Provided by: openssl_3.5.0-2ubuntu1_amd64 
NAME
openssl-s_client - SSL/TLS client program
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
This command implements a generic SSL/TLS client which connects to a remote host using SSL/TLS. It is a
very useful diagnostic tool for SSL servers.
OPTIONS
In addition to the options below, this command also supports the common and client only options
documented in the "Supported Command Line Commands" section of the SSL_CONF_cmd(3) manual page.
-help
Print out a usage message.
-ssl_config section
Use the specified section of the configuration file to configure the SSL_CTX object.
-connect host:port
This specifies the host and optional port to connect to. It is possible to select the host and port
using the optional target positional argument instead. If neither this nor the target positional
argument are specified then an attempt is made to connect to the local host on port 4433. 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
A file containing a list of certificates whose subject names will be sent to the server in the
certificate_authorities extension. Only supported for TLS 1.3
-dane_tlsa_domain domain
Enable RFC6698/RFC7671 DANE TLSA authentication and specify the TLSA base domain which becomes the
default SNI hint and the primary reference identifier for hostname checks. This must be used in
combination with at least one instance of the -dane_tlsa_rrdata option below.
When DANE authentication succeeds, the diagnostic output will include the lowest (closest to 0) depth
at which a TLSA record authenticated a chain certificate. When that TLSA record is a "2 1 0" trust
anchor public key that signed (rather than matched) the top-most certificate of the chain, the result
is reported as "TA public key verified". Otherwise, either the TLSA record "matched TA certificate"
at a positive depth or else "matched EE certificate" at depth 0.
-dane_tlsa_rrdata rrdata
Use one or more times to specify the RRDATA fields of the DANE TLSA RRset associated with the target
service. The rrdata value is specified in "presentation form", that is four whitespace separated
fields that specify the usage, selector, matching type and associated data, with the last of these
encoded in hexadecimal. Optional whitespace is ignored in the associated data field. For example:
$ openssl s_client -brief -starttls smtp \
-connect smtp.example.com:25 \
-dane_tlsa_domain smtp.example.com \
-dane_tlsa_rrdata "2 1 1
B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
-dane_tlsa_rrdata "2 1 1
60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
...
Verification: OK
Verified peername: smtp.example.com
DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
...
-dane_ee_no_namechecks
This disables server name checks when authenticating via DANE-EE(3) TLSA records. For some
applications, primarily web browsers, it is not safe to disable name checks due to "unknown key
share" attacks, in which a malicious server can convince a client that a connection to a victim
server is instead a secure connection to the malicious server. The malicious server may then be able
to violate cross-origin scripting restrictions. Thus, despite the text of RFC7671, name checks are
by default enabled for DANE-EE(3) TLSA records, and can be disabled in applications where it is safe
to do so. In particular, SMTP and XMPP clients should set this option as SRV and MX records already
make it possible for a remote domain to redirect client connections to any server of its choice, and
in any case SMTP and XMPP clients do not execute scripts downloaded from remote servers.
-reconnect
Reconnects to the same server 5 times using the same session ID, this can be used as a test that
session caching is working.
-showcerts
Displays the server certificate list as sent by the server: it only consists of certificates the
server has sent (in the order the server has sent them). It is not a verified chain.
-prexit
Print session information when the program exits. This will always attempt to print out information
even if the connection fails. Normally information will only be printed out once if the connection
succeeds. This option is useful because the cipher in use may be renegotiated or the connection may
fail because a client certificate is required or is requested only after an attempt is made to access
a certain URL. Note: the output produced by this option is not always accurate because a connection
might never have been established.
-no-interactive
This flag can be used to run the client in a non-interactive mode.
-state
Prints out the SSL session states.
-debug
Print extensive debugging information including a hex dump of all traffic.
-nocommands
Do not use interactive command letters.
-adv
Use advanced command mode.
-security_debug
Enable security debug messages.
-security_debug_verbose
Output more security debug output.
-msg
Show protocol messages.
-timeout
Enable send/receive timeout on DTLS connections.
-mtu size
Set MTU of the link layer to the specified size.
-no_ems
Disable Extended master secret negotiation.
-keymatexport label
Export keying material using the specified label.
-keymatexportlen len
Export the specified number of bytes of keying material; default is 20.
Show all protocol messages with hex dump.
-trace
Show verbose trace output of protocol messages.
-msgfile filename
File to send output of -msg or -trace to, default standard output.
-nbio_test
Tests nonblocking I/O
-nbio
Turns on nonblocking I/O
-crlf
This option translated a line feed from the terminal into CR+LF as required by some servers.
-ign_eof
Inhibit shutting down the connection when end of file is reached in the input.
-quiet
Inhibit printing of session and certificate information. This implicitly turns on -ign_eof as well.
-no_ign_eof
Shut down the connection when end of file is reached in the input. Can be used to override the
implicit -ign_eof after -quiet.
-psk_identity identity
Use the PSK identity identity when using a PSK cipher suite. The default value is "Client_identity"
(without the quotes).
-psk key
Use the PSK key key when using a PSK cipher suite. The key is given as a hexadecimal number without
leading 0x, for example -psk 1a2b3c4d. This option must be provided in order to use a PSK cipher.
-psk_session file
Use the pem encoded SSL_SESSION data stored in file as the basis of a PSK. Note that this will only
work if TLSv1.3 is negotiated.
-sctp
Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in conjunction with -dtls,
-dtls1 or -dtls1_2. This option is only available where OpenSSL has support for SCTP enabled.
-sctp_label_bug
Use the incorrect behaviour of older OpenSSL implementations when computing endpoint-pair shared
secrets for DTLS/SCTP. This allows communication with older broken implementations but breaks
interoperability with correct implementations. Must be used in conjunction with -sctp. This option is
only available where OpenSSL has support for SCTP enabled.
-fallback_scsv
Send TLS_FALLBACK_SCSV in the ClientHello.
-async
Switch on asynchronous mode. Cryptographic operations will be performed asynchronously. This will
only have an effect if an asynchronous capable engine is also used via the -engine option. For test
purposes the dummy async engine (dasync) can be used (if available).
-maxfraglen len
Enable Maximum Fragment Length Negotiation; allowed values are 512, 1024, 2048, and 4096.
-max_send_frag int
The maximum size of data fragment to send. See SSL_CTX_set_max_send_fragment(3) for further
information.
-split_send_frag int
The size used to split data for encrypt pipelines. If more data is written in one go than this value
then it will be split into multiple pipelines, up to the maximum number of pipelines defined by
max_pipelines. This only has an effect if a suitable cipher suite has been negotiated, an engine that
supports pipelining has been loaded, and max_pipelines is greater than 1. See
SSL_CTX_set_split_send_fragment(3) for further information.
-max_pipelines int
The maximum number of encrypt/decrypt pipelines to be used. This will only have an effect if an
engine has been loaded that supports pipelining (e.g. the dasync engine) and a suitable cipher suite
has been negotiated. The default value is 1. See SSL_CTX_set_max_pipelines(3) for further
information.
-read_buf int
The default read buffer size to be used for connections. This will only have an effect if the buffer
size is larger than the size that would otherwise be used and pipelining is in use (see
SSL_CTX_set_default_read_buffer_len(3) for further information).
-ignore_unexpected_eof
Some TLS implementations do not send the mandatory close_notify alert on shutdown. If the application
tries to wait for the close_notify alert but the peer closes the connection without sending it, an
error is generated. When this option is enabled the peer does not need to send the close_notify alert
and a closed connection will be treated as if the close_notify alert was received. For more
information on shutting down a connection, see SSL_shutdown(3).
-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 provide a brief summary of connection parameters instead of the normal verbose output.
-starttls protocol
Send the protocol-specific message(s) to switch to TLS for communication. protocol is a keyword for
the intended protocol. Currently, the only supported keywords are "smtp", "pop3", "imap", "ftp",
"xmpp", "xmpp-server", "irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".
-xmpphost hostname
This option, when used with "-starttls xmpp" or "-starttls xmpp-server", specifies the host for the
"to" attribute of the stream element. If this option is not specified, then the host specified with
"-connect" will be used.
This option is an alias of the -name option for "xmpp" and "xmpp-server".
-name hostname
This option is used to specify hostname information for various protocols used with -starttls option.
Currently only "xmpp", "xmpp-server", "smtp" and "lmtp" can utilize this -name option.
If this option is used with "-starttls xmpp" or "-starttls xmpp-server", if specifies the host for
the "to" attribute of the stream element. If this option is not specified, then the host specified
with "-connect" will be used.
If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies the name to use in the
"LMTP LHLO" or "SMTP EHLO" message, respectively. If this option is not specified, then
"mail.example.com" will be used.
-tlsextdebug
Print out a hex dump of any TLS extensions received from the server.
-sess_out filename
Output SSL session to filename.
-sess_in filename
Load SSL session from filename. The client will attempt to resume a connection from this session.
-serverinfo types
A list of comma-separated TLS Extension Types (numbers between 0 and 65535). Each type will be sent
as an empty ClientHello TLS Extension. The server's response (if any) will be encoded and displayed
as a PEM file.
-status
Sends a certificate status request to the server (OCSP stapling). The server response (if any) is
printed out.
-alpn protocols, -nextprotoneg protocols
These flags enable the Enable the Application-Layer Protocol Negotiation or Next Protocol Negotiation
(NPN) extension, respectively. ALPN is the IETF standard and replaces NPN. The protocols list is a
comma-separated list of protocol names that the client should advertise support for. The list should
contain the most desirable protocols first. Protocol names are printable ASCII strings, for example
"http/1.1" or "spdy/3". An empty list of protocols is treated specially and will cause the client to
advertise support for the TLS extension but disconnect just after receiving ServerHello with a list
of server supported protocols. The flag -nextprotoneg cannot be specified if -tls1_3 is used.
-ct, -noct
Use one of these two options to control whether Certificate Transparency (CT) is enabled (-ct) or
disabled (-noct). If CT is enabled, signed certificate timestamps (SCTs) will be requested from the
server and reported at handshake completion.
Enabling CT also enables OCSP stapling, as this is one possible delivery method for SCTs.
-ctlogfile
A file containing a list of known Certificate Transparency logs. See SSL_CTX_set_ctlog_list_file(3)
for the expected file format.
-keylogfile file
Appends TLS secrets to the specified keylog file such that external programs (like Wireshark) can
decrypt TLS connections.
-early_data file
Reads the contents of the specified file and attempts to send it as early data to the server. This
will only work with resumed sessions that support early data and when the server accepts the early
data.
-enable_pha
For TLSv1.3 only, send the Post-Handshake Authentication extension. This will happen whether or not a
certificate has been provided via -cert.
-use_srtp value
Offer SRTP key management, where value is a colon-separated profile list.
-srpuser value
Set the SRP username to the specified value. This option is deprecated.
-srppass value
Set the SRP password to the specified value. This option is deprecated.
-srp_lateuser
SRP username for the second ClientHello message. This option is deprecated.
-srp_moregroups This option is deprecated.
Tolerate other than the known g and N values.
-srp_strength number
Set the minimal acceptable length, in bits, for N. This option is deprecated.
-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.
-tfo
Enable creation of connections via TCP fast open (RFC7413).
-no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2, -no_tls1_3, -ssl3, -tls1, -tls1_1, -tls1_2, -tls1_3
See "TLS Version Options" in openssl(1).
-dtls, -dtls1, -dtls1_2
These specify the use of DTLS instead of TLS. See "TLS Version Options" in openssl(1).
-nameopt option
This specifies how the subject or issuer names are displayed. See openssl-namedisplay-options(1) for
details.
-xkey infile, -xcert file, -xchain file, -xchain_build file, -xcertform DER|PEM, -xkeyform DER|PEM
Set extended certificate verification options. See "Extended Verification Options" in
openssl-verification-options(1) for details.
-CAfile file, -no-CAfile, -CApath dir, -no-CApath, -CAstore uri, -no-CAstore
See "Trusted Certificate Options" in openssl-verification-options(1) for details.
-bugs, -comp, -no_comp, -no_ticket, -serverpref, -client_renegotiation, -legacy_renegotiation,
-no_renegotiation, -no_resumption_on_reneg, -legacy_server_connect, -no_legacy_server_connect, -no_etm
-allow_no_dhe_kex, -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.
-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).
-engine id
See "Engine Options" in openssl(1). This option is deprecated.
-ssl_client_engine id
Specify engine to be used for client certificate operations.
-allow_proxy_certs, -attime, -no_check_time, -check_ss_sig, -crl_check, -crl_check_all, -explicit_policy,
-extended_crl, -ignore_critical, -inhibit_any, -inhibit_map, -no_alt_chains, -partial_chain, -policy,
-policy_check, -policy_print, -purpose, -suiteB_128, -suiteB_128_only, -suiteB_192, -trusted_first,
-use_deltas, -auth_level, -verify_depth, -verify_email, -verify_hostname, -verify_ip, -verify_name,
-x509_strict -issuer_checks
Set various options of certificate chain verification. See "Verification Options" in
openssl-verification-options(1) for details.
Verification errors are displayed, for debugging, but the command will proceed unless the
-verify_return_error option is used.
-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.
-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.
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 "]".
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.
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.
Q End the current SSL connection and exit.
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. If end of file is reached then the connection will be closed down.
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.
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.
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 can be used to debug SSL servers. To connect to an SSL HTTP server the command:
openssl s_client -connect servername:443
would typically be used (https uses port 443). If the connection succeeds then an HTTP command can be
given such as "GET /" to retrieve a web page.
If the handshake fails then there are several possible causes, if it is nothing obvious like no client
certificate then the -bugs, -ssl3, -tls1, -no_ssl3, -no_tls1 options can be tried in case it is a buggy
server. In particular you should play with these options before submitting a bug report to an OpenSSL
mailing list.
A frequent problem when attempting to get client certificates working is that a web client complains it
has no certificates or gives an empty list to choose from. This is normally because the server is not
sending the clients certificate authority in its "acceptable CA list" when it requests a certificate. By
using this command, the CA list can be viewed and checked. However, some servers only request client
authentication after a specific URL is requested. To obtain the list in this case it is necessary to use
the -prexit option and send an HTTP request for an appropriate page.
If a certificate is specified on the command line using the -cert option it will not be used unless the
server specifically requests a client certificate. Therefore, merely including a client certificate on
the command line is no guarantee that the certificate works.
If there are problems verifying a server certificate then the -showcerts option can be used to show all
the certificates sent by the server.
This command is a test tool and is designed to continue the handshake after any certificate verification
errors. As a result it will accept any certificate chain (trusted or not) sent by the peer. Non-test
applications should not do this as it makes them vulnerable to a MITM attack. This behaviour can be
changed by with the -verify_return_error option: any verify errors are then returned aborting the
handshake.
The -bind option may be useful if the server or a firewall requires connections to come from some
particular address and or port.
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
openssl(1), openssl-sess_id(1), openssl-s_server(1), openssl-ciphers(1), SSL_CONF_cmd(3),
SSL_CTX_set_max_send_fragment(3), SSL_CTX_set_split_send_fragment(3), SSL_CTX_set_max_pipelines(3),
ossl_store-file(7)
HISTORY
The -no_alt_chains option was added in OpenSSL 1.1.0. The -name option was added in OpenSSL 1.1.1.
The -certform option has become obsolete in OpenSSL 3.0.0 and has no effect.
The -engine option was deprecated in OpenSSL 3.0.
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
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>.
3.5.0 2025-06-04 OPENSSL-S_CLIENT(1SSL)