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: 539 KB
How to install: sudo apt install stunnel4

  • adduser
  • init-system-helpers
  • libc6
  • libssl1.1
  • libsystemd0
  • libwrap0
  • lsb-base
  • netbase
  • openssl
  • perl
stunnel

TLS offloading and load-balancing proxy

root@kali:~# man stunnel
stunnel(8)                     stunnel TLS Proxy                    stunnel(8)

NAME
       stunnel - TLS offloading and load-balancing proxy

SYNOPSIS
       Unix:
           stunnel [FILE] | -fdN | -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
       (eay@cryptsoft.com)

OPTIONS
       FILE
           Use specified configuration file

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

       -help
           Print stunnel help menu

       -version
           Print stunnel version and compile time defaults

       -sockets
           Print default socket options

       -options
           Print supported 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.  Use debug = debug or debug = 7 for
           greatest debugging output.  The default is notice (5).

           The syslog facility 'daemon' will be used unless a facility name is
           supplied.  (Facilities are not supported on Win32.)

           Case is ignored for both facilities and levels.

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

           Entropy Gathering Daemon socket to use to feed the OpenSSL random
           number generator.

       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

       CApath = DIRECTORY
           Certificate Authority directory

           This is the directory in which stunnel will look for certificates
           when using the verifyChain or verifyPeer options.  Note that the
           certificates in this directory should be named XXXXXXXX.0 where
           XXXXXXXX is the hash value of the DER encoded subject of the cert.

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

           CApath path is relative to the chroot directory if specified.

       CAfile = CA_FILE
           Certificate Authority file

           This file contains multiple CA certificates, to 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
           email address of the peer certificate subject

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

           This option requires OpenSSL 1.0.2 or later.

       checkHost = HOST
           host of the peer certificate subject

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

           This option requires OpenSSL 1.0.2 or later.

       checkIP = IP
           IP address of the peer certificate subject

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

           This option requires OpenSSL 1.0.2 or later.

       ciphers = CIPHER_LIST
           select permitted 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 certificate verification

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

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

       OCSPflag = OCSP_FLAG
           specify OCSP responder flag

           Several OCSPflag can be used to specify multiple flags.

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

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

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

       options = SSL_OPTIONS
           OpenSSL library options

           The parameter is the OpenSSL option name as described in the
           SSL_CTX_set_options(3ssl) manual, but without SSL_OP_ prefix.
           stunnel -options lists the options found to be allowed in the
           current combination of stunnel and the OpenSSL library used to
           build it.

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

           For example, for compatibility with the erroneous Eudora 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.

           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
           reconnect a connect+exec section after it was disconnected

           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 slave service (a name-based virtual server)
           for Server Name Indication TLS extension (RFC 3546).

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

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

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

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

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

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

           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 to a remote host

       TIMEOUTidle = SECONDS
           time to keep an idle connection

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

           Supported values:

           none
               Disable transparent proxy support.  This is the default.

           source
               Re-write the address to appear as if a wrapped daemon is
               connecting from the 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.

           level 1
               Verify the peer certificate if present.

           level 2
               Verify the peer certificate.

           level 3
               Verify the peer against a locally installed certificate.

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

           default
               No verify.

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

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

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

           default: no

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

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

       SIGHUP
           Force a reload of the configuration file.

           Some global options will not be reloaded:

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

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

           [sni2]
           ; slave 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
       /etc/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
           <Michal.Trojnara@stunnel.org>

5.60                              2021.05.30                        stunnel(8)

stunnel3

Universal SSL tunnel

root@kali:~# stunnel3 --help
/usr/bin/stunnel3 version [unknown] calling Getopt::Std::getopts (version 1.12 [paranoid]),
running under Perl version 5.32.1.

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 5.60 on x86_64-pc-linux-gnu platform
[.] Compiled with OpenSSL 1.1.1k  25 Mar 2021
[.] Running  with OpenSSL 1.1.1l  24 Aug 2021
[.] 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 enabled: 0 methods
[ ] No PRNG seeding was required
[!] Inetd mode: TLS server needs a certificate
[!] Configuration failed
[ ] Deallocating temporary section defaults

stunnel4

TLS offloading and load-balancing proxy

root@kali:~# man stunnel4
stunnel(8)                     stunnel TLS Proxy                    stunnel(8)

NAME
       stunnel - TLS offloading and load-balancing proxy

SYNOPSIS
       Unix:
           stunnel [FILE] | -fdN | -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
       (eay@cryptsoft.com)

OPTIONS
       FILE
           Use specified configuration file

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

       -help
           Print stunnel help menu

       -version
           Print stunnel version and compile time defaults

       -sockets
           Print default socket options

       -options
           Print supported 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.  Use debug = debug or debug = 7 for
           greatest debugging output.  The default is notice (5).

           The syslog facility 'daemon' will be used unless a facility name is
           supplied.  (Facilities are not supported on Win32.)

           Case is ignored for both facilities and levels.

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

           Entropy Gathering Daemon socket to use to feed the OpenSSL random
           number generator.

       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

       CApath = DIRECTORY
           Certificate Authority directory

           This is the directory in which stunnel will look for certificates
           when using the verifyChain or verifyPeer options.  Note that the
           certificates in this directory should be named XXXXXXXX.0 where
           XXXXXXXX is the hash value of the DER encoded subject of the cert.

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

           CApath path is relative to the chroot directory if specified.

       CAfile = CA_FILE
           Certificate Authority file

           This file contains multiple CA certificates, to 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
           email address of the peer certificate subject

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

           This option requires OpenSSL 1.0.2 or later.

       checkHost = HOST
           host of the peer certificate subject

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

           This option requires OpenSSL 1.0.2 or later.

       checkIP = IP
           IP address of the peer certificate subject

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

           This option requires OpenSSL 1.0.2 or later.

       ciphers = CIPHER_LIST
           select permitted 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 certificate verification

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

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

       OCSPflag = OCSP_FLAG
           specify OCSP responder flag

           Several OCSPflag can be used to specify multiple flags.

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

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

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

       options = SSL_OPTIONS
           OpenSSL library options

           The parameter is the OpenSSL option name as described in the
           SSL_CTX_set_options(3ssl) manual, but without SSL_OP_ prefix.
           stunnel -options lists the options found to be allowed in the
           current combination of stunnel and the OpenSSL library used to
           build it.

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

           For example, for compatibility with the erroneous Eudora 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.

           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
           reconnect a connect+exec section after it was disconnected

           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 slave service (a name-based virtual server)
           for Server Name Indication TLS extension (RFC 3546).

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

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

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

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

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

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

           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 to a remote host

       TIMEOUTidle = SECONDS
           time to keep an idle connection

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

           Supported values:

           none
               Disable transparent proxy support.  This is the default.

           source
               Re-write the address to appear as if a wrapped daemon is
               connecting from the 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.

           level 1
               Verify the peer certificate if present.

           level 2
               Verify the peer certificate.

           level 3
               Verify the peer against a locally installed certificate.

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

           default
               No verify.

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

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

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

           default: no

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

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

       SIGHUP
           Force a reload of the configuration file.

           Some global options will not be reloaded:

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

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

           [sni2]
           ; slave 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
       /etc/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
           <Michal.Trojnara@stunnel.org>

5.60                              2021.05.30                        stunnel(8)

Updated on: 2021-Nov-26