stunnel TLS Proxy

- NAME
- SYNOPSIS
- DESCRIPTION
- OPTIONS
- CONFIGURATION FILE
  - GLOBAL OPTIONS
  - SERVICE-LEVEL OPTIONS
- RETURN VALUE
- SIGNALS
- EXAMPLES
- NOTES
  - RESTRICTIONS
  - INETD MODE
  - CERTIFICATES
  - RANDOMNESS
  - DH PARAMETERS
- FILES
- BUGS
- SEE ALSO
- AUTHOR

# NAME

stunnel - TLS offloading and load-balancing proxy

# SYNOPSIS

Unix:

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

WIN32:

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

# DESCRIPTION

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

stunnel can be used to add SSL functionality to commonly used Inetd daemons like POP-2, POP-3, and IMAP servers, to standalone daemons like NNTP, SMTP and HTTP, and in tunneling PPP over network sockets without changes to the source code.

This product includes cryptographic software written by Eric Young (eay@cryptsoft.com)

# OPTIONS

FILE

  Use specified configuration file

-fd N (Unix only)

  Read the config file from specified file descriptor

-help

  Print stunnel help menu

-version

  Print stunnel version and compile time defaults

-sockets

  Print default socket options

-options

  Print supported SSL options

-install (Windows NT and later only)

  Install NT Service

-uninstall (Windows NT and later only)

  Uninstall NT Service

-start (Windows NT and later only)

  Start NT Service

-stop (Windows NT and later only)

  Stop NT Service

-reload (Windows NT and later only)

  Reload the configuration file of the running NT Service

-reopen (Windows NT and later only)

  Reopen the log file of the running NT Service

-exit (Win32 only)

  Exit an already started stunnel

-quiet (Win32 only)

  Don't display any message boxes

# CONFIGURATION FILE

Each line of the configuration file can be either:

- An empty line (ignored).

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

- An 'option_name = option_value' pair.

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

An address parameter of an option may be either:

- A port number.

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

- A Unix socket path (Unix only).

## GLOBAL OPTIONS

chroot = DIRECTORY (Unix only)

  directory to chroot stunnel process

  chroot keeps stunnel in a chrooted jail. CApath, CRLpath, pid and exec are located inside the jail and the patches have to be relative to the directory specified with chroot.

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

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

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

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

compression = deflate | zlib

  select data compression algorithm

  default: no compression

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

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

debug = [FACILITY.]LEVEL

  debugging level

  Level is one of the syslog level names or numbers emerg (0), alert (1), crit (2), err (3), warning (4), notice (5), info (6), or debug (7). All logs for the specified level and all levels numerically less than it will be shown. Use debug = debug or debug = 7 for greatest debugging output. The default is notice (5).

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

  Case is ignored for both facilities and levels.

EGD = EGD_PATH (Unix only)

  path to Entropy Gathering Daemon socket

  Entropy Gathering Daemon socket to use to feed the OpenSSL random number generator. (Available only if compiled with OpenSSL 0.9.5a or higher)

engine = auto | ENGINE_ID

  select hardware engine

  default: software-only cryptography

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

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

          [service]
          engineId = pkcs11
          cert = id_45

engineCtrl = COMMAND[:PARAMETER]

  control hardware engine

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

engineDefault = TASK_LIST

  set OpenSSL tasks delegated to the current engine

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

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

fips = yes | no

  Enable or disable FIPS 140-2 mode.

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

  default: no (since version 5.00)

foreground = yes | quiet | no (Unix only)

  foreground mode

  Stay in foreground (don't fork).

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

  default: background in daemon mode

iconActive = ICON_FILE (GUI only)

  GUI icon to be displayed when there are established connections

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

iconError = ICON_FILE (GUI only)

  GUI icon to be displayed when no valid configuration is loaded

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

iconIdle = ICON_FILE (GUI only)

  GUI icon to be displayed when there are no established connections

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

log = append | overwrite

  log file handling

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

  default: append

output = FILE

  append log messages to a file

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

pid = FILE (Unix only)

  pid file location

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

  pid path is relative to the chroot directory if specified.

RNDbytes = BYTES

  bytes to read from random seed files

  Number of bytes of data read from random seed files. With SSL versions less than 0.9.5a, also determines how many bytes of data are considered sufficient to seed the PRNG. More recent OpenSSL versions have a builtin function to determine when sufficient randomness is available.

RNDfile = FILE

  path to file with random seed data

  The SSL library will use data from this file first to seed the random number generator.

RNDoverwrite = yes | no

  overwrite the random seed files with new random data

  default: yes

service = SERVICE (Unix only)

  stunnel service name

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

  default: stunnel

socket = a|l|r:OPTION=VALUE[:VALUE]

  Set an option on the accept/local/remote socket

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

  Examples:

          socket = l:SO_LINGER=1:60
              set one minute timeout for closing local socket
          socket = r:SO_OOBINLINE=yes
              place out-of-band data directly into the
              receive data stream for remote sockets
          socket = a:SO_REUSEADDR=no
              disable address reuse (enabled by default)
          socket = a:SO_BINDTODEVICE=lo
              only accept connections on loopback interface

syslog = yes | no (Unix only)

  enable logging via syslog

  default: yes

taskbar = yes | no (WIN32 only)

  enable the taskbar icon

  default: yes

## SERVICE-LEVEL OPTIONS

Each configuration section begins with a service name in square brackets. The service name is used for libwrap (TCP Wrappers) access control and lets you distinguish stunnel services in your log files.

Note that if you wish to run stunnel in inetd mode (where it is provided a network socket by a server such as inetd, xinetd, or tcpserver) then you should read the section entitled INETD MODE below.

accept = [HOST:]PORT

  accept connections on specified address

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

  To listen on all IPv6 addresses use:

          accept = :::PORT

CApath = DIRECTORY

  Certificate Authority directory

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

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

  CApath path is relative to the chroot directory if specified.

CAfile = CERT_FILE

  Certificate Authority file

  This file contains multiple CA certificates, to be used with the verifyChain and verifyPeer options.

cert = PEM_FILE

  certificate chain PEM file name

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

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

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

checkEmail = EMAIL

  email address of the peer certificate subject

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

  This option requires OpenSSL 1.0.2 or later.

checkHost = HOST

  host of the peer certificate subject

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

  This option requires OpenSSL 1.0.2 or later.

checkIP = IP

  IP address of the peer certificate subject

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

  This option requires OpenSSL 1.0.2 or later.

ciphers = CIPHER_LIST

  Select permitted SSL ciphers

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

client = yes | no

  client mode (remote service uses SSL)

  default: no (server mode)

config = COMMAND[:PARAMETER]

  OpenSSL configuration command

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

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

  This option requires OpenSSL 1.0.2 or later.

connect = [HOST:]PORT

  connect to a remote address

  If no host is specified, the host defaults to localhost.

  Multiple connect options are allowed in a single service section.

  If host resolves to multiple addresses and/or if multiple connect options are specified, then the remote address is chosen using a round-robin algorithm.

CRLpath = DIRECTORY

  Certificate Revocation Lists directory

  This is the directory in which stunnel will look for CRLs when using the verifyChain and verifyPeer options. Note that the CRLs in this directory should be named XXXXXXXX.r0 where XXXXXXXX is the hash value of the CRL.

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

  CRLpath path is relative to the chroot directory if specified.

CRLfile = CERT_FILE

  Certificate Revocation Lists file

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

curve = NID

  specify ECDH curve name

  To get a list of supported curves use:

          openssl ecparam -list_curves

  default: prime256v1

logId = TYPE

  connection identifier type

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

  Currently supported types:

  sequential

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

  unique

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

  thread

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

  default: sequential

debug = LEVEL

  debugging level

  Level is a one of the syslog level names or numbers emerg (0), alert (1), crit (2), err (3), warning (4), notice (5), info (6), or debug (7). All logs for the specified level and all levels numerically less than it will be shown. Use debug = debug or debug = 7 for greatest debugging output. The default is notice (5).

delay = yes | no

  delay DNS lookup for the connect option

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

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

  Delayed resolver inflicts failover = prio.

  default: no

engineId = ENGINE_ID

  select engine ID for the service

engineNum = ENGINE_NUMBER

  select engine number for the service

  The engines are numbered starting from 1.

exec = EXECUTABLE_PATH

  execute a local inetd-type program

  exec path is relative to the chroot directory if specified.

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

execArgs = $0 $1 $2 ...

  arguments for exec including the program name ($0)

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

failover = rr | prio

  Failover strategy for multiple "connect" targets.

          rr (round robin) - fair load distribution
          prio (priority) - use the order specified in config file

  default: rr

ident = USERNAME

  use IDENT (RFC 1413) username checking

include = DIRECTORY

  include all configuration file parts located in DIRECTORY

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

key = KEY_FILE

  private key for the certificate specified with cert option

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

          chmod 600 keyfile

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

  default: the value of the cert option

libwrap = yes | no

  Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.

  default: no (since version 5.00)

local = HOST

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

sni = SERVICE:SERVER_PATTERN (server mode)

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

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

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

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

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

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

sni = SERVER (client mode)

  Use the parameter as the value of TLS Server Name Indication (RFC 3546) extension.

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

OCSP = URL

  select OCSP responder for certificate verification

OCSPaia = yes | no

  validate certificates with their AIA OCSP responders

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

OCSPflag = OCSP_FLAG

  specify OCSP responder flag

  Several OCSPflag can be used to specify multiple flags.

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

OCSPnonce = yes | no

  send and verify the OCSP nonce extension

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

options = SSL_OPTIONS

  OpenSSL library options

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

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

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

          options = DONT_INSERT_EMPTY_FRAGMENTS

  default:

          options = NO_SSLv2
          options = NO_SSLv3

protocol = PROTO

  application protocol to negotiate SSL

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

  Currently supported protocols:

  cifs

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

  connect

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

    This protocol is only supported in client mode.

  imap

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

  nntp

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

    This protocol is only supported in client mode.

  pgsql

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

  pop3

    Based on RFC 2449 - POP3 Extension Mechanism

  proxy

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

  smtp

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

  socks

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

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

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

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

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

protocolAuthentication = AUTHENTICATION

  authentication type for the protocol negotiations

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

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

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

protocolDomain = DOMAIN

  domain for the protocol negotiations

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

protocolHost = HOST:PORT

  destination address for the protocol negotiations

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

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

protocolPassword = PASSWORD

  password for the protocol negotiations

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

protocolUsername = USERNAME

  username for the protocol negotiations

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

PSKidentity = IDENTITY

  PSK identity for the PSK client

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

  default: the first identity specified in the PSKsecrets file.

PSKsecrets = FILE

  file with PSK identities and corresponding keys

  Each line of the file in the following format:

          IDENTITY:KEY

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

pty = yes | no (Unix only)

  allocate a pseudoterminal for 'exec' option

redirect = [HOST:]PORT

  redirect SSL client connections on certificate-based authentication failures

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

renegotiation = yes | no

  support SSL renegotiation

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

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

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

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

  default: yes (if supported by OpenSSL)

reset = yes | no

  attempt to use the TCP RST flag to indicate an error

  This option is not supported on some platforms.

  default: yes

retry = yes | no

  reconnect a connect+exec section after it was disconnected

  default: no

requireCert = yes | no

  require a client certificate for verifyChain or verifyPeer

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

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

  default: no

setgid = GROUP (Unix only)

  Unix group id

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

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

setuid = USER (Unix only)

  Unix user id

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

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

sessionCacheSize = NUM_ENTRIES

  session cache size

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

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

sessionCacheTimeout = TIMEOUT

  session cache timeout

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

sessiond = HOST:PORT

  address of sessiond SSL cache server

sslVersion = SSL_VERSION

  select the SSL protocol version

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

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

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

stack = BYTES (except for FORK model)

  thread stack size

TIMEOUTbusy = SECONDS

  time to wait for expected data

TIMEOUTclose = SECONDS

  time to wait for close_notify (set to 0 for buggy MSIE)

TIMEOUTconnect = SECONDS

  time to wait to connect to a remote host

TIMEOUTidle = SECONDS

  time to keep an idle connection

transparent = none | source | destination | both (Unix only)

  enable transparent proxy support on selected platforms

  Supported values:

  none

    Disable transparent proxy support. This is the default.

  source

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

    This option is currently available in:

    Remote mode (connect option) on Linux >=2.6.28

      This configuration requires stunnel to be executed as root and without the setuid option.

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

              iptables -t mangle -N DIVERT
              iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
              iptables -t mangle -A DIVERT -j MARK --set-mark 1
              iptables -t mangle -A DIVERT -j ACCEPT
              ip rule add fwmark 1 lookup 100
              ip route add local 0.0.0.0/0 dev lo table 100
              echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter

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

    Remote mode (connect option) on Linux 2.2.x

      This configuration requires the kernel to be compiled with the transparent proxy option. Connected service must be installed on a separate host. Routing towards the clients has to go through the stunnel box.

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

    Remote mode (connect option) on FreeBSD >=8.0

      This configuration requires additional firewall and routing setup. stunnel must also to be executed as root and without the setuid option.

    Local mode (exec option)

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

  destination

    The original destination is used instead of the connect option.

    A service section for transparent destination may look like this:

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

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

    For a connect target installed on the same host:

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

    For a connect target installed on a remote host:

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

    The transparent destination option is currently only supported on Linux.

  both

    Use both source and destination transparent proxy.

  Two legacy options are also supported for backward compatibility:

  yes

    This option has been renamed to source.

  no

    This option has been renamed to none.

verify = LEVEL

  verify the peer certificate

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

  level 0

    Request and ignore the peer certificate.

  level 1

    Verify the peer certificate if present.

  level 2

    Verify the peer certificate.

  level 3

    Verify the peer against a locally installed certificate.

  level 4

    Ignore the chain and only verify the peer certificate.

  default

    No verify.

verifyChain = yes | no

  verify the peer certificate chain starting from the root CA

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

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

  default: no

verifyPeer = yes | no

  verify the peer certificate

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

  default: no

# RETURN VALUE

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

# SIGNALS

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

SIGHUP

  Force a reload of the configuration file.

  Some global options will not be reloaded:

  - chroot

  - foreground

  - pid

  - setgid

  - setuid

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

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

SIGUSR1

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

SIGTERM, SIGQUIT, SIGINT

  Shut stunnel down.

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

# EXAMPLES

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

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

or in remote mode:

        [imapd]
        accept = 993
        connect = 143

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

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

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

        [vpn]
        accept = 2020
        exec = /usr/sbin/pppd
        execArgs = pppd local
        pty = yes

If you want to use stunnel in inetd mode to launch your imapd process, you'd use this stunnel.conf. Note there must be no [service_name] section.

        exec = /usr/sbin/imapd
        execArgs = imapd

To setup SOCKS VPN configure the following client service:

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

The corresponding configuration on the vpn_server host:

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

Now test your configuration on the client machine with:

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

# NOTES

## RESTRICTIONS

stunnel cannot be used for the FTP daemon because of the nature of the FTP protocol which utilizes multiple ports for data transfers. There are available SSL-enabled versions of FTP and telnet daemons, however.

## INETD MODE

The most common use of stunnel is to listen on a network port and establish communication with either a new port via the connect option, or a new program via the exec option. However there is a special case when you wish to have some other program accept incoming connections and launch stunnel, for example with inetd, xinetd, or tcpserver.

For example, if you have the following line in inetd.conf:

        imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf

In these cases, the inetd-style program is responsible for binding a network socket (imaps above) and handing it to stunnel when a connection is received. Thus you do not want stunnel to have any accept option. All the Service Level Options should be placed in the global options section, and no [service_name] section will be present. See the EXAMPLES section for example configurations.

## CERTIFICATES

Each SSL-enabled daemon needs to present a valid X.509 certificate to the peer. It also needs a private key to decrypt the incoming data. The easiest way to obtain a certificate and a key is to generate them with the free OpenSSL package. You can find more information on certificates generation on pages listed below.

The order of contents of the .pem file is important. It should contain the unencrypted private key first, then a signed certificate (not certificate request). There should also be empty lines after the certificate and the private key. Any plaintext certificate information appended on the top of generated certificate should be discarded. So the file should look like this:

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

## RANDOMNESS

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

- The file specified with the RNDfile flag.

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

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

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

- The contents of the screen if running on Windows.

- The egd socket specified with the EGD flag.

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

- The /dev/urandom device.

With recent (OpenSSL 0.9.5a or later) version of SSL it will stop loading random data automatically when sufficient entropy has been gathered. With previous versions it will continue to gather from all the above sources since no SSL function exists to tell when enough data is available.

Note that on Windows machines that do not have console user interaction (mouse movements, creating windows, etc.) the screen contents are not variable enough to be sufficient, and you should provide a random file for use with the RNDfile flag.

Note that the file specified with the RNDfile flag should contain random data -- that means it should contain different information each time stunnel is run. This is handled automatically unless the RNDoverwrite flag is used. If you wish to update this file manually, the openssl rand command in recent versions of OpenSSL, would be useful.

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

## DH PARAMETERS

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

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

        openssl dhparam 2048 >> stunnel.pem

# FILES

/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

Michał Trojnara

  <Michal.Trojnara@stunnel.org>

stunnel TLS Proxy