OPENSSL-S_CLIENT(1SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

💖 OPENSSL-S_CLIENT 💖

bestie. sit down. hydrate. we're talking about openssl s_client, aka the SSL/TLS client program, straight outta the OpenSSL manual (that's OPENSSL-S_CLIENT(1SSL) if you're being formal, and you're not, because you're here with me 💅).

✨ NAME ✨

openssl-s_client — it's the SSL/TLS client program, okurrr? 🦋

🌸 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] [-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] [-ocsp_check_leaf] [-ocsp_check_all] [-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] [-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] [-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] [-expected-rpks] [host:port] [-ech_config_list]
[-ech_outer_alpn protocols] [-ech_grease] [-ech_grease_suite suite]
[-ech_grease_type type] [-ech_ignore_cid] [-ech_outer_sni value]
[-ech_no_outer_sni] [-ech_select config-index]

like. look at her. iconic. 👑

💜 DESCRIPTION 💜

okay so real talk: this command is a generic SSL/TLS client that connects to a remote host using SSL/TLS. that's the whole gig. and the way it's also a genuinely, absurdly, deliciously useful diagnostic tool for SSL servers? obsessed. 🔍✨

🎀 OPTIONS 🎀

lowkey important heads-up before we dive in: on top of everything listed below, this command also supports the common options and the client-only options documented in the "Supported Command Line Commands" section of the SSL_CONF_cmd(3) manual page. yw bestie 🫶

  • -help — prints out a usage message. bless.
  • -ssl_config section — use the specified section of the configuration file to configure the SSL_CTX object. so organized.
  • -connect host:port — this specifies the host and optional port to connect to. you can also pick the host and port using the optional target positional argument instead. and if neither this nor the target positional argument is specified, it attempts to connect to the local host on port 4433. and if the host string is an IPv6 address, it must be enclosed in "[" and "]" — no exceptions, she's strict 💅
  • -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. and 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 "]".
  • -ech_config_list value — specifies the ECHConfigList value to use for Encrypted Client Hello (ECH) for the TLS session. the value must be a base64 encoded ECHConfigList.

the ECHConfigList structure is defined in RFC 9849. 📜

  • -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. (not optional!!) ⚠️
  • -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 the -crl_check option is not provided. note that the maximum size of CRL is limited by the 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 — 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.

when any of -verifyCAfile, -verifyCApath, or -verifyCAstore is specified, they are loaded into a separate verification store (via SSL_CTX_set1_verify_cert_store(3)) and used for server certificate verification instead of the store built from -CAfile, -CApath, and -CAstore.

  • -chainCAfile file — a file in PEM format containing trusted certificates to use when attempting to build the client certificate chain.
  • -chainCApath directory — a directory full of trusted certs you wanna use to build the client cert chain you hand over to the server. It has to be in "hash format" btw — peep openssl-verify(1) for the whole lore. 🌸
  • -chainCAstore uri — same idea, but instead of a directory you're pointing at a store by URI, and it's used when you're trying to build that client cert chain. The URI can be a single cert or a whole lil collection of them. If it's a "file:" scheme URI, this acts like -chainCAfile or -chainCApath depending on whether the URI points at a directory or just one file. More deets on the "file:" scheme live in ossl_store-file(7). 🎀
  • -requestCAfile file — a file with 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 — switches 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 gotta pair it with at least one -dane_tlsa_rrdata option below, or else with at least one -expected-rpks option (which internally synthesises associated TLSA "3 1 0" records for you). 🦋

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 this one or more times to specify the RRDATA fields of the DANE TLSA RRset tied to the target service. The rrdata value goes in "presentation form" — that's four whitespace-separated fields: usage, selector, matching type, and the associated data, with that last one encoded in hexadecimal. Optional whitespace gets ignored inside the associated data field. Like so:
$ 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 one turns off server name checks when authenticating via DANE-EE(3) TLSA records. Okay so listen, for some apps — mostly web browsers — it is not safe to disable name checks, because of "unknown key share" attacks, where a malicious server convinces a client that a connection to a victim server is actually a secure connection to the malicious server. Then the bad server might be able to violate cross-origin scripting restrictions. So like, despite what the text of RFC7671 says, name checks are by default enabled for DANE-EE(3) TLSA records, and you can only turn them off in apps where it's safe. Fr fr: SMTP and XMPP clients should set this option, since SRV and MX records already let a remote domain redirect client connections to any server it wants, and anyway SMTP and XMPP clients don't run scripts downloaded from remote servers. 💅
  • -reconnect — reconnects to the same server 5 times using the same session ID. Consider it a vibe check for whether session caching is actually working. 🔁
  • -showcerts — shows you the server certificate list exactly as the server sent it. Heads up though: it only consists of certificates the server has sent (in the order the server sent them). It is not a verified chain. Don't get it twisted. 😭
  • -prexit — prints session information when the program exits. It'll always try to print out info even if the connection fails. Normally info gets printed out only 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 from this option is not always accurate because a connection might never have been established. 🙈
  • -no-interactive — flag you can use to run the client in a non-interactive mode.
  • -state — prints out the SSL session states. ✨
  • -debug — prints extensive debugging information, including a hex dump of all traffic. Spill everything, no cap.
  • -nocommands — do not use interactive command letters.
  • -adv — use advanced command mode. 👑
  • -security_debug — enables security debug messages.
  • -security_debug_verbose — outputs more security debug output.
  • -msg — shows protocol messages.
  • -timeout — enables send/receive timeout on DTLS connections.
  • -mtu size — sets the MTU of the link layer to the size you specify.
  • -no_ems — disables Extended master secret negotiation.
  • -keymatexport label — export keying material using the specified label.
  • -keymatexportlen len — export this many bytes of keying material; default is 20.
okay this next line is literally just floating in the source with no flag attached to it, so I'm giving it to you exactly as it appears, bestie, I refuse to guess: "Show all protocol messages with hex dump."
  • -trace — shows verbose trace output of protocol messages. 🦋
  • -msgfile filename — file to send output of -msg or -trace to; default is standard output.
  • -nbio_test — tests nonblocking I/O.
  • -nbio — turns on nonblocking I/O.
  • -crlf — this option translates 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 as well.
  • -quiet — inhibits printing of session and certificate information. This implicitly turns on -ign_eof and -nocommands as well.
  • -no_ign_eof — shuts 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 you're using a PSK cipher suite. The default value is "Client_identity" (without the quotes, obviously 💅).
  • -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. You literally must provide this option in order to use a PSK cipher. Non-negotiable.
  • -psk_session file — uses 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 — uses the incorrect behaviour of older OpenSSL implementations when computing endpoint-pair shared secrets for DTLS/SCTP. This lets you talk to older broken implementations, but it breaks interoperability with correct implementations. Must be used in conjunction with -sctp. Also, this option is only available where OpenSSL has support for SCTP enabled. 💀
  • -fallback_scsv — sends TLS_FALLBACK_SCSV in the ClientHello.
  • -async — switches on asynchronous mode. Cryptographic operations will be performed asynchronously. This only has an effect if an asynchronous capable provider is also used.
  • -maxfraglen len — enables Maximum Fragment Length Negotiation; allowed values are 512, 1024, 2048, and 4096. That's the whole menu, don't try ordering off it. 🙈
  • -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 — this is used for pipelines. There is currently no support for cipher pipelining in libssl. See SSL_CTX_set_max_send_fragment(3) for further information.
  • -max_pipelines int — this is used for pipelines. And again: there is currently no support for cipher pipelining in libssl. See SSL_CTX_set_max_send_fragment(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 info 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. (that's the wording, I'm not touching it)
  • -brief — only provide a brief summary of connection parameters instead of the normal verbose output. Snack-sized. 🍓
  • -starttls protocol — sends 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". Iconic lineup tbh.
  • -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 the -starttls option. Currently only "xmpp", "xmpp-server", "smtp" and "lmtp" can utilize this -name option. 💜

Okay so, continuing our little journey through the option zoo 💖 — if this one gets used alongside -starttls xmpp or -starttls xmpp-server, it sets the host for the to attribute of the stream element. And if you don't say anything at all? It just quietly uses whatever host you gave to -connect. Iconic behavior, honestly.

And if you're running it with -starttls lmtp or -starttls smtp, it instead names the thing to use in the LMTP LHLO or SMTP EHLO message, respectively. Leave it out and mail.example.com gets used, no thoughts, head empty. 🎀

  • -tlsextdebug — prints a straight-up hex dump of any TLS extensions the server sent back. Very "show me the receipts." 🔍
  • -sess_out filename — writes the SSL session out to filename. Save it for later, bestie.
  • -sess_in filename — loads an SSL session from filename. The client will then attempt to resume a connection from this session, like picking up a convo exactly where you left off.
  • -serverinfo types — a list of comma-separated TLS Extension Types (numbers between 0 and 65535). Each one gets yeeted as an empty ClientHello TLS Extension, and whatever the server sends back (if anything) gets encoded and displayed as a PEM file. ✨
  • -status — sends a certificate status request to the server (this is the infamous OCSP stapling), and the server's response (if any) is printed out. 💅
  • -ocsp_check_leaf — requires that server (end-entity) certificate status checking actually happens, and any OCSP response handed over during the TLS handshake (the so-called "OCSP stapling" one) gets tried first. If no valid and conclusive OCSP response can be found, CRL-based checking gets attempted as a fallback if it's enabled — otherwise the status check just fails, periodt.

This one implies -status.

  • -ocsp_check_all — exactly like the option before, but it also requires certificate status checking for the issuer chain of the server certificate (so: intermediate CA certificates, and specifically excluding the trust anchor).

This implies -status and -ocsp_check_leaf.

  • -alpn protocols, -nextprotoneg protocols — these two flags enable the Application-Layer Protocol Negotiation extension or the Next Protocol Negotiation (NPN) extension, respectively. ALPN is the IETF standard and it replaces NPN (NPN is giving legacy, ngl). The protocols list is a comma-separated list of protocol names the client should advertise support for, and you put your most desirable protocols first. Protocol names are printable ASCII strings, for example http/1.1 or spdy/3. 🦋
Heads up: -nextprotoneg cannot be specified if -tls1_3 is used. Pick a lane.
  • -ct, -noct — use one of these two to control whether Certificate Transparency (CT) is enabled (-ct) or disabled (-noct). With CT enabled, signed certificate timestamps (SCTs) get requested from the server and reported when the handshake completes. 🔥

Enabling CT also enables OCSP stapling, since that's one possible delivery method for SCTs. Two birds, one flag.

  • -ctlogfile — a file containing a list of known Certificate Transparency logs. See SSL_CTX_set_ctlog_list_file(3) for the expected file format, bestie.
  • -keylogfile file — appends TLS secrets to the specified keylog file so that external programs (like Wireshark, yes that Wireshark) can decrypt TLS connections. Spy era but make it ethical. 🕵️
  • -early_data file — reads the contents of the specified file and attempts to send it as early data to the server. This only works with resumed sessions that support early data and when the server actually accepts the early data. No cap, there are two gates here.
  • -enable_pha — TLSv1.3 only, sends the Post-Handshake Authentication extension. This happens whether or not a certificate has been provided via -cert. It's giving "I'm doing it anyway."
  • -use_srtp value — offers SRTP key management, where value is a colon-separated profile list.
  • -srpuser value — sets the SRP username to value.
This option is deprecated.
  • -srppass value — sets the SRP password to value.
This option is deprecated.
  • -srp_lateuser — SRP username for the second ClientHello message.
This option is deprecated.
  • -srp_moregroups — tolerates other than the known g and N values.
This option is deprecated.
  • -srp_strength number — sets the minimal acceptable length, in bits, for N.
This option is deprecated.
  • -ktls — enables 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.
  • -tfo — enables creation of connections via TCP fast open (RFC7413). Fast as in fr fr. ⚡
  • -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2, -no_tls1_3, -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 — specifies how the subject or issuer names get displayed. See openssl-namedisplay-options(1) for the deets.
  • -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. That's a whole squad right there. 👑
  • -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).
  • -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 anyway unless the -verify_return_error option is used. So yeah, it'll keep going while the errors just sit there judging you. 💀

  • -enable_server_rpk — enables 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 that simply prefer to use X.509 certificates, can still elect to send X.509 certificates as usual. Everyone gets to choose, love that for them. 🫶
  • -enable_client_rpk — enables 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. Their loss honestly.
  • -expected-rpks file — this option implies the -enable_server_rpk option and can be specified multiple times. Each PEM file should contain one or more public keys, private keys or certificates. With a private key or certificate, an attempt is made to extract the associated public key. Each resulting public key is added (via a call to SSL_add_expected_rpk(3)) as a valid raw public key that the server may present to be considered verified. If a server nevertheless presents an X.509 certificate, the enclosed public key is validated as though it were presented as a raw public key instead. Bestie does not care which wrapper you show up in, the key inside is what matters. 🔑

If the -dane_tlsa_domain and -dane_tlsa_rrdata options (with certificate usage DANE-EE(3) and selector SPKI(1)) are also used, then the specified TLSA records are used in combination with the keys found via the -expected-rpks option(s).

Verification success or failure is reported as a DANE verification success or failure, because verification of raw public keys is internally mapped to verification of DANE TLSA records derived from the specified keys.

  • host:port — instead of 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, it 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 ]. No exceptions, that's the vibe check. ⭐
  • -ech_outer_alpn protocols — when doing Encrypted Client Hello (ECH), this lets the caller specify ALPN values to use in the outer ClientHello. (A "normal" ALPN value specified via -alpn will be used in the inner ClientHello. Two outfits, one handshake. 💅)
  • -ech_grease — when you're not really doing Encrypted Client Hello (ECH), you can still emit a so-called GREASE value, which is essentially a random value, in order to try to ensure that server code is less likely to ossify. Basically throwing random noise into the world to keep things flexible. 🎲
  • -ech_grease_suite suite — when -ech_grease is specified, you can choose which ECH ciphersuite to use via this parameter.

The comma-separated suite string names an HPKE suite in the form of kem,kdf,aead, e.g. x25519,hkdf-sha256,aes256gcm, or it can use the numeric values (in decimal or hexadecimal form) from the HPKE specification — so 0x20,0x01,0x02 is the same as the previous example.

KEM values supported: p256 or 0x10; p384 or 0x11, p521 or 0x12, x25519 or 0x20, x448 or 0x21

KDF values supported: hkdf-sha256 or 0x01, hkdf-sha384 or 0x02, hkdf-sha512 or 0x03

AEAD values supported: aes128gcm or 0x01, aes256gcm or 0x02, chachapoly1305 or 0x03

  • -ech_grease_type type — lets the client set the TLS extension type for a GREASEd ECH value (currently equivalent to the ECH version number). The current default is 0xfe0d. 🌸
  • -ech_ignore_cid — Encrypted Client Hello (ECH) extensions contain a configuration identifier (cid) taken from the ECHConfigList usually found in the domain name system (DNS). Since those identifiers could be revealing, the client has the option to use a random value instead. Privacy queen behavior, we love to see it. 👑
  • -ech_outer_sni value — okay so when you're doing Encrypted Client Hello (ECH), this one lets the caller name a subject name indication (SNI) value to drop into the outer ClientHello, totally over-riding the public_name value from the relevant ECHConfigList 🫶
  • -ech_no_outer_sni — set this flag and it's giving silence, because no SNI gets emitted in the outer ClientHello at all. periodt.
  • -ech_select config-index — if an ECHConfigList is holding more than one ECHConfig, the client will lowkey just use the first one that works. This lets the caller pick which ECHConfig to use, with a zero-based index (so first is 0, bestie, not 1) 💅

🎀 CONNECTED COMMANDS (BASIC) 🎀

So if a connection actually gets established with an SSL/TLS server, then literally any data received from the server is displayed, and any key presses get sent to the server. If end of file is reached, the connection gets closed down. that's it, that's the vibe 🌸

When it's used interactively — which means neither -quiet nor -ign_eof have been given — and neither -adv nor -nocommands are given, then "Basic" command mode is entered. In this mode some very specific commands are recognized and they do special little operations ✨

The way these work: the command is a single letter that must appear at the start of a line. Everything else after that initial letter on the line? Ignored. bye. 👋

The commands are:

  • Q — End the current SSL connection and exit. girl, you're done here.
  • R — Renegotiate the SSL session. TLSv1.2 and below only 🫠
  • C — Attempt to reconnect to the server using a resumption handshake 🔁
  • 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

👑 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. And if end of file is reached, the connection gets closed down.

Special commands get supplied by wrapping them in braces, like "{help}" or "{quit}". These can show up anywhere in the text you type into s_client, and they are not sent to the server 🚫 Some commands can take an argument — you do that by ending the command name with ":" and then handing over the argument, e.g. "{keyup:req}". And some commands are only available when certain protocol versions have been negotiated, so check yourself before you wreck yourself 💀

If a newline appears at the end of a line entered into s_client, that newline also gets sent to the server. BUT if a command appears on a line all by itself with no other text on the same line, then the newline is suppressed and not sent. sneaky, but iconic 💅

The following commands are recognised:

  • help — Prints out summary help text about the available commands.
  • quit — Close the connection to the peer.
  • reconnect — Reconnect to the peer and attempt a resumption handshake.
  • 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".
  • reneg — Initiate a renegotiation with the server. (D)TLSv1.2 or below only.
  • fin — Indicate FIN on the current stream. QUIC only. Once FIN has been sent any further text entered for this stream is ignored.

💜 NOTES 💜

This command is your little debug buddy 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, iykyk). 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's nothing obvious like no client certificate, then the -bugs, -tls1, and -no_tls1 options can be tried in case it's a buggy server 🐛 In particular you should play with these options before submitting a bug report to an OpenSSL mailing list. pls, do the legwork first bestie 🙏

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 client's 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. So merely including a client certificate on the command line is no guarantee that the certificate works. it's giving "I brought snacks but nobody asked" 🍓

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

⚠️ okay this one is important, no cap: 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 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.

🌈 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. slay 💅

💀 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. yeah. it's giving duct tape and vibes 🫠

🌸 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 removed in OpenSSL 4.0.

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

The <-ocsp_check_leaf> and -ocsp_check_all options were added in OpenSSL 3.6.

The ech options were added in OpenSSL 4.0.

The -expected-rpks option was added in OpenSSL 4.0.

Copyright 2000-2026 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 bestieee 🥹💖 this one's from OpenSSL 4.0.3, dated 2026-09-29, and it goes by OPENSSL-S_CLIENT(1SSL) at the top of the page. go forth and debug responsibly, no cap 👑✨
📦 where this page comes from