NAME¶
s_client - SSL/TLS client program
SYNOPSIS¶
openssl s_client [
-connect host:port] [
-verify
depth] [
-cert filename] [
-certform DER|PEM] [
-key
filename] [
-keyform DER|PEM] [
-pass arg] [
-CApath
directory] [
-CAfile filename] [
-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] [
-rand file(s)]
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.
- -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.
- -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
- 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.
- -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, -no_ssl2,
-no_ssl3, -no_tls1, -no_tls1_1, -no_tls1_2
- 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 still 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.
- -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.
- -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.
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.
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
-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.
SEE ALSO¶
sess_id(1),
s_server(1),
ciphers(1)