S_CLIENT(1) OpenSSL S_CLIENT(1)
NAME
s_client - SSL/TLS client program
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] [-trusted_first] [-no_alt_chains] [-reconnect]
[-pause] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio]
[-crlf] [-ign_eof] [-no_ign_eof] [-quiet] [-ssl3] [-tls1] [-tls1_1]
[-tls1_2] [-dtls1] [-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2]
[-fallback_scsv] [-bugs] [-cipher cipherlist] [-krb5svc service]
[-serverpref] [-starttls protocol] [-xmpphost hostname] [-name
hostname] [-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,
-trusted_first -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.
-ssl3, -tls1, -tls1_1, -tls1_2, -dtls1, -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.
-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
its keytab.
-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",
"ftp", "xmpp", "xmpp-server", "irc", "postgres", "lmtp", "nntp",
"sieve" and "ldap".
-xmpphost hostname
This option, when used with "-starttls xmpp" or "-starttls xmpp-
server", specifies the host for the "to" attribute of the stream
element. If this option is not specified, then the host specified
with "-connect" will be used.
-name hostname
the host name to use with "-starttls smtp". If this option is not
specified, the default "openssl.client.net" will be used.
-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, -ssl3,
-tls1, -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.
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
sess_id(1), s_server(1), ciphers(1)
HISTORY
The -no_alt_chains options was first added to OpenSSL 1.0.2b.
1.0.2k 2023-11-13 S_CLIENT(1)
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