S_CLIENT(1SSL) OpenSSL S_CLIENT(1SSL)
openssl-s_client, s_client - SSL/TLS client program
openssl s_client [-help] [-connect host:port] [-proxy host:port] [-unix
path] [-4] [-6] [-servername name] [-verify depth]
[-verify_return_error] [-cert filename] [-certform DER|PEM] [-key
filename] [-keyform DER|PEM] [-pass arg] [-CApath directory] [-CAfile
filename] [-no-CAfile] [-no-CApath] [-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] [-verify_depth num] [-verify_email email]
[-verify_hostname hostname] [-verify_ip ip] [-verify_name name]
[-x509_strict] [-reconnect] [-showcerts] [-debug] [-msg] [-nbio_test]
[-state] [-nbio] [-crlf] [-ign_eof] [-no_ign_eof] [-quiet] [-ssl3]
[-tls1] [-tls1_1] [-tls1_2] [-no_ssl3] [-no_tls1] [-no_tls1_1]
[-no_tls1_2] [-dtls] [-dtls1] [-dtls1_2] [-fallback_scsv] [-async]
[-split_send_frag] [-max_pipelines] [-read_buf] [-bugs] [-comp]
[-no_comp] [-sigalgs sigalglist] [-curves curvelist] [-cipher
cipherlist] [-serverpref] [-starttls protocol] [-xmpphost hostname]
[-engine id] [-tlsextdebug] [-no_ticket] [-sess_out filename] [-sess_in
filename] [-rand file(s)] [-serverinfo types] [-status] [-alpn
protocols] [-nextprotoneg protocols] [-ct|noct] [-ctlogfile]
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
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.
Print out a usage message.
This specifies the host and optional port to connect to. If not
specified then an attempt is made to connect to the local host on
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.
Connect over the specified Unix-domain socket.
-4 Use IPv4 only.
-6 Use IPv6 only.
Set the TLS SNI (Server Name Indication) extension in the
The certificate to use, if one is requested by the server. The
default is not to use a certificate.
The certificate format to use: DER or PEM. PEM is the default.
The private key to use. If not specified then the certificate file
will be used.
The private format to use: DER or PEM. PEM is the default.
the private key password source. For more information about the
format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).
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.
Return verification errors instead of continuing. This will
typically abort the handshake with a fatal error.
The directory to use for server certificate verification. This
directory must be in "hash format", see verify for more
information. These are also used when building the client
A file containing trusted certificates to use during server
authentication and to use when attempting to build the client
Do not load the trusted CA certificates from the default file
Do not load the trusted CA certificates from the default directory
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
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.
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
$ 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
Verified peername: smtp.example.com
DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
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
-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.
reconnects to the same server 5 times using the same session ID,
this can be used as a test that session caching is working.
display the whole server certificate chain: normally only the
server certificate itself is displayed.
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
prints out the SSL session states.
print extensive debugging information including a hex dump of all
show all protocol messages with hex dump.
show verbose trace output of protocol messages. OpenSSL needs to be
compiled with enable-ssl-trace for this option to work.
file to send output of -msg or -trace to, default standard output.
tests non-blocking I/O
turns on non-blocking I/O
this option translated a line feed from the terminal into CR+LF as
required by some servers.
inhibit shutting down the connection when end of file is reached in
inhibit printing of session and certificate information. This
implicitly turns on -ign_eof as well.
shut down the connection when end of file is reached in the input.
Can be used to override the implicit -ign_eof after -quiet.
Use the PSK identity identity when using a PSK cipher suite. The
default value is "Client_identity" (without the quotes).
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
-ssl3, -tls1, -tls1_1, -tls1_2, -no_ssl3, -no_tls1, -no_tls1_1,
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
-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
Send TLS_FALLBACK_SCSV in the ClientHello.
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
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 ciphersuite
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.
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 ciphersuite has
been negotiated. The default value is 1. See
SSL_CTX_set_max_pipelines(3) for further information.
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).
there are several known bug in SSL and TLS implementations. Adding
this option enables various workarounds.
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.
Disables support for SSL/TLS compression. TLS compression is not
recommended and is off by default as of OpenSSL 1.1.0.
only provide a brief summary of connection parameters instead of
the normal verbose output.
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)
Specifies the list of supported curves to be sent by the client.
The curve is is ultimately selected by the server. For a list of
all curves, use:
$ openssl ecparam -list_curves
this allows the cipher list sent by the client to be modified.
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.
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", and "irc."
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.
print out a hex dump of any TLS extensions received from the
disable RFC4507bis session ticket support.
output SSL session to filename
load SSL session from filename. The client will attempt to resume a
connection from this session.
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.
a file or files containing random data used to seed the random
number generator, or an EGD socket (see RAND_egd(3)). Multiple
files can be specified separated by an OS-dependent character. The
separator is ; for MS-Windows, , for OpenVMS, and : for all others.
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.
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 extension, respectively.
ALPN is the IETF standard and replaces NPN. The protocols list is
a comma-separated protocol names that the client should advertise
support for. The list should contain most wanted protocols first.
Protocol names are printable ASCII strings, for example "http/1.1"
or "spdy/3". 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
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.
A file containing a list of known Certificate Transparency logs.
See SSL_CTX_set_ctlog_list_file(3) for the expected file format.
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. When used interactively (which means neither -quiet nor
-ign_eof have been given), the session will be renegotiated if the line
begins with an R, and if the line begins with a Q or if end of file is
reached, the connection will be closed down.
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
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 the whole chain.
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.
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.
SSL_CONF_cmd(3), sess_id(1), s_server(1), ciphers(1)
The -no_alt_chains options was first added to OpenSSL 1.1.0.
Copyright 2000-2016 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
1.1.0g 2017-11-04 S_CLIENT(1SSL)