s_client - SSL/TLS client program
The B<s_client> command implements a generic SSL/TLS client which connects
to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
=item B<-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.
=item B<-servername name>
Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
=item B<-cert certname>
The certificate to use, if one is requested by the server. The default is
not to use a certificate.
=item B<-certform format>
The certificate format to use: DER or PEM. PEM is the default.
=item B<-key keyfile>
The private key to use. If not specified then the certificate file will
=item B<-keyform format>
The private format to use: DER or PEM. PEM is the default.
=item B<-pass arg>
the private key password source. For more information about the format of B<arg>
see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.
=item B<-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.
=item B<-CApath directory>
The directory to use for server certificate verification. This directory
must be in "hash format", see B<verify> for more information. These are
also used when building the client certificate chain.
=item B<-CAfile file>
A file containing trusted certificates to use during server authentication
and to use when attempting to build the client certificate chain.
=item B<-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
L<B<verify>|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.
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
prints out the SSL session states.
print extensive debugging information including a hex dump of all traffic.
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
inhibit printing of session and certificate information. This implicitly
turns on B<-ign_eof> as well.
shut down the connection when end of file is reached in the input.
Can be used to override the implicit B<-ign_eof> after B<-quiet>.
=item B<-psk_identity identity>
Use the PSK identity B<identity> when using a PSK cipher suite.
=item B<-psk key>
Use the PSK key B<key> when using a PSK cipher suite. The key is
given as a hexadecimal number without leading 0x, for example -psk
=item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-tls1_1>, B<-tls1_2>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>
These options require or disable the use of the specified SSL or TLS protocols.
By default the initial handshake uses a I<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.
=item B<-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 B<ciphers>
command for more information.
use the server's cipher preferences; only used for SSLV2.
=item B<-starttls protocol>
send the protocol-specific message(s) to switch to TLS for communication.
B<protocol> is a keyword for the intended protocol. Currently, the only
supported keywords are "smtp", "pop3", "imap", and "ftp".
print out a hex dump of any TLS extensions received from the server.
disable RFC4507bis session ticket support.
=item B<-sess_out filename>
output SSL session to B<filename>
=item B<-sess_in sess.pem>
load SSL session from B<filename>. The client will attempt to resume a
connection from this session.
=item B<-engine id>
specifying an engine (by its unique B<id> string) will cause B<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.
=item B<-rand file(s)>
a file or files containing random data used to seed the random number
generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).
Multiple files can be specified separated by a OS-dependent character.
The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
=item B<-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.
=item B<-alpn protocols>, B<-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 B<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
reciving ServerHello with a list of server supported protocols.
=head1 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 B<-quiet> nor B<-ign_eof>
have been given), the session will be renegotiated if the line begins with an
B<R>, and if the line begins with a B<Q> or if end of file is reached, the
connection will be closed down.
B<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 B<-bugs>, B<-ssl2>,
B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried
in case it is a buggy server. In particular you should play with these
options B<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 B<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 B<-prexit> option and send an HTTP request
for an appropriate page.
If a certificate is specified on the command line using the B<-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
B<-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
The B<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 B<not> do this as it makes them vulnerable to a MITM
attack. This behaviour can be changed by with the B<-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 B<-prexit> option is a bit of a hack. We should really report
information whenever a session is renegotiated.
=head1 SEE ALSO
L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>
The -no_alt_chains options was first added to OpenSSL 1.0.2b.