Packages and Binaries:

stunnel4

The stunnel program is designed to work as SSL encryption wrapper between remote client and local (inetd-startable) or remote server. The concept is that having non-SSL aware daemons running on your system you can easily setup them to communicate with clients over secure SSL channel.

stunnel can be used to add SSL functionality to commonly used inetd daemons like POP-2, POP-3 and IMAP servers without any changes in the programs’ code.

This package contains a wrapper script for compatibility with stunnel 3.x

Installed size: 549 KB
How to install: sudo apt install stunnel4

Dependencies:
  • adduser
  • init-system-helpers
  • libc6
  • libssl3t64
  • libsystemd0
  • libwrap0
  • netbase
  • openssl
  • perl
  • systemd | systemd-standalone-sysusers | systemd-sysusers
stunnel

TLS offloading and load-balancing proxy

root@kali:~# man stunnel
stunnel(8)                     stunnel4 TLS Proxy                    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 TLS encryption wrapper
       between remote clients and local (inetd-startable)  or  remote  servers.
       The  concept is that having non-TLS aware daemons running on your system
       you can easily set them up to communicate with clients over  secure  TLS
       channels.

       stunnel  can  be  used  to  add TLS 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
       ([email protected])

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 TLS 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:

       o   An empty line (ignored).

       o   A comment starting with ';' (ignored).

       o   An 'option_name = option_value' pair.

       o   '[service_name]' indicating a start of a service definition.

       An address parameter of an option may be either:

       o   A port number.

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

       o   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.:

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

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

           o   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.

       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.

           The debug = debug (or the equivalent <debug = 7>) level produces for
           the most verbose log output.  This logging level is only meant to be
           understood  by  stunnel developers, and not by users.  Please either
           use the debug level when requested to do so by an stunnel developer,
           or when you intend to get confused.

           The default logging level is notice (5).

           The syslog 'daemon' facility 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.

       engine = auto | ENGINE_ID
           select hardware or software cryptographic engine

           default: software-only cryptography

           See  Examples  section  for  an  engine  configuration  to  use  the
           certificate  and  the corresponding private key from a cryptographic
           device.

       engineCtrl = COMMAND[:PARAMETER]
           control hardware engine

       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

       RNDfile = FILE
           path to file with random seed data

           The  OpenSSL  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

       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

       CAengine = ENGINE-SPECIFIC_CA_CERTIFICATE_IDENTIFIER
           load a trusted CA certificate from an engine

           The loaded CA certificates will be used  with  the  verifyChain  and
           verifyPeer options.

           Multiple CAengine options are allowed in a single service section.

           Currently supported engines: pkcs11, cng.

       CApath = CA_DIRECTORY
           load trusted CA certificates from a directory

           The  loaded  CA  certificates  will be used with the verifyChain and
           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 or later.

           CApath path is relative to the chroot directory if specified.

       CAfile = CA_FILE
           load trusted CA certificates from a file

           The loaded CA certificates will be used  with  the  verifyChain  and
           verifyPeer options.

       cert = CERT_FILE
           certificate chain file name

           The  parameter  specifies  the  file containing certificates used by
           stunnel to authenticate itself against the remote client or  server.
           The  file  should  contain the whole certificate chain starting from
           the actual server/client certificate,  and  ending  with  the  self-
           signed  root  CA certificate.  The file must be either in PEM or P12
           format.

           A certificate chain 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
           verify the email address of the end-entity (leaf)  peer  certificate
           subject

           Certificates  are  accepted  if no subject checks were specified, or
           the email address of the end-entity (leaf) peer certificate  matches
           any of the email addresses specified with checkEmail.

           Multiple checkEmail options are allowed in a single service section.

           This option requires OpenSSL 1.0.2 or later.

       checkHost = HOST
           verify the host of the end-entity (leaf) peer certificate subject

           Certificates  are  accepted  if no subject checks were specified, or
           the host name of the end-entity (leaf) peer certificate matches  any
           of the hosts specified with checkHost.

           Multiple checkHost options are allowed in a single service section.

           This option requires OpenSSL 1.0.2 or later.

       checkIP = IP
           verify  the  IP  address  of  the end-entity (leaf) peer certificate
           subject

           Certificates are accepted if no subject checks  were  specified,  or
           the IP address of the end-entity (leaf) peer certificate matches any
           of the IP addresses specified with checkIP.

           Multiple checkIP options are allowed in a single service section.

           This option requires OpenSSL 1.0.2 or later.

       ciphers = CIPHER_LIST
           select permitted TLS ciphers (TLSv1.2 and below)

           This option does not impact TLSv1.3 ciphersuites.

           A   colon-delimited  list  of  the  ciphers  to  allow  in  the  TLS
           connection, for example DES-CBC3-SHA:IDEA-CBC-MD5.

       ciphersuites = CIPHERSUITES_LIST
           select permitted TLSv1.3 ciphersuites

           A colon-delimited list of TLSv1.3 ciphersuites  names  in  order  of
           preference.

           This option requires OpenSSL 1.1.1 or later.

           default:
           TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256

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

           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.

           Use curves option instead of enabling config = Curves:list_curves to
           support elliptic curves.

           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 = CRL_FILE
           Certificate Revocation Lists file

           This file contains multiple CRLs,  used  with  the  verifyChain  and
           verifyPeer options.

       curves = list
           ECDH curves separated with ':'

           Only a single curve name is allowed for OpenSSL older than 1.1.1.

           To get a list of supported curves use:

               openssl ecparam -list_curves

           default:

               X25519:P-256:X448:P-521:P-384 (OpenSSL 1.1.1 or later)

               prime256v1 (OpenSSL older than 1.1.1)

       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.

           process
               The operating system process identifier (PID) may be  useful  in
               the inetd mode.

           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.  The default is notice (5).

           While  the  debug  =  debug  or  debug  = 7 level generates the most
           verbose  output,  it  is  only  intended  to  be  used  by   stunnel
           developers.   Please  only use this value if you are a developer, or
           you intend to send your logs to our technical  support.   Otherwise,
           the generated logs will be confusing.

       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: prio

       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. The recommended filename convention is

           for global options:

                   00-global.conf

           for local service-level options:

                   01-service.conf

                   02-service.conf

       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.

       OCSP = URL
           select  OCSP  responder  for  the end-entity (leaf) peer 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.

       OCSPrequire = yes | no
           require a conclusive OCSP response

           Disable this option to allow a connection even though no  conclusive
           OCSP  response  was  retrieved from stapling and a direct request to
           the OCSP responder.

           default: yes

       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  TLS
           implementation, the following option can be used:

               options = DONT_INSERT_EMPTY_FRAGMENTS

           default:

               options = NO_SSLv2
               options = NO_SSLv3

           Use sslVersionMax  or  sslVersionMin  option  instead  of  disabling
           specific  TLS  protocol versions when compiled with OpenSSL 1.1.0 or
           later.

       protocol = PROTO
           application protocol to negotiate TLS

           This option enables initial, protocol-specific  negotiation  of  the
           TLS  encryption.   The  protocol  option should not be used with TLS
           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.

           capwin
               http://www.capwin.org/ application support

           capwinctrl
               http://www.capwin.org/ application support

               This protocol is only supported in client mode.

           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

           ldap
               Based on RFC 2830 - Lightweight Directory Access Protocol  (v3):
               Extension for Transport Layer Security

           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
               Passing  of  the  original  client IP address with HAProxy PROXY
               protocol                        version                        1
               https://www.haproxy.org/download/1.8/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 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.

       protocolHeader = HEADER
           header for the protocol negotiations

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

       protocolHost = ADDRESS
           host address for the protocol negotiations

           For the  'connect'  protocol  negotiations,  protocolHost  specifies
           HOST:PORT  of  the final TLS server to be connected to by the proxy.
           The proxy server directly connected by  stunnel  must  be  specified
           with the connect option.

           For  the  'smtp'  protocol  negotiations,  protocolHost controls the
           client SMTP HELO/EHLO value.

       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

           Hexadecimal keys are automatically converted to binary  form.   Keys
           are required to be at least 16 bytes long, which implies at least 32
           characters  for hexadecimal keys.  The file should neither be world-
           readable nor world-writable.

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

       redirect = [HOST:]PORT
           redirect TLS 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 TLS renegotiation

           Applications of the TLS 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 TLS 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 | DELAY
           reconnect a connect+exec section after it was disconnected

           The  DELAY  value  specifies  the  number  of  milliseconds   before
           retrying.  "retry = yes" has the same effect as "retry = 1000".

           default: no

       securityLevel = LEVEL
           set the security level

           The meaning of each level is described below:

           level 0
               Everything is permitted.

           level 1
               The  security  level  corresponds  to  a  minimum  of 80 bits of
               security. Any parameters offering below 80 bits of security  are
               excluded.  As  a  result  RSA, DSA and DH keys shorter than 1024
               bits and ECC keys shorter than  160  bits  are  prohibited.  All
               export  cipher  suites  are prohibited since they all offer less
               than 80 bits of security.  SSL  version  2  is  prohibited.  Any
               cipher suite using MD5 for the MAC is also prohibited.

           level 2
               Security level set to 112 bits of security. As a result RSA, DSA
               and DH keys shorter than 2048 bits and ECC keys shorter than 224
               bits  are  prohibited. In addition to the level 1 exclusions any
               cipher suite using RC4 is also prohibited. SSL version 3 is also
               not allowed. Compression is disabled.

           level 3
               Security level set to 128 bits of security. As a result RSA, DSA
               and DH keys shorter than 3072 bits and ECC keys shorter than 256
               bits are prohibited. In  addition  to  the  level  2  exclusions
               cipher  suites  not offering forward secrecy are prohibited. TLS
               versions below  1.1  are  not  permitted.  Session  tickets  are
               disabled.

           level 4
               Security level set to 192 bits of security. As a result RSA, DSA
               and DH keys shorter than 7680 bits and ECC keys shorter than 384
               bits  are  prohibited.  Cipher suites using SHA1 for the MAC are
               prohibited. TLS versions below 1.2 are not permitted.

           level 5
               Security level set to 256 bits of security. As a result RSA, DSA
               and DH keys shorter than 15360 bits and ECC  keys  shorter  than
               512 bits are prohibited.

           default: 2

           The  securityLevel  option  is  only  available  when  compiled with
           OpenSSL 1.1.0 and later.

       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 TLS sessions.

       sessionResume = yes | no
           allow or disallow session resumption

           default: yes

       sessiond = HOST:PORT
           address of sessiond TLS cache server

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

           SERVICE_NAME  specifies  the  primary  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 secondary  services  are
           normally specified for a single primary service.  The sni option can
           also be specified more than once within a single secondary service.

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

           The connect option of the secondary  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  primary
           service  name  after  TCP  connection  is  accepted,  and  with  the
           secondary 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.

           Empty SERVER_NAME disables sending the SNI extension.

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

       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

       sslVersion = SSL_VERSION
           select the TLS protocol version

           Supported versions: all,  SSLv2,  SSLv3,  TLSv1,  TLSv1.1,  TLSv1.2,
           TLSv1.3

           Availability  of  specific  protocols  depends on the linked OpenSSL
           library.  Older versions of OpenSSL do not support TLSv1.1,  TLSv1.2
           and TLSv1.3.  Newer versions of OpenSSL do not support SSLv2.

           Obsolete SSLv2 and SSLv3 are currently disabled by default.

           Setting the option

               sslVersion = SSL_VERSION

           is equivalent to options

               sslVersionMax = SSL_VERSION
               sslVersionMin = SSL_VERSION

           when compiled with OpenSSL 1.1.0 and later.

       sslVersionMax = SSL_VERSION
           maximum supported protocol versions

           Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3

           all  enable protocol versions up to the highest version supported by
           the linked OpenSSL library.

           Availability of specific protocols depends  on  the  linked  OpenSSL
           library.

           The  sslVersionMax  option  is  only  available  when  compiled with
           OpenSSL 1.1.0 and later.

           default: all

       sslVersionMin = SSL_VERSION
           minimum supported protocol versions

           Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3

           all enable protocol versions down to the lowest version supported by
           the linked OpenSSL library.

           Availability of specific protocols depends  on  the  linked  OpenSSL
           library.

           The  sslVersionMin  option  is  only  available  when  compiled with
           OpenSSL 1.1.0 and later.

           default: TLSv1

       stack = BYTES (except for FORK model)
           CPU stack size of created threads

           Excessive  thread  stack  size  increases  virtual   memory   usage.
           Insufficient thread stack size may cause application crashes.

           default: 65536 bytes (sufficient for all platforms we tested)

       ticketKeySecret = SECRET
           hexadecimal  symmetric  key  used for session ticket confidentiality
           protection

           Session tickets defined in RFC  5077  provide  an  enhanced  session
           resumption capability, where the server-side caching is not required
           to maintain per session state.

           Combining  ticketKeySecret  and  ticketMacSecret  options  allow  to
           resume a negotiated session on other cluster nodes, or to  resume  a
           negotiated session after server restart.

           The  key is required to be either 16 or 32 bytes long, which implies
           exactly 32 or 64 hexadecimal digits.  Colons may optionally be  used
           between two-character hexadecimal bytes.

           This option only works in server mode.

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

           Disabling NO_TICKET option is required for  the  ticket  support  in
           OpenSSL  older than 1.1.1, but note that this option is incompatible
           with the redirect option.

       ticketMacSecret = SECRET
           hexadecimal  symmetric  key  used  for  session   ticket   integrity
           protection

           The  key is required to be either 16 or 32 bytes long, which implies
           exactly 32 or 64 hexadecimal digits.  Colons may optionally be  used
           between two-character hexadecimal bytes.

           This option only works in server mode.

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

       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

       TIMEOUTocsp = SECONDS
           time to wait to connect an OCSP responder

       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  TLS 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 chain.

           level 1
               Verify the peer certificate chain if present.

           level 2
               Verify the peer certificate chain.

           level 3
               Verify the peer certificate chain and the end-entity (leaf) peer
               certificate against a locally installed certificate.

           level 4
               Ignore the peer certificate chain and only verify the end-entity
               (leaf) peer certificate against a locally installed 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 checkIP.

           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 end-entity (leaf) peer certificate

           The  end-entity (leaf) 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:

           o   chroot

           o   foreground

           o   pid

           o   setgid

           o   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.

       SIGUSR2
           Log the list of active connections.

       SIGTERM, SIGQUIT, SIGINT
           Shut stunnel down.

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

EXAMPLES
       In order to provide TLS 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 a TLS-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/

       An example server mode SNI configuration:

           [virtual]
           ; primary service
           accept = 443
           cert =  default.pem
           connect = default.internal.mydomain.com:8080

           [sni1]
           ; secondary service 1
           sni = virtual:server1.mydomain.com
           cert = server1.pem
           connect = server1.internal.mydomain.com:8081

           [sni2]
           ; secondary service 2
           sni = virtual:server2.mydomain.com
           cert = server2.pem
           connect = server2.internal.mydomain.com:8082
           verifyPeer = yes
           CAfile = server2-allowed-clients.pem

       An  example  of  advanced engine configuration allows for authentication
       with private keys stored  in  the  Windows  certificate  store  (Windows
       only).   With  the  CAPI  engine  you  don't need to manually select the
       client key to use.  The client key is automatically  selected  based  on
       the list of CAs trusted by the server.

           engine = capi

           [service]
           engineId = capi
           client = yes
           accept = 127.0.0.1:8080
           connect = example.com:8443

       An  example  of advanced engine configuration to use the certificate and
       the corresponding private key from a pkcs11 engine:

           engine = pkcs11
           engineCtrl = MODULE_PATH:opensc-pkcs11.so
           engineCtrl = PIN:123456

           [service]
           engineId = pkcs11
           client = yes
           accept = 127.0.0.1:8080
           connect = example.com:843
           cert = pkcs11:token=MyToken;object=MyCert
           key = pkcs11:token=MyToken;object=MyKey

       An example of advanced engine configuration to use the  certificate  and
       the corresponding private key from a SoftHSM token:

           engine = pkcs11
           engineCtrl = MODULE_PATH:softhsm2.dll
           engineCtrl = PIN:12345

           [service]
           engineId = pkcs11
           client = yes
           accept = 127.0.0.1:8080
           connect = example.com:843
           cert = pkcs11:token=MyToken;object=KeyCert

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 TLS-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 TLS-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 .pem file should contain the unencrypted private key  and  a  signed
       certificate  (not  certificate  request).   So the file should look like
       this:

           -----BEGIN RSA PRIVATE KEY-----
           [encoded key]
           -----END RSA PRIVATE KEY-----
           -----BEGIN CERTIFICATE-----
           [encoded certificate]
           -----END CERTIFICATE-----

   RANDOMNESS
       stunnel needs to seed the PRNG (pseudo-random number generator) in order
       for TLS to use good randomness.  The following  sources  are  loaded  in
       order until sufficient random data has been gathered:

       o   The file specified with the RNDfile flag.

       o   The file specified by the RANDFILE environment variable, if set.

       o   The file .rnd in your home directory, if RANDFILE not set.

       o   The file specified with '--with-random' at compile time.

       o   The contents of the screen if running on Windows.

       o   The egd socket specified with the EGD flag.

       o   The egd socket specified with '--with-egd-sock' at compile time.

       o   The /dev/urandom device.

       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
       Michal Trojnara
           <[email protected]>

5.72                               2024.04.11                        stunnel(8)

stunnel3

Universal SSL tunnel

root@kali:~# stunnel3 --help
/usr/bin/stunnel3 version [unknown] calling Getopt::Std::getopts (version 1.13 [paranoid]),
running under Perl version 5.38.2.

Usage: stunnel3 [-OPTIONS [-MORE_OPTIONS]] [--] [PROGRAM_ARG1 ...]

The following single-character options are accepted:
	With arguments: -D -O -o -C -p -v -a -A -t -N -u -n -E -R -B -I -d -s -g -P -r -L -l
	Boolean (without arguments): -c -T -W -f

Options may be merged together.  -- stops processing of options.
Space is not required between options and their arguments.
  [Now continuing due to backward compatibility and excessive paranoia.
   See 'perldoc Getopt::Std' about $Getopt::Std::STANDARD_HELP_VERSION.]
[ ] Initializing inetd mode configuration
[ ] Clients allowed=500
[.] stunnel 5.72 on x86_64-pc-linux-gnu platform
[.] Compiled with OpenSSL 3.2.2-dev 
[.] Running  with OpenSSL 3.2.2 4 Jun 2024
[.] Threading:PTHREAD Sockets:POLL,IPv6,SYSTEMD TLS:ENGINE,OCSP,PSK,SNI Auth:LIBWRAP
[ ] errno: (*__errno_location ())
[ ] Initializing inetd mode configuration
[.] Reading configuration from descriptor 3
[.] FIPS mode disabled
[ ] Compression disabled
[ ] No PRNG seeding was required
[!] Inetd mode: TLS server needs a certificate
[!] Configuration failed
[ ] Deallocating temporary section defaults
[ ] Cleaning up context [stunnel]

stunnel4

TLS offloading and load-balancing proxy

root@kali:~# man stunnel4
stunnel(8)                     stunnel4 TLS Proxy                    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 TLS encryption wrapper
       between remote clients and local (inetd-startable)  or  remote  servers.
       The  concept is that having non-TLS aware daemons running on your system
       you can easily set them up to communicate with clients over  secure  TLS
       channels.

       stunnel  can  be  used  to  add TLS 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
       ([email protected])

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 TLS 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:

       o   An empty line (ignored).

       o   A comment starting with ';' (ignored).

       o   An 'option_name = option_value' pair.

       o   '[service_name]' indicating a start of a service definition.

       An address parameter of an option may be either:

       o   A port number.

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

       o   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.:

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

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

           o   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.

       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.

           The debug = debug (or the equivalent <debug = 7>) level produces for
           the most verbose log output.  This logging level is only meant to be
           understood  by  stunnel developers, and not by users.  Please either
           use the debug level when requested to do so by an stunnel developer,
           or when you intend to get confused.

           The default logging level is notice (5).

           The syslog 'daemon' facility 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.

       engine = auto | ENGINE_ID
           select hardware or software cryptographic engine

           default: software-only cryptography

           See  Examples  section  for  an  engine  configuration  to  use  the
           certificate  and  the corresponding private key from a cryptographic
           device.

       engineCtrl = COMMAND[:PARAMETER]
           control hardware engine

       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

       RNDfile = FILE
           path to file with random seed data

           The  OpenSSL  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

       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

       CAengine = ENGINE-SPECIFIC_CA_CERTIFICATE_IDENTIFIER
           load a trusted CA certificate from an engine

           The loaded CA certificates will be used  with  the  verifyChain  and
           verifyPeer options.

           Multiple CAengine options are allowed in a single service section.

           Currently supported engines: pkcs11, cng.

       CApath = CA_DIRECTORY
           load trusted CA certificates from a directory

           The  loaded  CA  certificates  will be used with the verifyChain and
           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 or later.

           CApath path is relative to the chroot directory if specified.

       CAfile = CA_FILE
           load trusted CA certificates from a file

           The loaded CA certificates will be used  with  the  verifyChain  and
           verifyPeer options.

       cert = CERT_FILE
           certificate chain file name

           The  parameter  specifies  the  file containing certificates used by
           stunnel to authenticate itself against the remote client or  server.
           The  file  should  contain the whole certificate chain starting from
           the actual server/client certificate,  and  ending  with  the  self-
           signed  root  CA certificate.  The file must be either in PEM or P12
           format.

           A certificate chain 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
           verify the email address of the end-entity (leaf)  peer  certificate
           subject

           Certificates  are  accepted  if no subject checks were specified, or
           the email address of the end-entity (leaf) peer certificate  matches
           any of the email addresses specified with checkEmail.

           Multiple checkEmail options are allowed in a single service section.

           This option requires OpenSSL 1.0.2 or later.

       checkHost = HOST
           verify the host of the end-entity (leaf) peer certificate subject

           Certificates  are  accepted  if no subject checks were specified, or
           the host name of the end-entity (leaf) peer certificate matches  any
           of the hosts specified with checkHost.

           Multiple checkHost options are allowed in a single service section.

           This option requires OpenSSL 1.0.2 or later.

       checkIP = IP
           verify  the  IP  address  of  the end-entity (leaf) peer certificate
           subject

           Certificates are accepted if no subject checks  were  specified,  or
           the IP address of the end-entity (leaf) peer certificate matches any
           of the IP addresses specified with checkIP.

           Multiple checkIP options are allowed in a single service section.

           This option requires OpenSSL 1.0.2 or later.

       ciphers = CIPHER_LIST
           select permitted TLS ciphers (TLSv1.2 and below)

           This option does not impact TLSv1.3 ciphersuites.

           A   colon-delimited  list  of  the  ciphers  to  allow  in  the  TLS
           connection, for example DES-CBC3-SHA:IDEA-CBC-MD5.

       ciphersuites = CIPHERSUITES_LIST
           select permitted TLSv1.3 ciphersuites

           A colon-delimited list of TLSv1.3 ciphersuites  names  in  order  of
           preference.

           This option requires OpenSSL 1.1.1 or later.

           default:
           TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256

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

           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.

           Use curves option instead of enabling config = Curves:list_curves to
           support elliptic curves.

           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 = CRL_FILE
           Certificate Revocation Lists file

           This file contains multiple CRLs,  used  with  the  verifyChain  and
           verifyPeer options.

       curves = list
           ECDH curves separated with ':'

           Only a single curve name is allowed for OpenSSL older than 1.1.1.

           To get a list of supported curves use:

               openssl ecparam -list_curves

           default:

               X25519:P-256:X448:P-521:P-384 (OpenSSL 1.1.1 or later)

               prime256v1 (OpenSSL older than 1.1.1)

       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.

           process
               The operating system process identifier (PID) may be  useful  in
               the inetd mode.

           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.  The default is notice (5).

           While  the  debug  =  debug  or  debug  = 7 level generates the most
           verbose  output,  it  is  only  intended  to  be  used  by   stunnel
           developers.   Please  only use this value if you are a developer, or
           you intend to send your logs to our technical  support.   Otherwise,
           the generated logs will be confusing.

       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: prio

       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. The recommended filename convention is

           for global options:

                   00-global.conf

           for local service-level options:

                   01-service.conf

                   02-service.conf

       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.

       OCSP = URL
           select  OCSP  responder  for  the end-entity (leaf) peer 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.

       OCSPrequire = yes | no
           require a conclusive OCSP response

           Disable this option to allow a connection even though no  conclusive
           OCSP  response  was  retrieved from stapling and a direct request to
           the OCSP responder.

           default: yes

       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  TLS
           implementation, the following option can be used:

               options = DONT_INSERT_EMPTY_FRAGMENTS

           default:

               options = NO_SSLv2
               options = NO_SSLv3

           Use sslVersionMax  or  sslVersionMin  option  instead  of  disabling
           specific  TLS  protocol versions when compiled with OpenSSL 1.1.0 or
           later.

       protocol = PROTO
           application protocol to negotiate TLS

           This option enables initial, protocol-specific  negotiation  of  the
           TLS  encryption.   The  protocol  option should not be used with TLS
           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.

           capwin
               http://www.capwin.org/ application support

           capwinctrl
               http://www.capwin.org/ application support

               This protocol is only supported in client mode.

           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

           ldap
               Based on RFC 2830 - Lightweight Directory Access Protocol  (v3):
               Extension for Transport Layer Security

           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
               Passing  of  the  original  client IP address with HAProxy PROXY
               protocol                        version                        1
               https://www.haproxy.org/download/1.8/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 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.

       protocolHeader = HEADER
           header for the protocol negotiations

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

       protocolHost = ADDRESS
           host address for the protocol negotiations

           For the  'connect'  protocol  negotiations,  protocolHost  specifies
           HOST:PORT  of  the final TLS server to be connected to by the proxy.
           The proxy server directly connected by  stunnel  must  be  specified
           with the connect option.

           For  the  'smtp'  protocol  negotiations,  protocolHost controls the
           client SMTP HELO/EHLO value.

       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

           Hexadecimal keys are automatically converted to binary  form.   Keys
           are required to be at least 16 bytes long, which implies at least 32
           characters  for hexadecimal keys.  The file should neither be world-
           readable nor world-writable.

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

       redirect = [HOST:]PORT
           redirect TLS 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 TLS renegotiation

           Applications of the TLS 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 TLS 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 | DELAY
           reconnect a connect+exec section after it was disconnected

           The  DELAY  value  specifies  the  number  of  milliseconds   before
           retrying.  "retry = yes" has the same effect as "retry = 1000".

           default: no

       securityLevel = LEVEL
           set the security level

           The meaning of each level is described below:

           level 0
               Everything is permitted.

           level 1
               The  security  level  corresponds  to  a  minimum  of 80 bits of
               security. Any parameters offering below 80 bits of security  are
               excluded.  As  a  result  RSA, DSA and DH keys shorter than 1024
               bits and ECC keys shorter than  160  bits  are  prohibited.  All
               export  cipher  suites  are prohibited since they all offer less
               than 80 bits of security.  SSL  version  2  is  prohibited.  Any
               cipher suite using MD5 for the MAC is also prohibited.

           level 2
               Security level set to 112 bits of security. As a result RSA, DSA
               and DH keys shorter than 2048 bits and ECC keys shorter than 224
               bits  are  prohibited. In addition to the level 1 exclusions any
               cipher suite using RC4 is also prohibited. SSL version 3 is also
               not allowed. Compression is disabled.

           level 3
               Security level set to 128 bits of security. As a result RSA, DSA
               and DH keys shorter than 3072 bits and ECC keys shorter than 256
               bits are prohibited. In  addition  to  the  level  2  exclusions
               cipher  suites  not offering forward secrecy are prohibited. TLS
               versions below  1.1  are  not  permitted.  Session  tickets  are
               disabled.

           level 4
               Security level set to 192 bits of security. As a result RSA, DSA
               and DH keys shorter than 7680 bits and ECC keys shorter than 384
               bits  are  prohibited.  Cipher suites using SHA1 for the MAC are
               prohibited. TLS versions below 1.2 are not permitted.

           level 5
               Security level set to 256 bits of security. As a result RSA, DSA
               and DH keys shorter than 15360 bits and ECC  keys  shorter  than
               512 bits are prohibited.

           default: 2

           The  securityLevel  option  is  only  available  when  compiled with
           OpenSSL 1.1.0 and later.

       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 TLS sessions.

       sessionResume = yes | no
           allow or disallow session resumption

           default: yes

       sessiond = HOST:PORT
           address of sessiond TLS cache server

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

           SERVICE_NAME  specifies  the  primary  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 secondary  services  are
           normally specified for a single primary service.  The sni option can
           also be specified more than once within a single secondary service.

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

           The connect option of the secondary  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  primary
           service  name  after  TCP  connection  is  accepted,  and  with  the
           secondary 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.

           Empty SERVER_NAME disables sending the SNI extension.

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

       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

       sslVersion = SSL_VERSION
           select the TLS protocol version

           Supported versions: all,  SSLv2,  SSLv3,  TLSv1,  TLSv1.1,  TLSv1.2,
           TLSv1.3

           Availability  of  specific  protocols  depends on the linked OpenSSL
           library.  Older versions of OpenSSL do not support TLSv1.1,  TLSv1.2
           and TLSv1.3.  Newer versions of OpenSSL do not support SSLv2.

           Obsolete SSLv2 and SSLv3 are currently disabled by default.

           Setting the option

               sslVersion = SSL_VERSION

           is equivalent to options

               sslVersionMax = SSL_VERSION
               sslVersionMin = SSL_VERSION

           when compiled with OpenSSL 1.1.0 and later.

       sslVersionMax = SSL_VERSION
           maximum supported protocol versions

           Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3

           all  enable protocol versions up to the highest version supported by
           the linked OpenSSL library.

           Availability of specific protocols depends  on  the  linked  OpenSSL
           library.

           The  sslVersionMax  option  is  only  available  when  compiled with
           OpenSSL 1.1.0 and later.

           default: all

       sslVersionMin = SSL_VERSION
           minimum supported protocol versions

           Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3

           all enable protocol versions down to the lowest version supported by
           the linked OpenSSL library.

           Availability of specific protocols depends  on  the  linked  OpenSSL
           library.

           The  sslVersionMin  option  is  only  available  when  compiled with
           OpenSSL 1.1.0 and later.

           default: TLSv1

       stack = BYTES (except for FORK model)
           CPU stack size of created threads

           Excessive  thread  stack  size  increases  virtual   memory   usage.
           Insufficient thread stack size may cause application crashes.

           default: 65536 bytes (sufficient for all platforms we tested)

       ticketKeySecret = SECRET
           hexadecimal  symmetric  key  used for session ticket confidentiality
           protection

           Session tickets defined in RFC  5077  provide  an  enhanced  session
           resumption capability, where the server-side caching is not required
           to maintain per session state.

           Combining  ticketKeySecret  and  ticketMacSecret  options  allow  to
           resume a negotiated session on other cluster nodes, or to  resume  a
           negotiated session after server restart.

           The  key is required to be either 16 or 32 bytes long, which implies
           exactly 32 or 64 hexadecimal digits.  Colons may optionally be  used
           between two-character hexadecimal bytes.

           This option only works in server mode.

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

           Disabling NO_TICKET option is required for  the  ticket  support  in
           OpenSSL  older than 1.1.1, but note that this option is incompatible
           with the redirect option.

       ticketMacSecret = SECRET
           hexadecimal  symmetric  key  used  for  session   ticket   integrity
           protection

           The  key is required to be either 16 or 32 bytes long, which implies
           exactly 32 or 64 hexadecimal digits.  Colons may optionally be  used
           between two-character hexadecimal bytes.

           This option only works in server mode.

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

       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

       TIMEOUTocsp = SECONDS
           time to wait to connect an OCSP responder

       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  TLS 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 chain.

           level 1
               Verify the peer certificate chain if present.

           level 2
               Verify the peer certificate chain.

           level 3
               Verify the peer certificate chain and the end-entity (leaf) peer
               certificate against a locally installed certificate.

           level 4
               Ignore the peer certificate chain and only verify the end-entity
               (leaf) peer certificate against a locally installed 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 checkIP.

           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 end-entity (leaf) peer certificate

           The  end-entity (leaf) 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:

           o   chroot

           o   foreground

           o   pid

           o   setgid

           o   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.

       SIGUSR2
           Log the list of active connections.

       SIGTERM, SIGQUIT, SIGINT
           Shut stunnel down.

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

EXAMPLES
       In order to provide TLS 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 a TLS-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/

       An example server mode SNI configuration:

           [virtual]
           ; primary service
           accept = 443
           cert =  default.pem
           connect = default.internal.mydomain.com:8080

           [sni1]
           ; secondary service 1
           sni = virtual:server1.mydomain.com
           cert = server1.pem
           connect = server1.internal.mydomain.com:8081

           [sni2]
           ; secondary service 2
           sni = virtual:server2.mydomain.com
           cert = server2.pem
           connect = server2.internal.mydomain.com:8082
           verifyPeer = yes
           CAfile = server2-allowed-clients.pem

       An  example  of  advanced engine configuration allows for authentication
       with private keys stored  in  the  Windows  certificate  store  (Windows
       only).   With  the  CAPI  engine  you  don't need to manually select the
       client key to use.  The client key is automatically  selected  based  on
       the list of CAs trusted by the server.

           engine = capi

           [service]
           engineId = capi
           client = yes
           accept = 127.0.0.1:8080
           connect = example.com:8443

       An  example  of advanced engine configuration to use the certificate and
       the corresponding private key from a pkcs11 engine:

           engine = pkcs11
           engineCtrl = MODULE_PATH:opensc-pkcs11.so
           engineCtrl = PIN:123456

           [service]
           engineId = pkcs11
           client = yes
           accept = 127.0.0.1:8080
           connect = example.com:843
           cert = pkcs11:token=MyToken;object=MyCert
           key = pkcs11:token=MyToken;object=MyKey

       An example of advanced engine configuration to use the  certificate  and
       the corresponding private key from a SoftHSM token:

           engine = pkcs11
           engineCtrl = MODULE_PATH:softhsm2.dll
           engineCtrl = PIN:12345

           [service]
           engineId = pkcs11
           client = yes
           accept = 127.0.0.1:8080
           connect = example.com:843
           cert = pkcs11:token=MyToken;object=KeyCert

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 TLS-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 TLS-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 .pem file should contain the unencrypted private key  and  a  signed
       certificate  (not  certificate  request).   So the file should look like
       this:

           -----BEGIN RSA PRIVATE KEY-----
           [encoded key]
           -----END RSA PRIVATE KEY-----
           -----BEGIN CERTIFICATE-----
           [encoded certificate]
           -----END CERTIFICATE-----

   RANDOMNESS
       stunnel needs to seed the PRNG (pseudo-random number generator) in order
       for TLS to use good randomness.  The following  sources  are  loaded  in
       order until sufficient random data has been gathered:

       o   The file specified with the RNDfile flag.

       o   The file specified by the RANDFILE environment variable, if set.

       o   The file .rnd in your home directory, if RANDFILE not set.

       o   The file specified with '--with-random' at compile time.

       o   The contents of the screen if running on Windows.

       o   The egd socket specified with the EGD flag.

       o   The egd socket specified with '--with-egd-sock' at compile time.

       o   The /dev/urandom device.

       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
       Michal Trojnara
           <[email protected]>

5.72                               2024.04.11                        stunnel(8)

Updated on: 2024-Aug-06