Provided by: stunnel4_5.30-1_amd64 bug

NAME

       stunnel - TLS offloading and load-balancing proxy

SYNOPSIS

       Unix:
           stunnel [FILE] | -fd N | -help | -version | -sockets | -options

       WIN32:
           stunnel [ [ -install | -uninstall | -start | -stop |
               -reload | -reopen | -exit ] [-quiet] [FILE] ] |
               -help | -version | -sockets | -options

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

       FILE
           Use specified configuration file

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

       -help
           Print stunnel help menu

       -version
           Print stunnel version and compile time defaults

       -sockets
           Print default socket options

       -options
           Print supported SSL options

       -install (Windows NT and later only)
           Install NT Service

       -uninstall (Windows NT and later only)
           Uninstall NT Service

       -start (Windows NT and later only)
           Start NT Service

       -stop (Windows NT and later only)
           Stop NT Service

       -reload (Windows NT and later only)
           Reload the configuration file of the running NT Service

       -reopen (Windows NT and later only)
           Reopen the log file of the running NT Service

       -exit (Win32 only)
           Exit an already started stunnel

       -quiet (Win32 only)
           Don't display any message boxes

CONFIGURATION FILE

       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.

       An address parameter of an option may be either:

       ·   A port number.

       ·   A colon-separated pair of IP address (either IPv4, IPv6, or domain name) and port
           number.

       ·   A Unix socket path (Unix only).

   GLOBAL OPTIONS
       chroot = DIRECTORY (Unix only)
           directory to chroot stunnel process

           chroot keeps stunnel in a 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.

           Several functions of the operating system also need their files to be located within
           the chroot jail, e.g.:

           ·   Delayed resolver typically needs /etc/nsswitch.conf and /etc/resolv.conf.

           ·   Local time in log files needs /etc/timezone.

           ·   Some other functions may need devices, e.g. /dev/zero or /dev/null.

       compression = deflate | zlib
           select data compression algorithm

           default: no compression

           deflate is the standard compression method as described in RFC 1951.

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

       debug = [FACILITY.]LEVEL
           debugging level

           Level is 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 the 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

           An example of advanced engine configuration to use the certificate and the
           corresponding private key from an OpenSC PKCS#11 engine:

               engine = dynamic
               engineCtrl = SO_PATH:/usr/local/lib/engines/libpkcs11.so
               engineCtrl = LOAD
               engineCtrl = MODULE_PATH:opensc-pkcs11.so
               engineCtrl = PIN:123456

               [service]
               engineId = pkcs11
               cert = id_45

       engineCtrl = COMMAND[:PARAMETER]
           control hardware engine

           A special command "LOAD" can be used to load engine cryptographic module.

       engineDefault = TASK_LIST
           set OpenSSL tasks delegated to the current engine

           The parameter specifies a comma-separated list of task to be delegated to the current
           engine.

           The following tasks may be available, if supported by the engine: ALL, RSA, DSA, ECDH,
           ECDSA, DH, RAND, CIPHERS, DIGESTS, PKEY, PKEY_CRYPTO, PKEY_ASN1.

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

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

           default: no (since version 5.00)

       foreground = yes | quiet | no (Unix only)
           foreground mode

           Stay in foreground (don't fork).

           With the yes parameter it also logs to stderr in addition to the destinations
           specified with syslog and output.

           default: background in daemon mode

       iconActive = ICON_FILE (GUI only)
           GUI icon to be displayed when there are established connections

           On Windows platform the parameter should be an .ico file containing a 16x16 pixel
           image.

       iconError = ICON_FILE (GUI only)
           GUI icon to be displayed when no valid configuration is loaded

           On Windows platform the parameter should be an .ico file containing a 16x16 pixel
           image.

       iconIdle = ICON_FILE (GUI only)
           GUI icon to be displayed when there are no established connections

           On Windows platform the parameter should be an .ico file containing a 16x16 pixel
           image.

       log = append | overwrite
           log file handling

           This option allows you to choose whether the log file (specified with the output
           option) is appended or overwritten when opened or re-opened.

           default: append

       output = FILE
           append log messages to a file

           /dev/stdout device can be used to send log messages to the standard output (for
           example to log them with daemontools splogger).

       pid = FILE (Unix only)
           pid file location

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

           pid path is relative to the 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 = SERVICE (Unix only)
           stunnel service name

           The specified service name is used for syslog and as the inetd mode service name for
           TCP Wrappers.  While this option can technically be specified in the service sections,
           it is only useful in global options.

           default: stunnel

       socket = a|l|r:OPTION=VALUE[:VALUE]
           Set an option on the accept/local/remote socket

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

           Examples:

               socket = l:SO_LINGER=1:60
                   set one minute timeout for closing local socket
               socket = r:SO_OOBINLINE=yes
                   place out-of-band data directly into the
                   receive data stream for remote sockets
               socket = a:SO_REUSEADDR=no
                   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

   SERVICE-LEVEL OPTIONS
       Each configuration section begins with a 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 address

           If no host specified, defaults to all IPv4 addresses for the local host.

           To listen on all IPv6 addresses use:

               accept = :::PORT

       CApath = DIRECTORY
           Certificate Authority directory

           This is the directory in which stunnel will look for certificates when using the
           verify option.  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 hash algorithm has been changed in OpenSSL 1.0.0.  It is required to c_rehash the
           directory on upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x.

           CApath path is relative to the chroot directory if specified.

       CAfile = CERT_FILE
           Certificate Authority file

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

       cert = PEM_FILE
           certificate chain PEM file name

           The certificates must be in PEM format, and must be from the actual server/client
           certificate to the self-signed root CA certificate.

           A certificate is required in server mode, and optional in client mode.

           This parameter is also used as the certificate identifier when a hardware engine is
           enabled.

       checkEmail = EMAIL
           email address of the peer certificate subject

           Multiple checkEmail options are allowed in a single service section.  Certificates are
           accepted if no checkEmail option was specified, or the email address of the peer
           certificate matches any of the email addresses specified with checkEmail.

           This option requires OpenSSL 1.0.2 or later.

       checkHost = HOST
           host of the peer certificate subject

           Multiple checkHost options are allowed in a single service section.  Certificates are
           accepted if no checkHost option was specified, or the host name of the peer
           certificate matches any of the hosts specified with checkHost.

           This option requires OpenSSL 1.0.2 or later.

       checkIP = IP
           IP address of the peer certificate subject

           Multiple checkIP options are allowed in a single service section.  Certificates are
           accepted if no checkIP option was specified, or the IP address of the peer certificate
           matches any of the IP addresses specified with checkIP.

           This option requires OpenSSL 1.0.2 or later.

       ciphers = CIPHER_LIST
           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)

       config = COMMAND[:PARAMETER]
           OpenSSL configuration command

           The OpenSSL configuration command is executed with the specified parameter.  This
           allows any configuration commands to be invoked from the stunnel configuration file.
           Supported commands are described on the SSL_CONF_cmd(3ssl) manual page.

           Several config lines can be used to specify multiple configuration commands.

           This option requires OpenSSL 1.0.2 or later.

       connect = [HOST:]PORT
           connect to a remote address

           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
           option. Note that the CRLs in this directory should be named XXXXXXXX.r0 where
           XXXXXXXX is the hash value of the CRL.

           The hash algorithm has been changed in OpenSSL 1.0.0.  It is required to c_rehash the
           directory on upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x.

           CRLpath path is relative to the chroot directory if specified.

       CRLfile = CERT_FILE
           Certificate Revocation Lists file

           This file contains multiple CRLs, used with the verify option.

       curve = NID
           specify ECDH curve name

           To get a list of supported curves use:

               openssl ecparam -list_curves

           default: prime256v1

       logId = TYPE
           connection identifier type

           This identifier allows you to distinguish log entries generated for each of the
           connections.

           Currently supported types:

           sequential
               The numeric sequential identifier is only unique within a single instance of
               stunnel, but very compact.  It is most useful for manual log analysis.

           unique
               This alphanumeric identifier is globally unique, but longer than the sequential
               number.  It is most useful for automated log analysis.

           thread
               The operating system thread identifier is neither unique (even within a single
               instance of stunnel) nor short.  It is most useful for debugging software or
               configuration issues.

           default: sequential

       debug = 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).

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

           This option is useful for dynamic DNS, or when DNS is not available during stunnel
           startup (road warrior VPN, dial-up configurations).

           Delayed resolver mode is automatically engaged when stunnel fails to resolve on
           startup any of the connect targets for a service.

           Delayed resolver inflicts failover = prio.

           default: no

       engineId = ENGINE_ID
           select engine ID for the service

       engineNum = ENGINE_NUMBER
           select engine number for the service

           The engines are numbered starting from 1.

       exec = EXECUTABLE_PATH
           execute a local inetd-type program

           exec path is relative to the chroot directory if specified.

           The following environmental variables are set on Unix platforms: REMOTE_HOST,
           REMOTE_PORT, SSL_CLIENT_DN, SSL_CLIENT_I_DN.

       execArgs = $0 $1 $2 ...
           arguments for exec including the program name ($0)

           Quoting is currently not supported.  Arguments are separated with an arbitrary amount
           of whitespace.

       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

       include = DIRECTORY
           include all configuration file parts located in DIRECTORY

           The files are included in the ascending alphabetical order of their names.

       key = KEY_FILE
           private key for the certificate specified with cert option

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

               chmod 600 keyfile

           This parameter is also used as the private key identifier when a hardware engine is
           enabled.

           default: the value of the cert option

       libwrap = yes | no
           Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.

           default: no (since version 5.00)

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

       sni = SERVICE:SERVER_PATTERN (server mode)
           Use the service as a slave service (a name-based virtual server) for Server Name
           Indication TLS extension (RFC 3546).

           service_name specifies the master service that accepts client connections with the
           accept option.  server_name_pattern specifies the host name to be redirected.  The
           pattern may start with the '*' character, e.g. '*.example.com'.  Multiple slave
           services are normally specified for a single master service.  The sni option can also
           be specified more than once within a single slave service.

           This service, as well as the master service, may not be configured in client mode.

           The connect option of the slave service is ignored when the protocol option is
           specified, as protocol connects to the remote host before TLS handshake.

           Libwrap checks (Unix only) are performed twice: with the master service name after TCP
           connection is accepted, and with the slave service name during the TLS handshake.

           The sni option is only available when compiled with OpenSSL 1.0.0 and later.

       sni = SERVER (client mode)
           Use the parameter as the value of TLS Server Name Indication (RFC 3546) extension.

           The sni option is only available when compiled with OpenSSL 1.0.0 and later.

       OCSP = URL
           select OCSP responder for certificate verification

       OCSPaia = yes | no
           validate certificates with their AIA OCSP responders

           This option enables stunnel to validate certificates with the list of OCSP responder
           URLs retrieved from their AIA (Authority Information Access) extension.

       OCSPflag = OCSP_FLAG
           specify OCSP responder flag

           Several OCSPflag can be used to specify multiple flags.

           currently supported flags: NOCERTS, NOINTERN NOSIGS, NOCHAIN, NOVERIFY, NOEXPLICIT,
           NOCASIGN, NODELEGATED, NOCHECKS, TRUSTOTHER, RESPID_KEY, NOTIME

       OCSPnonce = yes | no
           send and verify the OCSP nonce extension

           This option protects the OCSP protocol against replay attacks.  Due to its
           computational overhead, the nonce extension is usually only supported on internal
           (e.g. corporate) responders, and not on public OCSP responders.

       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.  stunnel -options lists the options found to be
           allowed in the current combination of stunnel and the OpenSSL library used to build
           it.

           Several option lines can be used to specify multiple options.  An option name can be
           prepended with a dash ("-") to disable the option.

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

               options = DONT_INSERT_EMPTY_FRAGMENTS

           default:

               options = NO_SSLv2
               options = NO_SSLv3

       protocol = PROTO
           application protocol to negotiate SSL

           This option enables initial, protocol-specific negotiation of the SSL/TLS encryption.
           The protocol option should not be used with SSL encryption on a separate port.

           Currently supported protocols:

           cifs
               Proprietary (undocummented) extension of CIFS protocol implemented in Samba.
               Support for this extension was dropped in Samba 3.0.0.

           connect
               Based on RFC 2817 - Upgrading to TLS Within HTTP/1.1, section 5.2 - Requesting a
               Tunnel with CONNECT

               This protocol is only supported in client mode.

           imap
               Based on RFC 2595 - Using TLS with IMAP, POP3 and ACAP

           nntp
               Based on RFC 4642 - Using Transport Layer Security (TLS) with Network News
               Transfer Protocol (NNTP)

               This protocol is only supported in client mode.

           pgsql
               Based on http://www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982

           pop3
               Based on RFC 2449 - POP3 Extension Mechanism

           proxy
               Haproxy client IP address
               http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt

           smtp
               Based on RFC 2487 - SMTP Service Extension for Secure SMTP over TLS

           socks
               SOCKS versions 4, 4a, and 5 are supported.  The SOCKS protocol itself is
               encapsulated within SSL/TLS encryption layer to protect the final destination
               address.

               http://www.openssh.com/txt/socks4.protocol

               http://www.openssh.com/txt/socks4a.protocol

               The BIND command of the SOCKS protocol is not supported.  The USERID parameter is
               ignored.

               See Examples section for sample configuration files for VPN based on SOCKS
               encryption.

       protocolAuthentication = AUTHENTICATION
           authentication type for the protocol negotiations

           Currently, this option is only supported in the client-side 'connect' and 'smtp'
           protocols.

           Supported authentication types for the 'connect' protocol are 'basic' or 'ntlm'.  The
           default 'connect' authentication type is 'basic'.

           Supported authentication types for the 'smtp' protocol are 'plain' or 'login'.  The
           default 'smtp' authentication type is 'plain'.

       protocolDomain = DOMAIN
           domain for the protocol negotiations

           Currently, this option is only supported in the client-side 'connect' protocol.

       protocolHost = HOST:PORT
           destination address for the protocol negotiations

           protocolHost specifies the final SSL server to be connected to by the proxy, and not
           the proxy server directly connected by stunnel.  The proxy server should be specified
           with the 'connect' option.

           Currently the protocol destination address only applies to the 'connect' protocol.

       protocolPassword = PASSWORD
           password for the protocol negotiations

           Currently, this option is only supported in the client-side 'connect' and 'smtp'
           protocols.

       protocolUsername = USERNAME
           username for the protocol negotiations

           Currently, this option is only supported in the client-side 'connect' and 'smtp'
           protocols.

       PSKidentity = IDENTITY
           PSK identity for the PSK client

           PSKidentity can be used on stunnel clients to select the PSK identity used for
           authentication.  This option is ignored in server sections.

           default: the first identity specified in the PSKsecrets file.

       PSKsecrets = FILE
           file with PSK identities and corresponding keys

           Each line of the file in the following format:

               IDENTITY:KEY

           The key is required to be at least 20 characters long.  The file should not be world-
           readable nor world-writable.

       pty = yes | no (Unix only)
           allocate a pseudoterminal for 'exec' option

       redirect = [HOST:]PORT
           redirect SSL client connections on certificate-based authentication failures

           This option only works in server mode.  Some protocol negotiations are also
           incompatible with the redirect option.

       renegotiation = yes | no
           support SSL renegotiation

           Applications of the SSL renegotiation include some authentication scenarios, or re-
           keying long lasting connections.

           On the other hand this feature can facilitate a trivial CPU-exhaustion DoS attack:

           http://vincent.bernat.im/en/blog/2011-ssl-dos-mitigation.html

           Please note that disabling SSL renegotiation does not fully mitigate this issue.

           default: yes (if supported by OpenSSL)

       reset = yes | no
           attempt to use the TCP RST flag to indicate an error

           This option is not supported on some platforms.

           default: yes

       retry = yes | no
           reconnect a connect+exec section after it was disconnected

           default: no

       setgid = GROUP (Unix only)
           Unix group id

           As a global option: setgid() to the specified group in daemon mode and clear all other
           groups.

           As a service-level option: set the group of the Unix socket specified with "accept".

       setuid = USER (Unix only)
           Unix user id

           As a global option: setuid() to the specified user in daemon mode.

           As a service-level option: set the owner of the Unix socket specified with "accept".

       sessionCacheSize = NUM_ENTRIES
           session cache size

           sessionCacheSize specifies the maximum number of the internal session cache entries.

           The value of 0 can be used for unlimited size.  It is not recommended for production
           use due to the risk of a memory exhaustion DoS attack.

       sessionCacheTimeout = TIMEOUT
           session cache timeout

           This is the number of seconds to keep cached SSL sessions.

       sessiond = HOST:PORT
           address of sessiond SSL cache server

       sslVersion = SSL_VERSION
           select the SSL protocol version

           Supported values: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2

           Availability of specific protocols depends on the linked OpenSSL library.  Older
           versions of OpenSSL do not support TLSv1.1 and TLSv1.2.  Newer versions of OpenSSL do
           not support SSLv2.

           Obsolete SSLv2 and SSLv3 are currently disabled by default.  See the options option
           documentation for details.

       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 to a remote host

       TIMEOUTidle = SECONDS
           time to keep an idle connection

       transparent = none | source | destination | both (Unix only)
           enable transparent proxy support on selected platforms

           Supported values:

           none
               Disable transparent proxy support.  This is the default.

           source
               Re-write the address to appear as if a wrapped daemon is connecting from the SSL
               client machine instead of the machine running stunnel.

               This option is currently available in:

               Remote mode (connect option) on Linux >=2.6.28
                   This configuration requires stunnel to be executed as root and without the
                   setuid option.

                   This configuration 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 0.0.0.0/0 dev lo table 100
                       echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter

                   stunnel must also to be executed as root and without the setuid option.

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

                   stunnel must also to be executed as root and without the setuid option.

               Remote mode (connect option) on FreeBSD >=8.0
                   This configuration requires additional firewall and routing setup.  stunnel
                   must also to be executed as root and without the setuid option.

               Local mode (exec option)
                   This configuration works by pre-loading the libstunnel.so shared library.
                   _RLD_LIST environment variable is used on Tru64, and LD_PRELOAD variable on
                   other platforms.

           destination
               The original destination is used instead of the connect option.

               A service section for transparent destination may look like this:

                   [transparent]
                   client = yes
                   accept = <stunnel_port>
                   transparent = destination

               This configuration requires iptables setup to work, possibly in /etc/rc.local or
               equivalent file.

               For a connect target installed on the same host:

                   /sbin/iptables -t nat -I OUTPUT -p tcp --dport <redirected_port> \
                       -m ! --uid-owner <stunnel_user_id> \
                       -j DNAT --to-destination <local_ip>:<stunnel_port>

               For a connect target installed on a remote host:

                   /sbin/iptables -I INPUT -i eth0 -p tcp --dport <stunnel_port> -j ACCEPT
                   /sbin/iptables -t nat -I PREROUTING -p tcp --dport <redirected_port> \
                       -i eth0 -j DNAT --to-destination <local_ip>:<stunnel_port>

               The transparent destination option is currently only supported on Linux.

           both
               Use both source and destination transparent proxy.

           Two legacy options are also supported for backward compatibility:

           yes This option has been renamed to source.

           no  This option has been renamed to none.

       verify = LEVEL
           verify the peer certificate

           level 0
               Request and ignore the peer certificate.

           level 1
               Verify the peer certificate if present.

           level 2
               Verify the peer certificate.

           level 3
               Verify the peer with locally installed certificate.

           level 4
               Ignore the CA chain and only verify the peer 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.

RETURN VALUE

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

SIGNALS

       The following signals can be used to control stunnel in Unix environment:

       SIGHUP
           Force a reload of the configuration file.

           Some global options will not be reloaded:

           ·   chroot

           ·   foreground

           ·   pid

           ·   setgid

           ·   setuid

           The use of the 'setuid' option will also prevent stunnel from binding to privileged
           (<1024) ports during configuration reloading.

           When the 'chroot' option is used, stunnel will look for all its files (including the
           configuration file, certificates, the log file and the pid file) within the chroot
           jail.

       SIGUSR1
           Close and reopen the stunnel log file.  This function can be used for log rotation.

       SIGTERM, SIGQUIT, SIGINT
           Shut stunnel down.

       The result of sending any other signals to the server is undefined.

EXAMPLES

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

           [imapd]
           accept = 993
           exec = /usr/sbin/imapd
           execArgs = imapd

       or in remote mode:

           [imapd]
           accept = 993
           connect = 143

       In order to let your local e-mail client connect to an SSL-enabled imapd service on
       another server, configure the e-mail client to connect to localhost on port 119 and use:

           [imap]
           client = yes
           accept = 143
           connect = servername:993

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

           [vpn]
           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] section.

           exec = /usr/sbin/imapd
           execArgs = imapd

       To setup SOCKS VPN configure the following client service:

           [socks_client]
           client = yes
           accept = 127.0.0.1:1080
           connect = vpn_server:9080
           verify = 4
           CAfile = stunnel.pem

       The corresponding configuration on the vpn_server host:

           [socks_server]
           protocol = socks
           accept = 9080
           cert = stunnel.pem
           key = stunnel.key

       Now test your configuration on the client machine with:

           curl --socks4a localhost http://www.example.com/

NOTES

   RESTRICTIONS
       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.

   INETD MODE
       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.

   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.

       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 also
       be empty lines after the certificate and the private key.  Any 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 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 or later) 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 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 useful.

       Important note: If /dev/urandom is available, OpenSSL often seeds the PRNG with it while
       checking the random state.  On systems with /dev/urandom OpenSSL is likely to use it even
       though it is listed at the very bottom of the list above.  This is the behaviour of
       OpenSSL and not stunnel.

   DH PARAMETERS
       stunnel 4.40 and later contains hardcoded 2048-bit DH parameters.  Starting with stunnel
       5.18, these hardcoded DH parameters are replaced every 24 hours with autogenerated
       temporary DH parameters.  DH parameter generation may take several minutes.

       Alternatively, it is possible to specify static DH parameters in the certificate file,
       which disables generating temporary DH parameters:

           openssl dhparam 2048 >> stunnel.pem

FILES

       @sysconfdir@/stunnel/stunnel.conf
           stunnel configuration file

BUGS

       The execArgs option and the Win32 command line do not support quoting.

SEE ALSO

       tcpd(8)
           access control facility for internet services

       inetd(8)
           internet 'super-server'

       http://www.stunnel.org/
           stunnel homepage

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

AUTHOR

       Michał Trojnara
           <Michal.Trojnara@mirt.net>