Section: OpenSSL (1SSL)
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s_client - SSL/TLS client program
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)
Print out a usage message.
- -connect host:port
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 port 4433.
- -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.
Use IPv4 only.
Use IPv6 only.
- -servername name
Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
- -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
- -keyform format
The private format to use: DER or PEM. PEM is the default.
- -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.
Return verification errors instead of continuing. This will typically
abort the handshake with a fatal error.
- -CApath directory
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 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.
Do not load the trusted CA certificates from the default file location
Do not load the trusted CA certificates from the default directory location
- -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
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
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
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
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.
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 been
prints out the SSL session states.
print extensive debugging information including a hex dump of all traffic.
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 the
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.
- -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
This option must be provided in order to use a PSK cipher.
- -ssl3, -tls1, -tls1_1, -tls1_2, -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2
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
When a specific TLS version is required, only that version will be offered to
and accepted from the server.
- -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
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 available).
- -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 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.
- -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 ciphersuite 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
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
Disables support for SSL/TLS compression.
TLS compression is not recommended and is off by default as of
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
is ultimately selected by the server. For a list of all curves, use:
$ openssl ecparam -list_curves
- -cipher cipherlist
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.
- -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'',
- -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.
print out a hex dump of any TLS extensions received from the server.
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(s)
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
- -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
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
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
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.
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
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 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 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.
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
- CONNECTED COMMANDS
- SEE ALSO
This document was created by
using the manual pages.
Time: 00:00:39 GMT, September 25, 2018