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       stunnel - universal SSL tunnel


           stunnel [<filename>] | -fdn | -help | -version | -sockets

           stunnel [ [-install | -uninstall | -start | -stop]
               [-quiet] [<filename>] ] | -help | -version | -sockets


       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


           Use specified configuration file

       -fd n (Unix only)
           Read the config file from specified file descriptor

           Print stunnel help menu

           Print stunnel version and compile time defaults

           Print default socket options

       -install (NT/2000/XP only)
           Install NT Service

       -uninstall (NT/2000/XP only)
           Uninstall NT Service

       -start (NT/2000/XP only)
           Start NT Service

       -stop (NT/2000/XP only)
           Stop NT Service

       -quiet (NT/2000/XP only)
           Don’t display a message box when successfully installed or
           uninstalled NT service


       Each line of the configuration file can be either:

       ·   an empty line (ignored)

       ·   a comment starting with ’;’ (ignored)

       ·   an ’option_name = option_value’ pair

       ·   ’[service_name]’ indicating a start of a service definition

       chroot = directory (Unix only)
           directory to chroot stunnel process

           chroot keeps stunnel in chrooted jail.  CApath, CRLpath, pid and
           exec are located inside the jail and the patches have to be
           relative to the directory specified with chroot.

           To have libwrap (TCP Wrappers) control effective in a chrooted
           environment you also have to copy its configuration files
           (/etc/hosts.allow and /etc/hosts.deny) there.

       compression = zlib | rle
           select data compression algorithm

           default: no compression

           zlib compression of OpenSSL 0.9.8 or above is not backward
           compatible with OpenSSL 0.9.7.

           rle compression is currently not implemented by the OpenSSL

       debug = [facility.]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 debug = debug or debug
           = 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 Win32.)

           Case is ignored for both facilities and levels.

       EGD = egd path (Unix only)
           path to Entropy Gathering Daemon socket

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

       engine = auto | <engine id>
           select hardware engine

           default: software-only cryptography

           There’s an example in ’EXAMPLES’ section.

       engineCtrl = command[:parameter]
           control hardware engine

           Special commands "LOAD" and "INIT" can be used to load and
           initialize the engine cryptogaphic module.

       fips = yes | no
           Enable or disable FIPS 140-2 mode.

           This option allows to disable entering FIPS mode if stunnel was
           compiled with FIPS 140-2 support.

           default: yes

       foreground = yes | no (Unix only)
           foreground mode

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

           default: background in daemon mode

       output = file
           append log messages to a file instead of using syslog

           /dev/stdout device can be used to redirect log messages to the
           standard output (for example to log them with daemontools

       pid = file (Unix only)
           pid file location

           If the argument is empty, then no pid file will be created.

           pid path is relative to chroot directory if specified.

       RNDbytes = bytes
           bytes to read from random seed files

           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.

       RNDfile = file
           path to file with random seed data

           The SSL library will use data from this file first to seed the
           random number generator.

       RNDoverwrite = yes | no
           overwrite the random seed files with new random data

           default: yes

       service = servicename
           use specified string as the service name

           On Unix: inetd mode service name for TCP Wrapper library.

           On NT/2000/XP: NT service name in the Control Panel.

           default: stunnel

       setgid = groupname (Unix only)
           setgid() to groupname in daemon mode and clears all other groups

       setuid = username (Unix only)
           setuid() to username in daemon mode

       socket = 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.


               socket = l:SO_LINGER=1:60
                   set one minute timeout for closing local socket
               socket = r:TCP_NODELAY=1
                   turn off the Nagle algorithm for remote sockets
               socket = r:SO_OOBINLINE=1
                   place out-of-band data directly into the
                   receive data stream for remote sockets
               socket = a:SO_REUSEADDR=0
                   disable address reuse (enabled by default)
               socket = a:SO_BINDTODEVICE=lo
                   only accept connections on loopback interface

       syslog = yes | no (Unix only)
           enable logging via syslog

           default: yes

       taskbar = yes | no (WIN32 only)
           enable the taskbar icon

           default: yes

       Each configuration section begins with service name in square brackets.
       The service name is used for libwrap (TCP Wrappers) access control and
       lets you distinguish stunnel services in your log files.

       Note that if you wish to run stunnel in inetd mode (where it is
       provided a network socket by a server such as inetd, xinetd, or
       tcpserver) then you should read the section entitled INETD MODE below.

       accept = [host:]port
           accept connections on specified host:port

           If no host specified, defaults to all IP addresses for the local

       CApath = directory
           Certificate Authority directory

           This is the directory in which stunnel will look for certificates
           when using the verify. Note that the certificates in this directory
           should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the
           DER encoded subject of the cert (the first 4 bytes of the MD5 hash
           in least significant byte order).

           CApath path is relative to chroot directory if specified.

       CAfile = certfile
           Certificate Authority file

           This file contains multiple CA certificates, used with the verify.

       cert = pemfile
           certificate chain PEM file name

           A PEM is always needed in server mode.  Specifying this flag in
           client mode will use this 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).

       ciphers = 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

       client = yes | no
           client mode (remote service uses SSL)

           default: no (server mode)

       connect = [host:]port
           connect to a remote host:port

           If no host is specified, the host defaults to localhost.

           Multiple connect options are allowed in a single service section.

           If host resolves to multiple addresses and/or if multiple connect
           options are specified, then the remote address is chosen using a
           round-robin algorithm.

       CRLpath = directory
           Certificate Revocation Lists directory

           This is the directory in which stunnel will look for CRLs when
           using the verify. Note that the CRLs in this directory should be
           named XXXXXXXX.0 where XXXXXXXX is the hash value of the CRL.

           CRLpath path is relative to chroot directory if specified.

       CRLfile = certfile
           Certificate Revocation Lists file

           This file contains multiple CRLs, used with the verify.

       delay = yes | no
           delay DNS lookup for ’connect’ option

       engineNum = engine number
           select engine number to read private key

           The engines are numbered starting from 1.

       exec = executable_path (Unix only)
           execute local inetd-type program

           exec path is relative to chroot directory if specified.

       execargs = $0 $1 $2 ... (Unix only)
           arguments for exec including program name ($0)

           Quoting is currently not supported.  Arguments are separated with
           arbitrary number of whitespaces.

       failover = rr | prio
           Failover strategy for multiple "connect" targets.

               rr (round robin) - fair load distribution
               prio (priority) - use the order specified in config file

           default: rr

       ident = username
           use IDENT (RFC 1413) username checking

       key = keyfile
           private key for certificate specified with cert option

           Private key is needed to authenticate certificate owner.  Since
           this file should be kept secret it should only be readable to its
           owner.  On Unix systems you can use the following command:

               chmod 600 keyfile

           default: value of cert option

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

       OCSP = url
           select OCSP server for certificate verification

       OCSPflag = flag
           specify OCSP server flag

           Several OCSPflag can be used to specify multiple flags.

           currently supported flags: NOCERTS, NOINTERN NOSIGS, NOCHAIN,

       options = SSL_options
           OpenSSL library options

           The parameter is the OpenSSL option name as described in the
           SSL_CTX_set_options(3ssl) manual, but without SSL_OP_ prefix.
           Several options can be used to specify multiple options.

           For example for compatibility with erroneous Eudora SSL
           implementation the following option can be used:

               options = DONT_INSERT_EMPTY_FRAGMENTS

       protocol = proto
           application protocol to negotiate SSL

           currently supported: cifs, connect, imap, nntp, pop3, smtp, pgsql

       protocolAuthentication = auth_type
           authentication type for protocol negotiations

           currently supported: basic, NTLM

           Currently authentication type only applies to ’connect’ protocol.

           default: basic

       protocolHost = host:port
           destination address for protocol negotiations

       protocolPassword = password
           password for protocol negotiations

       protocolUsername = username
           username for protocol negotiations

       pty = yes | no (Unix only)
           allocate pseudo terminal for ’exec’ option

       retry = yes | no (Unix only)
           reconnect a connect+exec section after it’s disconnected

           default: no

       session = timeout
           session cache timeout

       sessiond = host:port
           address of sessiond SSL cache server

       sslVersion = version
           select version of SSL protocol

           Allowed options: all, SSLv2, SSLv3, TLSv1

       stack = bytes (except for FORK model)
           thread stack size

       TIMEOUTbusy = seconds
           time to wait for expected data

       TIMEOUTclose = seconds
           time to wait for close_notify (set to 0 for buggy MSIE)

       TIMEOUTconnect = seconds
           time to wait to connect a remote host

       TIMEOUTidle = seconds
           time to keep an idle connection

       transparent = yes | no (Unix only)
           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.

           This option is currently available in:

               remote mode (I<connect> option) on Linux >=2.6.28
               remote mode (I<connect> option) 2.2.x
               local mode (I<exec> option)

           Remote mode (either 2.2.x and >=2.6.28) requires stunnel to be
           executed as root.  setuid option will also break this

           Linux >=2.6.28 requires the following setup for iptables and
           routing (possibly in /etc/rc.local or equivalent file):

               iptables -t mangle -N DIVERT
               iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
               iptables -t mangle -A DIVERT -j MARK --set-mark 1
               iptables -t mangle -A DIVERT -j ACCEPT
               ip rule add fwmark 1 lookup 100
               ip route add local dev lo table 100

           Linux 2.2.x requires kernel to be compiled with transparent proxy
           option.  Connected service must be installed on a separate host.
           Routing towards the clients has to go through the stunnel box.

           Local mode works by LD_PRELOADing shared library.

       verify = 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

           It is important to understand, that this option was solely designed
           for access control and not for authorization.  Specifically for
           level 2 every non-revoked certificate is accepted regardless of its
           Common Name.  For this reason a dedicated CA should be used with
           level 2, and not a generic CA commonly used for webservers.  Level
           3 is preferred for point-to-point connections.


       stunnel returns zero on success, non-zero on error.


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

           accept = 993
           exec = /usr/sbin/imapd
           execargs = imapd

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

           accept = 2020
           exec = /usr/sbin/pppd
           execargs = pppd local
           pty = yes

       If you want to use stunnel in inetd mode to launch your imapd process,
       you’d use this stunnel.conf.  Note there must be no [service_name]

           exec = /usr/sbin/imapd
           execargs = imapd

       Here is an example of advanced engine configuration to read private key
       from an OpenSC engine




           stunnel configuration file

           stunnel certificate and private key


       Option execargs does not support quoting.


       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 most common use of stunnel is to listen on a network port and
       establish communication with either a new port via the connect option,
       or a new program via the exec option.  However there is a special case
       when you wish to have some other program accept incoming connections
       and launch stunnel, for example with inetd, xinetd, or tcpserver.

       For example, if you have the following line in inetd.conf:

           imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf

       In these cases, the inetd-style program is responsible for binding a
       network socket (imaps above) and handing it to stunnel when a
       connection is received.  Thus you do not want stunnel to have any
       accept option.  All the Service Level Options should be placed in the
       global options section, and no [service_name] section will be present.
       See the EXAMPLES section for example configurations.

       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.

       The order of contents of the .pem file is 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 RNDfile 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 EGD 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

       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 RNDfile flag.

       Note that the file specified with the RNDfile flag should contain
       random data -- that means it should contain different information each
       time stunnel is run.  This is handled automatically unless the
       RNDoverwrite 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 stunnels
       behaviour, it’s OpenSSLs.


           access control facility for internet services

           internet ’super-server’
           stunnel homepage
           stunnel Frequently Asked Questions
           OpenSSL project website


       Michal Trojnara