NAME
s_client - SSL/TLS client program
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
openssl s_client [
-connect host:port] [
-servername
name] [
-verify depth] [
-verify_return_error] [
-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] [
-no_ign_eof] [
-quiet] [
-ssl2] [
-ssl3] [
-tls1] [
-no_ssl2] [
-no_ssl3] [
-no_tls1] [
-no_tls1_1] [
-no_tls1_2] [
-fallback_scsv] [
-bugs] [
-cipher cipherlist] [
-serverpref] [
-starttls protocol] [
-engine id] [
-tlsextdebug] [
-no_ticket] [
-sess_out filename] [
-sess_in filename] [
-rand file(s)] [
-serverinfo types]
[
-status] [
-alpn protocols] [
-nextprotoneg protocols]
DESCRIPTION
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
SSL servers.
OPTIONS
- -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.
- -servername name
- Set the TLS SNI (Server Name Indication) extension in the
ClientHello message.
- -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 used.
- -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.
- -verify_return_error
- Return verification errors instead of continuing. This will
typically abort the handshake with a fatal error.
- -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.
- -purpose, -ignore_critical, -issuer_checks, -crl_check,
-crl_check_all, -policy_check, -extended_crl, -x509_strict, -policy
-check_ss_sig -no_alt_chains
- Set various certificate chain valiadition option. See the
verify manual page for details.
- -reconnect
- reconnects to the same server 5 times using the same
session ID, this can be used as a test that session caching is
working.
- -pause
- pauses 1 second between each read and write call.
- -showcerts
- display the whole server certificate chain: normally only
the server certificate itself is displayed.
- -prexit
- 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.
- -state
- prints out the SSL session states.
- -debug
- print extensive debugging information including a hex dump
of all traffic.
- -msg
- show all protocol messages with hex dump.
- -nbio_test
- tests non-blocking I/O
- -nbio
- turns on non-blocking I/O
- -crlf
- this option translated a line feed from the terminal into
CR+LF as required by some servers.
- -ign_eof
- inhibit shutting down the connection when end of file is
reached in the input.
- -quiet
- inhibit printing of session and certificate information.
This implicitly turns on -ign_eof as well.
- -no_ign_eof
- shut down the connection when end of file is reached in the
input. Can be used to override the implicit -ign_eof after
-quiet.
- -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 1a2b3c4d.
- -ssl2, -ssl3, -tls1, -tls1_1,
-tls1_2, -no_ssl2, -no_ssl3, -no_tls1,
-no_tls1_1, -no_tls1_2
- These options require or disable the use of the specified
SSL or TLS protocols. By default the initial handshake uses a
version-flexible method which will negotiate the highest mutually
supported protocol version.
- -fallback_scsv
- Send TLS_FALLBACK_SCSV in the ClientHello.
- -bugs
- 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.
- -serverpref
- use the server's cipher preferences; only used for
SSLV2.
- -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".
- -tlsextdebug
- print out a hex dump of any TLS extensions received from
the server.
- -no_ticket
- 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 others.
- -serverinfo types
- a list of comma-separated TLS Extension Types (numbers
between 0 and 65535). Each type will be sent as an empty ClientHello TLS
Extension. The server's response (if any) will be encoded and displayed as
a PEM file.
- -status
- sends a certificate status request to the server (OCSP
stapling). The server response (if any) is printed out.
- -alpn protocols, -nextprotoneg protocols
- these flags enable the Enable the Application-Layer
Protocol Negotiation or Next Protocol Negotiation extension, respectively.
ALPN is the IETF standard and replaces NPN. The protocols list is a
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
protocols.
CONNECTED COMMANDS
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.
NOTES
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
-no_sslv2 option.
The
s_client utility is a test tool and is designed to continue the
handshake after any certificate verification errors. As a result it will
accept any certificate chain (trusted or not) sent by the peer. None test
applications should
not do this as it makes them vulnerable to a MITM
attack. This behaviour can be changed by with the
-verify_return_error
option: any verify errors are then returned aborting the handshake.
BUGS
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
-prexit option is a bit of a hack. We should really report
information whenever a session is renegotiated.
SEE ALSO
openssl_sess_id(1),
openssl_s_server(1),
openssl_ciphers(1)
HISTORY
The -no_alt_chains options was first added to OpenSSL 1.0.2b.