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S_CLIENT(1SSL) OpenSSL S_CLIENT(1SSL)

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] [-sctp_label_bug] [-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 "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 -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 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
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.

-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.

-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).

-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 bugs 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 added in OpenSSL 1.1.0. The -name option
was added in OpenSSL 1.1.1.

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>.

1.1.1f 2024-07-30 S_CLIENT(1SSL)