STUNNEL(8)

NAME
stunnel - universal SSL tunnel

SYNOPSIS
stunnel [-c | -T] [-D level] [-C cipherlist] [-p pemfile] [-v level] [-A certfile] [-S sources] [-a directory] [-t timeout] [-u ident_username] [-s setuid_user] [-g setgid_group] [-n protocol] [-P { dir/ | filename | none } ] [-B bytes] [-R randfile] [-W] [-E socket] [-d [host:]port [-f] ] [ -r [host:]port | { -l | -L } program [-- progname args] ]

DESCRIPTION
The stunnel program is designed to work as SSL encryption wrapper between remote clients and local (inetd-startable) or remote servers. The concept is that having non-SSL aware daemons running on your system you can easily set them up to communicate with clients over secure SSL channels.

stunnel can be used to add SSL functionality to commonly used inetd daemons like POP-2, POP-3, and IMAP servers, to standalone daemons like NNTP, SMTP and HTTP, and in tunneling PPP over network sockets without changes to the source code.

This product includes cryptographic software written by Eric Young (eay@cryptsoft.com)

OPTIONS
-h Print stunnel help menu

-V Print stunnel version and compile time defaults

-D level Debugging level
    Level is a number between 0 (no logging at all) and 7 (show lots of debugging info)

-C cipherlist Select permitted SSL ciphers
    A colon delimited list of the ciphers to allow in the SSL connection. For example DES-CBC3-SHA:IDEA-CBC-MD5

-c client mode (remote service uses SSL)
    default: server mode

-T transparent proxy mode
    Available only in server mode, re-write address to appear as if connection is originating from actual SSL client machine. Only available on some OS versions.

-p pemfile key/certificate PEM file name
    A PEM is always needed in server mode (see stunnel -V output for default). Specifing this flag in client mode will use this key/certificate as a client side certificate. Using client side certs is optional.

-v level verify peer certificate
    level 1 - verify peer certificate
              if present
    level 2 - verify peer certificate
    level 3 - verify peer with locally
              installed certificate
    default: no verify

-a directory client certificate directory
    This is the directory in which stunnel will look for certificates when using the -v options. Note that the certificates in this directory should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the cert. This directory will override the stunnel compile-time default, if any.
    See stunnel -V output for default.

-A certfile Certificate Authority file
    This file contains multiple CA certificates, used with the -v options. This file will override the stunnel compile-time default, if any.
    See stunnel -V output for default.

-S sources Certificate source defaults
    Both stunnel and the SSL library used to compile stunnel have default locations to look for your certificate file (-A option) and certificate directories (-a option). The -S flag allows you to control which of these default sources, if any, should be used.
      0 = ignore all default sources
      1 = use ssl library defaults
      2 = use stunnel defaults
      3 = use both ssl library and stunnel defaults
    (See stunnel -V output for the default.)

    Note that the -A and -a flags overwrite, not supliment, the stunnel defaults, whereas the ssl library defaults, if enabled, are used in addition to the other sources.

    In general, to avoid hurting one's brain, use -S 0 and explicitly set -A and/or -a as desired.

-t timeout session cache timeout
    default: 300 s.

-u ident_username Use IDENT (RFC 1413) username checking

-n proto Negotiate SSL with specified protocol
    currenty supported: smtp

-E socket
    Entropy Gathering Daemon socket to use to feed OpenSSL random number generator. (Available only if compiled with OpenSSL 0.9.5a or higher)

-R filename
    File containing random input. The SSL library will use data from this file first to seed the random number generator.

-W Do not overwrite the random seed files with new random data.

-B bytes
    Number of bytes of data read from random seed files. With SSL versions less than 0.9.5a, also determines how many bytes of data are considered sufficient to seed the PRNG. More recent OpenSSL versions have a builtin function to determine when sufficient randomness is available.

-d [host:]port daemon mode
    Listen for connections on [host:]port. If no host specified, defaults to all IP addresses for the local host.
    default: inetd mode

-f foreground mode
    Stay in foreground (don't fork) and log to stderr instead of via syslog.
    default: background in daemon mode

-l program [-- programname [arg1 arg2 arg3...] ]
    execute local inetd-type program.

-L program [-- programname [arg1 arg2 arg3...] ]
    open local pty and execute program.

-s username
    setuid() to username in daemon mode

-g groupname
    setgid() to groupname in daemon mode. Clears all other groups.

-P { dir/ | file | none } Pid file location
    If the argument is a pathname ending in a slash, then a pid file named "stunnel.servicename.pid" will be created in the specified directory. If the argument is a filename (no trailing slash), then that filename will be used for the pid. If the argument is 'none', then no pid file will be created.

-r [host:]port connect to remote service
    If no host specified, defaults to localhost.

EXAMPLES
In order to provide SSL encapsulation to your local imapd service, use

    stunnel -d 993 -l /usr/sbin/imapd -- imapd

If you want to provide tunneling to your pppd daemon on port 2020, use something like

    stunnel -d 2020 -L /usr/sbin/pppd -- pppd local

CERTIFICATES
Each SSL enabled daemon needs to present a valid X.509 certificate to the peer. It also needs a private key to decrypt the incoming data. The easiest way to obtain a certificate and a key is to generate them with the free openssl package. You can find more information on certificates generation on pages listed below.

Two things are important when generating certificate-key pairs for stunnel. The private key cannot be encrypted, because the server has no way to obtain the password from the user. To produce an unencrypted key add the -nodes option when running the req command from the openssl kit.

The order of contents of the .pem file is also important. It should contain the unencrypted private key first, then a signed certificate (not certificate request). There should be also empty lines after certificate and private key. Plaintext certificate information appended on the top of generated certificate should be discarded. So the file should look like this:

-----BEGIN RSA PRIVATE KEY-----
[encoded key]
-----END RSA PRIVATE KEY-----
[empty line]
-----BEGIN CERTIFICATE-----
[encoded certificate]
-----END CERTIFICATE-----
[empty line]

RANDOMNESS
stunnel needs to seed the PRNG (pseudo random number generator) in order for SSL to use good randomness. The following sources are loaded in order until sufficient random data has been gathered:

    The file specified with the -R flag.
    The file specified by the RANDFILE environment variable, if set.
    The file .rnd in your home directory, iff RANDFILE not set.
    The file specified with '--with-random' at compile time.
    The contents of the screen if running on Windows.
    The egd socket specified with the -E flag.
    The egd socket specified with '--with-egd-sock' at compile time.
    The /dev/urandom device.

With recent (>=OpenSSL 0.9.5a) version of SSL it will stop loading random data automatically when sufficient entropy has been gathered. With previous versions it will continue to gather from all the above sources since no SSL function exists to tell when enough data is available.

Note that on Windows machines that do not have console user interaction (mouse movements, creating windows, etc) the screen contents are not variable enough to be sufficient, and you should provide a random file for use with the -R flag.

Note that the file specified with the -R flag should contain random data -- that means it should contain different information each time stunnel is run. This is handled automatically unless the -W flag is used. If you wish to update this file manually, the openssl rand command in recent versions of OpenSSL, would be useful.

One important note -- if /dev/urandom is available, OpenSSL has a habit of seeding the PRNG with it even when checking the random state, so on systems with /dev/urandom you're likely to use it even though it's listed at the very bottom of the list above. This isn't stunnel's behaviour, it's OpenSSLs.

LIMITATIONS
stunnel cannot be used for the FTP daemon because of the nature of the FTP protocol which utilizes multiple ports for data transfers. There are available SSL enabled versions of FTP and telnet daemons, however.

SEE ALSO
tcpd(8), inetd(8)

http://stunnel.mirt.net/ Stunnel homepage

http://www.openssl.org OpenSSL project website

AUTHORS
Michal Trojnara <Michal.Trojnara@mirt.net>

Adam Hernik <adas@infocentrum.com>

Pawel Krawczyk <kravietz@ceti.com.pl>

PTY support by Dirk O. Siebnich <dok@vossnet.de>