/usr/man/cat.1/s_client.1
S_CLIENT(1) OpenSSL S_CLIENT(1)
NAME
openssl-s_client, s_client - SSL/TLS client program
SYNOPSIS
openssl s_client [-connect host:port] [-servername name]
[-verify depth] [-verify_return_error] [-cert filename]
[-certform DER|PEM] [-key filename] [-keyform DER|PEM]
[-pass arg] [-CApath directory] [-CAfile filename]
[-no_alt_chains] [-reconnect] [-pause] [-showcerts] [-debug]
[-msg] [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof]
[-no_ign_eof] [-quiet] [-ssl2] [-ssl3] [-tls1] [-no_ssl2]
[-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2]
[-fallback_scsv] [-bugs] [-sigalgs sigalglist] [-curves
curvelist] [-cipher cipherlist] [-serverpref] [-starttls
protocol] [-engine id] [-tlsextdebug] [-no_ticket]
[-sess_out filename] [-sess_in filename] [-rand file(s)]
[-serverinfo types] [-status] [-alpn protocols]
[-nextprotoneg protocols]
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
-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.
-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 be used.
-keyform format
The private format to use: DER or PEM. PEM is the
default.
-pass arg
the private key password source. For more information
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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.
-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.
-crl_check_all, -policy_check, -extended_crl, -x509_strict,
-policy -check_ss_sig -no_alt_chains
-purpose, -ignore_critical, -issuer_checks, -crl_check,
Set various certificate chain valiadition option. See
the verify 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.
-pause
pauses 1 second between each read and write call.
-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
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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.
-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.
-no_tls1, -no_tls1_1, -no_tls1_2
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-ssl2, -ssl3, -tls1, -tls1_1, -tls1_2, -no_ssl2, -no_ssl3,
These options require or disable the use of the
specified SSL or TLS protocols. By default the initial
handshake uses a version-flexible method which will
negotiate the highest mutually supported protocol
version.
-fallback_scsv
Send TLS_FALLBACK_SCSV in the ClientHello.
-bugs
there are several known bug in SSL and TLS
implementations. Adding this option enables various
workarounds.
-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.
-serverpref
use the server's cipher preferences; only used for
SSLV2.
-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" and "xmpp".
-tlsextdebug
print out a hex dump of any TLS extensions received from
the server.
-no_ticket
disable RFC4507bis session ticket support.
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-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 a OS-dependent character. The separator is ; for
MS-Windows, , for OpenVMS, and : for all others.
-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
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 reciving ServerHello with a list of server
supported protocols.
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. 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
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reached, the connection will be closed down.
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, -ssl2, -ssl3, -tls1, -no_ssl2, -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.
Since the SSLv23 client hello cannot include compression
methods or extensions these will only be supported if its
use is disabled, for example by using the -no_sslv2 option.
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.
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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
sess_id(1), s_server(1), ciphers(1)
HISTORY
The -no_alt_chains options was first added to OpenSSL
1.0.2b.
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See also openssl-s_client(1)
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