify_quiet Restrict verify output to errors -brief Restrict output to brief summary of connection parameters -prexit Print session information when the program exits -security_debug Enable security debug messages -security_debug_verbose Output more security debug output -cert_chain infile Certificate chain file (in PEM format) -chainCApath dir Use dir as certificate store path to build CA certificate chain -verifyCApath dir Use dir as certificate store path to verify CA certificate -build_chain Build certificate chain -chainCAfile infile CA file for certificate chain (PEM format) -verifyCAfile infile CA file for certificate verification (PEM format) -nocommands Do not use interactive command letters -servername val Set TLS extension servername (SNI) in ClientHello (default) -noservername Do not send the server name (SNI) extension in the ClientHello -tlsextdebug Hex dump of all TLS extensions received -status Request certificate status from server -serverinfo val types Send empty ClientHello extensions (comma-separated numbers) -alpn val Enable ALPN extension, considering named protocols supported (comma-separated list) -async Support asynchronous operation -ssl_config val Use specified configuration file -max_send_frag +int Maximum Size of send frames -split_send_frag +int Size used to split data for encrypt pipelines -max_pipelines +int Maximum number of encrypt/decrypt pipelines to be used -read_buf +int Default read buffer size to be used for connections -no_ssl3 Just disable SSLv3 -no_tls1 Just disable TLSv1 -no_tls1_1 Just disable TLSv1.1 -no_tls1_2 Just disable TLSv1.2 -no_tls1_3 Just disable TLSv1.3 -bugs Turn on SSL bug compatibility -no_comp Disable SSL/TLS compression (default) -comp Use SSL/TLS-level compression -no_ticket Disable use of TLS session tickets -serverpref Use server's cipher preferences -legacy_renegotiation Enable use of legacy renegotiation (dangerous) -no_renegotiation Disable all renegotiation. -legacy_server_connect Allow initial connection to servers that don't support RI -no_resumption_on_reneg Disallow session resumption on renegotiation -no_legacy_server_connect Disallow initial connection to servers that don't support RI -allow_no_dhe_kex In TLSv1.3 allow non-(ec)dhe based key exchange on resumption -prioritize_chacha Prioritize ChaCha ciphers when preferred by clients -strict Enforce strict certificate checks as per TLS standard -sigalgs val Signature algorithms to support (colon-separated list) -client_sigalgs val Signature algorithms to support for client certificate authentication (colon-separated list) -groups val Groups to advertise (colon-separated list) -curves val Groups to advertise (colon-separated list) -named_curve val Elliptic curve used for ECDHE (server-side only) -cipher val Specify TLSv1.2 and below cipher list to be used -ciphersuites val Specify TLSv1.3 ciphersuites to be used -min_protocol val Specify the minimum protocol version to be used -max_protocol val Specify the maximum protocol version to be used -record_padding val Block size to pad TLS 1.3 records to. -debug_broken_protocol Perform all sorts of protocol violations for testing purposes -no_middlebox Disable TLSv1.3 middlebox compat mode -policy val adds policy to the acceptable policy set -purpose val certificate chain purpose -verify_name val verification policy name -verify_depth int chain depth limit -auth_level int chain authentication security level -attime intmax verification epoch time -verify_hostname val expected peer hostname -verify_email val expected peer email -verify_ip val expected peer IP address -ignore_critical permit unhandled critical extensions -issuer_checks (deprecated) -crl_check check leaf certificate revocation -crl_check_all check full chain revocation -policy_check perform rfc5280 policy checks -explicit_policy set policy variable require-explicit-policy -inhibit_any set policy variable inhibit-any-policy -inhibit_map set policy variable inhibit-policy-mapping -x509_strict disable certificate compatibility work-arounds -extended_crl enable extended CRL features -use_deltas use delta CRLs -policy_print print policy processing diagnostics -check_ss_sig check root CA self-signatures -trusted_first search trust store first (default) -suiteB_128_only Suite B 128-bit-only mode -suiteB_128 Suite B 128-bit mode allowing 192-bit algorithms -suiteB_192 Suite B 192-bit-only mode -partial_chain accept chains anchored by intermediate trust-store CAs -no_alt_chains (deprecated) -no_check_time ignore certificate validity time -allow_proxy_certs allow the use of proxy certificates -xkey infile key for Extended certificates -xcert infile cert for Extended certificates -xchain infile chain for Extended certificates -xchain_build build certificate chain for the extended certificates -xcertform PEM|DER format of Extended certificate (PEM or DER) PEM default -xkeyform PEM|DER format of Extended certificate's key (PEM or DER) PEM default -tls1 Just use TLSv1 -tls1_1 Just use TLSv1.1 -tls1_2 Just use TLSv1.2 -tls1_3 Just use TLSv1.3 -dtls Use any version of DTLS -timeout Enable send/receive timeout on DTLS connections -mtu +int Set the link layer MTU -dtls1 Just use DTLSv1 -dtls1_2 Just use DTLSv1.2 -nbio Use non-blocking IO -psk_identity val PSK identity -psk val PSK in hex (without 0x) -psk_session infile File to read PSK SSL session from -srpuser val SRP authentication for 'user' -srppass val Password for 'user' -srp_lateuser SRP username into second ClientHello message -srp_moregroups Tolerate other than the known g N values. -srp_strength +int Minimal length in bits for N -nextprotoneg val Enable NPN extension, considering named protocols supported (comma-separated list) -engine val Use engine, possibly a hardware device -ssl_client_engine val Specify engine to be used for client certificate operations -ct Request and parse SCTs (also enables OCSP stapling) -noct Do not request or parse SCTs (default) -ctlogfile infile CT log list CONF file -keylogfile outfile Write TLS secrets to file -early_data infile File to send as early data -enable_pha Enable post-handshake-authentication
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