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.

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.

Connect over the specified Unix-domain socket.

Use IPv4 only.

Use IPv6 only.

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

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

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

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

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

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

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.

A file containing trusted certificates to use during server authentication and to use when attempting to build the client certificate chain.

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

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

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.

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
...

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.

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 not a verified chain.

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.

prints out the SSL session states.

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

show all protocol messages with hex dump.

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

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

tests non-blocking I/O

turns on non-blocking I/O

this option translated a line feed from the terminal into CR+LF as required by some servers.

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

inhibit printing of session and certificate information. This implicitly turns on -ign_eof as well.

shut down the connection when end of file is reached in the input. Can be used to override the implicit -ign_eof after -quiet.

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

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.

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.

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.

Send TLS_FALLBACK_SCSV in the ClientHello.

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).

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.

Disables support for SSL/TLS compression. TLS compression is not recommended and is off by default as of OpenSSL 1.1.0.

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

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.

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."

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.

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

disable RFC4507bis session ticket support.

output SSL session to filename

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

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.

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

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.

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.