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>]
50 [B<-serverinfo types>]
54 The B<s_client> command implements a generic SSL/TLS client which connects
55 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
62 =item B<-connect host:port>
64 This specifies the host and optional port to connect to. If not specified
65 then an attempt is made to connect to the local host on port 4433.
67 =item B<-servername name>
69 Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
71 =item B<-cert certname>
73 The certificate to use, if one is requested by the server. The default is
74 not to use a certificate.
76 =item B<-certform format>
78 The certificate format to use: DER or PEM. PEM is the default.
82 The private key to use. If not specified then the certificate file will
85 =item B<-keyform format>
87 The private format to use: DER or PEM. PEM is the default.
91 the private key password source. For more information about the format of B<arg>
92 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.
94 =item B<-verify depth>
96 The verify depth to use. This specifies the maximum length of the
97 server certificate chain and turns on server certificate verification.
98 Currently the verify operation continues after errors so all the problems
99 with a certificate chain can be seen. As a side effect the connection
100 will never fail due to a server certificate verify failure.
102 =item B<-verify_return_error>
104 Return verification errors instead of continuing. This will typically
105 abort the handshake with a fatal error.
107 =item B<-CApath directory>
109 The directory to use for server certificate verification. This directory
110 must be in "hash format", see B<verify> for more information. These are
111 also used when building the client certificate chain.
113 =item B<-CAfile file>
115 A file containing trusted certificates to use during server authentication
116 and to use when attempting to build the client certificate chain.
118 =item B<-purpose, -ignore_critical, -issuer_checks, -crl_check, -crl_check_all, -policy_check, -extended_crl, -x509_strict, -policy -check_ss_sig>
120 Set various certificate chain valiadition option. See the
121 L<B<verify>|verify(1)> manual page for details.
125 reconnects to the same server 5 times using the same session ID, this can
126 be used as a test that session caching is working.
130 pauses 1 second between each read and write call.
134 display the whole server certificate chain: normally only the server
135 certificate itself is displayed.
139 print session information when the program exits. This will always attempt
140 to print out information even if the connection fails. Normally information
141 will only be printed out once if the connection succeeds. This option is useful
142 because the cipher in use may be renegotiated or the connection may fail
143 because a client certificate is required or is requested only after an
144 attempt is made to access a certain URL. Note: the output produced by this
145 option is not always accurate because a connection might never have been
150 prints out the SSL session states.
154 print extensive debugging information including a hex dump of all traffic.
158 show all protocol messages with hex dump.
162 tests non-blocking I/O
166 turns on non-blocking I/O
170 this option translated a line feed from the terminal into CR+LF as required
175 inhibit shutting down the connection when end of file is reached in the
180 inhibit printing of session and certificate information. This implicitly
181 turns on B<-ign_eof> as well.
185 shut down the connection when end of file is reached in the input.
186 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
188 =item B<-psk_identity identity>
190 Use the PSK identity B<identity> when using a PSK cipher suite.
194 Use the PSK key B<key> when using a PSK cipher suite. The key is
195 given as a hexadecimal number without leading 0x, for example -psk
198 =item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>
200 these options disable the use of certain SSL or TLS protocols. By default
201 the initial handshake uses a method which should be compatible with all
202 servers and permit them to use SSL v3, SSL v2 or TLS as appropriate.
204 Unfortunately there are a lot of ancient and broken servers in use which
205 cannot handle this technique and will fail to connect. Some servers only
206 work if TLS is turned off with the B<-no_tls> option others will only
207 support SSL v2 and may need the B<-ssl2> option.
211 there are several known bug in SSL and TLS implementations. Adding this
212 option enables various workarounds.
214 =item B<-cipher cipherlist>
216 this allows the cipher list sent by the client to be modified. Although
217 the server determines which cipher suite is used it should take the first
218 supported cipher in the list sent by the client. See the B<ciphers>
219 command for more information.
223 use the server's cipher preferences; only used for SSLV2.
225 =item B<-starttls protocol>
227 send the protocol-specific message(s) to switch to TLS for communication.
228 B<protocol> is a keyword for the intended protocol. Currently, the only
229 supported keywords are "smtp", "pop3", "imap", and "ftp".
231 =item B<-tlsextdebug>
233 print out a hex dump of any TLS extensions received from the server.
237 disable RFC4507bis session ticket support.
239 =item B<-sess_out filename>
241 output SSL session to B<filename>
243 =item B<-sess_in sess.pem>
245 load SSL session from B<filename>. The client will attempt to resume a
246 connection from this session.
250 specifying an engine (by its unique B<id> string) will cause B<s_client>
251 to attempt to obtain a functional reference to the specified engine,
252 thus initialising it if needed. The engine will then be set as the default
253 for all available algorithms.
255 =item B<-rand file(s)>
257 a file or files containing random data used to seed the random number
258 generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).
259 Multiple files can be specified separated by a OS-dependent character.
260 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
263 =item B<-serverinfo types>
265 a list of comma-separated TLS Extension Types (numbers between 0 and
266 65535). Each type will be sent as an empty ClientHello TLS Extension.
267 The server's response (if any) will be encoded and displayed as a PEM
272 =head1 CONNECTED COMMANDS
274 If a connection is established with an SSL server then any data received
275 from the server is displayed and any key presses will be sent to the
276 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
277 have been given), the session will be renegotiated if the line begins with an
278 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
279 connection will be closed down.
283 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
286 openssl s_client -connect servername:443
288 would typically be used (https uses port 443). If the connection succeeds
289 then an HTTP command can be given such as "GET /" to retrieve a web page.
291 If the handshake fails then there are several possible causes, if it is
292 nothing obvious like no client certificate then the B<-bugs>, B<-ssl2>,
293 B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried
294 in case it is a buggy server. In particular you should play with these
295 options B<before> submitting a bug report to an OpenSSL mailing list.
297 A frequent problem when attempting to get client certificates working
298 is that a web client complains it has no certificates or gives an empty
299 list to choose from. This is normally because the server is not sending
300 the clients certificate authority in its "acceptable CA list" when it
301 requests a certificate. By using B<s_client> the CA list can be viewed
302 and checked. However some servers only request client authentication
303 after a specific URL is requested. To obtain the list in this case it
304 is necessary to use the B<-prexit> option and send an HTTP request
305 for an appropriate page.
307 If a certificate is specified on the command line using the B<-cert>
308 option it will not be used unless the server specifically requests
309 a client certificate. Therefor merely including a client certificate
310 on the command line is no guarantee that the certificate works.
312 If there are problems verifying a server certificate then the
313 B<-showcerts> option can be used to show the whole chain.
315 Since the SSLv23 client hello cannot include compression methods or extensions
316 these will only be supported if its use is disabled, for example by using the
319 The B<s_client> utility is a test tool and is designed to continue the
320 handshake after any certificate verification errors. As a result it will
321 accept any certificate chain (trusted or not) sent by the peer. None test
322 applications should B<not> do this as it makes them vulnerable to a MITM
323 attack. This behaviour can be changed by with the B<-verify_return_error>
324 option: any verify errors are then returned aborting the handshake.
328 Because this program has a lot of options and also because some of
329 the techniques used are rather old, the C source of s_client is rather
330 hard to read and not a model of how things should be done. A typical
331 SSL client program would be much simpler.
333 The B<-prexit> option is a bit of a hack. We should really report
334 information whenever a session is renegotiated.
338 L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>