s_client - SSL/TLS client program
[ -connect host:port
] [ -verify
] [ -cert filename
] [ -certform DER|PEM
] [ -key
] [ -keyform DER|PEM
] [ -pass arg
] [ -CApath
] [ -CAfile filename
] [ -trusted_first
] [ -keytab filename
] [ -reconnect
] [ -showcerts
] [ -debug
] [ -msg
] [ -state
] [ -nbio
] [ -crlf
] [ -no_ign_eof
] [ -quiet
] [ -ssl2
] [ -tls1
] [ -tls1_1
] [ -tls1_2
] [ -no_ssl2
] [ -no_ssl3
] [ -no_tls1
] [ -no_tls1_2
] [ -fallback_scsv
] [ -cipher cipherlist
] [ -starttls protocol
] [ -tlsextdebug
] [ -no_ticket
] [ -sess_out
] [ -sess_in filename
] [ -rand file(s)
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 be
- -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
- -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
- -CAfile file
- A file containing trusted certificates to use during server authentication
and to use when attempting to build the client certificate chain.
- -purpose, -ignore_critical, -issuer_checks, -crl_check, -crl_check_all,
-policy_check, -extended_crl, -x509_strict, -policy -check_ss_sig,
- Set various certificate chain valiadition option. See the verify
manual page for details.
- -krb5svc service
- the Kerberos service name to use (default "host"). This means
s_server will expect a ticket for the principal
service/hostname@REALM, and will need keys for that principal in
- -keytab filename
- the Kerberos "keytab" (key table) file, containing keys for the
s_server service principal (Kerberos identity; see -krb5svc).
- 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 established.
- 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 the
- shut down the connection when end of file is reached in the input. Can be
used to override the implicit -ign_eof after -quiet.
- inhibit printing of session and certificate information. This implicitly
turns on -ign_eof as well.
- -psk_identity identity
- Use the PSK identity identity when using a PSK cipher suite.
- -psk key
- Use the PSK key key when using a PSK cipher suite. The key is given
as a hexadecimal number without leading 0x, for example -psk
- -ssl2, -ssl3, -tls1, -tls1_1, -tls1_2,
-dtls1, -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.
- Send TLS_FALLBACK_SCSV in the ClientHello.
- 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.
- disable RFC4507bis session ticket support.
- -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 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.
- -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 all
- -nextprotoneg protocols
- enable Next Protocol Negotiation TLS extension and provide a list of
comma-separated protocol names that the client should advertise support
for. The list should contain most wanted protocols first. Protocol names
are printable ASCII strings, for example "http/1.1" or
"spdy/3". Empty list of protocols is treated specially and will
cause the client to advertise support for the TLS extension but disconnect
just after reciving ServerHello with a list of server supported
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
been given), the session will be renegotiated if the line begins with an
, and if the line begins with a Q
or if end of file is reached,
the connection will be closed down.
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
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
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
option and send an HTTP request for an appropriate page.
If a certificate is specified on the command line using the -cert
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
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.
option should really exit if the server verification fails.
option is a bit of a hack. We should really report
information whenever a session is renegotiated.