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stunnel(8) stunnel TLS Proxy stunnel(8)

NAME
stunnel - TLS offloading and load-balancing proxy

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:

• 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.

debug = [FACILITY.]LEVEL
debugging level

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

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

The default logging level is notice (5).

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

Case is ignored for both facilities and levels.

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

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

engine = auto | ENGINE_ID
select hardware or software cryptographic engine

default: software-only cryptography

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

engineCtrl = COMMAND[:PARAMETER]
control hardware engine

engineDefault = TASK_LIST
set OpenSSL tasks delegated to the current engine

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

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

fips = yes | no
enable or disable FIPS 140-2 mode.

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

default: no (since version 5.00)

foreground = yes | quiet | no (Unix only)
foreground mode

Stay in foreground (don't fork).

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

default: background in daemon mode

iconActive = ICON_FILE (GUI only)
GUI icon to be displayed when there are established connections

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

iconError = ICON_FILE (GUI only)
GUI icon to be displayed when no valid configuration is loaded

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

iconIdle = ICON_FILE (GUI only)
GUI icon to be displayed when there are no established connections

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

log = append | overwrite
log file handling

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

default: append

output = FILE
append log messages to a file

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

pid = FILE (Unix only)
pid file location

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

pid path is relative to the chroot directory if specified.

provider = PROVIDER_ID
Specifies the identifier of the provider to be used. PROVIDER_ID
is a unique identifier referring to a specific cryptographic
service provider.

This option requires OpenSSL 3.0 or later.

providerParameter = PROVIDER_ID:PARAMETER=VALUE
Sets a specific parameter for the given provider. PROVIDER_ID
identifies the provider, PARAMETER is the parameter name, and VALUE
is its value. This option allows customization of the selected
cryptographic service provider's configuration.

This option requires OpenSSL 3.5 or later.

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

setEnv = VAR_NAME=VALUE
Change or add an environment variable for child processes. If
VAR_NAME already exists, its value will be updated; otherwise, a
new variable will be created. This modification applies only to
spawned child processes and does not affect the current
environment.

syslog = yes | no (Unix only)
enable logging via syslog

default: yes

taskbar = yes | no (WIN32 only)
enable the taskbar icon

default: yes

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

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

accept = [HOST:]PORT
accept connections on specified address

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

To listen on all IPv6 addresses use:

accept = :::PORT

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

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

Multiple CAengine options are allowed in a single service section.

Currently supported engines: pkcs11, cng.

CApath = CA_DIRECTORY
load trusted CA certificates from a directory

The loaded CA certificates will be used with the verifyChain and
verifyPeer options. Note that the certificates in this directory
should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the
DER encoded subject of the cert.

This parameter can also be used to provide the root CA certificate
needed to validate OCSP stapling in server mode.

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

CApath path is relative to the chroot directory if specified.

CAfile = CA_FILE
load trusted CA certificates from a file

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

This parameter can also be used to provide the root CA certificate
needed to validate OCSP stapling in server mode.

CAstore = URI_CA
load trusted CA certificates from a resource specified by a URI

This option allows loading CA certificates from an external source
supported by the OSSL_STORE framework, such as a PKCS#11 module
(e.g., hardware token), the system certificate store, or a remote
resource.

This option can be used independently of CAfile and CAdir, and
similarly provides certificates used by verifyChain or verifyPeer,
as well as for validating OCSP stapling in server mode.

This option requires OpenSSL 3.0 or later.

cert = CERT_FILE | URI
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 (PKCS#11
URI) when a hardware engine or provider is enabled.

Note: The provider requires OpenSSL 3.0 or later

checkEmail = EMAIL
verify the email address of the end-entity (leaf) peer certificate
subject

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

Multiple checkEmail options are allowed in a single service
section.

This option requires OpenSSL 1.0.2 or later.

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

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

Multiple checkHost options are allowed in a single service section.

This option requires OpenSSL 1.0.2 or later.

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

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

Multiple checkIP options are allowed in a single service section.

This option requires OpenSSL 1.0.2 or later.

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

This option does not impact TLSv1.3 ciphersuites.

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

ciphersuites = CIPHERSUITES_LIST
select permitted TLSv1.3 ciphersuites

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

The ciphersuites option ignores unknown ciphers when compiled with
OpenSSL 3.0 or later.

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 | URI
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 (PKCS#11
URI) when a hardware engine or provider is enabled.

Note: The provider requires OpenSSL 3.0 or later

default: the value of the cert option

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

default: no (since version 5.00)

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

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

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

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

OCSPflag = OCSP_FLAG
specify OCSP responder flag

Several OCSPflag can be used to specify multiple flags.

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

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

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

OCSPrequire = yes | no
require a conclusive OCSP response

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

default: yes

options = SSL_OPTIONS
OpenSSL library options

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

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

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

options = DONT_INSERT_EMPTY_FRAGMENTS

default:

options = NO_SSLv2
options = NO_SSLv3

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

protocol = PROTO
application protocol to negotiate TLS

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

Currently supported protocols:

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

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

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

This protocol is only supported in client mode.

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

This protocol is only supported in client mode.

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

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

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

This protocol is only supported in client mode.

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

pop3
Based on RFC 2449 - POP3 Extension Mechanism

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

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

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

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

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

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

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

protocolAuthentication = AUTHENTICATION
authentication type for the protocol negotiations

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

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

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

protocolDomain = DOMAIN
domain for the protocol negotiations

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

protocolHeader = HEADER
header for the protocol negotiations

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

protocolHost = ADDRESS
host address for the protocol negotiations

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

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

protocolPassword = PASSWORD
password for the protocol negotiations

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

protocolUsername = USERNAME
username for the protocol negotiations

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

PSKidentity = IDENTITY
PSK identity for the PSK client

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

default: the first identity specified in the PSKsecrets file.

PSKsecrets = FILE
file with PSK identities and corresponding keys

Each line of the file in the following format:

IDENTITY:KEY

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

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

redirect = [HOST:]PORT
redirect TLS client connections on certificate-based authentication
failures

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

renegotiation = yes | no
support TLS renegotiation

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

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

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

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

default: yes (if supported by OpenSSL)

reset = yes | no
attempt to use the TCP RST flag to indicate an error

This option is not supported on some platforms.

default: yes

retry = yes | no | DELAY
reconnect a connect+exec section after it was disconnected

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

default: no

securityLevel = LEVEL
set the security level

The meaning of each level is described below:

level 0
Everything is permitted.

level 1
The security level corresponds to a minimum of 80 bits of
security. Any parameters offering below 80 bits of security are
excluded. As a result RSA, DSA and DH keys shorter than 1024
bits and ECC keys shorter than 160 bits are prohibited. All
export cipher suites are prohibited since they all offer less
than 80 bits of security. SSL version 2 is prohibited. Any
cipher suite using MD5 for the MAC is also prohibited.
Additionally, SSLv3, TLS 1.0, TLS 1.1 are all disabled for
OpenSSL 3.0 and later.

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.
Compression is disabled. SSL version 3 is also not allowed for
OpenSSL older than 3.0.

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. Session tickets are disabled. TLS versions below
1.1 are not permitted for OpenSSL older than 3.0.

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
for OpenSSL older than 3.0.

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

default: 2

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

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

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

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

default: no

setgid = GROUP (Unix only)
Unix group id

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

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

setuid = USER (Unix only)
Unix user id

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

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

sessionCacheSize = NUM_ENTRIES
session cache size

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

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

sessionCacheTimeout = TIMEOUT
session cache timeout

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

sessionResume = yes | no
allow or disallow session resumption

default: yes

sessiond = HOST:PORT
address of sessiond TLS cache server

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

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

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

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

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

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

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

Empty SERVER_NAME disables sending the SNI extension.

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

socket = a|l|r:OPTION=VALUE[:VALUE]
Set an option on the accept/local/remote socket

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

Examples:

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

sslVersion = SSL_VERSION
select the TLS protocol version

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

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

Obsolete SSLv2 and SSLv3 are currently disabled by default.

Setting the option

sslVersion = SSL_VERSION

is equivalent to options

sslVersionMax = SSL_VERSION
sslVersionMin = SSL_VERSION

when compiled with OpenSSL 1.1.0 and later.

sslVersionMax = SSL_VERSION
maximum supported protocol versions

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

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

Availability of specific protocols depends on the linked OpenSSL
library.

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

default: all

sslVersionMin = SSL_VERSION
minimum supported protocol versions

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

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

Availability of specific protocols depends on the linked OpenSSL
library.

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

default: TLSv1

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

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

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

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

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

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

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

This option only works in server mode.

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

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

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

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

This option only works in server mode.

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

TIMEOUTbusy = SECONDS
time to wait for expected data

TIMEOUTclose = SECONDS
time to wait for close_notify (set to 0 for buggy MSIE)

TIMEOUTconnect = SECONDS
time to wait to connect a remote host

TIMEOUTidle = SECONDS
time to keep an idle connection

TIMEOUTocsp = SECONDS
time to wait to connect an OCSP responder

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

Supported values:

none
Disable transparent proxy support. This is the default.

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

This option is currently available in:

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

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

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

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

Remote mode (connect option) on Linux 2.2.x
This configuration requires the kernel to be compiled with
the transparent proxy option. Connected service must be
installed on a separate host. Routing towards the clients
has to go through the stunnel box.

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

Remote mode (connect option) on FreeBSD >=8.0
This configuration requires additional firewall and routing
setup. stunnel must also to be executed as root and
without the setuid option.

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

destination
The original destination is used instead of the connect option.

A service section for transparent destination may look like
this:

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

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

For a connect target installed on the same host:

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

For a connect target installed on a remote host:

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

The transparent destination option is currently only supported
on Linux.

both
Use both source and destination transparent proxy.

Two legacy options are also supported for backward compatibility:

yes This option has been renamed to source.

no This option has been renamed to none.

verify = LEVEL
verify the peer certificate

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

level 0
Request and ignore the peer certificate chain.

level 1
Verify the peer certificate chain if present.

level 2
Verify the peer certificate chain.

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

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

default
No verify.

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

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

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

default: no

verifyPeer = yes | no
verify the end-entity (leaf) peer certificate

The end-entity (leaf) peer certificate needs to be stored either in
the file specified with CAfile, or in the directory specified with
CApath.

default: no

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

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

SIGHUP
Force a reload of the configuration file.

Some global options will not be reloaded:

• 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.

SIGUSR2
Log the list of active connections.

SIGTERM, SIGQUIT, SIGINT
Shut stunnel down.

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

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

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

or in remote mode:

[imapd]
accept = 993
connect = 143

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

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

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

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

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

exec = /usr/sbin/imapd
execArgs = imapd

To setup SOCKS VPN configure the following client service:

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

The corresponding configuration on the vpn_server host:

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

Now test your configuration on the client machine with:

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

An example server mode SNI configuration:

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

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

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

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

engine = capi

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

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

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

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

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

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

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

An example of advanced provider configuration to use the certificate
and the corresponding private key from a `pkcs11prov` provider:

Note: requires OpenSSL 3.0 or later

setEnv = PKCS11_MODULE_PATH=opensc-pkcs11.dll
setEnv = PKCS11_PIN:123456
provider = pkcs11prov

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

Note: requires OpenSSL 3.5 or later

provider = pkcs11prov
providerParameter = pkcs11prov:pkcs11_module=opensc-pkcs11.dll
providerParameter = pkcs11prov:pin=123456

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

On Windows, the PKCS#11 library must be placed in the `ossl-modules`
directory, or a full absolute path must be provided in
`PKCS11_MODULE_PATH` or `pkcs11_module` parameter.

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/local/bin/stunnel stunnel /usr/local/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:

• 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.

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

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

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

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

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

openssl dhparam 2048 >> stunnel.pem

FILES
@sysconfdir@/stunnel/stunnel.conf
stunnel configuration file

BUGS
The execArgs option and the Win32 command line do not support quoting.

SEE ALSO
tcpd(8)
access control facility for internet services

inetd(8)
internet 'super-server'

http://www.stunnel.org/
stunnel homepage

http://www.openssl.org/
OpenSSL project website

AUTHOR
Micha Trojnara
<Michal.Trojnara@stunnel.org>

5.75 2025.05.16 stunnel(8)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=stunnel&sektion=8&manpath=FreeBSD+Ports+15.0>

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