openssl s_client [-connect host:port] [-verify depth] [-cert filename]
[-certform DER|PEM] [-key filename] [-keyform DER|PEM] [-pass arg]
[-CApath directory] [-CAfile filename] [-no_alt_chains] [-reconnect]
[-pause] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio]
[-crlf] [-ign_eof] [-quiet] [-ssl2] [-ssl3] [-tls1] [-no_ssl2]
[-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2] [-fallback_scsv]
[-bugs] [-cipher cipherlist] [-starttls protocol] [-engine id]
[-tlsextdebug] [-no_ticket] [-sess_out filename] [-sess_in filename]
The s_client command implements a generic SSL/TLS client which connects
to a remote host using SSL/TLS. It is a very useful diagnostic tool for
This specifies the host and optional port to connect to. If not
specified then an attempt is made to connect to the local host on
The certificate to use, if one is requested by the server. The
default is not to use a certificate.
The certificate format to use: DER or PEM. PEM is the default.
The private key to use. If not specified then the certificate file
will be used.
The private format to use: DER or PEM. PEM is the default.
the private key password source. For more information about the
format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).
The verify depth to use. This specifies the maximum length of the
server certificate chain and turns on server certificate
verification. Currently the verify operation continues after
errors so all the problems with a certificate chain can be seen. As
a side effect the connection will never fail due to a server
certificate verify failure.
The directory to use for server certificate verification. This
directory must be in "hash format", see verify for more
information. These are also used when building the client
reconnects to the same server 5 times using the same session ID,
this can be used as a test that session caching is working.
pauses 1 second between each read and write call.
display the whole server certificate chain: normally only the
server certificate itself is displayed.
print session information when the program exits. This will always
attempt to print out information even if the connection fails.
Normally information will only be printed out once if the
connection succeeds. This option is useful because the cipher in
use may be renegotiated or the connection may fail because a client
certificate is required or is requested only after an attempt is
made to access a certain URL. Note: the output produced by this
option is not always accurate because a connection might never have
prints out the SSL session states.
print extensive debugging information including a hex dump of all
show all protocol messages with hex dump.
tests non-blocking I/O
turns on non-blocking I/O
this option translated a line feed from the terminal into CR+LF as
required by some servers.
inhibit shutting down the connection when end of file is reached in
inhibit printing of session and certificate information. This
implicitly turns on -ign_eof as well.
Use the PSK identity identity when using a PSK cipher suite.
which cannot handle this technique and will fail to connect. Some
servers only work if TLS is turned off.
Send TLS_FALLBACK_SCSV in the ClientHello.
there are several known bug in SSL and TLS implementations. Adding
this option enables various workarounds.
this allows the cipher list sent by the client to be modified.
Although the server determines which cipher suite is used it should
take the first supported cipher in the list sent by the client. See
the ciphers command for more information.
send the protocol-specific message(s) to switch to TLS for
communication. protocol is a keyword for the intended protocol.
Currently, the only supported keywords are "smtp", "pop3", "imap",
print out a hex dump of any TLS extensions received from the
disable RFC4507bis session ticket support.
output SSL session to filename
load SSL session from filename. The client will attempt to resume a
connection from this session.
specifying an engine (by its unique id string) will cause s_client
to attempt to obtain a functional reference to the specified
engine, thus initialising it if needed. The engine will then be set
as the default for all available algorithms.
a file or files containing random data used to seed the random
number generator, or an EGD socket (see RAND_egd(3)). Multiple
files can be specified separated by a OS-dependent character. The
separator is ; for MS-Windows, , for OpenVMS, and : for all others.
If a connection is established with an SSL server then any data
received from the server is displayed and any key presses will be sent
to the server. When used interactively (which means neither -quiet nor
-ign_eof have been given), the session will be renegotiated if the line
begins with an R, and if the line begins with a Q or if end of file is
If the handshake fails then there are several possible causes, if it is
nothing obvious like no client certificate then the -bugs, -ssl2,
-ssl3, -tls1, -no_ssl2, -no_ssl3, -no_tls1 options can be tried in case
it is a buggy server. In particular you should play with these options
before submitting a bug report to an OpenSSL mailing list.
A frequent problem when attempting to get client certificates working
is that a web client complains it has no certificates or gives an empty
list to choose from. This is normally because the server is not sending
the clients certificate authority in its "acceptable CA list" when it
requests a certificate. By using s_client the CA list can be viewed and
checked. However some servers only request client authentication after
a specific URL is requested. To obtain the list in this case it is
necessary to use the -prexit option and send an HTTP request for an
If a certificate is specified on the command line using the -cert
option it will not be used unless the server specifically requests a
client certificate. Therefor merely including a client certificate on
the command line is no guarantee that the certificate works.
If there are problems verifying a server certificate then the
-showcerts option can be used to show the whole chain.
Since the SSLv23 client hello cannot include compression methods or
extensions these will only be supported if its use is disabled, for
example by using the -no_sslv2 option.
Because this program has a lot of options and also because some of the
techniques used are rather old, the C source of s_client is rather hard
to read and not a model of how things should be done. A typical SSL
client program would be much simpler.
The -verify option should really exit if the server verification fails.
The -prexit option is a bit of a hack. We should really report
information whenever a session is renegotiated.
sess_id(1), s_server(1), ciphers(1)
The -no_alt_chains options was first added to OpenSSL 1.0.1n and
1.0.1f 2018-12-04 S_CLIENT(1SSL)
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