stunnel4 • man page

stunnel4 (8)

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NAME

stunnel - TLS offloading and load-balancing proxy

SYNOPSIS

Unix:: stunnel [
FILE
] | -fd N | -help | -version | -sockets | -options
WIN32:: stunnel [ [ -install | -uninstall | -start | -stop |
-reload | -reopen | -exit ] [-quiet] [
FILE
] ] |
-help | -version | -sockets | -options

DESCRIPTION

The stunnel program is designed to work as

TLS

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

TLS

channels.

stunnel can be used to add

TLS

functionality to commonly used Inetd daemons like

POP-2, POP-3,

and

IMAP

servers, to standalone daemons like

NNTP, SMTP

and

HTTP,

and in tunneling

PPP

over network sockets without changes to the source code.

This product includes cryptographic software written by Eric Young (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. 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 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

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

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

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

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

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

address of the peer certificate matches any of the

addresses specified with checkIP.

This option requires OpenSSL 1.0.2 or later.

ciphers =
CIPHER_LIST

Select permitted

TLS

ciphers

A colon-delimited list of the ciphers to allow in the

TLS

connection, for example

DES-CBC3-SHA:IDEA-CBC-MD5.

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.

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.
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. 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

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

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

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

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

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 www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982

pop3

Based on

RFC 2449 -

POP3

Extension Mechanism

proxy

Haproxy client

address 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

TLS

encryption layer to protect the final destination address.

www.openssh.com/txt/socks4.protocol

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

TLS

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

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:

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

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.

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.

sslVersion =
SSL_VERSION

select the

TLS

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

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

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

The self-signed root

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

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 an 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

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 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:

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

parameters. Starting with stunnel 5.18, these hardcoded

parameters are replaced every 24 hours with autogenerated temporary

parameters.

parameter generation may take several minutes.

Alternatively, it is possible to specify static

parameters in the certificate file, which disables generating temporary

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.

AUTHOR

Michał Trojnara: <Michal.Trojnara@stunnel.org>

stunnel4 • man page

stunnel4 • man page

stunnel4 (8)

Leading comments

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

Installed via

Man Section

extra • Version

extra • Source

extra • Book

References

stunnel4 • man page

stunnel4 • man page

stunnel4 (8)

Leading comments

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

Installed via

Man Section

extra • Version

extra • Source

extra • Book

References

SERVICE-LEVEL
OPTIONS