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>]
44 [B<-cipher cipherlist>]
46 [B<-starttls protocol>]
50 [B<-sess_out filename>]
51 [B<-sess_in filename>]
53 [B<-serverinfo types>]
55 [B<-nextprotoneg protocols>]
59 The B<s_client> command implements a generic SSL/TLS client which connects
60 to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for
67 =item B<-connect host:port>
69 This specifies the host and optional port to connect to. If not specified
70 then an attempt is made to connect to the local host on port 4433.
72 =item B<-servername name>
74 Set the TLS SNI (Server Name Indication) extension in the ClientHello message.
76 =item B<-cert certname>
78 The certificate to use, if one is requested by the server. The default is
79 not to use a certificate.
81 =item B<-certform format>
83 The certificate format to use: DER or PEM. PEM is the default.
87 The private key to use. If not specified then the certificate file will
90 =item B<-keyform format>
92 The private format to use: DER or PEM. PEM is the default.
96 the private key password source. For more information about the format of B<arg>
97 see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)|openssl(1)>.
99 =item B<-verify depth>
101 The verify depth to use. This specifies the maximum length of the
102 server certificate chain and turns on server certificate verification.
103 Currently the verify operation continues after errors so all the problems
104 with a certificate chain can be seen. As a side effect the connection
105 will never fail due to a server certificate verify failure.
107 =item B<-verify_return_error>
109 Return verification errors instead of continuing. This will typically
110 abort the handshake with a fatal error.
112 =item B<-CApath directory>
114 The directory to use for server certificate verification. This directory
115 must be in "hash format", see B<verify> for more information. These are
116 also used when building the client certificate chain.
118 =item B<-CAfile file>
120 A file containing trusted certificates to use during server authentication
121 and to use when attempting to build the client certificate chain.
123 =item B<-purpose, -ignore_critical, -issuer_checks, -crl_check, -crl_check_all, -policy_check, -extended_crl, -x509_strict, -policy -check_ss_sig>
125 Set various certificate chain valiadition option. See the
126 L<B<verify>|verify(1)> manual page for details.
130 reconnects to the same server 5 times using the same session ID, this can
131 be used as a test that session caching is working.
135 pauses 1 second between each read and write call.
139 display the whole server certificate chain: normally only the server
140 certificate itself is displayed.
144 print session information when the program exits. This will always attempt
145 to print out information even if the connection fails. Normally information
146 will only be printed out once if the connection succeeds. This option is useful
147 because the cipher in use may be renegotiated or the connection may fail
148 because a client certificate is required or is requested only after an
149 attempt is made to access a certain URL. Note: the output produced by this
150 option is not always accurate because a connection might never have been
155 prints out the SSL session states.
159 print extensive debugging information including a hex dump of all traffic.
163 show all protocol messages with hex dump.
167 tests non-blocking I/O
171 turns on non-blocking I/O
175 this option translated a line feed from the terminal into CR+LF as required
180 inhibit shutting down the connection when end of file is reached in the
185 inhibit printing of session and certificate information. This implicitly
186 turns on B<-ign_eof> as well.
190 shut down the connection when end of file is reached in the input.
191 Can be used to override the implicit B<-ign_eof> after B<-quiet>.
193 =item B<-psk_identity identity>
195 Use the PSK identity B<identity> when using a PSK cipher suite.
199 Use the PSK key B<key> when using a PSK cipher suite. The key is
200 given as a hexadecimal number without leading 0x, for example -psk
203 =item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>
205 these options disable the use of certain SSL or TLS protocols. By default
206 the initial handshake uses a method which should be compatible with all
207 servers and permit them to use SSL v3, SSL v2 or TLS as appropriate.
209 Unfortunately there are still ancient and broken servers in use which
210 cannot handle this technique and will fail to connect. Some servers only
211 work if TLS is turned off.
213 =item B<-fallback_scsv>
215 Send TLS_FALLBACK_SCSV in the ClientHello.
219 there are several known bug in SSL and TLS implementations. Adding this
220 option enables various workarounds.
222 =item B<-cipher cipherlist>
224 this allows the cipher list sent by the client to be modified. Although
225 the server determines which cipher suite is used it should take the first
226 supported cipher in the list sent by the client. See the B<ciphers>
227 command for more information.
231 use the server's cipher preferences; only used for SSLV2.
233 =item B<-starttls protocol>
235 send the protocol-specific message(s) to switch to TLS for communication.
236 B<protocol> is a keyword for the intended protocol. Currently, the only
237 supported keywords are "smtp", "pop3", "imap", and "ftp".
239 =item B<-tlsextdebug>
241 print out a hex dump of any TLS extensions received from the server.
245 disable RFC4507bis session ticket support.
247 =item B<-sess_out filename>
249 output SSL session to B<filename>
251 =item B<-sess_in sess.pem>
253 load SSL session from B<filename>. The client will attempt to resume a
254 connection from this session.
258 specifying an engine (by its unique B<id> string) will cause B<s_client>
259 to attempt to obtain a functional reference to the specified engine,
260 thus initialising it if needed. The engine will then be set as the default
261 for all available algorithms.
263 =item B<-rand file(s)>
265 a file or files containing random data used to seed the random number
266 generator, or an EGD socket (see L<RAND_egd(3)|RAND_egd(3)>).
267 Multiple files can be specified separated by a OS-dependent character.
268 The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for
271 =item B<-serverinfo types>
273 a list of comma-separated TLS Extension Types (numbers between 0 and
274 65535). Each type will be sent as an empty ClientHello TLS Extension.
275 The server's response (if any) will be encoded and displayed as a PEM
280 sends a certificate status request to the server (OCSP stapling). The server
281 response (if any) is printed out.
283 =item B<-nextprotoneg protocols>
285 enable Next Protocol Negotiation TLS extension and provide a list of
286 comma-separated protocol names that the client should advertise
287 support for. The list should contain most wanted protocols first.
288 Protocol names are printable ASCII strings, for example "http/1.1" or
290 Empty list of protocols is treated specially and will cause the client to
291 advertise support for the TLS extension but disconnect just after
292 reciving ServerHello with a list of server supported protocols.
296 =head1 CONNECTED COMMANDS
298 If a connection is established with an SSL server then any data received
299 from the server is displayed and any key presses will be sent to the
300 server. When used interactively (which means neither B<-quiet> nor B<-ign_eof>
301 have been given), the session will be renegotiated if the line begins with an
302 B<R>, and if the line begins with a B<Q> or if end of file is reached, the
303 connection will be closed down.
307 B<s_client> can be used to debug SSL servers. To connect to an SSL HTTP
310 openssl s_client -connect servername:443
312 would typically be used (https uses port 443). If the connection succeeds
313 then an HTTP command can be given such as "GET /" to retrieve a web page.
315 If the handshake fails then there are several possible causes, if it is
316 nothing obvious like no client certificate then the B<-bugs>, B<-ssl2>,
317 B<-ssl3>, B<-tls1>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1> options can be tried
318 in case it is a buggy server. In particular you should play with these
319 options B<before> submitting a bug report to an OpenSSL mailing list.
321 A frequent problem when attempting to get client certificates working
322 is that a web client complains it has no certificates or gives an empty
323 list to choose from. This is normally because the server is not sending
324 the clients certificate authority in its "acceptable CA list" when it
325 requests a certificate. By using B<s_client> the CA list can be viewed
326 and checked. However some servers only request client authentication
327 after a specific URL is requested. To obtain the list in this case it
328 is necessary to use the B<-prexit> option and send an HTTP request
329 for an appropriate page.
331 If a certificate is specified on the command line using the B<-cert>
332 option it will not be used unless the server specifically requests
333 a client certificate. Therefor merely including a client certificate
334 on the command line is no guarantee that the certificate works.
336 If there are problems verifying a server certificate then the
337 B<-showcerts> option can be used to show the whole chain.
339 Since the SSLv23 client hello cannot include compression methods or extensions
340 these will only be supported if its use is disabled, for example by using the
343 The B<s_client> utility is a test tool and is designed to continue the
344 handshake after any certificate verification errors. As a result it will
345 accept any certificate chain (trusted or not) sent by the peer. None test
346 applications should B<not> do this as it makes them vulnerable to a MITM
347 attack. This behaviour can be changed by with the B<-verify_return_error>
348 option: any verify errors are then returned aborting the handshake.
352 Because this program has a lot of options and also because some of
353 the techniques used are rather old, the C source of s_client is rather
354 hard to read and not a model of how things should be done. A typical
355 SSL client program would be much simpler.
357 The B<-prexit> option is a bit of a hack. We should really report
358 information whenever a session is renegotiated.
362 L<sess_id(1)|sess_id(1)>, L<s_server(1)|s_server(1)>, L<ciphers(1)|ciphers(1)>