Section: OpenSSL (1SSL)
Return to Main Contents
s_client - SSL/TLS client program
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
- -connect host:port
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 port 4433.
- -cert certname
The certificate to use, if one is requested by the server. The default is
not to use a certificate.
- -certform format
The certificate format to use: DER or PEM. PEM is the default.
- -key keyfile
The private key to use. If not specified then the certificate file will
- -keyform format
The private format to use: DER or PEM. PEM is the default.
- -pass arg
the private key password source. For more information about the format of arg
see the PASS PHRASE ARGUMENTS section in openssl(1).
- -verify depth
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.
- -CApath directory
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 certificate chain.
- -CAfile file
A file containing trusted certificates to use during server authentication
and to use when attempting to build the client certificate chain.
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 been
prints out the SSL session states.
print extensive debugging information including a hex dump of all traffic.
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 the
inhibit printing of session and certificate information. This implicitly
turns on -ign_eof as well.
- -ssl2, -ssl3, -tls1, -no_ssl2, -no_ssl3, -no_tls1
these options disable the use of certain SSL or TLS protocols. By default
the initial handshake uses a method which should be compatible with all
servers and permit them to use SSL v3, SSL v2 or TLS as appropriate.
Unfortunately there are a lot of ancient and broken servers in use which
cannot handle this technique and will fail to connect. Some servers only
work if TLS is turned off with the -no_tls option others will only
support SSL v2 and may need the -ssl2 option.
there are several known bug in SSL and TLS implementations. Adding this
option enables various workarounds.
- -cipher cipherlist
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.
- -starttls protocol
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, and ftp.
print out a hex dump of any TLS extensions received from the server. Note: this
option is only available if extension support is explicitly enabled at compile
disable RFC4507bis session ticket support. Note: this option is only available
if extension support is explicitly enabled at compile time
- -sess_out filename
output SSL session to filename
- -sess_in sess.pem
load SSL session from filename. The client will attempt to resume a
connection from this session.
- -engine id
specifying an engine (by it's 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.
- -rand file(s)
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
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 reached, the
connection will be closed down.
s_client can be used to debug SSL servers. To connect to an SSL HTTP
server the command:
openssl s_client -connect servername:443
would typically be used (https uses port 443). If the connection succeeds
then an HTTP command can be given such as GET / to retrieve a web page.
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 appropriate page.
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
TLS extensions are only supported in OpenSSL 0.9.8 if they are explictly
enabled at compile time using for example the enable-tlsext switch.
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
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)
- CONNECTED COMMANDS
- SEE ALSO
This document was created by
using the manual pages.