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.\" ========================================================================
.\"
.IX Title "S_CLIENT 1"
.TH S_CLIENT 1 "2022-10-06" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
openssl\-s_client,
s_client \- SSL/TLS client program
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
\&\fBopenssl\fR \fBs_client\fR
[\fB\-connect host:port\fR]
[\fB\-servername name\fR]
[\fB\-verify depth\fR]
[\fB\-verify_return_error\fR]
[\fB\-cert filename\fR]
[\fB\-certform DER|PEM\fR]
[\fB\-key filename\fR]
[\fB\-keyform DER|PEM\fR]
[\fB\-pass arg\fR]
[\fB\-CApath directory\fR]
[\fB\-CAfile filename\fR]
[\fB\-trusted_first\fR]
[\fB\-no_alt_chains\fR]
[\fB\-reconnect\fR]
[\fB\-pause\fR]
[\fB\-showcerts\fR]
[\fB\-debug\fR]
[\fB\-msg\fR]
[\fB\-nbio_test\fR]
[\fB\-state\fR]
[\fB\-nbio\fR]
[\fB\-crlf\fR]
[\fB\-ign_eof\fR]
[\fB\-no_ign_eof\fR]
[\fB\-quiet\fR]
[\fB\-ssl2\fR]
[\fB\-ssl3\fR]
[\fB\-tls1\fR]
[\fB\-tls1_1\fR]
[\fB\-tls1_2\fR]
[\fB\-dtls1\fR]
[\fB\-no_ssl2\fR]
[\fB\-no_ssl3\fR]
[\fB\-no_tls1\fR]
[\fB\-no_tls1_1\fR]
[\fB\-no_tls1_2\fR]
[\fB\-fallback_scsv\fR]
[\fB\-bugs\fR]
[\fB\-sigalgs sigalglist\fR]
[\fB\-curves curvelist\fR]
[\fB\-cipher cipherlist\fR]
[\fB\-krb5svc service\fR]
[\fB\-serverpref\fR]
[\fB\-starttls protocol\fR]
[\fB\-engine id\fR]
[\fB\-tlsextdebug\fR]
[\fB\-no_ticket\fR]
[\fB\-sess_out filename\fR]
[\fB\-sess_in filename\fR]
[\fB\-rand file(s)\fR]
[\fB\-serverinfo types\fR]
[\fB\-status\fR]
[\fB\-alpn protocols\fR]
[\fB\-nextprotoneg protocols\fR]
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \fBs_client\fR command implements a generic \s-1SSL/TLS\s0 client which connects
to a remote host using \s-1SSL/TLS.\s0 It is a \fIvery\fR useful diagnostic tool for
\&\s-1SSL\s0 servers.
.SH "OPTIONS"
.IX Header "OPTIONS"
.IP "\fB\-connect host:port\fR" 4
.IX Item "-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.
.IP "\fB\-servername name\fR" 4
.IX Item "-servername name"
Set the \s-1TLS SNI\s0 (Server Name Indication) extension in the ClientHello message.
.IP "\fB\-cert certname\fR" 4
.IX Item "-cert certname"
The certificate to use, if one is requested by the server. The default is
not to use a certificate.
.IP "\fB\-certform format\fR" 4
.IX Item "-certform format"
The certificate format to use: \s-1DER\s0 or \s-1PEM. PEM\s0 is the default.
.IP "\fB\-key keyfile\fR" 4
.IX Item "-key keyfile"
The private key to use. If not specified then the certificate file will
be used.
.IP "\fB\-keyform format\fR" 4
.IX Item "-keyform format"
The private format to use: \s-1DER\s0 or \s-1PEM. PEM\s0 is the default.
.IP "\fB\-pass arg\fR" 4
.IX Item "-pass arg"
the private key password source. For more information about the format of \fBarg\fR
see the \fB\s-1PASS PHRASE ARGUMENTS\s0\fR section in \fBopenssl\fR\|(1).
.IP "\fB\-verify depth\fR" 4
.IX Item "-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.
.IP "\fB\-verify_return_error\fR" 4
.IX Item "-verify_return_error"
Return verification errors instead of continuing. This will typically
abort the handshake with a fatal error.
.IP "\fB\-CApath directory\fR" 4
.IX Item "-CApath directory"
The directory to use for server certificate verification. This directory
must be in \*(L"hash format\*(R", see \fBverify\fR for more information. These are
also used when building the client certificate chain.
.IP "\fB\-CAfile file\fR" 4
.IX Item "-CAfile file"
A file containing trusted certificates to use during server authentication
and to use when attempting to build the client certificate chain.
.IP "\fB\-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\fR" 4
.IX Item "-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
\&\fBverify\fR manual page for details.
.IP "\fB\-reconnect\fR" 4
.IX Item "-reconnect"
reconnects to the same server 5 times using the same session \s-1ID,\s0 this can
be used as a test that session caching is working.
.IP "\fB\-pause\fR" 4
.IX Item "-pause"
pauses 1 second between each read and write call.
.IP "\fB\-showcerts\fR" 4
.IX Item "-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
\&\fBnot\fR a verified chain.
.IP "\fB\-prexit\fR" 4
.IX Item "-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 \s-1URL.\s0 Note: the output produced by this
option is not always accurate because a connection might never have been
established.
.IP "\fB\-state\fR" 4
.IX Item "-state"
prints out the \s-1SSL\s0 session states.
.IP "\fB\-debug\fR" 4
.IX Item "-debug"
print extensive debugging information including a hex dump of all traffic.
.IP "\fB\-msg\fR" 4
.IX Item "-msg"
show all protocol messages with hex dump.
.IP "\fB\-nbio_test\fR" 4
.IX Item "-nbio_test"
tests non-blocking I/O
.IP "\fB\-nbio\fR" 4
.IX Item "-nbio"
turns on non-blocking I/O
.IP "\fB\-crlf\fR" 4
.IX Item "-crlf"
this option translated a line feed from the terminal into \s-1CR+LF\s0 as required
by some servers.
.IP "\fB\-ign_eof\fR" 4
.IX Item "-ign_eof"
inhibit shutting down the connection when end of file is reached in the
input.
.IP "\fB\-quiet\fR" 4
.IX Item "-quiet"
inhibit printing of session and certificate information.  This implicitly
turns on \fB\-ign_eof\fR as well.
.IP "\fB\-no_ign_eof\fR" 4
.IX Item "-no_ign_eof"
shut down the connection when end of file is reached in the input.
Can be used to override the implicit \fB\-ign_eof\fR after \fB\-quiet\fR.
.IP "\fB\-psk_identity identity\fR" 4
.IX Item "-psk_identity identity"
Use the \s-1PSK\s0 identity \fBidentity\fR when using a \s-1PSK\s0 cipher suite.
The default value is \*(L"Client_identity\*(R" (without the quotes).
.IP "\fB\-psk key\fR" 4
.IX Item "-psk key"
Use the \s-1PSK\s0 key \fBkey\fR when using a \s-1PSK\s0 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 \s-1PSK\s0 cipher.
.IP "\fB\-ssl2\fR, \fB\-ssl3\fR, \fB\-tls1\fR, \fB\-tls1_1\fR, \fB\-tls1_2\fR, \fB\-dtls1\fR, \fB\-no_ssl2\fR, \fB\-no_ssl3\fR, \fB\-no_tls1\fR, \fB\-no_tls1_1\fR, \fB\-no_tls1_2\fR" 4
.IX Item "-ssl2, -ssl3, -tls1, -tls1_1, -tls1_2, -dtls1, -no_ssl2, -no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2"
These options require or disable the use of the specified \s-1SSL\s0 or \s-1TLS\s0 protocols.
By default the initial handshake uses a \fIversion-flexible\fR method which will
negotiate the highest mutually supported protocol version.
.IP "\fB\-fallback_scsv\fR" 4
.IX Item "-fallback_scsv"
Send \s-1TLS_FALLBACK_SCSV\s0 in the ClientHello.
.IP "\fB\-bugs\fR" 4
.IX Item "-bugs"
there are several known bug in \s-1SSL\s0 and \s-1TLS\s0 implementations. Adding this
option enables various workarounds.
.IP "\fB\-sigalgs sigalglist\fR" 4
.IX Item "-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 \fBSSL_CTX_set1_sigalgs\fR\|(3)
.IP "\fB\-curves curvelist\fR" 4
.IX Item "-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:
.Sp
.Vb 1
\&    $ openssl ecparam \-list_curves
.Ve
.IP "\fB\-cipher cipherlist\fR" 4
.IX Item "-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 \fBciphers\fR
command for more information.
.IP "\fB\-serverpref\fR" 4
.IX Item "-serverpref"
use the server's cipher preferences; only used for \s-1SSLV2.\s0
.IP "\fB\-krb5svc service\fR" 4
.IX Item "-krb5svc service"
the Kerberos service name to use (default \*(L"host\*(R"). This means s_server
will expect a ticket for the principal \fIservice\fR/hostname@REALM, and will
need keys for that principal in its keytab.
.IP "\fB\-starttls protocol\fR" 4
.IX Item "-starttls protocol"
send the protocol-specific message(s) to switch to \s-1TLS\s0 for communication.
\&\fBprotocol\fR is a keyword for the intended protocol.  Currently, the only
supported keywords are \*(L"smtp\*(R", \*(L"pop3\*(R", \*(L"imap\*(R", \*(L"ftp\*(R" and \*(L"xmpp\*(R".
.IP "\fB\-tlsextdebug\fR" 4
.IX Item "-tlsextdebug"
print out a hex dump of any \s-1TLS\s0 extensions received from the server.
.IP "\fB\-no_ticket\fR" 4
.IX Item "-no_ticket"
disable RFC4507bis session ticket support.
.IP "\fB\-sess_out filename\fR" 4
.IX Item "-sess_out filename"
output \s-1SSL\s0 session to \fBfilename\fR
.IP "\fB\-sess_in sess.pem\fR" 4
.IX Item "-sess_in sess.pem"
load \s-1SSL\s0 session from \fBfilename\fR. The client will attempt to resume a
connection from this session.
.IP "\fB\-engine id\fR" 4
.IX Item "-engine id"
specifying an engine (by its unique \fBid\fR string) will cause \fBs_client\fR
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.
.IP "\fB\-rand file(s)\fR" 4
.IX Item "-rand file(s)"
a file or files containing random data used to seed the random number
generator, or an \s-1EGD\s0 socket (see \fBRAND_egd\fR\|(3)).
Multiple files can be specified separated by a OS-dependent character.
The separator is \fB;\fR for MS-Windows, \fB,\fR for OpenVMS, and \fB:\fR for
all others.
.IP "\fB\-serverinfo types\fR" 4
.IX Item "-serverinfo types"
a list of comma-separated \s-1TLS\s0 Extension Types (numbers between 0 and 
65535).  Each type will be sent as an empty ClientHello \s-1TLS\s0 Extension.
The server's response (if any) will be encoded and displayed as a \s-1PEM\s0
file.
.IP "\fB\-status\fR" 4
.IX Item "-status"
sends a certificate status request to the server (\s-1OCSP\s0 stapling). The server
response (if any) is printed out.
.IP "\fB\-alpn protocols\fR, \fB\-nextprotoneg protocols\fR" 4
.IX Item "-alpn protocols, -nextprotoneg protocols"
these flags enable the 
Enable the Application-Layer Protocol Negotiation or Next Protocol
Negotiation extension, respectively. \s-1ALPN\s0 is the \s-1IETF\s0 standard and
replaces \s-1NPN.\s0
The \fBprotocols\fR 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 \s-1ASCII\s0 strings, for example \*(L"http/1.1\*(R" or
\&\*(L"spdy/3\*(R".
Empty list of protocols is treated specially and will cause the client to
advertise support for the \s-1TLS\s0 extension but disconnect just after
reciving ServerHello with a list of server supported protocols.
.SH "CONNECTED COMMANDS"
.IX Header "CONNECTED COMMANDS"
If a connection is established with an \s-1SSL\s0 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 \fB\-quiet\fR nor \fB\-ign_eof\fR
have been given), the session will be renegotiated if the line begins with an
\&\fBR\fR, and if the line begins with a \fBQ\fR or if end of file is reached, the
connection will be closed down.
.SH "NOTES"
.IX Header "NOTES"
\&\fBs_client\fR can be used to debug \s-1SSL\s0 servers. To connect to an \s-1SSL HTTP\s0
server the command:
.PP
.Vb 1
\& openssl s_client \-connect servername:443
.Ve
.PP
would typically be used (https uses port 443). If the connection succeeds
then an \s-1HTTP\s0 command can be given such as \*(L"\s-1GET /\*(R"\s0 to retrieve a web page.
.PP
If the handshake fails then there are several possible causes, if it is
nothing obvious like no client certificate then the \fB\-bugs\fR, \fB\-ssl2\fR,
\&\fB\-ssl3\fR, \fB\-tls1\fR, \fB\-no_ssl2\fR, \fB\-no_ssl3\fR, \fB\-no_tls1\fR options can be tried
in case it is a buggy server. In particular you should play with these
options \fBbefore\fR submitting a bug report to an OpenSSL mailing list.
.PP
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 \*(L"acceptable \s-1CA\s0 list\*(R" when it
requests a certificate. By using \fBs_client\fR the \s-1CA\s0 list can be viewed
and checked. However some servers only request client authentication
after a specific \s-1URL\s0 is requested. To obtain the list in this case it
is necessary to use the \fB\-prexit\fR option and send an \s-1HTTP\s0 request
for an appropriate page.
.PP
If a certificate is specified on the command line using the \fB\-cert\fR
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.
.PP
If there are problems verifying a server certificate then the
\&\fB\-showcerts\fR option can be used to show all the certificates sent by the
server.
.PP
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
\&\fB\-no_sslv2\fR option.
.PP
The \fBs_client\fR 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 \fBnot\fR do this as it makes them vulnerable to a \s-1MITM\s0
attack. This behaviour can be changed by with the \fB\-verify_return_error\fR
option: any verify errors are then returned aborting the handshake.
.SH "BUGS"
.IX Header "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
\&\s-1SSL\s0 client program would be much simpler.
.PP
The \fB\-prexit\fR option is a bit of a hack. We should really report
information whenever a session is renegotiated.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBsess_id\fR\|(1), \fBs_server\fR\|(1), \fBciphers\fR\|(1)
.SH "HISTORY"
.IX Header "HISTORY"
The \-no_alt_chains options was first added to OpenSSL 1.0.2b.