Provided by: stunnel4_4.42-1_amd64 bug


       stunnel - universal SSL tunnel


           stunnel [<filename>] | -fd n | -help | -version | -sockets

           stunnel [ [-install | -uninstall | -start | -stop] | -exit]
               [-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 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 (


           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

       -exit (Win32 only)
           Exit an already started stunnel

       -quiet (NT/2000/XP only)
           Don't display any message boxes


       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

       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

           rle compression is currently not implemented by the OpenSSL library.

       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

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



       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 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 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

       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.

           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


               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

       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 host.

       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 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 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

       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.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 chroot directory if specified.

       CRLfile = certfile
           Certificate Revocation Lists file

           This file contains multiple CRLs, used with the verify.

       curve = nid
           specify ECDH curve name

           To get a list of supported cuves use:

               openssl ecparam -list_curves

           default: prime256v1

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

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

       engineNum = engine number
           select engine number to read private key

           The engines are numbered starting from 1.

       exec = executable_path
           execute local inetd-type program

           exec path is relative to chroot directory if specified.

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

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

       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

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

           default: yes

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

       sni = service_name:server_name
           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 accept
           option.  server_name specifies the host name to be redirected.  Multiple slave
           services are normally specified for a single master service.  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.
           connect option of the slave service is ignored when protocol option is specified, as
           protocol connects remote host before TLS handshake.  Libwrap checks (Unix only) are
           performed twice: with master service name after TCP connection is accepted, and with
           slave service name during TLS handshake.

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

       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, NOVERIFY, NOEXPLICIT,

       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

           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 (e.g. starttls or stls)

           protocol option should not be used with SSL encryption on a separate port.

           Currently supported protocols:

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

               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.

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

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

               This protocol is only supported in client mode.

               Based on RFC 2449 - POP3 Extension Mechanism

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

               Based on

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

           Supported values:

               Disable transparent proxy support.  This is the default.

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

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

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

               Remote mode (connect option) on Linux 2.2.x
                   This configuration 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.

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

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

               Original destination is used instead of connect option.

               A service section for transparent destination may look like this:


               This configuration requires the following setup for iptables (possibly in
               /etc/rc.local or equivalent file):

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

               Transparent destination option is currently only supported on Linux.

               Use both source and destination transparent proxy.

           Two legacy options are also supported for backward compatibility:

           yes This options has been renamed to source.

           no  This options has been renamed to none.

       verify = level
           verify peer certificate

           level 0 - request and ignore 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.


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

           Force a reload of the configuration file.

           Some global options will not be reloaded:

           ·   chroot

           ·   fips

           ·   foreground

           ·   pid

           ·   setgid

           ·   setuid

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

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

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

           Shut stunnel down.

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


       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] section.

           exec = /usr/sbin/imapd
           execargs = imapd


       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 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.

       One important note -- if /dev/urandom is available, OpenSSL has a habit of seeding the
       PRNG with it even when checking the random state, so on systems with /dev/urandom you're
       likely to use it even though it's listed at the very bottom of the list above.  This isn't
       stunnel's behaviour, it's OpenSSLs.

       Stunnel 4.40 and later contains hardcoded 2048-bit DH parameters.

       It is also possible to specify DH parameters in the certificate file:

           openssl dhparam 2048 >> stunnel.pem

       DH parameter generation may take several minutes.


           stunnel configuration file


       Option execargs does not support quoting.


           access control facility for internet services

           internet 'super-server'
           stunnel homepage
           OpenSSL project website


       Michal Trojnara