stunnel(8)


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 verifyChain or verifyPeer options.  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, to be used with the
       verifyChain and verifyPeer options.

   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 verifyChain and verifyPeer options. 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 verifyChain and
       verifyPeer options.

   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_NAME:SERVER_NAME_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_NAME (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

   requireCert = yes | no
       require a client certificate for verifyChain or verifyPeer

       With requireCert set to no, the stunnel server accepts client
       connections that did not present a certificate.

       Both verifyChain = yes and verifyPeer = yes imply requireCert =
       yes.

       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

       This option is obsolete and should be replaced with the verifyChain
       and verifyPeer options.

       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 against a locally installed certificate.

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

       default
           No verify.

   verifyChain = yes | no
       verify the peer certificate chain starting from the root CA

       For server certificate verification it is essential to also require
       a specific certificate with checkHost or verifyPeer.

       The self-signed root CA certificate needs to be stored either in
       the file specified with CAfile, or in the directory specified with
       CApath.

       default: no

   verifyPeer = yes | no
       verify the peer certificate

       The peer certificate needs to be stored either in the file
       specified with CAfile, or in the directory specified with CApath.

       default: no

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
       verifyPeer = yes
       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@stunnel.org>





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