stunnel(8) - version 3


       stunnel [-c | -T] [-D [facility.]level] [-O
       a|l|r:option=value[:value]] [-o file] [-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] [-I host] [-d [host:]port [-f] ]
       [ -r [host:]port | { -l | -L } program [--
       progname args] ]


       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

       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 (


       -h  Print stunnel help menu

       -V  Print stunnel version and compile time defaults

       -D level
           Debugging level

           Level is a one of the syslog level names or numbers
           emerg (0), alert (1), crit (2), err (3), warning (4),
           notice (5), info (6), or debug (7).  All logs for the
           specified level and all levels numerically less than
           it will be shown.  Use -D debug or -D 7 for greatest
           debugging output.  The default is notice (5).

           The syslog facility 'daemon' will be used unless a
           facility name is supplied.  (Facilities are not
           supported on windows.)

           Case is ignored for both facilities and levels.

       -O a|l|r:option=value[:value]
           Set an option on accept/local/remote socket

           -O a:SO_REUSEADDR=0 - disable address reuse (enabled
           by default)

           -O a:SO_BINDTODEVICE=lo - only accept connections on
           loopback interface

           See 'stunnel -V' for options supported on your system
           and their default values.

       -o file
           Append log messages to a file.

       -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

           Re-write address to appear as if wrapped daemon is
           connecting from the SSL client machine instead of the
           machine running stunnel. Available only on some
           operating systems (Linux only, we believe) and then
           only in server mode. Note that this option will not
           combine with proxy mode (-r) unless the client's
           default route to the target machine lies through the
           host running stunnel, which cannot be localhost.

       -p pemfile
           private key and certificate chain PEM file name

           A PEM is always needed in server mode (see stunnel -V
           output for default). Specifying this flag in client
           mode will use this key and certificate chain as a
           client side certificate chain.  Using client side
           certs is optional. The certificates must be in PEM
           format and must be sorted starting with the
           certificate to the highest level (root CA).

       -v level
           verify peer certificate

       o           level 1 - verify peer certificate if present

       o           level 2 - verify peer certificate

       o           level 3 - verify peer with locally installed
           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
           supplement, 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 seconds.

       -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
           Negotiate SSL with specified protocol

       -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.

       -I host
           IP of the outgoing interface is used as source for
           remote connections.  Use this option to bind a static
           local IP address, instead.

       -d [host:]port
           daemon mode

           Listen for connections on [host:]port. If no host
           specified, defaults to all IP addresses for the local

           default: inetd mode

       -f  foreground mode

           Stay in foreground (don't fork) and log to stderr
           instead of via syslog (unless -o is specified).

           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

       -P { dir/ | file | none }
           Pid file location

           If the argument is a pathname ending in a slash, then
           a pid file named "" 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',

       port 2020, use something like

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


       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]


       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:

       o   The file specified with the -R flag.

       o   The file specified by the RANDFILE environment
           variable, if set.

       o   The file .rnd in your home directory, if RANDFILE not

       o   The file specified with '--with-random' at compile
       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.


       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.


       the tcpd(8) manpage
           access control facility for internet services

       the inetd(8) manpage
           internet ``super-server''
           Stunnel homepage
           Stunnel Frequently Asked Questions
           OpenSSL project website


       Michal Trojnara

3rd Berkeley Distribution                          STUNNEL.OUT(1)

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