s_client manpage

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S_CLIENT(1)			    OpenSSL			   S_CLIENT(1)

       s_client - SSL/TLS client program

       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] [-trusted_first] [-no_alt_chains] [-reconnect]
       [-pause] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio]
       [-crlf] [-ign_eof] [-no_ign_eof] [-quiet] [-ssl3] [-tls1] [-tls1_1]
       [-tls1_2] [-dtls1] [-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2]
       [-fallback_scsv] [-bugs] [-cipher cipherlist] [-krb5svc service]
       [-serverpref] [-starttls protocol] [-xmpphost hostname] [-name
       hostname] [-engine id] [-tlsextdebug] [-no_ticket] [-sess_out filename]
       [-sess_in filename] [-rand file(s)] [-serverinfo types] [-status]
       [-alpn protocols] [-nextprotoneg protocols]

       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.

       -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.

	   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,
       -trusted_first -no_alt_chains
	   Set various certificate chain valiadition option. See the verify
	   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.

	   pauses 1 second between each read and write call.

	   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 established.

	   prints out the SSL session states.

	   print extensive debugging information including a hex dump of all

	   show all protocol messages with hex dump.

	   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 input.

	   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.

       -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

       -ssl3, -tls1, -tls1_1, -tls1_2, -dtls1, -no_ssl3, -no_tls1, -no_tls1_1,
	   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.

	   Send TLS_FALLBACK_SCSV in the ClientHello.

	   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.

	   use the server's cipher preferences; only used for SSLV2.

       -krb5svc service
	   the Kerberos service name to use (default "host"). This means
	   s_server will expect a ticket for the principal
	   service/hostname@REALM, and will need keys for that principal in
	   its keytab.

       -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", "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.

       -name hostname
	   the host name to use with "-starttls smtp".	If this option is not
	   specified, the default "openssl.client.net" will be used.

	   print out a hex dump of any TLS extensions received from the

	   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.

	   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.

       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 1.0.2b.

1.0.2k				  2022-07-22			   S_CLIENT(1)