openssl-s_client (1)

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NAME

openssl-s_client, s_client - SSL/TLS client program

SYNOPSIS

openssl s_client [-help] [-connect host:port] [-proxy host:port] [-unix path] [-4] [-6] [-servername name] [-verify depth] [-verify_return_error] [-cert filename] [-certform DER|PEM] [-key filename] [-keyform DER|PEM] [-pass arg] [-CApath directory] [-CAfile filename] [-no-CAfile] [-no-CApath] [-dane_tlsa_domain domain] [-dane_tlsa_rrdata rrdata] [-dane_ee_no_namechecks] [-attime timestamp] [-check_ss_sig] [-crl_check] [-crl_check_all] [-explicit_policy] [-extended_crl] [-ignore_critical] [-inhibit_any] [-inhibit_map] [-no_check_time] [-partial_chain] [-policy arg] [-policy_check] [-policy_print] [-purpose purpose] [-suiteB_128] [-suiteB_128_only] [-suiteB_192] [-trusted_first] [-no_alt_chains] [-use_deltas] [-auth_level num] [-verify_depth num] [-verify_email email] [-verify_hostname hostname] [-verify_ip ip] [-verify_name name] [-x509_strict] [-reconnect] [-showcerts] [-debug] [-msg] [-nbio_test] [-state] [-nbio] [-crlf] [-ign_eof] [-no_ign_eof] [-quiet] [-ssl3] [-tls1] [-tls1_1] [-tls1_2] [-no_ssl3] [-no_tls1] [-no_tls1_1] [-no_tls1_2] [-dtls] [-dtls1] [-dtls1_2] [-fallback_scsv] [-async] [-split_send_frag] [-max_pipelines] [-read_buf] [-bugs] [-comp] [-no_comp] [-sigalgs sigalglist] [-curves curvelist] [-cipher cipherlist] [-serverpref] [-starttls protocol] [-xmpphost hostname] [-engine id] [-tlsextdebug] [-no_ticket] [-sess_out filename] [-sess_in filename] [-rand file(s)] [-serverinfo types] [-status] [-alpn protocols] [-nextprotoneg protocols] [-ct|noct] [-ctlogfile]

DESCRIPTION

The s_client command implements a generic client which connects to a remote host using It is a very useful diagnostic tool for servers.

OPTIONS

In addition to the options below the s_client utility also supports the common and client only options documented in the in the ``Supported Command Line Commands'' section of the SSL_CONF_cmd(3) manual page.

-help

Print out a usage message.

-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.

-proxy host:port

When used with the -connect flag, the program uses the host and port specified with this flag and issues an

HTTP CONNECT

command to connect to the desired server.

-unix path

Connect over the specified Unix-domain socket.

-4

Use IPv4 only.

-6

Use IPv6 only.

-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.

-no-CAfile

Do not load the trusted

CA

certificates from the default file location

-no-CApath

Do not load the trusted

CA

certificates from the default directory location

-dane_tlsa_domain domain

Enable

RFC6698/RFC7671 DANE TLSA

authentication and specify the

TLSA

base domain which becomes the default

SNI

hint and the primary reference identifier for hostname checks. This must be used in combination with at least one instance of the -dane_tlsa_rrdata option below.

When

DANE

authentication succeeds, the diagnostic output will include the lowest (closest to 0) depth at which a

TLSA

record authenticated a chain certificate. When that

TLSA

record is a ``2 1 0'' trust anchor public key that signed (rather than matched) the top-most certificate of the chain, the result is reported as ``

TA

public key verified''. Otherwise, either the

TLSA

record ``matched

TA

certificate'' at a positive depth or else ``matched

EE

certificate'' at depth 0.

-dane_tlsa_rrdata rrdata

Use one or more times to specify the

RRDATA

fields of the

DANE TLSA

RRset associated with the target service. The rrdata value is specied in ``presentation form'', that is four whitespace separated fields that specify the usage, selector, matching type and associated data, with the last of these encoded in hexadecimal. Optional whitespace is ignored in the associated data field. For example:

  $ openssl s_client -brief -starttls smtp \
    -connect smtp.example.com:25 \
    -dane_tlsa_domain smtp.example.com \
    -dane_tlsa_rrdata "2 1 1
      B111DD8A1C2091A89BD4FD60C57F0716CCE50FEEFF8137CDBEE0326E 02CF362B" \
    -dane_tlsa_rrdata "2 1 1
      60B87575447DCBA2A36B7D11AC09FB24A9DB406FEE12D2CC90180517 616E8A18"
  ...
  Verification: OK
  Verified peername: smtp.example.com
  DANE TLSA 2 1 1 ...ee12d2cc90180517616e8a18 matched TA certificate at depth 1
  ...

-dane_ee_no_namechecks

This disables server name checks when authenticating via

DANE-EE

(3)

TLSA

records. For some applications, primarily web browsers, it is not safe to disable name checks due to ``unknown key share'' attacks, in which a malicious server can convince a client that a connection to a victim server is instead a secure connection to the malicious server. The malicious server may then be able to violate cross-origin scripting restrictions. Thus, despite the text of

RFC7671,

name checks are by default enabled for

DANE-EE

(3)

TLSA

records, and can be disabled in applications where it is safe to do so. In particular,

SMTP

and

XMPP

clients should set this option as

SRV

and

MX

records already make it possible for a remote domain to redirect client connections to any server of its choice, and in any case

SMTP

and

XMPP

clients do not execute scripts downloaded from remote servers.

-attime, -check_ss_sig, -crl_check, -crl_check_all, -explicit_policy, -extended_crl, -ignore_critical, -inhibit_any, -inhibit_map, -no_alt_chains, -no_check_time, -partial_chain, -policy, -policy_check, -policy_print, -purpose, -suiteB_128, -suiteB_128_only, -suiteB_192, -trusted_first, -use_deltas, -auth_level, -verify_depth, -verify_email, -verify_hostname, -verify_ip, -verify_name, -x509_strict

Set various certificate chain validation options. See the verify(1) 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.

-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.

-trace

show verbose trace output of protocol messages. OpenSSL needs to be compiled with enable-ssl-trace for this option to work.

-msgfile

file to send output of -msg or -trace to, default standard output.

-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. The default value is ``Client_identity'' (without the quotes).

-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. This option must be provided in order to use a

PSK

cipher.

-ssl3, -tls1, -tls1_1, -tls1_2, -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 s_client will negotiate the highest mutually supported protocol version. When a specific

TLS

version is required, only that version will be offered to and accepted from the server.

-dtls, -dtls1, -dtls1_2

These options make s_client use

DTLS

protocols instead of

TLS.

With -dtls, s_client will negotiate any supported

DTLS

protocol version, whilst -dtls1 and -dtls1_2 will only support

DTLS1.0

and

DTLS1.2

respectively.

-fallback_scsv

Send

TLS_FALLBACK_SCSV

in the ClientHello.

-async

switch on asynchronous mode. Cryptographic operations will be performed asynchronously. This will only have an effect if an asynchronous capable engine is also used via the -engine option. For test purposes the dummy async engine (dasync) can be used (if available).

-split_send_frag int

The size used to split data for encrypt pipelines. If more data is written in one go than this value then it will be split into multiple pipelines, up to the maximum number of pipelines defined by max_pipelines. This only has an effect if a suitable ciphersuite has been negotiated, an engine that supports pipelining has been loaded, and max_pipelines is greater than 1. See SSL_CTX_set_split_send_fragment(3) for further information.

-max_pipelines int

The maximum number of encrypt/decrypt pipelines to be used. This will only have an effect if an engine has been loaded that supports pipelining (e.g. the dasync engine) and a suitable ciphersuite has been negotiated. The default value is 1. See SSL_CTX_set_max_pipelines(3) for further information.

-read_buf int

The default read buffer size to be used for connections. This will only have an effect if the buffer size is larger than the size that would otherwise be used and pipelining is in use (see SSL_CTX_set_default_read_buffer_len(3) for further information).

-bugs

there are several known bug in

SSL

and

TLS

implementations. Adding this option enables various workarounds.

-comp

Enables support for

SSL/TLS

compression. This option was introduced in OpenSSL 1.1.0.

TLS

compression is not recommended and is off by default as of OpenSSL 1.1.0.

-no_comp

Disables support for

SSL/TLS

compression.

TLS

compression is not recommended and is off by default as of OpenSSL 1.1.0.

-brief

only provide a brief summary of connection parameters instead of the normal verbose output.

-sigalgs sigalglist

Specifies the list of signature algorithms that are sent by the client. The server selects one entry in the list based on its preferences. For example strings, see SSL_CTX_set1_sigalgs(3)

-curves curvelist

Specifies the list of supported curves to be sent by the client. The curve is is ultimately selected by the server. For a list of all curves, use:

    $ openssl ecparam -list_curves

-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'', ``ftp'', ``xmpp'', ``xmpp-server'', and ``irc.''

-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.

-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 an 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 receiving ServerHello with a list of server supported protocols.

-ct|noct

Use one of these two options to control whether Certificate Transparency (

CT

) is enabled (-ct) or disabled (-noct). If

CT

is enabled, signed certificate timestamps (SCTs) will be requested from the server and reported at handshake completion.

Enabling

CT

also enables

OCSP

stapling, as this is one possible delivery method for SCTs.

-ctlogfile

A file containing a list of known Certificate Transparency logs. See SSL_CTX_set_ctlog_list_file(3) for the expected file format.

CONNECTED COMMANDS

If a connection is established with an 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 servers. To connect to an server the command:

 openssl s_client -connect servername:443

would typically be used (https uses port 443). If the connection succeeds then an

command can be given such as `` 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

list'' when it requests a certificate. By using s_client the list can be viewed and checked. However some servers only request client authentication after a specific is requested. To obtain the list in this case it is necessary to use the -prexit option and send an 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

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 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

SSL_CONF_cmd(3), sess_id(1), s_server(1), ciphers(1)

HISTORY

The -no_alt_chains options was first added to OpenSSL 1.1.0.

COPYRIGHT

Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the ``License''). You may not use this file except in compliance with the License. You can obtain a copy in the file

in the source distribution or at <www.openssl.org/source/license.html>.