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>]
41 [B<-cipher cipherlist>]
43 [B<-starttls protocol>]
47 [B<-sess_out filename>]
48 [B<-sess_in filename>]
53 The B<s_client> command implements a generic SSL/TLS client which connects
54 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
61 =item B<-connect host:port>
63 This specifies the host and optional port to connect to. If not specified
64 then an attempt is made to connect to the local host on port 4433.
66 =item B<-servername name>
68 Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
70 =item B<-cert certname>
72 The certificate to use, if one is requested by the server. The default is
73 not to use a certificate.
75 =item B<-certform format>
77 The certificate format to use: DER or PEM. PEM is the default.
81 The private key to use. If not specified then the certificate file will
84 =item B<-keyform format>
86 The private format to use: DER or PEM. PEM is the default.
90 the private key password source. For more information about the format of B<arg>
91 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.
93 =item B<-verify depth>
95 The verify depth to use. This specifies the maximum length of the
96 server certificate chain and turns on server certificate verification.
97 Currently the verify operation continues after errors so all the problems
98 with a certificate chain can be seen. As a side effect the connection
99 will never fail due to a server certificate verify failure.
101 =item B<-verify_return_error>
103 Return verification errors instead of continuing. This will typically
104 abort the handshake with a fatal error.
106 =item B<-CApath directory>
108 The directory to use for server certificate verification. This directory
109 must be in "hash format", see B<verify> for more information. These are
110 also used when building the client certificate chain.
112 =item B<-CAfile file>
114 A file containing trusted certificates to use during server authentication
115 and to use when attempting to build the client certificate chain.
117 =item B<-purpose, -ignore_critical, -issuer_checks, -crl_check, -crl_check_all, -policy_check, -extended_crl, -x509_strict, -policy -check_ss_sig>
119 Set various certificate chain valiadition option. See the
120 L<B<verify>|verify(1)> manual page for details.
124 reconnects to the same server 5 times using the same session ID, this can
125 be used as a test that session caching is working.
129 pauses 1 second between each read and write call.
133 display the whole server certificate chain: normally only the server
134 certificate itself is displayed.
138 print session information when the program exits. This will always attempt
139 to print out information even if the connection fails. Normally information
140 will only be printed out once if the connection succeeds. This option is useful
141 because the cipher in use may be renegotiated or the connection may fail
142 because a client certificate is required or is requested only after an
143 attempt is made to access a certain URL. Note: the output produced by this
144 option is not always accurate because a connection might never have been
149 prints out the SSL session states.
153 print extensive debugging information including a hex dump of all traffic.
157 show all protocol messages with hex dump.
161 tests non-blocking I/O
165 turns on non-blocking I/O
169 this option translated a line feed from the terminal into CR+LF as required
174 inhibit shutting down the connection when end of file is reached in the
179 inhibit printing of session and certificate information. This implicitly
180 turns on B<-ign_eof> as well.
184 shut down the connection when end of file is reached in the input.
185 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
187 =item B<-psk_identity identity>
189 Use the PSK identity B<identity> when using a PSK cipher suite.
193 Use the PSK key B<key> when using a PSK cipher suite. The key is
194 given as a hexadecimal number without leading 0x, for example -psk
197 =item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>
199 these options disable the use of certain SSL or TLS protocols. By default
200 the initial handshake uses a method which should be compatible with all
201 servers and permit them to use SSL v3, SSL v2 or TLS as appropriate.
203 Unfortunately there are a lot of ancient and broken servers in use which
204 cannot handle this technique and will fail to connect. Some servers only
205 work if TLS is turned off with the B<-no_tls> option others will only
206 support SSL v2 and may need the B<-ssl2> option.
210 there are several known bug in SSL and TLS implementations. Adding this
211 option enables various workarounds.
213 =item B<-cipher cipherlist>
215 this allows the cipher list sent by the client to be modified. Although
216 the server determines which cipher suite is used it should take the first
217 supported cipher in the list sent by the client. See the B<ciphers>
218 command for more information.
222 use the server's cipher preferences; only used for SSLV2.
224 =item B<-starttls protocol>
226 send the protocol-specific message(s) to switch to TLS for communication.
227 B<protocol> is a keyword for the intended protocol. Currently, the only
228 supported keywords are "smtp", "pop3", "imap", and "ftp".
230 =item B<-tlsextdebug>
232 print out a hex dump of any TLS extensions received from the server.
236 disable RFC4507bis session ticket support.
238 =item B<-sess_out filename>
240 output SSL session to B<filename>
242 =item B<-sess_in sess.pem>
244 load SSL session from B<filename>. The client will attempt to resume a
245 connection from this session.
249 specifying an engine (by its unique B<id> string) will cause B<s_client>
250 to attempt to obtain a functional reference to the specified engine,
251 thus initialising it if needed. The engine will then be set as the default
252 for all available algorithms.
254 =item B<-rand file(s)>
256 a file or files containing random data used to seed the random number
257 generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).
258 Multiple files can be specified separated by a OS-dependent character.
259 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
264 =head1 CONNECTED COMMANDS
266 If a connection is established with an SSL server then any data received
267 from the server is displayed and any key presses will be sent to the
268 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
269 have been given), the session will be renegotiated if the line begins with an
270 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
271 connection will be closed down.
275 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
278 openssl s_client -connect servername:443
280 would typically be used (https uses port 443). If the connection succeeds
281 then an HTTP command can be given such as "GET /" to retrieve a web page.
283 If the handshake fails then there are several possible causes, if it is
284 nothing obvious like no client certificate then the B<-bugs>, B<-ssl2>,
285 B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried
286 in case it is a buggy server. In particular you should play with these
287 options B<before> submitting a bug report to an OpenSSL mailing list.
289 A frequent problem when attempting to get client certificates working
290 is that a web client complains it has no certificates or gives an empty
291 list to choose from. This is normally because the server is not sending
292 the clients certificate authority in its "acceptable CA list" when it
293 requests a certificate. By using B<s_client> the CA list can be viewed
294 and checked. However some servers only request client authentication
295 after a specific URL is requested. To obtain the list in this case it
296 is necessary to use the B<-prexit> option and send an HTTP request
297 for an appropriate page.
299 If a certificate is specified on the command line using the B<-cert>
300 option it will not be used unless the server specifically requests
301 a client certificate. Therefor merely including a client certificate
302 on the command line is no guarantee that the certificate works.
304 If there are problems verifying a server certificate then the
305 B<-showcerts> option can be used to show the whole chain.
307 Since the SSLv23 client hello cannot include compression methods or extensions
308 these will only be supported if its use is disabled, for example by using the
311 The B<s_client> utility is a test tool and is designed to continue the
312 handshake after any certificate verification errors. As a result it will
313 accept any certificate chain (trusted or not) sent by the peer. None test
314 applications should B<not> do this as it makes them vulnerable to a MITM
315 attack. This behaviour can be changed by with the B<-verify_return_error>
316 option: any verify errors are then returned aborting the handshake.
320 Because this program has a lot of options and also because some of
321 the techniques used are rather old, the C source of s_client is rather
322 hard to read and not a model of how things should be done. A typical
323 SSL client program would be much simpler.
325 The B<-prexit> option is a bit of a hack. We should really report
326 information whenever a session is renegotiated.
330 L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>