- sslclient [m[blue]dacsoptionsm] [-caf | --ca_cert_file filename]
[-cad | --ca_cert_dir dirname]
- [-ccf | --cert_chain_file filename]
- [-C | --ciphers cipherstring]
- [[-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 ]...]
- [-vd | --verify_depth depth]
- [-vt | --verify_type none | peer] [--] server [:port ]
- [-ccf | --cert_chain_file filename]
The sslclient utility acts as an SSL client. After establishing a bidirectional SSL connection with an SSL server, it forwards its standard input to the SSL server and writes data produced by the SSL 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_portportnum), that is used; failing that, the m[blue]default SSL port for https (443)m 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 functionality is provided by m[blue]OpenSSLm.
sslclient recognizes these options:
- This identifies filename as a file of CA certificates in PEM format. This is the CAfile argument to the m[blue]OpenSSLmm[blue]SSL_CTX_load_verify_locations()m function. It is similar to m[blue]mod_ssl'smm[blue]SSLCACertificateFilem directive, except that it is used to verify the server's SSL certificate.
- This identifies dirname as a directory containing CA certificates in PEM format, one certificate per file. This is the CApath argument to the m[blue]OpenSSLmm[blue]SSL_CTX_load_verify_locations()m function. It is similar to m[blue]mod_ssl'smm[blue]SSLCACertificatePathm directive, except that it is used to verify the server's certificate.
This causes the client certificate chain to be loaded from
filename, a file containing certificates in PEM format. This is the
argument to the
function. It is similar to
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.
- This sets the list of ciphers to be used to cipherstring. This is the str argument to the m[blue]OpenSSLmm[blue]SSL_CTX_set_cipher_list()m function. It is similar to m[blue]mod_ssl'smm[blue]SSLCipherSuitem directive.
- This flag tells sslclient to use default locations for finding CA certificates. It results in a call to the m[blue]OpenSSLmSSL_CTX_set_default_verify_paths() function.
- Print a usage synopsis.
- This sets sslclient's private key to the first private key found in filename. This is the file argument to the m[blue]OpenSSLmSSL_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.
- 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.
- Unless appended to the server argument, portnum is the port number to use, overriding the default port (443).
- Seed material for the PRNG is read from filename. This is the filename argument to the m[blue]OpenSSLmRAND_load_file() function.
- 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 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 m[blue]belowm for the matching algorithm.
- This sets the maximum depth for certificate chain verification to depth. This is the depth argument to the m[blue]OpenSSLmSSL_CTX_set_verify_depth() function.
- This sets the verification mode to type, which must be either none or peer (case insensitive). This is the mode argument to the m[blue]OpenSSLmSSL_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 example.com and prints to stdout the server's response to a request for the home page:
% perl -e 'printf "GET / HTTP/1.0\n\n";' | sslclient example.com:443
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 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. This mode might run on a firewall host to forward an approved incoming SSL connection (presumably authenticated by a client certificate, and possibly by a DACS ruleset) to a service running on an interior host, for instance.
Distributed Systems Software (m[blue]www.dss.cam)
Copyright2003-2013 Distributed Systems Software. See the m[blue]LICENSEm file that accompanies the distribution for licensing information.
- default SSL port for https (443)
- SSL/TLS Strong Encryption: An Introduction
- Netscape SSL 3.0 Specification
- RFC 2246