SSLCLIENT(1) DACS Tools and Utilities SSLCLIENT(1)


sslclient — an SSL/TLS client


sslclient [dacsoptions] [ -caf | --ca_cert_file filename ]
[ -cad | --ca_cert_dir dirname ]
[ -ccf | --cert_chain_file filename ]
[ -C | --ciphers cipherstring ]
[--disable-sni] [ [-dvp] | [--default_verify_paths] cipherstring ]
[ -h | --help ] [ -kf | --key_file filename ]
[ -kft | --key_file_type pem | asn1 ]
[ -p | -sp | [--server_port] portnum ]
[ -r | --random filename ]
[[ -sm | --server_match regex ] ...]
[ -sni | --enable-sni ]
[ -vd | --verify_depth depth ]
[ -vp | --verify_peer method ]
[ -vt | --verify_type none | peer ] [--] server [:port ]


This program is part of the DACS suite. It can be used with the usual DACS command line options (dacsoptions), provided they all appear before the program-specific flags (note that the -un flag can be used to suppress configuration file processing). sslclient is also used by the dacshttp(1) command and by requests generated internally by DACS components.

The sslclient utility acts as an SSL/TLS client. After establishing a bidirectional SSL/TLS connection with an SSL/TLS server, it forwards its standard input to the SSL/TLS server and writes data produced by the SSL/TLS server to sslclient's standard output.

sslclient connects to server (a domain name or IP address). If a port number suffix is given (port), it is used; otherwise, if a port number is specified as a separate command line argument (--server_port portnum), that is used; failing that, the default SSL/TLS port for https (443) is used.

The program reads from its standard input and the server asynchronously (using non-blocking I/O). Note that the server side might need to see end-of-file on its input before its output is returned to sslclient.

This program's underlying SSL/TLS functionality is provided by OpenSSL.


sslclient recognizes these options:

-caf filename
--ca_cert_file filename

This identifies filename as a file of CA certificates in PEM format. This is the CAfile argument to the OpenSSL SSL_CTX_load_verify_locations() function. It is similar to mod_ssl's SSLCACertificateFile directive, except that it is used to verify the server's SSL certificate.

-cad dirname
--ca_cert_dir dirname

This identifies dirname as a directory containing CA certificates in PEM format, one certificate per file. This is the CApath argument to the OpenSSL SSL_CTX_load_verify_locations() function. It is similar to mod_ssl's SSLCACertificatePath directive, except that it is used to verify the server's certificate.

-ccf filename
--cert_chain_file filename

This causes the client certificate chain to be loaded from filename, a file containing certificates in PEM format. This is the file argument to the OpenSSL SSL_CTX_use_certificate_chain_file() function. It is similar to mod_ssl's SSLCACertificateChainFile directive, except that it is used for the client's chain.


If you want the client certificate to be sent you must also specify the -kf flag.

-C cipherstring
--ciphers cipherstring

This sets the list of SSL/TLS ciphers to be used to cipherstring. This is the str argument to the OpenSSL SSL_CTX_set_cipher_list() function. It is similar to mod_ssl's SSLCipherSuite directive. Also see the --with-default-cipher-list build option.


This flag tells sslclient to use default locations for finding CA certificates. It results in a call to the OpenSSL SSL_CTX_set_default_verify_paths() function.


This flag tells sslclient not to use Server Name Indication (SNI), a TLS extension.


Print a usage synopsis, which includes the default cipher list.

-kf filename
--key_file filename

This sets sslclient's private key to the first private key found in filename. This is the file argument to the OpenSSL SSL_CTX_usePrivateKey_file() function. The default private key file type is PEM. If the key has been encrypted, the program will prompt for the passphrase.

-kft type
--key_file_type type

The private key file type is set to type, which must be either pem or asn1 (case insensitive). The default private key file type is PEM.

-p portnum
-sp portnum
--server_port portnum

Unless appended to the server argument, portnum is the port number to use, overriding the default port (443).

-r filename
--random filename

Seed material for the PRNG is read from filename. This is the filename argument to the OpenSSL RAND_load_file() function.

-sm regex
--server_match regex

This argument, which may be repeated, specifies a constraint on the server's identity by matching an attribute value in the server's certificate against regex. These tests are made immediately after an SSL/TLS connection is established. Each regex is an IEEE Std 1003.2 ("POSIX.2") regular expression with extended expressions and case insensitivity (REG_EXTENDED | REG_ICASE). See below for the matching algorithm.


When it is provided by its OpenSSL library, the Server Name Indication (SNI) TLS extension is used by default, so it should not be necessary to specify this flag. Refer to RFC 6066 for details.

-vd depth
--verify_depth depth

This sets the maximum depth for certificate chain verification to depth. This is the depth argument to the OpenSSL SSL_CTX_set_verify_depth() function.

-vp method
--verify_peer method

This sets the method for verifying the identity of an SSL/TLS peer, overriding the default (see the --with-default-verify-peer build option). The method may be dacs (to select the original "server match" algorithm that uses the -sm flag), openssl (for the verification/validation methods provided by OpenSSL), or any of none, disable, disabled, or off (any of which turn off peer identity verification). Refer to Server Identity Verification for details.

-vt type
--verify_type type

This sets the verification mode to type, which must be either none or peer (case insensitive). This is the mode argument to the OpenSSL SSL_CTX_set_verify() function.


This argument explicitly marks the end of the flags.

The DACS -v (or --verbose) flag causes the program to show some of the server's SSL certificate, print feedback about regular expression matching, and so on. If sslclient is not doing what you expect, try using this flag.

Server Identity Verification

If the server presents a valid SSL (X.509) certificate, a set of checks is applied to it to help ensure that sslclient is communicating with the intended entity. Verification is successful and checking is terminated as soon as any test is successful. If no test succeeds, the program terminates immediately.


You can use a command like the following one to display an X.509 certificate to stdout in text form:

% openssl x509 -noout -text < cert.crt

Here, cert.crt is the certificate to display.

The server certificate's subjectAltName extension fields have the format field-name:field-value. For each such field, tests are made in the following sequence:

  1. the entire field is matched against each of the regular expressions given on the command line.

  2. if the previous test failed and field-name is "DNS" (exact match), it is compared case insensitively to the server's name (as given on the command line).

  3. if the previous test failed and if the field-name is "IP Address" (exact match), it is compared to the server's name (exact match), which is assumed to be an IP address (as given on the command line).

If the above procedure is unsuccessful and the server certificate's commonName attribute value is available, it is matched against each of the regular expressions given on the command line.


The following command line attempts to connect to port 443 at and prints to stdout the server's response to a request for the home page:

% printf "GET HTTP/1.0\r\n\r\n" | sslclient


When connecting to a web server, note that the request-line and every header-field should be terminated by a CRLF (carriage return, line feed/newline), otherwise the web server may respond with a 400 (Bad Request) error or a 301 (Moved Permanently) redirect. Apparently, Apache has become more strict in this regard. In particular, this may trip you up if you use sslclient interactively, since your input will end with only a newline. Refer to RFC 7230, Section 3.


When used with DACS logging configured, messages are directed to a log file, otherwise error messages and verbose output are written to stderr. The program exits 0 if everything was fine, 1 if an error occurred.


A wrapper mode of operation might be useful.

It would also be useful to have a mode where it listens for an SSL/TLS connection for input (rather than its standard input) and then relays data over that connection to a specified server, possibly but not necessarily via SSL/TLS. This mode might run on a firewall host to forward an approved incoming SSL/TLS connection (presumably authenticated by a client certificate, and possibly by a DACS ruleset) to a service running on an interior host, for instance.


dacshttp(1), openssl(1), s_client(1), stunnel(1), curl(1), sslwrap(1), and others, and regex(3).

A variety of reference material on SSL/TLS is available. Perhaps best is Network Security with OpenSSL by John Viega, Matt Messier, and Pravir Chandra, O'Reilly & Associates, Inc., 2002. Also useful are SSL/TLS Strong Encryption: An Introduction, Netscape SSL 3.0 Specification, RFC 2246, and RFC 6066.


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Copyright 2003-2023 Distributed Systems Software. See the LICENSE file that accompanies the distribution for licensing information.

DACS Version 1.4.50 21-Jul-2023 SSLCLIENT(1)

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$Id: sslclient.1.xml 3241 2023-02-08 23:23:28Z brachman $