Provided by: openssl_1.1.1-1ubuntu2.1~18.04.23_amd64 
NAME
openssl-s_client, s_client - SSL/TLS client program
SYNOPSIS
openssl s_client [-help] [-connect host:port] [-bind host:port] [-proxy host:port] [-unix path] [-4] [-6]
[-servername name] [-noservername] [-verify depth] [-verify_return_error] [-cert filename] [-certform
DER|PEM] [-key filename] [-keyform DER|PEM] [-cert_chain filename] [-build_chain] [-xkey] [-xcert]
[-xchain] [-xchain_build] [-xcertform PEM|DER] [-xkeyform PEM|DER] [-pass arg] [-CApath directory]
[-CAfile filename] [-chainCApath directory] [-chainCAfile filename] [-no-CAfile] [-no-CApath]
[-requestCAfile filename] [-dane_tlsa_domain domain] [-dane_tlsa_rrdata rrdata] [-dane_ee_no_namechecks]
[-attime timestamp] [-check_ss_sig] [-crl_check] [-crl_check_all] [-explicit_policy] [-extended_crl]
[-ignore_critical] [-inhibit_any] [-inhibit_map] [-no_check_time] [-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] [-nameopt option] [-verify_depth num]
[-verify_email email] [-verify_hostname hostname] [-verify_ip ip] [-verify_name name] [-build_chain]
[-x509_strict] [-reconnect] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof]
[-no_ign_eof] [-psk_identity identity] [-psk key] [-psk_session file] [-quiet] [-ssl3] [-tls1] [-tls1_1]
[-tls1_2] [-tls1_3] [-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2] [-no_tls1_3] [-dtls] [-dtls1]
[-dtls1_2] [-sctp] [-fallback_scsv] [-async] [-max_send_frag] [-split_send_frag] [-max_pipelines]
[-read_buf] [-bugs] [-comp] [-no_comp] [-allow_no_dhe_kex] [-sigalgs sigalglist] [-curves curvelist]
[-cipher cipherlist] [-ciphersuites val] [-serverpref] [-starttls protocol] [-xmpphost hostname] [-name
hostname] [-engine id] [-tlsextdebug] [-no_ticket] [-sess_out filename] [-sess_in filename] [-rand
file...] [-writerand file] [-serverinfo types] [-status] [-alpn protocols] [-nextprotoneg protocols]
[-ct] [-noct] [-ctlogfile] [-keylogfile file] [-early_data file] [-enable_pha] [target]
DESCRIPTION
The s_client 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 the s_client utility also supports the common and client only options
documented in the in the "Supported Command Line Commands" section of the SSL_CONF_cmd(3) manual page.
-help
Print out a usage message.
-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.
-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.
-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.
-unix path
Connect over the specified Unix-domain socket.
-4 Use IPv4 only.
-6 Use IPv6 only.
-servername name
Set the TLS SNI (Server Name Indication) extension in the ClientHello message to the given value. If
both this option and the -noservername are not given, the TLS SNI extension is still set to the
hostname provided to the -connect option, or "localhost" if -connect has not been supplied. This is
default since OpenSSL 1.1.1.
Even though SNI name should normally be a DNS name and not an IP address, this option will not make
the distinction when parsing -connect and will send IP address if one passed.
-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 certname
The certificate to use, if one is requested by the server. The default is not to use a certificate.
-certform format
The certificate format to use: DER or PEM. PEM is the default.
-key keyfile
The private key to use. If not specified then the certificate file will be used.
-keyform format
The private format to use: DER or PEM. PEM is the default.
-cert_chain
A file containing trusted certificates to use when attempting to build the client/server certificate
chain related to the certificate specified via the -cert option.
-build_chain
Specify whether the application should build the certificate chain to be provided to the server.
-xkey infile, -xcert infile, -xchain
Specify an extra certificate, private key and certificate chain. These behave in the same manner as
the -cert, -key and -cert_chain options. When specified, the callback returning the first valid
chain will be in use by the client.
-xchain_build
Specify whether the application should build the certificate chain to be provided to the server for
the extra certificates provided via -xkey infile, -xcert infile, -xchain options.
-xcertform PEM|DER, -xkeyform PEM|DER
Extra certificate and private key format respectively.
-pass arg
the private key password source. For more information about the format of arg see the PASS PHRASE
ARGUMENTS section in openssl(1).
-verify depth
The verify depth to use. This specifies the maximum length of the server certificate chain and turns
on server certificate verification. Currently 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.
-verify_return_error
Return verification errors instead of continuing. This will typically abort the handshake with a
fatal error.
-nameopt option
Option which determines how the subject or issuer names are displayed. The option argument can be a
single option or multiple options separated by commas. Alternatively the -nameopt switch may be used
more than once to set multiple options. See the x509(1) manual page for details.
-CApath directory
The directory to use for server certificate verification. This directory must be in "hash format",
see verify(1) for more information. These are also used when building the client certificate chain.
-CAfile file
A file containing trusted certificates to use during server authentication and to use when attempting
to build the client certificate chain.
-chainCApath directory
The directory to use for building the chain provided to the server. This directory must be in "hash
format", see verify(1) for more information.
-chainCAfile file
A file containing trusted certificates to use when attempting to build the client certificate chain.
-no-CAfile
Do not load the trusted CA certificates from the default file location
-no-CApath
Do not load the trusted CA certificates from the default directory location
-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 specied 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.
-attime, -check_ss_sig, -crl_check, -crl_check_all, -explicit_policy, -extended_crl, -ignore_critical,
-inhibit_any, -inhibit_map, -no_alt_chains, -no_check_time, -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
Set various certificate chain validation options. See the verify(1) manual page for details.
-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.
-state
Prints out the SSL session states.
-debug
Print extensive debugging information including a hex dump of all traffic.
-msg
Show all protocol messages with hex dump.
-trace
Show verbose trace output of protocol messages. OpenSSL needs to be compiled with enable-ssl-trace
for this option to work.
-msgfile
File to send output of -msg or -trace to, default standard output.
-nbio_test
Tests non-blocking I/O
-nbio
Turns on non-blocking 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.
-ssl3, -tls1, -tls1_1, -tls1_2, -tls1_3, -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2, -no_tls1_3
These options require or disable the use of the specified SSL or TLS protocols. By default s_client
will negotiate the highest mutually supported protocol version. When a specific TLS version is
required, only that version will be offered to and accepted from the server. Note that not all
protocols and flags may be available, depending on how OpenSSL was built.
-dtls, -dtls1, -dtls1_2
These options make s_client use DTLS protocols instead of TLS. With -dtls, s_client will negotiate
any supported DTLS protocol version, whilst -dtls1 and -dtls1_2 will only support DTLS1.0 and DTLS1.2
respectively.
-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.
-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).
-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).
-bugs
There are several known bug in SSL and TLS implementations. Adding this option enables various
workarounds.
-comp
Enables support for SSL/TLS compression. This option was introduced in OpenSSL 1.1.0. TLS
compression is not recommended and is off by default as of OpenSSL 1.1.0.
-no_comp
Disables support for SSL/TLS compression. TLS compression is not recommended and is off by default
as of OpenSSL 1.1.0.
-brief
Only provide a brief summary of connection parameters instead of the normal verbose output.
-sigalgs sigalglist
Specifies the list of signature algorithms that are sent by the client. The server selects one entry
in the list based on its preferences. For example strings, see SSL_CTX_set1_sigalgs(3)
-curves curvelist
Specifies the list of supported curves to be sent by the client. The curve is ultimately selected by
the server. For a list of all curves, use:
$ openssl ecparam -list_curves
-cipher cipherlist
This allows the TLSv1.2 and below cipher list sent by the client to be modified. This list will be
combined with any TLSv1.3 ciphersuites that have been configured. Although the server determines
which ciphersuite is used it should take the first supported cipher in the list sent by the client.
See the ciphers command for more information.
-ciphersuites val
This allows the TLSv1.3 ciphersuites sent by the client to be modified. This list will be combined
with any TLSv1.2 and below ciphersuites that have been configured. Although the server determines
which cipher suite is used it should take the first supported cipher in the list sent by the client.
See the ciphers command for more information. The format for this list is a simple colon (":")
separated list of TLSv1.3 ciphersuite names.
-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.
-no_ticket
Disable RFC4507bis session ticket support.
-sess_out filename
Output SSL session to filename.
-sess_in sess.pem
Load SSL session from filename. The client will attempt to resume a connection from this session.
-engine id
Specifying an engine (by its unique id string) will cause s_client to attempt to obtain a functional
reference to the specified engine, thus initialising it if needed. The engine will then be set as the
default for all available algorithms.
-rand file...
A file or files containing random data used to seed the random number generator. Multiple files can
be specified separated by an OS-dependent character. The separator is ; for MS-Windows, , for
OpenVMS, and : for all others.
[-writerand file]
Writes random data to the specified file upon exit. This can be used with a subsequent -rand flag.
-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.
[target]
Rather than providing -connect, the target hostname 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.
CONNECTED COMMANDS
If a connection is established with an SSL 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), then certain
commands are also recognized which perform special operations. These commands are a letter which must
appear at the start of a line. They are listed below.
Q End the current SSL connection and exit.
R Renegotiate the SSL session (TLSv1.2 and below only).
B Send a heartbeat message to the server (DTLS only)
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)
NOTES
s_client 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 s_client 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. Therefor 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.
The s_client utility 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.
None 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.
BUGS
Because this program has a lot of options and also because some of the techniques used are rather old,
the C source of s_client 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
SSL_CONF_cmd(3), sess_id(1), s_server(1), ciphers(1), SSL_CTX_set_max_send_fragment(3),
SSL_CTX_set_split_send_fragment(3), SSL_CTX_set_max_pipelines(3)
HISTORY
The -no_alt_chains option was first added to OpenSSL 1.1.0. The -name option was added in OpenSSL 1.1.1.
COPYRIGHT
Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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>.
1.1.1 2023-05-24 S_CLIENT(1SSL)