6 s_client - SSL/TLS client program
10 B<openssl> B<s_client>
11 [B<-connect host:port>]
14 [B<-verify_return_error>]
16 [B<-certform DER|PEM>]
20 [B<-CApath directory>]
45 [B<-cipher cipherlist>]
47 [B<-starttls protocol>]
51 [B<-sess_out filename>]
52 [B<-sess_in filename>]
54 [B<-serverinfo types>]
57 [B<-nextprotoneg protocols>]
61 The B<s_client> command implements a generic SSL/TLS client which connects
62 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
69 =item B<-connect host:port>
71 This specifies the host and optional port to connect to. If not specified
72 then an attempt is made to connect to the local host on port 4433.
74 =item B<-servername name>
76 Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
78 =item B<-cert certname>
80 The certificate to use, if one is requested by the server. The default is
81 not to use a certificate.
83 =item B<-certform format>
85 The certificate format to use: DER or PEM. PEM is the default.
89 The private key to use. If not specified then the certificate file will
92 =item B<-keyform format>
94 The private format to use: DER or PEM. PEM is the default.
98 the private key password source. For more information about the format of B<arg>
99 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.
101 =item B<-verify depth>
103 The verify depth to use. This specifies the maximum length of the
104 server certificate chain and turns on server certificate verification.
105 Currently the verify operation continues after errors so all the problems
106 with a certificate chain can be seen. As a side effect the connection
107 will never fail due to a server certificate verify failure.
109 =item B<-verify_return_error>
111 Return verification errors instead of continuing. This will typically
112 abort the handshake with a fatal error.
114 =item B<-CApath directory>
116 The directory to use for server certificate verification. This directory
117 must be in "hash format", see B<verify> for more information. These are
118 also used when building the client certificate chain.
120 =item B<-CAfile file>
122 A file containing trusted certificates to use during server authentication
123 and to use when attempting to build the client certificate chain.
125 =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>
127 Set various certificate chain valiadition option. See the
128 L<B<verify>|verify(1)> manual page for details.
132 reconnects to the same server 5 times using the same session ID, this can
133 be used as a test that session caching is working.
137 pauses 1 second between each read and write call.
141 display the whole server certificate chain: normally only the server
142 certificate itself is displayed.
146 print session information when the program exits. This will always attempt
147 to print out information even if the connection fails. Normally information
148 will only be printed out once if the connection succeeds. This option is useful
149 because the cipher in use may be renegotiated or the connection may fail
150 because a client certificate is required or is requested only after an
151 attempt is made to access a certain URL. Note: the output produced by this
152 option is not always accurate because a connection might never have been
157 prints out the SSL session states.
161 print extensive debugging information including a hex dump of all traffic.
165 show all protocol messages with hex dump.
169 tests non-blocking I/O
173 turns on non-blocking I/O
177 this option translated a line feed from the terminal into CR+LF as required
182 inhibit shutting down the connection when end of file is reached in the
187 inhibit printing of session and certificate information. This implicitly
188 turns on B<-ign_eof> as well.
192 shut down the connection when end of file is reached in the input.
193 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
195 =item B<-psk_identity identity>
197 Use the PSK identity B<identity> when using a PSK cipher suite.
201 Use the PSK key B<key> when using a PSK cipher suite. The key is
202 given as a hexadecimal number without leading 0x, for example -psk
205 =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>
207 These options require or disable the use of the specified SSL or TLS protocols.
208 By default the initial handshake uses a I<version-flexible> method which will
209 negotiate the highest mutually supported protocol version.
211 =item B<-fallback_scsv>
213 Send TLS_FALLBACK_SCSV in the ClientHello.
217 there are several known bug in SSL and TLS implementations. Adding this
218 option enables various workarounds.
220 =item B<-cipher cipherlist>
222 this allows the cipher list sent by the client to be modified. Although
223 the server determines which cipher suite is used it should take the first
224 supported cipher in the list sent by the client. See the B<ciphers>
225 command for more information.
229 use the server's cipher preferences; only used for SSLV2.
231 =item B<-starttls protocol>
233 send the protocol-specific message(s) to switch to TLS for communication.
234 B<protocol> is a keyword for the intended protocol. Currently, the only
235 supported keywords are "smtp", "pop3", "imap", and "ftp".
237 =item B<-tlsextdebug>
239 print out a hex dump of any TLS extensions received from the server.
243 disable RFC4507bis session ticket support.
245 =item B<-sess_out filename>
247 output SSL session to B<filename>
249 =item B<-sess_in sess.pem>
251 load SSL session from B<filename>. The client will attempt to resume a
252 connection from this session.
256 specifying an engine (by its unique B<id> string) will cause B<s_client>
257 to attempt to obtain a functional reference to the specified engine,
258 thus initialising it if needed. The engine will then be set as the default
259 for all available algorithms.
261 =item B<-rand file(s)>
263 a file or files containing random data used to seed the random number
264 generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).
265 Multiple files can be specified separated by a OS-dependent character.
266 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
269 =item B<-serverinfo types>
271 a list of comma-separated TLS Extension Types (numbers between 0 and
272 65535). Each type will be sent as an empty ClientHello TLS Extension.
273 The server's response (if any) will be encoded and displayed as a PEM
278 sends a certificate status request to the server (OCSP stapling). The server
279 response (if any) is printed out.
281 =item B<-alpn protocols>, B<-nextprotoneg protocols>
283 these flags enable the
284 Enable the Application-Layer Protocol Negotiation or Next Protocol
285 Negotiation extension, respectively. ALPN is the IETF standard and
287 The B<protocols> list is a
288 comma-separated protocol names that the client should advertise
289 support for. The list should contain most wanted protocols first.
290 Protocol names are printable ASCII strings, for example "http/1.1" or
292 Empty list of protocols is treated specially and will cause the client to
293 advertise support for the TLS extension but disconnect just after
294 reciving ServerHello with a list of server supported protocols.
298 =head1 CONNECTED COMMANDS
300 If a connection is established with an SSL server then any data received
301 from the server is displayed and any key presses will be sent to the
302 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
303 have been given), the session will be renegotiated if the line begins with an
304 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
305 connection will be closed down.
309 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
312 openssl s_client -connect servername:443
314 would typically be used (https uses port 443). If the connection succeeds
315 then an HTTP command can be given such as "GET /" to retrieve a web page.
317 If the handshake fails then there are several possible causes, if it is
318 nothing obvious like no client certificate then the B<-bugs>, B<-ssl2>,
319 B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried
320 in case it is a buggy server. In particular you should play with these
321 options B<before> submitting a bug report to an OpenSSL mailing list.
323 A frequent problem when attempting to get client certificates working
324 is that a web client complains it has no certificates or gives an empty
325 list to choose from. This is normally because the server is not sending
326 the clients certificate authority in its "acceptable CA list" when it
327 requests a certificate. By using B<s_client> the CA list can be viewed
328 and checked. However some servers only request client authentication
329 after a specific URL is requested. To obtain the list in this case it
330 is necessary to use the B<-prexit> option and send an HTTP request
331 for an appropriate page.
333 If a certificate is specified on the command line using the B<-cert>
334 option it will not be used unless the server specifically requests
335 a client certificate. Therefor merely including a client certificate
336 on the command line is no guarantee that the certificate works.
338 If there are problems verifying a server certificate then the
339 B<-showcerts> option can be used to show the whole chain.
341 Since the SSLv23 client hello cannot include compression methods or extensions
342 these will only be supported if its use is disabled, for example by using the
345 The B<s_client> utility is a test tool and is designed to continue the
346 handshake after any certificate verification errors. As a result it will
347 accept any certificate chain (trusted or not) sent by the peer. None test
348 applications should B<not> do this as it makes them vulnerable to a MITM
349 attack. This behaviour can be changed by with the B<-verify_return_error>
350 option: any verify errors are then returned aborting the handshake.
354 Because this program has a lot of options and also because some of
355 the techniques used are rather old, the C source of s_client is rather
356 hard to read and not a model of how things should be done. A typical
357 SSL client program would be much simpler.
359 The B<-prexit> option is a bit of a hack. We should really report
360 information whenever a session is renegotiated.
364 L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>
368 The -no_alt_chains options was first added to OpenSSL 1.0.2b.