OPENSSL-S_CLIENT(1ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 NAME 💅

openssl-s_client — the SSL/TLS client program. This is the OPENSSL-S_CLIENT(1ossl) manual page, bestie: same facts, way more main-character energy. 💖

✨ SYNOPSIS ✨

okay so here’s the whole invocation shape, no cap:

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] [-quic]
[-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]
[-no-interactive] [-debug] [-trace] [-nocommands] [-adv]
[-security_debug] [-security_debug_verbose] [-msg] [-timeout] [-mtu
size] [-no_ems] [-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] [-no_tx_cert_comp] [-no_rx_cert_comp] [-brief]
[-starttls protocol] [-xmpphost hostname] [-name hostname]
[-tlsextdebug] [-sess_out filename] [-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] [-ktls] [-tfo] [-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]
[-prefer_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] [-provparam [name:]key=value]
[-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] [-enable_server_rpk]
[-enable_client_rpk] [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 — like a stethoscope for TLS vibes, fr fr. 🩺✨

🎀 OPTIONS 🎀

Heads up, bestie: 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. If the host string is an IPv6 address, it must be enclosed in [ and ]. 🎯
  • -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. If the host string is an IPv6 address, it must be enclosed in [ and ]. 🪢
  • -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. If the host string is an IPv6 address, it must be enclosed in [ and ]. 🌉
  • -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.
  • -quic — Connect using the QUIC protocol. If specified then the -alpn option must also be provided.
  • -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. Note that this option is ignored if -crl_check option is not provided. Note that the maximum size of CRL is limited by X509_CRL_load_http(3) function.
  • -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 certificate 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. Unless the -verify_return_error option is given, 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.

By default, validation of server certificates and their chain is done w.r.t. the (D)TLS Server ("sslserver") purpose. For details see "Certificate Extensions" in openssl-verification-options(1). 🧐

  • -verify_return_error — Turns on server certificate verification, like with -verify, but returns 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 — Hands over a list of certs whose subject names get sent to the server in the certificate_authorities extension. Only supported for TLS 1.3, bestie. 💅
  • -dane_tlsa_domain domain — Turns on RFC6698/RFC7671 DANE TLSA authentication and sets the TLSA base domain. That base domain becomes the default SNI hint and the primary reference identifier for hostname checks. You must also use at least one -dane_tlsa_rrdata — no solo runs, okay? 🫶 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 — 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. So yeah — not a casual browser toggle, iykyk. 💀
  • -reconnect — Reconnects to the same server 5 times using the same session ID. It's literally a vibe check for session caching. If it works, slay. ✨
  • -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 — read that again, bestie. 👑
  • -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. 😭
  • -no-interactive — This flag can be used to run the client in a non-interactive mode. No chatting back, just vibes. 🎀
  • -state — Prints out the SSL session states. Receipts for the handshake, lowkey. 🧚
  • -debug — Print extensive debugging information including a hex dump of all traffic. It's giving “show me everything,” and honestly? Obsessed. 🔥
  • -nocommands — Do not use interactive command letters. The command letters are not invited to the group chat. 💅
  • -adv — Use advanced command mode. Main character energy for your interactive session. 👑
  • -security_debug — Enable security debug messages. Let the security tea spill. ☕
  • -security_debug_verbose — Output more security debug output. More tea, more receipts, no cap. 🫶
  • -msg — Show protocol messages. The raw conversation, bestie. 👀
  • -timeout — Enable send/receive timeout on DTLS connections. So you're not waiting forever like a read receipt that never comes. 💀
  • -mtu size — Set MTU of the link layer to the specified size. Tiny packet energy, if that's what the link wants. 🦋
  • -no_ems — Disable Extended master secret negotiation. Just say no to EMS, periodt. 💅
  • -keymatexport label — Export keying material using the specified label. Label it and yeet it. ✨
  • -keymatexportlen len — Export the specified number of bytes of keying material; default is 20. Twenty bytes is the default little snack. 🍓

Also, this exact line is just floating in the manual right here: Show all protocol messages with hex dump. I'm not moving it, bestie — it lives there now. 💀

  • -trace — Show verbose trace output of protocol messages. Maximum protocol yappage, and I mean that affectionately. 🌸
  • -msgfile filename — File to send output of -msg or -trace to, default standard output. Redirect the yapping. 🎀
  • -nbio_test — Tests nonblocking I/O. A little stress test for the nonblocking girlies. 🧚
  • -nbio — Turns on nonblocking I/O. No blocking, no waiting, no bad vibes. ⭐
  • -crlf — This option translated a line feed from the terminal into CR+LF as required by some servers. Some servers are extra like that. 💅
  • -ign_eof — Inhibit shutting down the connection when end of file is reached in the input. Stay on the line, bestie. 🫶
  • -quiet — Inhibit printing of session and certificate information. This implicitly turns on -ign_eof as well. Silence is golden, but read the fine print. 👀
  • -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. The override button ate. 🔥
  • -psk_identity identity — Use the PSK identity identity when using a PSK cipher suite. The default value is "Client_identity" (without the quotes). Default identity is iconic, ngl. 👑
  • -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. No key, no PSK, fr fr. 🔑
  • -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. TLSv1.3 only, bestie — don't try it with the wrong vibe. 💜
  • -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. Niche transport, but make it fashion. 🦋
  • -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. It's literally the “talk to the broken old one” button — correct implementations will side-eye you. 💀
  • -fallback_scsv — Send TLS_FALLBACK_SCSV in the ClientHello. A tiny little safety signal, slay. ✨
  • -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). Async girlies, assemble. 🫶
  • -maxfraglen len — Enable Maximum Fragment Length Negotiation; allowed values are 512, 1024, 2048, and 4096. Pick your fragment size, bestie. 🎀
  • -max_send_frag int — The maximum size of data fragment to send. See SSL_CTX_set_max_send_fragment(3) for further information. Big send energy, but capped. 💅
  • -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. Pipeline choreography, honestly. 🌈
  • -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. One pipeline is the default, but you can serve more looks. 👑
  • -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). Buffer size? More like buffer slay. 🍓
  • -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). Graceful shutdown who? We ignore the unexpected, bestie. 🙈
  • -no_tx_cert_comp — Disables support for sending TLSv1.3 compressed certificates. No compressed certs on the way out. 💜
  • -no_rx_cert_comp — Disables support for receiving TLSv1.3 compressed certificate. No compressed certs on the way in either. 🦋
  • -brief — Only provide a brief summary of connection parameters instead of the normal verbose output. Short and cute, like a text that actually gets to the point. ✨
  • -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". StartTLS is the glow-up moment for the protocol. 💅
  • -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". XMPP girlies, this one's for you. 🫶
  • -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, it 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. Names matter, bestie — use the right one. 🎀
  • -tlsextdebug — prints out a hex dump of any TLS extensions received from the server. yes bestie, all the raw bytes, laid bare 💅
  • -sess_out filename — outputs the SSL session to filename. gotta save that girl for later ✨
  • -sess_in filename — loads an SSL session from filename. the client will attempt to resume a connection from this session. it's giving main character re-entry energy 🎀
  • -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. we're just shooting our shot and see what comes back 💌
  • -status — sends a certificate status request to the server (OCSP stapling). the server response (if any) is printed out. asking the server "so… what's the tea on that cert?" ☕
  • -alpn protocols, -nextprotoneg protocols — these flags 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. no cap, you literally can't mix those two 💀
  • -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. so if you turn on CT you also lowkey turn on OCSP stapling, they're besties like that 🫶
  • -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. bestie this is the skeleton key for your Wireshark era 🔑
  • -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. no promises, it's giving "shoot your shot" 🏹
  • -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. it's giving bold, unapologetic energy 💅
  • -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. girl it's retired, don't come for her 💀
  • -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.
  • -ktls — enable Kernel TLS for sending and receiving. this option was introduced in OpenSSL 3.2.0. Kernel TLS is off by default as of OpenSSL 3.2.0. new era, new defaults, we love a glow-up ✨
  • -tfo — enable creation of connections via TCP fast open (RFC7413). zoom zoom 🏎️
  • -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). pick your era, bestie, but choose wisely 🕰️
  • -dtls, -dtls1, -dtls1_2 — these specify the use of DTLS instead of TLS. see "TLS Version Options" in openssl(1). DTLS said "I'm not like other protocols" 💁‍♀️
  • -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, -prefer_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. it's a whole syllabus, but the manpage holds all the answers, iykyk 📚
  • -rand files, -writerand file — see "Random State Options" in openssl(1) for details.
  • -provider name, -provider-path path, -provparam [name:]key=value, -propquery propq — see "Provider Options" in openssl(1), provider(7), and property(7). the whole provider friend group, go read their lore 🫶
  • -engine id — see "Engine Options" in openssl(1). this option is deprecated. she's a legacy icon, respect her history but don't expect her to show up 💀
  • -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. so like, it'll tell you what's wrong but keep going anyway unless you specifically said "no bestie, stop" 🛑

  • -enable_server_rpk — enable support for receiving raw public keys (RFC7250) from the server. use of X.509 certificates by the server becomes optional, and servers that support raw public keys may elect to use them. servers that don't support raw public keys or prefer to use X.509 certificates can still elect to send X.509 certificates as usual. everyone can choose their own path, we love options ✨
  • -enable_client_rpk — enable support for sending raw public keys (RFC7250) to the server. a raw public key will be sent by the client, if solicited by the server, provided a suitable key and public certificate pair is configured. some servers may nevertheless not request any client credentials, or may request a certificate. it's giving "only if you ask nicely" energy 🙈
  • host:port — rather than providing -connect, the target host 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 the host string is an IPv6 address, it must be enclosed in "[" and "]". putting your address in brackets is the IPv6 equivalent of wearing a cute jacket, it just completes the look 💅

✨ CONNECTED COMMANDS (BASIC) ✨

so once you're actually connected to an SSL/TLS server, bestie, here's the deal: 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. simple as that 🎀

when used interactively (which means neither -quiet nor -ign_eof have been given), and neither of -adv or -nocommands are given, then "Basic" command mode is entered. in this mode certain commands are recognized which perform special operations. these commands are a letter which must appear at the start of a line. all further data after the initial letter on the line is ignored. the commands are listed below. it's giving secret menu, but make it letters 👑

  • Q — end the current SSL connection and exit. goodbye bestie, it's been real 🫶
  • R — renegotiate the SSL session (TLSv1.2 and below only). living her best retro life 🕰️
  • C — attempt to reconnect to the server using a resumption handshake. re-entry era ✨
  • k — send a key update message to the server (TLSv1.3 only). freshness update, no cap 🔄
  • K — send a key update message to the server and request one back (TLSv1.3 only). reciprocal key update energy, we love mutual effort 💖

✨ CONNECTED COMMANDS (ADVANCED) ✨

if -adv has been given, then "advanced" command mode is entered. as with basic mode, if a connection is established with an SSL/TLS 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. same vibe, upgraded interface 👑

special commands can be supplied by enclosing them in braces, e.g. "{help}" or "{quit}". these commands can appear anywhere in the text entered into s_client, but they are not sent to the server. some commands can take an argument by ending the command name with ":" and then providing the argument, e.g. "{keyup:req}". some commands are only available when certain protocol versions have been negotiated. it's giving hidden features, and honestly? obsessed 💅

if a newline appears at the end of a line entered into s_client, then this is also sent to the server. if a command appears on a line on its own with no other text on the same line, then the newline is suppressed and not sent to the server. subtle but important, bestie, don't let that newline sneak through 🫣

the following commands are recognised.

  • help — prints out summary help text about the available commands. when in doubt, {help} it out 🆘
  • quit — close the connection to the peer. bye girl, slay your exit 💅
  • reconnect — reconnect to the peer and attempt a resumption handshake. we're back and we're resuming ✨
  • keyup — send a Key Update message. TLSv1.3 only. this command takes an optional argument. if the argument "req" is supplied, then the peer is also requested to update its keys; otherwise if "noreq" is supplied, the peer is not requested to update its keys. the default is "req". communicate your needs clearly, bestie 🗣️
  • reneg — initiate a renegotiation with the server. (D)TLSv1.2 or below only. a classic move for a classic protocol 🎩
  • fin — indicate FIN on the current stream. QUIC only. once FIN has been sent, any further text entered for this stream is ignored. that stream has left the chat, periodt 💀

💬 NOTES 💬

Okay bestie, so here's the tea on what this command is actually for — it's a debug tool for SSL servers, periodt. If you wanna connect to an SSL HTTP server, the classic move is:

openssl s_client -connect servername:443

because https lives on port 443, iykyk. Connection goes through? Slay. Then you can just type an HTTP command like "GET /" and pull a whole web page. Iconic.

Now, if the handshake fails 😭 there's a whole list of possible causes. If nothing is super obvious — like, there's no missing client certificate situation — then you can try the -bugs, -ssl3, -tls1, -no_ssl3, -no_tls1 options in case the server is just buggy. Lowkey important: play with these options before you go submitting a bug report to an OpenSSL mailing list. Do the homework first, bestie 🫶.

A super frequent problem when you're trying to get client certificates working: the web client complains it has no certificates, or it hands you an empty list to choose from. That's normally because the server isn't sending the client's certificate authority in its "acceptable CA list" when it requests a certificate. The good news? With this command you can view and check that CA list directly 👑. Small catch though — some servers only request client authentication after a specific URL gets requested. To get the list in that case, you gotta use the -prexit option and send an HTTP request for an appropriate page.

Also, real talk: if you specify a certificate on the command line with -cert, it will not be used unless the server specifically requests a client certificate. So just including a client cert on the command line is no guarantee that the certificate works. Not me learning that the hard way 💀.

Problems verifying a server certificate? The -showcerts option shows you all the certificates the server sent. Bless.

Bestie, heads up, this one matters: this command is a test tool and it's designed to continue the handshake after any certificate verification errors. That means it will accept any certificate chain (trusted or not) sent by the peer. Non-test applications should not do this — it makes them vulnerable to a MITM attack. You can change this behaviour with the -verify_return_error option: then any verify errors get returned and the handshake gets aborted. 🔥

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

📎 Note on Non-Interactive Use 📎

So when s_client runs in a non-interactive environment (think a cron job, or a script with no valid stdin), it may close the connection prematurely — especially with TLS 1.3 😭. To prevent that, you can use the -ign_eof flag, which keeps s_client running even after it hits EOF from stdin.

Like, for example:

openssl s_client -connect <server address>:443 -tls1_3
                 -sess_out /path/to/tls_session_params_file
                 -ign_eof </dev/null

But — and this is the plot twist — relying solely on -ign_eof can lead to issues if the server keeps the connection open expecting the client to close first. In those cases, the client may hang indefinitely 🙈. And this behaviour is not uncommon, particularly with protocols where the server waits for a graceful disconnect from the client.

Example time: when connecting to an SMTP server, the session may pause if the server expects a QUIT command before closing:

$ openssl s_client -brief -ign_eof -starttls smtp
                   -connect <server address>:25 </dev/null
CONNECTION ESTABLISHED
Protocol version: TLSv1.3
Ciphersuite: TLS_AES_256_GCM_SHA384
...
250 CHUNKING
[long pause]

Not the [long pause] 😭😭

To dodge hangs like that, it's better to use an application-level command to start a clean disconnect. For SMTP, you can send a QUIT command:

printf 'QUIT\r\n' | openssl s_client -connect <server address>:25
                                     -starttls smtp -brief -ign_eof

Same energy for HTTP/1.1 connections — include a Connection: close header and the server will close the connection after responding:

printf 'GET / HTTP/1.1\r\nHost: <server address>\r\nConnection: close\r\n\r\n'
    | openssl s_client -connect <server address>:443 -brief

These approaches help manage the connection closure gracefully and prevent hangs caused by the server waiting for the client to initiate the disconnect. Chef's kiss 🎀.

💀 BUGS 💀

Okay so, not the cutest part. 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 is not a model of how things should be done. A typical SSL client program would be much simpler. Fr fr.

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

👀 SEE ALSO 👀

Go read these too, they're giving main character energy:

🦋 HISTORY 🦋

A little lore for you, bestie ✨:

  • 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.
  • The -enable_client_rpk, -enable_server_rpk, -no_rx_cert_comp, -no_tx_cert_comp, and -tfo options were added in OpenSSL 3.2.

Copyright 2000-2025 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 <https://www.openssl.org/source/license.html>.

And that's the whole page, bestie! 🫶 This one's from version 3.5.0, dated 2026-09-25 — go forth and debug your SSL servers responsibly, mwah 💅💖

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