-help

Print out a usage message.

-connect

This specifies the host and optional port to connect to. If not specified then an attempt is made toconnect to the local host on port 4433.

-proxy

When used with the -connect flag, the program uses the host and port specified with this flag and issuesan HTTP CONNECT command to connect to the desired server.

-unix

Connect over the specified Unix-domain socket.

-servername

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

-cert

The certificate to use, if one is requested by the server. The default is not to use a certificate.

-certform

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

-key

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

-keyform

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

-pass

the private key password source. For more information about the format of arg see the PASS PHRASEARGUMENTS section in openssl(1).

-verify

The verify depth to use. This specifies the maximum length of the server certificate chain and turns onserver certificate verification. Currently the verify operation continues after errors so all theproblems with a certificate chain can be seen. As a side effect the connection will never fail due to aserver certificate verify failure.

-verify_return_error

Return verification errors instead of continuing. This will typically abort the handshake with a fatalerror.

-CApath

The directory to use for server certificate verification. This directory must be in "hash format", seeverify for more information. These are also used when building the client certificate chain.

-CAfile

A file containing trusted certificates to use during server authentication and to use when attempting tobuild 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

Enable RFC6698/RFC7671 DANE TLSA authentication and specify the TLSA base domain which becomes the defaultSNI hint and the primary reference identifier for hostname checks. This must be used in combination withat 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 atwhich a TLSA record authenticated a chain certificate. When that TLSA record is a "2 1 0" trust anchorpublic key that signed (rather than matched) the top-most certificate of the chain, the result is reportedas "TA public key verified". Otherwise, either the TLSA record "matched TA certificate" at a positivedepth or else "matched EE certificate" at depth 0.

-dane_tlsa_rrdata

Use one or more times to specify the RRDATA fields of the DANE TLSA RRset associated with the targetservice. The rrdata value is specied in "presentation form", that is four whitespace separated fieldsthat specify the usage, selector, matching type and associated data, with the last of these encoded inhexadecimal. Optional whitespace is ignored in the associated data field. For example:$ openssl s_client -brief -starttls smtp \

-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 whicha malicious server can convince a client that a connection to a victim server is instead a secureconnection to the malicious server. The malicious server may then be able to violate cross-originscripting restrictions. Thus, despite the text of RFC7671, name checks are by default enabled forDANE-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 remotedomain to redirect client connections to any server of its choice, and in any case SMTP and XMPP clientsdo 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

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 sessioncaching 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 evenif 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 aclient 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 beenestablished.

-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 forthis 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

-psk_identity

Use the PSK identity identity when using a PSK cipher suite. The default value is "Client_identity"(without the quotes).

-psk

Use the PSK key key when using a PSK cipher suite. The key is given as a hexadecimal number withoutleading 0x, for example -psk 1a2b3c4d. This option must be provided in order to use a PSK cipher.

-ssl3

These options require or disable the use of the specified SSL or TLS protocols. By default s_client willnegotiate the highest mutually supported protocol version. When a specific TLS version is required, onlythat version will be offered to and accepted from the server.

-dtls

These options make s_client use DTLS protocols instead of TLS. With -dtls, s_client will negotiate anysupported DTLS protocol version, whilst -dtls1 and -dtls1_2 will only support DTLS1.0 and DTLS1.2respectively.

-fallback_scsv

Send TLS_FALLBACK_SCSV in the ClientHello.

-async

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

-split_send_frag

The size used to split data for encrypt pipelines. If more data is written in one go than this value thenit 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 pipelininghas been loaded, and max_pipelines is greater than 1. See SSL_CTX_set_split_send_fragment(3) for furtherinformation.

-max_pipelines

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

-read_buf

The default read buffer size to be used for connections. This will only have an effect if the buffer sizeis larger than the size that would otherwise be used and pipelining is in use (seeSSL_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 variousworkarounds.

-comp

Enables support for SSL/TLS compression. This option was introduced in OpenSSL 1.1.0. TLS compression isnot 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 ofOpenSSL 1.1.0.

-brief

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

-sigalgs

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

-curves

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

-cipher

this allows the cipher list sent by the client to be modified. Although the server determines which ciphersuite is used it should take the first supported cipher in the list sent by the client. See the cipherscommand for more information.

-starttls

send the protocol-specific message(s) to switch to TLS for communication. protocol is a keyword for theintended protocol. Currently, the only supported keywords are "smtp", "pop3", "imap", "ftp", "xmpp","xmpp-server", and "irc."

-xmpphost

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

output SSL session to filename

-sess_in

load SSL session from filename. The client will attempt to resume a connection from this session.

-engine

specifying an engine (by its unique id string) will cause s_client to attempt to obtain a functionalreference to the specified engine, thus initialising it if needed. The engine will then be set as thedefault for all available algorithms.

-rand

a file or files containing random data used to seed the random number generator, or an EGD socket (seeRAND_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

a list of comma-separated TLS Extension Types (numbers between 0 and 65535). Each type will be sent as anempty ClientHello TLS Extension. The server's response (if any) will be encoded and displayed as a PEMfile.

-status

sends a certificate status request to the server (OCSP stapling). The server response (if any) is printedout.

-alpn

these flags enable the Enable the Application-Layer Protocol Negotiation or Next Protocol Negotiationextension, 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 wantedprotocols first. Protocol names are printable ASCII strings, for example "http/1.1" or "spdy/3". Emptylist of protocols is treated specially and will cause the client to advertise support for the TLSextension 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

-ctlogfile

A file containing a list of known Certificate Transparency logs. See SSL_CTX_set_ctlog_list_file(3) forthe expected file format.CONNECTED COMMANDSIf a connection is established with an SSL server then any data received from the server is displayed and anykey presses will be sent to the server. When used interactively (which means neither -quiet nor -ign_eof havebeen given), the session will be renegotiated if the line begins with an R, and if the line begins with a Q orif end of file is reached, the connection will be closed down.NOTESs_client can be used to debug SSL servers. To connect to an SSL HTTP server the command:openssl s_client -connect servername:443would typically be used (https uses port 443). If the connection succeeds then an HTTP command can be givensuch as "GET /" to retrieve a web page.If the handshake fails then there are several possible causes, if it is nothing obvious like no clientcertificate then the -bugs, -ssl3, -tls1, -no_ssl3, -no_tls1 options can be tried in case it is a buggyserver. In particular you should play with these options before submitting a bug report to an OpenSSL mailinglist.A frequent problem when attempting to get client certificates working is that a web client complains it has nocertificates or gives an empty list to choose from. This is normally because the server is not sending theclients certificate authority in its "acceptable CA list" when it requests a certificate. By using s_clientthe CA list can be viewed and checked. However some servers only request client authentication after aspecific URL is requested. To obtain the list in this case it is necessary to use the -prexit option and sendan 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 serverspecifically requests a client certificate. Therefor merely including a client certificate on the command lineis no guarantee that the certificate works.If there are problems verifying a server certificate then the -showcerts option can be used to show the wholechain.The s_client utility is a test tool and is designed to continue the handshake after any certificateverification errors. As a result it will accept any certificate chain (trusted or not) sent by the peer. Nonetest applications should not do this as it makes them vulnerable to a MITM attack. This behaviour can bechanged by with the -verify_return_error option: any verify errors are then returned aborting the handshake.BUGSBecause this program has a lot of options and also because some of the techniques used are rather old, the Csource of s_client is rather hard to read and not a model of how things should be done. A typical SSL clientprogram would be much simpler.The -prexit option is a bit of a hack. We should really report information whenever a session is renegotiated.