Provided by: openssl_1.0.2g-1ubuntu4.20_amd64 
NAME
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] [-cipher cipherlist] [-serverpref]
[-starttls protocol] [-engine id] [-tlsextdebug] [-no_ticket] [-sess_out filename] [-sess_in filename]
[-rand file(s)] [-serverinfo types] [-status] [-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 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.
-purpose, -ignore_critical, -issuer_checks, -crl_check, -crl_check_all, -policy_check, -extended_crl,
-x509_strict, -policy -check_ss_sig -no_alt_chains
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
display the whole server certificate chain: normally only the server certificate itself is displayed.
-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.
-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.
-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.
-ssl2, -ssl3, -tls1, -tls1_1, -tls1_2, -no_ssl2, -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 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.
-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", and "ftp".
-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(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.
-nextprotoneg protocols
enable Next Protocol Negotiation TLS extension and provide a list of 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 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 the
whole chain.
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.
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.