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openssl-1.0.2f/doc/apps/s_client.pod
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openssl-1.0.2f/doc/apps/s_client.pod
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=pod
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=head1 NAME
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s_client - SSL/TLS client program
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=head1 SYNOPSIS
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B<openssl> B<s_client>
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[B<-connect host:port>]
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[B<-servername name>]
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[B<-verify depth>]
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[B<-verify_return_error>]
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[B<-cert filename>]
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[B<-certform DER|PEM>]
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[B<-key filename>]
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[B<-keyform DER|PEM>]
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[B<-pass arg>]
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[B<-CApath directory>]
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[B<-CAfile filename>]
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[B<-no_alt_chains>]
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[B<-reconnect>]
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[B<-pause>]
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[B<-showcerts>]
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[B<-debug>]
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[B<-msg>]
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[B<-nbio_test>]
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[B<-state>]
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[B<-nbio>]
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[B<-crlf>]
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[B<-ign_eof>]
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[B<-no_ign_eof>]
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[B<-quiet>]
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[B<-ssl2>]
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[B<-ssl3>]
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[B<-tls1>]
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[B<-no_ssl2>]
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[B<-no_ssl3>]
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[B<-no_tls1>]
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[B<-no_tls1_1>]
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[B<-no_tls1_2>]
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[B<-fallback_scsv>]
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[B<-bugs>]
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[B<-cipher cipherlist>]
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[B<-serverpref>]
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[B<-starttls protocol>]
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[B<-engine id>]
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[B<-tlsextdebug>]
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[B<-no_ticket>]
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[B<-sess_out filename>]
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[B<-sess_in filename>]
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[B<-rand file(s)>]
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[B<-serverinfo types>]
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[B<-status>]
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[B<-nextprotoneg protocols>]
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=head1 DESCRIPTION
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The B<s_client> command implements a generic SSL/TLS client which connects
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to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
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SSL servers.
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=head1 OPTIONS
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=over 4
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=item B<-connect host:port>
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This specifies the host and optional port to connect to. If not specified
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then an attempt is made to connect to the local host on port 4433.
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=item B<-servername name>
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Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
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=item B<-cert certname>
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The certificate to use, if one is requested by the server. The default is
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not to use a certificate.
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=item B<-certform format>
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The certificate format to use: DER or PEM. PEM is the default.
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=item B<-key keyfile>
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The private key to use. If not specified then the certificate file will
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be used.
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=item B<-keyform format>
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The private format to use: DER or PEM. PEM is the default.
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=item B<-pass arg>
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the private key password source. For more information about the format of B<arg>
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see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.
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=item B<-verify depth>
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The verify depth to use. This specifies the maximum length of the
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server certificate chain and turns on server certificate verification.
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Currently the verify operation continues after errors so all the problems
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with a certificate chain can be seen. As a side effect the connection
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will never fail due to a server certificate verify failure.
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=item B<-verify_return_error>
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Return verification errors instead of continuing. This will typically
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abort the handshake with a fatal error.
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=item B<-CApath directory>
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The directory to use for server certificate verification. This directory
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must be in "hash format", see B<verify> for more information. These are
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also used when building the client certificate chain.
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=item B<-CAfile file>
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A file containing trusted certificates to use during server authentication
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and to use when attempting to build the client certificate chain.
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=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>
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Set various certificate chain valiadition option. See the
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L<B<verify>|verify(1)> manual page for details.
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=item B<-reconnect>
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reconnects to the same server 5 times using the same session ID, this can
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be used as a test that session caching is working.
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=item B<-pause>
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pauses 1 second between each read and write call.
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=item B<-showcerts>
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display the whole server certificate chain: normally only the server
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certificate itself is displayed.
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=item B<-prexit>
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print session information when the program exits. This will always attempt
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to print out information even if the connection fails. Normally information
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will only be printed out once if the connection succeeds. This option is useful
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because the cipher in use may be renegotiated or the connection may fail
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because a client certificate is required or is requested only after an
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attempt is made to access a certain URL. Note: the output produced by this
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option is not always accurate because a connection might never have been
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established.
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=item B<-state>
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prints out the SSL session states.
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=item B<-debug>
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print extensive debugging information including a hex dump of all traffic.
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=item B<-msg>
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show all protocol messages with hex dump.
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=item B<-nbio_test>
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tests non-blocking I/O
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=item B<-nbio>
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turns on non-blocking I/O
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=item B<-crlf>
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this option translated a line feed from the terminal into CR+LF as required
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by some servers.
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=item B<-ign_eof>
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inhibit shutting down the connection when end of file is reached in the
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input.
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=item B<-quiet>
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inhibit printing of session and certificate information. This implicitly
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turns on B<-ign_eof> as well.
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=item B<-no_ign_eof>
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shut down the connection when end of file is reached in the input.
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Can be used to override the implicit B<-ign_eof> after B<-quiet>.
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=item B<-psk_identity identity>
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Use the PSK identity B<identity> when using a PSK cipher suite.
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=item B<-psk key>
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Use the PSK key B<key> when using a PSK cipher suite. The key is
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given as a hexadecimal number without leading 0x, for example -psk
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1a2b3c4d.
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=item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>
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these options disable the use of certain SSL or TLS protocols. By default
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the initial handshake uses a method which should be compatible with all
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servers and permit them to use SSL v3, SSL v2 or TLS as appropriate.
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Unfortunately there are still ancient and broken servers in use which
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cannot handle this technique and will fail to connect. Some servers only
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work if TLS is turned off.
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=item B<-fallback_scsv>
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Send TLS_FALLBACK_SCSV in the ClientHello.
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=item B<-bugs>
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there are several known bug in SSL and TLS implementations. Adding this
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option enables various workarounds.
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=item B<-cipher cipherlist>
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this allows the cipher list sent by the client to be modified. Although
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the server determines which cipher suite is used it should take the first
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supported cipher in the list sent by the client. See the B<ciphers>
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command for more information.
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=item B<-serverpref>
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use the server's cipher preferences; only used for SSLV2.
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=item B<-starttls protocol>
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send the protocol-specific message(s) to switch to TLS for communication.
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B<protocol> is a keyword for the intended protocol. Currently, the only
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supported keywords are "smtp", "pop3", "imap", and "ftp".
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=item B<-tlsextdebug>
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print out a hex dump of any TLS extensions received from the server.
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=item B<-no_ticket>
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disable RFC4507bis session ticket support.
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=item B<-sess_out filename>
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output SSL session to B<filename>
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=item B<-sess_in sess.pem>
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load SSL session from B<filename>. The client will attempt to resume a
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connection from this session.
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=item B<-engine id>
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specifying an engine (by its unique B<id> string) will cause B<s_client>
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to attempt to obtain a functional reference to the specified engine,
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thus initialising it if needed. The engine will then be set as the default
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for all available algorithms.
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=item B<-rand file(s)>
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a file or files containing random data used to seed the random number
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generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).
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Multiple files can be specified separated by a OS-dependent character.
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The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
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all others.
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=item B<-serverinfo types>
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a list of comma-separated TLS Extension Types (numbers between 0 and
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65535). Each type will be sent as an empty ClientHello TLS Extension.
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The server's response (if any) will be encoded and displayed as a PEM
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file.
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=item B<-status>
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sends a certificate status request to the server (OCSP stapling). The server
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response (if any) is printed out.
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=item B<-nextprotoneg protocols>
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enable Next Protocol Negotiation TLS extension and provide a list of
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comma-separated protocol names that the client should advertise
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support for. The list should contain most wanted protocols first.
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Protocol names are printable ASCII strings, for example "http/1.1" or
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"spdy/3".
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Empty list of protocols is treated specially and will cause the client to
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advertise support for the TLS extension but disconnect just after
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reciving ServerHello with a list of server supported protocols.
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=back
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=head1 CONNECTED COMMANDS
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If a connection is established with an SSL server then any data received
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from the server is displayed and any key presses will be sent to the
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server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
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have been given), the session will be renegotiated if the line begins with an
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B<R>, and if the line begins with a B<Q> or if end of file is reached, the
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connection will be closed down.
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=head1 NOTES
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B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
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server the command:
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openssl s_client -connect servername:443
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would typically be used (https uses port 443). If the connection succeeds
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then an HTTP command can be given such as "GET /" to retrieve a web page.
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If the handshake fails then there are several possible causes, if it is
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nothing obvious like no client certificate then the B<-bugs>, B<-ssl2>,
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B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried
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in case it is a buggy server. In particular you should play with these
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options B<before> submitting a bug report to an OpenSSL mailing list.
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A frequent problem when attempting to get client certificates working
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is that a web client complains it has no certificates or gives an empty
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list to choose from. This is normally because the server is not sending
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the clients certificate authority in its "acceptable CA list" when it
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requests a certificate. By using B<s_client> the CA list can be viewed
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and checked. However some servers only request client authentication
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after a specific URL is requested. To obtain the list in this case it
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is necessary to use the B<-prexit> option and send an HTTP request
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for an appropriate page.
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If a certificate is specified on the command line using the B<-cert>
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option it will not be used unless the server specifically requests
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a client certificate. Therefor merely including a client certificate
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on the command line is no guarantee that the certificate works.
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If there are problems verifying a server certificate then the
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B<-showcerts> option can be used to show the whole chain.
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Since the SSLv23 client hello cannot include compression methods or extensions
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these will only be supported if its use is disabled, for example by using the
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B<-no_sslv2> option.
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The B<s_client> utility is a test tool and is designed to continue the
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handshake after any certificate verification errors. As a result it will
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accept any certificate chain (trusted or not) sent by the peer. None test
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applications should B<not> do this as it makes them vulnerable to a MITM
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attack. This behaviour can be changed by with the B<-verify_return_error>
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option: any verify errors are then returned aborting the handshake.
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=head1 BUGS
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Because this program has a lot of options and also because some of
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the techniques used are rather old, the C source of s_client is rather
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hard to read and not a model of how things should be done. A typical
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SSL client program would be much simpler.
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The B<-prexit> option is a bit of a hack. We should really report
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information whenever a session is renegotiated.
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=head1 SEE ALSO
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L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>
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=head1 HISTORY
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The -no_alt_chains options was first added to OpenSSL 1.0.2b.
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=cut
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