1. NAME

stunnel - universal SSL tunnel

2. SYNOPSIS

stunnel [-c | -T] [-D [facility.]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] ]

3. 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)

4. OPTIONS

-h

Print stunnel help menu

-V

Print stunnel version and compile time defaults

-D level

-C cipherlist

-c

-T

-p pemfile

stunnel -V

-v level

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

-v

stunnel -V

-A certfile

-v

stunnel -V

-S sources

-A

-a

-S

        0 = ignore all default sources
        1 = use ssl library defaults
        2 = use stunnel defaults
        3 = use both ssl library and stunnel defaults

stunnel -V

-A

-a

-S 0

-A

-a

-t timeout

-N servicename

Service name to use for tcpwrappers. If not specified then a tcpwrapper service name will be generated automatically for you. This will also be used when auto-generating pid filenames.

-u ident_username

Use IDENT (RFC 1413) username checking

-n proto

-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

-f

-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 }

stunnel.

.pid

-r [host:]port

5. 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

6. 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]

7. 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, if 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.

8. 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.