=pod

=head1 NAME

openssl-s_client,

s_client - SSL/TLS client program

=head1 SYNOPSIS

B<openssl> B<s_client>

[B<-help>]

[B<-connect host:port>]

[B<-bind host:port>]

[B<-proxy host:port>]

[B<-unix path>]

[B<-4>]

[B<-6>]

[B<-servername name>]

[B<-noservername>]

[B<-verify depth>]

[B<-verify_return_error>]

[B<-cert filename>]

[B<-certform DER|PEM>]

[B<-key filename>]

[B<-keyform DER|PEM>]

[B<-cert_chain filename>]

[B<-build_chain>]

[B<-xkey>]

[B<-xcert>]

[B<-xchain>]

[B<-xchain_build>]

[B<-xcertform PEM|DER>]

[B<-xkeyform PEM|DER>]

[B<-pass arg>]

[B<-CApath directory>]

[B<-CAfile filename>]

[B<-chainCApath directory>]

[B<-chainCAfile filename>]

[B<-no-CAfile>]

[B<-no-CApath>]

[B<-requestCAfile filename>]

[B<-dane_tlsa_domain domain>]

[B<-dane_tlsa_rrdata rrdata>]

[B<-dane_ee_no_namechecks>]

[B<-attime timestamp>]

[B<-check_ss_sig>]

[B<-crl_check>]

[B<-crl_check_all>]

[B<-explicit_policy>]

[B<-extended_crl>]

[B<-ignore_critical>]

[B<-inhibit_any>]

[B<-inhibit_map>]

[B<-no_check_time>]

[B<-partial_chain>]

[B<-policy arg>]

[B<-policy_check>]

[B<-policy_print>]

[B<-purpose purpose>]

[B<-suiteB_128>]

[B<-suiteB_128_only>]

[B<-suiteB_192>]

[B<-trusted_first>]

[B<-no_alt_chains>]

[B<-use_deltas>]

[B<-auth_level num>]

[B<-nameopt option>]

[B<-verify_depth num>]

[B<-verify_email email>]

[B<-verify_hostname hostname>]

[B<-verify_ip ip>]

[B<-verify_name name>]

[B<-build_chain>]

[B<-x509_strict>]

[B<-reconnect>]

[B<-showcerts>]

[B<-debug>]

[B<-msg>]

[B<-nbio_test>]

[B<-state>]

[B<-nbio>]

[B<-crlf>]

[B<-ign_eof>]

[B<-no_ign_eof>]

[B<-psk_identity identity>]

[B<-psk key>]

[B<-psk_session file>]

[B<-quiet>]

[B<-ssl3>]

[B<-tls1>]

[B<-tls1_1>]

[B<-tls1_2>]

[B<-tls1_3>]

[B<-no_ssl3>]

[B<-no_tls1>]

[B<-no_tls1_1>]

[B<-no_tls1_2>]

[B<-no_tls1_3>]

[B<-dtls>]

[B<-dtls1>]

[B<-dtls1_2>]

[B<-sctp>]

[B<-sctp_label_bug>]

[B<-fallback_scsv>]

[B<-async>]

[B<-max_send_frag>]

[B<-split_send_frag>]

[B<-max_pipelines>]

[B<-read_buf>]

[B<-bugs>]

[B<-comp>]

[B<-no_comp>]

[B<-allow_no_dhe_kex>]

[B<-sigalgs sigalglist>]

[B<-curves curvelist>]

[B<-cipher cipherlist>]

[B<-ciphersuites val>]

[B<-serverpref>]

[B<-starttls protocol>]

[B<-xmpphost hostname>]

[B<-name hostname>]

[B<-engine id>]

[B<-tlsextdebug>]

[B<-no_ticket>]

[B<-sess_out filename>]

[B<-sess_in filename>]

[B<-rand file...>]

[B<-writerand file>]

[B<-serverinfo types>]

[B<-status>]

[B<-alpn protocols>]

[B<-nextprotoneg protocols>]

[B<-ct>]

[B<-noct>]

[B<-ctlogfile>]

[B<-keylogfile file>]

[B<-early_data file>]

[B<-enable_pha>]

[B<target>]

=head1 DESCRIPTION

The B<s_client> command implements a generic SSL/TLS client which connects

to a remote host using SSL/TLS. It is a I<very> useful diagnostic tool for

SSL servers.

=head1 OPTIONS

In addition to the options below the B<s_client> utility also supports the

common and client only options documented

in the "Supported Command Line Commands" section of the L<SSL_CONF_cmd(3)>

manual page.

=over 4

=item B<-help>

Print out a usage message.

=item B<-connect host:port>

This specifies the host and optional port to connect to. It is possible to

select the host and port using the optional target positional argument instead.

If neither this nor the target positional argument are specified then an attempt

is made to connect to the local host on port 4433.

=item B<-bind host:port>]

This specifies the host address and or port to bind as the source for the

connection.  For Unix-domain sockets the port is ignored and the host is

used as the source socket address.

=item B<-proxy host:port>

When used with the B<-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.

=item B<-unix path>

Connect over the specified Unix-domain socket.

=item B<-4>

Use IPv4 only.

=item B<-6>

Use IPv6 only.

=item B<-servername name>

Set the TLS SNI (Server Name Indication) extension in the ClientHello message to

the given value. 

If B<-servername> is not provided, the TLS SNI extension will be populated with 

the name given to B<-connect> if it follows a DNS name format. If B<-connect> is 

not provided either, the SNI is set to "localhost".

This is the default since OpenSSL 1.1.1.

Even though SNI should normally be a DNS name and not an IP address, if 

B<-servername> is provided then that name will be sent, regardless of whether 

it is a DNS name or not.

This option cannot be used in conjunction with B<-noservername>.

=item B<-noservername>

Suppresses sending of the SNI (Server Name Indication) extension in the

ClientHello message. Cannot be used in conjunction with the B<-servername> or

<-dane_tlsa_domain> options.

=item B<-cert certname>

The certificate to use, if one is requested by the server. The default is

not to use a certificate.

=item B<-certform format>

The certificate format to use: DER or PEM. PEM is the default.

=item B<-key keyfile>

The private key to use. If not specified then the certificate file will

be used.

=item B<-keyform format>

The private format to use: DER or PEM. PEM is the default.

=item B<-cert_chain>

A file containing trusted certificates to use when attempting to build the

client/server certificate chain related to the certificate specified via the

B<-cert> option.

=item B<-build_chain>

Specify whether the application should build the certificate chain to be

provided to the server.

=item B<-xkey infile>, B<-xcert infile>, B<-xchain>

Specify an extra certificate, private key and certificate chain. These behave

in the same manner as the B<-cert>, B<-key> and B<-cert_chain> options.  When

specified, the callback returning the first valid chain will be in use by the

client.

=item B<-xchain_build>

Specify whether the application should build the certificate chain to be

provided to the server for the extra certificates provided via B<-xkey infile>,

B<-xcert infile>, B<-xchain> options.

=item B<-xcertform PEM|DER>, B<-xkeyform PEM|DER>

Extra certificate and private key format respectively.

=item B<-pass arg>

the private key password source. For more information about the format of B<arg>

see the B<PASS PHRASE ARGUMENTS> section in L<openssl(1)>.

=item B<-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.

=item B<-verify_return_error>

Return verification errors instead of continuing. This will typically

abort the handshake with a fatal error.

=item B<-nameopt option>

Option which determines how the subject or issuer names are displayed. The

B<option> argument can be a single option or multiple options separated by

commas.  Alternatively the B<-nameopt> switch may be used more than once to

set multiple options. See the L<x509(1)> manual page for details.

=item B<-CApath directory>

The directory to use for server certificate verification. This directory

must be in "hash format", see L<verify(1)> for more information. These are

also used when building the client certificate chain.

=item B<-CAfile file>

A file containing trusted certificates to use during server authentication

and to use when attempting to build the client certificate chain.

=item B<-chainCApath directory>

The directory to use for building the chain provided to the server. This

directory must be in "hash format", see L<verify(1)> for more information.

=item B<-chainCAfile file>

A file containing trusted certificates to use when attempting to build the

client certificate chain.

=item B<-no-CAfile>

Do not load the trusted CA certificates from the default file location

=item B<-no-CApath>

Do not load the trusted CA certificates from the default directory location

=item B<-requestCAfile file>

A file containing a list of certificates whose subject names will be sent

to the server in the B<certificate_authorities> extension. Only supported

for TLS 1.3

=item B<-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 B<-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.

=item B<-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 B<rrdata> value is

specified 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

  ...

=item B<-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.

=item B<-attime>, B<-check_ss_sig>, B<-crl_check>, B<-crl_check_all>,

B<-explicit_policy>, B<-extended_crl>, B<-ignore_critical>, B<-inhibit_any>,

B<-inhibit_map>, B<-no_alt_chains>, B<-no_check_time>, B<-partial_chain>, B<-policy>,

B<-policy_check>, B<-policy_print>, B<-purpose>, B<-suiteB_128>,

B<-suiteB_128_only>, B<-suiteB_192>, B<-trusted_first>, B<-use_deltas>,

B<-auth_level>, B<-verify_depth>, B<-verify_email>, B<-verify_hostname>,

B<-verify_ip>, B<-verify_name>, B<-x509_strict>

Set various certificate chain validation options. See the

L<verify(1)> manual page for details.

=item B<-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.

=item B<-showcerts>

Displays the server certificate list as sent by the server: it only consists of

certificates the server has sent (in the order the server has sent them). It is

B<not> a verified chain.

=item B<-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.

=item B<-state>

Prints out the SSL session states.

=item B<-debug>

Print extensive debugging information including a hex dump of all traffic.

=item B<-msg>

Show all protocol messages with hex dump.

=item B<-trace>

Show verbose trace output of protocol messages. OpenSSL needs to be compiled

with B<enable-ssl-trace> for this option to work.

=item B<-msgfile>

File to send output of B<-msg> or B<-trace> to, default standard output.

=item B<-nbio_test>

Tests non-blocking I/O

=item B<-nbio>

Turns on non-blocking I/O

=item B<-crlf>

This option translated a line feed from the terminal into CR+LF as required

by some servers.

=item B<-ign_eof>

Inhibit shutting down the connection when end of file is reached in the

input.

=item B<-quiet>

Inhibit printing of session and certificate information.  This implicitly

turns on B<-ign_eof> as well.

=item B<-no_ign_eof>

Shut down the connection when end of file is reached in the input.

Can be used to override the implicit B<-ign_eof> after B<-quiet>.

=item B<-psk_identity identity>

Use the PSK identity B<identity> when using a PSK cipher suite.

The default value is "Client_identity" (without the quotes).

=item B<-psk key>

Use the PSK key B<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.

=item B<-psk_session file>

Use the pem encoded SSL_SESSION data stored in B<file> as the basis of a PSK.

Note that this will only work if TLSv1.3 is negotiated.

=item B<-ssl3>, B<-tls1>, B<-tls1_1>, B<-tls1_2>, B<-tls1_3>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>, B<-no_tls1_3>

These options require or disable the use of the specified SSL or TLS protocols.

By default B<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.

Note that not all protocols and flags may be available, depending on how

OpenSSL was built.

=item B<-dtls>, B<-dtls1>, B<-dtls1_2>

These options make B<s_client> use DTLS protocols instead of TLS.

With B<-dtls>, B<s_client> will negotiate any supported DTLS protocol version,

whilst B<-dtls1> and B<-dtls1_2> will only support DTLS1.0 and DTLS1.2

respectively.

=item B<-sctp>

Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in

conjunction with B<-dtls>, B<-dtls1> or B<-dtls1_2>. This option is only

available where OpenSSL has support for SCTP enabled.

=item B<-sctp_label_bug>

Use the incorrect behaviour of older OpenSSL implementations when computing

endpoint-pair shared secrets for DTLS/SCTP. This allows communication with

older broken implementations but breaks interoperability with correct

implementations. Must be used in conjunction with B<-sctp>. This option is only

available where OpenSSL has support for SCTP enabled.

=item B<-fallback_scsv>

Send TLS_FALLBACK_SCSV in the ClientHello.

=item B<-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 B<-engine> option. For test purposes the dummy async engine

(dasync) can be used (if available).

=item B<-max_send_frag int>

The maximum size of data fragment to send.

See L<SSL_CTX_set_max_send_fragment(3)> for further information.

=item B<-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 cipher suite has been negotiated, an engine that supports pipelining

has been loaded, and max_pipelines is greater than 1. See

L<SSL_CTX_set_split_send_fragment(3)> for further information.

=item B<-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 cipher suite has been negotiated. The default value is 1.

See L<SSL_CTX_set_max_pipelines(3)> for further information.

=item B<-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 L<SSL_CTX_set_default_read_buffer_len(3)> for

further information).

=item B<-bugs>

There are several known bugs in SSL and TLS implementations. Adding this

option enables various workarounds.

=item B<-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.

=item B<-no_comp>

Disables support for SSL/TLS compression.

TLS compression is not recommended and is off by default as of

OpenSSL 1.1.0.

=item B<-brief>

Only provide a brief summary of connection parameters instead of the

normal verbose output.

=item B<-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 L<SSL_CTX_set1_sigalgs(3)>

=item B<-curves curvelist>

Specifies the list of supported curves to be sent by the client. The curve is

ultimately selected by the server. For a list of all curves, use:

    $ openssl ecparam -list_curves

=item B<-cipher cipherlist>

This allows the TLSv1.2 and below cipher list sent by the client to be modified.

This list will be combined with any TLSv1.3 ciphersuites that have been

configured. Although the server determines which ciphersuite is used it should

take the first supported cipher in the list sent by the client. See the

B<ciphers> command for more information.

=item B<-ciphersuites val>

This allows the TLSv1.3 ciphersuites sent by the client to be modified. This

list will be combined with any TLSv1.2 and below ciphersuites that have been

configured. 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

B<ciphers> command for more information. The format for this list is a simple

colon (":") separated list of TLSv1.3 ciphersuite names.

=item B<-starttls protocol>

Send the protocol-specific message(s) to switch to TLS for communication.

B<protocol> is a keyword for the intended protocol.  Currently, the only

supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp", "xmpp-server",

"irc", "postgres", "mysql", "lmtp", "nntp", "sieve" and "ldap".

=item B<-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.

This option is an alias of the B<-name> option for "xmpp" and "xmpp-server".

=item B<-name hostname>

This option is used to specify hostname information for various protocols

used with B<-starttls> option. Currently only "xmpp", "xmpp-server",

"smtp" and "lmtp" can utilize this B<-name> option.

If this option is used with "-starttls xmpp" or "-starttls xmpp-server",

if 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.

If this option is used with "-starttls lmtp" or "-starttls smtp", it specifies

the name to use in the "LMTP LHLO" or "SMTP EHLO" message, respectively. If

this option is not specified, then "mail.example.com" will be used.

=item B<-tlsextdebug>

Print out a hex dump of any TLS extensions received from the server.

=item B<-no_ticket>

Disable RFC4507bis session ticket support.

=item B<-sess_out filename>

Output SSL session to B<filename>.

=item B<-sess_in sess.pem>

Load SSL session from B<filename>. The client will attempt to resume a

connection from this session.

=item B<-engine id>

Specifying an engine (by its unique B<id> string) will cause B<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.

=item B<-rand file...>

A file or files containing random data used to seed the random number

generator.

Multiple files can be specified separated by an OS-dependent character.

The separator is B<;> for MS-Windows, B<,> for OpenVMS, and B<:> for

all others.

=item [B<-writerand file>]

Writes random data to the specified I<file> upon exit.

This can be used with a subsequent B<-rand> flag.

=item B<-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.

=item B<-status>

Sends a certificate status request to the server (OCSP stapling). The server

response (if any) is printed out.

=item B<-alpn protocols>, B<-nextprotoneg protocols>

These flags enable the Enable the Application-Layer Protocol Negotiation

or Next Protocol Negotiation (NPN) extension, respectively. ALPN is the

IETF standard and replaces NPN.

The B<protocols> list is a comma-separated list of protocol names that

the client should advertise support for. The list should contain the most

desirable protocols first.  Protocol names are printable ASCII strings,

for example "http/1.1" or "spdy/3".

An 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.

The flag B<-nextprotoneg> cannot be specified if B<-tls1_3> is used.

=item B<-ct>, B<-noct>

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

is enabled (B<-ct>) or disabled (B<-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.

=item B<-ctlogfile>

A file containing a list of known Certificate Transparency logs. See

L<SSL_CTX_set_ctlog_list_file(3)> for the expected file format.

=item B<-keylogfile file>

Appends TLS secrets to the specified keylog file such that external programs

(like Wireshark) can decrypt TLS connections.

=item B<-early_data file>

Reads the contents of the specified file and attempts to send it as early data

to the server. This will only work with resumed sessions that support early

data and when the server accepts the early data.

=item B<-enable_pha>

For TLSv1.3 only, send the Post-Handshake Authentication extension. This will

happen whether or not a certificate has been provided via B<-cert>.

=item B<[target]>

Rather than providing B<-connect>, the target hostname and optional port may

be provided as a single positional argument after all options. If neither this

nor B<-connect> are provided, falls back to attempting to connect to localhost

on port 4433.

=back

=head1 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. If end of file is reached then the connection will be closed down. When

used interactively (which means neither B<-quiet> nor B<-ign_eof> have been

given), then certain commands are also recognized which perform special

operations. These commands are a letter which must appear at the start of a

line. They are listed below.

=over 4

=item B<Q>

End the current SSL connection and exit.

=item B<R>

Renegotiate the SSL session (TLSv1.2 and below only).

=item B

Send a heartbeat message to the server (DTLS only)

=item B<k>

Send a key update message to the server (TLSv1.3 only)

=item B<K>

Send a key update message to the server and request one back (TLSv1.3 only)

=back

=head1 NOTES

B<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 B<-bugs>,

B<-ssl3>, B<-tls1>, B<-no_ssl3>, B<-no_tls1> options can be tried

in case it is a buggy server. In particular you should play with these

options B<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 B<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 B<-prexit> option and send an HTTP request

for an appropriate page.

If a certificate is specified on the command line using the B<-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

B<-showcerts> option can be used to show all the certificates sent by the

server.

The B<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 B<not> do this as it makes them vulnerable to a MITM

attack. This behaviour can be changed by with the B<-verify_return_error>

option: any verify errors are then returned aborting the handshake.

The B<-bind> option may be useful if the server or a firewall requires

connections to come from some particular address and or port.

=head1 BUGS

Because this program has a lot of options and also because some of the

techniques used are rather old, the C source of B<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 B<-prexit> option is a bit of a hack. We should really report

information whenever a session is renegotiated.

=head1 SEE ALSO

L<SSL_CONF_cmd(3)>, L<sess_id(1)>, L<s_server(1)>, L<ciphers(1)>,

L<SSL_CTX_set_max_send_fragment(3)>, L<SSL_CTX_set_split_send_fragment(3)>,

L<SSL_CTX_set_max_pipelines(3)>

=head1 HISTORY

The B<-no_alt_chains> option was added in OpenSSL 1.1.0.

The B<-name> option was added in OpenSSL 1.1.1.

=head1 COPYRIGHT

Copyright 2000-2020 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 LICENSE in the source distribution or at

L<https://www.openssl.org/source/license.html>.

=cut