Use the specified section of the configuration file to configure the SSL_CTX object.

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.

Host to connect to; use -connect instead.

Connect to the specified port; use -connect instead.

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.

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.

When used with the -proxy flag, the program will attempt to authenticate with the specified proxy using basic (base64) authentication. NB: Basic authentication is insecure; the credentials are sent to the proxy in easily reversible base64 encoding before any TLS/SSL session is established. Therefore these credentials are easily recovered by anyone able to sniff/trace the network. Use with caution.

Use IPv4 only.

Use IPv6 only.

Set the TLS SNI (Server Name Indication) extension in the ClientHello message to the given value. If -servername is not provided, the TLS SNI extension will be populated with the name given to -connect if it follows a DNS name format. If -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 -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 -noservername.

Suppresses sending of the SNI (Server Name Indication) extension in the ClientHello message. Cannot be used in conjunction with the -servername or <-dane_tlsa_domain> options.

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

The chain for the client certificate may be specified using -cert_chain.

Specify whether the application should build the client certificate chain to be provided to the server.

CRL file to use to check the server's certificate.

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

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

Limit verify output to only errors.

A file in PEM format containing trusted certificates to use for verifying the server's certificate.

A file in PEM format containing trusted certificates to use when attempting to build the client certificate chain.

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 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
  ...

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.

Reconnects to the same server 5 times using the same session ID, this can be used as a test that session caching is working.

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.

Do not use interactive command letters.

Enable security debug messages.

Output more security debug output.

Show protocol messages.

Enable send/receive timeout on DTLS connections.

Set MTU of the link layer to the specified size.

Export keying material using the specified label.

Export the specified number of bytes of keying material; default is 20.

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.

Use the pem encoded SSL_SESSION data stored in file as the basis of a PSK. Note that this will only work if TLSv1.3 is negotiated.

Use SCTP for the transport protocol instead of UDP in DTLS. Must be used in conjunction with -dtls, -dtls1 or -dtls1_2. This option is only available where OpenSSL has support for SCTP enabled.

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 -sctp. This option is only available where OpenSSL has support for SCTP enabled.

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).

Enable Maximum Fragment Length Negotiation; allowed values are 512, 1024, 2048, and 4096.

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 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 -name option for "xmpp" and "xmpp-server".

This option is used to specify hostname information for various protocols used with -starttls option. Currently only "xmpp", "xmpp-server", "smtp" and "lmtp" can utilize this -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.

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.

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.

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 (NPN) extension, respectively. ALPN is the IETF standard and replaces NPN. The 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 -nextprotoneg cannot be specified if -tls1_3 is used.

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.

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.

For TLSv1.3 only, send the Post-Handshake Authentication extension. This will happen whether or not a certificate has been provided via -cert.

Offer SRTP key management, where value is a colon-separated profile list.

Set the SRP username to the specified value.

Set the SRP password to the specified value.

SRP username for the second ClientHello message.

Tolerate other than the known g and N values.

Set the minimal acceptable length, in bits, for N.