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       stunnel - universal SSL tunnel


       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 { filename │ ’’ } ] [-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

       -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

           The values for linger option are l_onof:l_linger. The values for
           time are tv_sec:tv_usec.


           -O l:SO_LINGER=1:60 - set one minute timeout for closing local

           -O r:TCP_NODELAY=1 - turn off the Nagle algorithm for remote

           -O r:SO_OOBINLINE=1 - place out-of-band data directly into the
           receive data stream for remote sockets

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

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

           The available options and their defaults are:
               Option          Accept    Local     Remote    OS default
               SO_DEBUG            --        --        --             0
               SO_DONTROUTE        --        --        --             0
               SO_KEEPALIVE        --        --        --             0
               SO_LINGER           --        --        --    0:0
               SO_OOBINLINE        --        --        --             0
               SO_RCVBUF           --        --        --         87380
               SO_SNDBUF           --        --        --         16384
               SO_RCVLOWAT         --        --        --             1
               SO_SNDLOWAT         --        --        --             1
               SO_RCVTIMEO         --        --        --         0:0
               SO_SNDTIMEO         --        --        --         0:0
               SO_REUSEADDR             1    --        --             0
               SO_BINDTODEVICE     --        --        --        --
               IP_TOS              --        --        --             0
               IP_TTL              --        --        --            64
               TCP_NODELAY         --        --        --             0

       -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 (by default located in
           /etc/stunnel/stunnel.pem). 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

           ·       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.

       -A certfile
           Certificate Authority file

           This file contains multiple CA certificates, used with the -v

       -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

           currently supported: smtp, pop3, nntp

       -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.

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

       -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 (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 groups.

       -P { file │ ’’ }
           Pid file location

           If the argument is a filename, then that filename will be used for
           the pid. If the argument is empty (’’, not missing), then no pid
           file will be created.

       -r [host:]port
           connect to remote service

           If no host specified, defaults to localhost.


       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


       If Stunnel is used to create local processes using the -l or -L
       options, it will set the following environment variables

           The IP address of the remote end of the connection.

           The DN (Distinguished Name, aka subject name) of the peer
           certificate, if a certificate was present and verified.

           The Issuer’s DN of the peer’s certificate, if a certificate was
           present and verified.


       ·   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:

           ·       The file specified with the -R flag.

           ·       The file specified by the RANDFILE environment variable, if

           ·       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

           ·       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

           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,


           tcpd(8) access control facility for internet services

                   internet ‘‘super-server’’

                   Stunnel homepage

                   Stunnel Frequently Asked Questions

                   OpenSSL project website


           Michal Trojnara

                                  2003-08-01                        STUNNEL(8)