OPENSSL-S_CLIENT(1ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

🌸 NAME 🌸

openssl-s_client — it's the SSL/TLS client program, bestie. This is the OPENSSL-S_CLIENT(1ossl) manual page, and it's literally here to help you connect to a remote host using SSL/TLS. Iconic diagnostic tool for SSL servers? Yes. Main character energy? Also yes.

✨ SYNOPSIS ✨

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 💖

Okay so this command is a generic SSL/TLS client that connects to a remote host using SSL/TLS. It is a very useful diagnostic tool for SSL servers. Like, genuinely useful — not just cute. 🦋

🎀 OPTIONS 🎀

Also, on top of the options below, this command supports the common and client only options documented in the "Supported Command Line Commands" section of the SSL_CONF_cmd(3) manual page.

  • -help — prints out a usage message. Your little “what do I do” bestie.
  • -ssl_config section — uses the specified section of the configuration file to configure the SSL_CTX object.
  • -connect host:port — tells it the host and optional port to connect to. You can also pick the host and port using the optional target positional argument instead. If neither this nor the target positional argument is specified, it tries 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. It's giving legacy, honestly.
  • -port port — connect to the specified port; use -connect instead. Same vibe.
  • -bind host:port — 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 — sets 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 — points at a file full of certificates, and the subject names of those certs get sent to the server inside the certificate_authorities extension. Heads up: only works with TLS 1.3, bestie 💅
  • -dane_tlsa_domain domain — turns on RFC6698/RFC7671 DANE TLSA authentication and sets the TLSA base domain. That domain becomes the default SNI hint AND the primary reference identifier for hostname checks. You must pair it with at least one -dane_tlsa_rrdata or it's giving nothing.

When DANE auth actually succeeds, the diagnostic output tells you the lowest depth (closest to 0) where a TLSA record authenticated a chain certificate. If that TLSA record is a "2 1 0" trust anchor public key that signed (not matched) the top-most cert in the chain, it's reported as "TA public key verified". Otherwise it's either "matched TA certificate" at a positive depth, or "matched EE certificate" at depth 0. Slay, now you know which one you got ✨

  • -dane_tlsa_rrdata rrdata — use it one or more times to spell out the RRDATA fields of the DANE TLSA RRset for the target service. The rrdata value is in "presentation form" — four whitespace-separated fields: usage, selector, matching type, and the associated data, with that last one hex-encoded. Whitespace inside the associated data field is ignored, no cap. Example era:
$ 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 — kills server name checks when you're authenticating via DANE-EE(3) TLSA records. Ok so real talk: for some apps — web browsers mainly — turning off name checks is not safe because of "unknown key share" attacks, where a malicious server can convince a client that a connection to a victim server is actually a secure connection to the malicious server. Then that malicious server might be able to violate cross-origin scripting restrictions. Yikes. So even though RFC7671 says otherwise, name checks are on by default for DANE-EE(3) TLSA records, and you can turn them off in apps where it's safe. Specifically: SMTP and XMPP clients should set this option, because SRV and MX records already let a remote domain redirect client connections to any server of its choice, and anyway SMTP/XMPP clients don't execute scripts downloaded from remote servers. 💀
  • -reconnect — reconnects to the same server 5 times using the same session ID. It's literally a session-caching test, iykyk.
  • -showcerts — displays the server certificate list exactly as the server sent it: only the certs the server has sent, in the order the server sent them. This is not a verified chain, don't get it twisted 🫶
  • -prexit — prints session info when the program exits. It'll always try to print info even if the connection fails. Normally info only gets printed once, and only if the connection succeeds. This one's useful because the cipher in use might get renegotiated, or the connection might fail because a client certificate is required — or only requested after an attempt to access some URL. Note: the output from this option is not always accurate, because a connection might never have been established. 😭
  • -no-interactive — flag for running the client in non-interactive mode.
  • -state — prints out the SSL session states.
  • -debug — prints extensive debugging info, including a hex dump of all the traffic.
  • -nocommands — do not use interactive command letters.
  • -adv — use advanced command mode.
  • -security_debug — enable security debug messages.
  • -security_debug_verbose — output even more security debug output.
  • -msg — show protocol messages.
  • -timeout — enable the send/receive timeout on DTLS connections.
  • -mtu size — set the MTU of the link layer to size.
  • -no_ems — disable Extended master secret negotiation.
  • -keymatexport label — export keying material using the given label.
  • -keymatexportlen len — export len bytes of keying material; default is 20.
Random floating line in the original text, right here: "Show all protocol messages with hex dump." No option attached to it in this stretch of the page — but it's a detail, so it stays 💖
  • -trace — show verbose trace output of protocol messages.
  • -msgfile filename — the file to send -msg or -trace output to; defaults to 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 — inhibits shutting down the connection when end of file is reached in the input. This implicitly turns on -nocommands too.
  • -quiet — inhibits printing of session and certificate information. This implicitly turns on -ign_eof and -nocommands too.
  • -no_ign_eof — shuts down the connection when end of file is reached in the input. Use it to override the implicit -ign_eof you get after -quiet 🌸
  • -psk_identity identity — use the PSK identity when using a PSK cipher suite. Default value is "Client_identity" (no quotes).
  • -psk key — use the PSK key when using a PSK cipher suite. The key is a hexadecimal number with no leading 0x, like -psk 1a2b3c4d. You must provide this option to use a PSK cipher, periodt 🔥
  • -psk_session file — use the PEM-encoded SSL_SESSION data stored in file as the basis of a PSK. Note: this only works if TLSv1.3 is negotiated.
  • -sctp — use SCTP as the transport protocol instead of UDP in DTLS. Must be used together with -dtls, -dtls1 or -dtls1_2. Only available where OpenSSL has SCTP support enabled.
  • -sctp_label_bug — use the incorrect behaviour of older OpenSSL implementations when computing endpoint-pair shared secrets for DTLS/SCTP. This lets you talk to old broken implementations, but it breaks interoperability with correct ones. Must be used with -sctp. Only available where OpenSSL has SCTP support enabled. 🩹
  • -fallback_scsv — send TLS_FALLBACK_SCSV in the ClientHello.
  • -async — switch on asynchronous mode, so cryptographic operations get performed asynchronously. This only does anything if you're also using an async-capable engine via the -engine option. For testing, the dummy async engine (dasync) can be used (if it's available).
  • -maxfraglen len — enable Maximum Fragment Length Negotiation. Allowed values: 512, 1024, 2048, and 4096.
  • -max_send_frag int — the max size of a data fragment to send. See SSL_CTX_set_max_send_fragment(3) for more.
  • -split_send_frag int — the size used to split data for encrypt pipelines. Write more than this in one go and it gets split into multiple pipelines, up to the max number defined by max_pipelines. Only has an effect if a suitable cipher suite got negotiated, an engine that supports pipelining is loaded, and max_pipelines is greater than 1. See SSL_CTX_set_split_send_fragment(3) for more.
  • -max_pipelines int — the maximum number of encrypt/decrypt pipelines to use. Only has an effect if an engine supporting pipelining is loaded (e.g. the dasync engine) and a suitable cipher suite got negotiated. Default is 1. See SSL_CTX_set_max_pipelines(3) for more.
  • -read_buf int — the default read buffer size for connections. Only has an effect if the buffer size is larger than the size that'd otherwise be used and pipelining is in use (see SSL_CTX_set_default_read_buffer_len(3) for more).
  • -ignore_unexpected_eof — some TLS implementations don't send the mandatory close_notify alert on shutdown 😤 If the app waits for close_notify but the peer just closes the connection without it, you get an error. Enable this and the peer doesn't need to send close_notify — a closed connection gets treated as if it was received. For more on shutting down a connection, see SSL_shutdown(3).
  • -no_tx_cert_comp — disables support for sending TLSv1.3 compressed certificates.
  • -no_rx_cert_comp — disables support for receiving TLSv1.3 compressed certificate.
  • -brief — only give a brief summary of connection parameters instead of the normal verbose output. Main character energy, minimal words ✨
  • -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". That's the whole list, no extras 👑
  • -xmpphost hostname — when used with -starttls xmpp or -starttls xmpp-server, specifies the host for the "to" attribute of the stream element. If you skip it, the host given with -connect gets used. This option is an alias of -name for "xmpp" and "xmpp-server".
  • -name hostname — use this to specify hostname info for various protocols used with the -starttls option. Currently only "xmpp", "xmpp-server", "smtp" and "lmtp" can use this -name option. If it's used with -starttls xmpp or -starttls xmpp-server, it specifies the host for the "to" attribute of the stream element (yep, the original wording here literally says "if specifies" 🙈). If you don't specify it, the host given with -connect gets used.

okay so 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. Hex dump bestie, it's giving forensic main character energy. 🔍
  • -sess_out filename — Output the SSL session to filename. Save that session for later, periodt. 💾
  • -sess_in filename — Load the SSL session from filename. The client will attempt to resume a connection from this session. Reusing a session? obsessed. 🔁
  • -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. Numbers and PEM files, very demure. ✨
  • -status — Sends a certificate status request to the server (OCSP stapling). The server response (if any) is printed out. OCSP stapling, no cap. 🎀
  • -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 -nextprotoneg flag cannot be specified if -tls1_3 is used. ALPN ate, NPN is the legacy vibe. 💅
  • -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. CT on, SCTs incoming. 🧚
  • -ctlogfile — A file containing a list of known Certificate Transparency logs. See SSL_CTX_set_ctlog_list_file(3) for the expected file format. CT log lore, iykyk. 📜
  • -keylogfile file — Appends TLS secrets to the specified keylog file such that external programs (like Wireshark) can decrypt TLS connections. Wireshark said let me in. 🔓
  • -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. Early data is a whole vibe, but only if the server says yes. 🌸
  • -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. PHA said post-handshake, not post-cancellation. ✨
  • -use_srtp value — Offer SRTP key management, where value is a colon-separated profile list. SRTP profiles, colon-separated, very precise. 🎀
  • -srpuser value — Set the SRP username to the specified value. This option is deprecated. Deprecated, bestie, don't get attached. 💀
  • -srppass value — Set the SRP password to the specified value. This option is deprecated. Also deprecated, fr fr. 🙈
  • -srp_lateuser — SRP username for the second ClientHello message. This option is deprecated. Second ClientHello energy, but deprecated. 🫠
  • -srp_moregroups — Tolerate other than the known g and N values. This option is deprecated. Deprecated and tolerant, a messy icon. 💅
  • -srp_strength number — Set the minimal acceptable length, in bits, for N. This option is deprecated. Deprecated bit-length gatekeeping. 😭
  • -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. Kernel TLS, introduced in 3.2.0, off by default as of 3.2.0. The way that's so specific. 🔥
  • -tfo — Enable creation of connections via TCP fast open (RFC7413). 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). TLS version choices, the original vibe check. 🌈
  • -dtls, -dtls1, -dtls1_2 — These specify the use of DTLS instead of TLS. See "TLS Version Options" in openssl(1). DTLS said not today, TLS. ⭐
  • -nameopt option — This specifies how the subject or issuer names are displayed. See openssl-namedisplay-options(1) for details. Name display options, because names deserve main character treatment. 💖
  • -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. Extended verification, extra spicy cert checks. 🔍
  • -CAfile file, -no-CAfile, -CApath dir, -no-CApath, -CAstore uri, -no-CAstore — See "Trusted Certificate Options" in openssl-verification-options(1) for details. Trusted certs, the blueprint. 📁
  • -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. This group is a whole roster, but every single one is here. 👑
  • -rand files, -writerand file — See "Random State Options" in openssl(1) for details. Random state, keeping it chaotic but documented. 🎲
  • -provider name, -provider-path path, -provparam [name:]key=value, -propquery propq — See "Provider Options" in openssl(1), provider(7), and property(7). Provider options, the plug-in besties. 🧩
  • -engine id — See "Engine Options" in openssl(1). This option is deprecated. Engine options, deprecated era. 💀
  • -ssl_client_engine id — Specify the engine to be used for client certificate operations. Client cert engine, very specific assignment. 🛠️
  • -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. Verification errors show up, but the command keeps going unless you use -verify_return_error. That's the warning, bestie. ⚠️
  • -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. Raw public keys, RFC7250, optional X.509, server's choice. 🦋
  • -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. Client RPK, only if solicited, only with the right key+cert pair. Some servers just won't ask. That's their vibe. 🌈
  • 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 ]. Positional host:port, localhost:4433 fallback, IPv6 needs brackets. Iconic. 🎀

🌸 CONNECTED COMMANDS (BASIC) 🌸

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. Like a group chat, but with TLS. 💬

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. Basic mode: one letter at the start of a line, everything after it is ignored. Periodt. 💅

  • Q — End the current SSL connection and exit. Q for quit, obviously. 👋
  • R — Renegotiate the SSL session (TLSv1.2 and below only). Renegotiation, but only TLSv1.2 and below. Respect the version. 🔁
  • C — Attempt to reconnect to the server using a resumption handshake. Reconnect with resumption, the comeback era. 🫶
  • k — Send a key update message to the server (TLSv1.3 only). Key update, TLSv1.3 only, lowercase k. 🔑
  • K — Send a key update message to the server and request one back (TLSv1.3 only). Uppercase K says update your keys too, bestie. 🔐

🦋 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. Advanced mode said glow-up. ✨

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. Braces, colon arguments, version-gated commands — advanced mode is the main character. 👑

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. Newline lore: sent with text, suppressed for a solo command. iykyk. 📨

The following commands are recognised.

  • help — Prints out summary help text about the available commands. Help, because even icons need a little help sometimes. 🆘
  • quit — Close the connection to the peer. Quit, close the peer connection, bye. 👋
  • reconnect — Reconnect to the peer and attempt a resumption handshake. Reconnect + resumption handshake, the comeback arc. 🔁
  • 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. Key update, TLSv1.3 only, optional req or noreq, default req. Slay. 🔑
  • reneg — Initiate a renegotiation with the server. (D)TLSv1.2 or below only. Renegotiation, but only (D)TLSv1.2 or below. No cap. 🔄
  • fin — Indicate FIN on the current stream. QUIC only. Once FIN has been sent any further text entered for this stream is ignored. FIN, QUIC only. After FIN, further text for this stream is ignored. The end, literally. 🏁

📝 NOTES 📝

okay so bestie, this command is literally a debug tool for 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. fr fr, do the vibe check first.

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 ⭐. no cap.

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. This is the serious part, bestie — do not ignore it.

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 🌀

When s_client is run in a non-interactive environment (e.g., a cron job or a script without a valid stdin), it may close the connection prematurely, especially with TLS 1.3 😭. To prevent this, you can use the -ign_eof flag, which keeps s_client running even after reaching EOF from stdin.

For example:

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

However, relying solely on -ign_eof can lead to issues if the server keeps the connection open, expecting the client to close first 🙈. In such cases, the client may hang indefinitely. This behavior is not uncommon, particularly with protocols where the server waits for a graceful disconnect from the client.

For example, 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]

To avoid such hangs, it's better to use an application-level command to initiate 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

Similarly, for HTTP/1.1 connections, including a Connection: close header ensures the server closes 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 💖.

🐛 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 👀

🕰️ 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.

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 is from OpenSSL 3.5.6, dated 2026-04-07, and it’s literally OPENSSL-S_CLIENT(1ossl) energy 💅✨👑

📦 where this page comes from