Provided by: stunnel4_5.72-1build2_amd64 
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.
The debug = debug (or the equivalent <debug = 7>) level produces for the most verbose log output.
This logging level is only meant to be understood by stunnel developers, and not by users. Please
either use the debug level when requested to do so by an stunnel developer, or when you intend to get
confused.
The default logging level is notice (5).
The syslog 'daemon' facility will be used unless a facility name is supplied. (Facilities are not
supported on Win32.)
Case is ignored for both facilities and levels.
EGD = EGD_PATH (Unix only)
path to Entropy Gathering Daemon socket
Entropy Gathering Daemon socket to use to feed the OpenSSL random number generator.
engine = auto | ENGINE_ID
select hardware or software cryptographic engine
default: software-only cryptography
See Examples section for an engine configuration to use the certificate and the corresponding private
key from a cryptographic device.
engineCtrl = COMMAND[:PARAMETER]
control hardware engine
engineDefault = TASK_LIST
set OpenSSL tasks delegated to the current engine
The parameter specifies a comma-separated list of task to be delegated to the current engine.
The following tasks may be available, if supported by the engine: ALL, RSA, DSA, ECDH, ECDSA, DH,
RAND, CIPHERS, DIGESTS, PKEY, PKEY_CRYPTO, PKEY_ASN1.
fips = yes | no
enable or disable FIPS 140-2 mode.
This option allows you to disable entering FIPS mode if stunnel was compiled with FIPS 140-2 support.
default: no (since version 5.00)
foreground = yes | quiet | no (Unix only)
foreground mode
Stay in foreground (don't fork).
With the yes parameter it also logs to stderr in addition to the destinations specified with syslog
and output.
default: background in daemon mode
iconActive = ICON_FILE (GUI only)
GUI icon to be displayed when there are established connections
On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.
iconError = ICON_FILE (GUI only)
GUI icon to be displayed when no valid configuration is loaded
On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.
iconIdle = ICON_FILE (GUI only)
GUI icon to be displayed when there are no established connections
On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.
log = append | overwrite
log file handling
This option allows you to choose whether the log file (specified with the output option) is appended
or overwritten when opened or re-opened.
default: append
output = FILE
append log messages to a file
/dev/stdout device can be used to send log messages to the standard output (for example to log them
with daemontools splogger).
pid = FILE (Unix only)
pid file location
If the argument is empty, then no pid file will be created.
pid path is relative to the chroot directory if specified.
RNDbytes = BYTES
bytes to read from random seed files
RNDfile = FILE
path to file with random seed data
The OpenSSL library will use data from this file first to seed the random number generator.
RNDoverwrite = yes | no
overwrite the random seed files with new random data
default: yes
service = SERVICE (Unix only)
stunnel service name
The specified service name is used for syslog and as the inetd mode service name for TCP Wrappers.
While this option can technically be specified in the service sections, it is only useful in global
options.
default: stunnel
syslog = yes | no (Unix only)
enable logging via syslog
default: yes
taskbar = yes | no (WIN32 only)
enable the taskbar icon
default: yes
SERVICE-LEVEL OPTIONS
Each configuration section begins with a service name in square brackets. The service name is used for
libwrap (TCP Wrappers) access control and lets you distinguish stunnel services in your log files.
Note that if you wish to run stunnel in inetd mode (where it is provided a network socket by a server
such as inetd, xinetd, or tcpserver) then you should read the section entitled INETD MODE below.
accept = [HOST:]PORT
accept connections on specified address
If no host specified, defaults to all IPv4 addresses for the local host.
To listen on all IPv6 addresses use:
accept = :::PORT
CAengine = ENGINE-SPECIFIC_CA_CERTIFICATE_IDENTIFIER
load a trusted CA certificate from an engine
The loaded CA certificates will be used with the verifyChain and verifyPeer options.
Multiple CAengine options are allowed in a single service section.
Currently supported engines: pkcs11, cng.
CApath = CA_DIRECTORY
load trusted CA certificates from a directory
The loaded CA certificates will be used with the verifyChain and verifyPeer options. Note that the
certificates in this directory should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the DER
encoded subject of the cert.
The hash algorithm has been changed in OpenSSL 1.0.0. It is required to c_rehash the directory on
upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x or later.
CApath path is relative to the chroot directory if specified.
CAfile = CA_FILE
load trusted CA certificates from a file
The loaded CA certificates will be used with the verifyChain and verifyPeer options.
cert = CERT_FILE
certificate chain file name
The parameter specifies the file containing certificates used by stunnel to authenticate itself
against the remote client or server. The file should contain the whole certificate chain starting
from the actual server/client certificate, and ending with the self-signed root CA certificate. The
file must be either in PEM or P12 format.
A certificate chain is required in server mode, and optional in client mode.
This parameter is also used as the certificate identifier when a hardware engine is enabled.
checkEmail = EMAIL
verify the email address of the end-entity (leaf) peer certificate subject
Certificates are accepted if no subject checks were specified, or the email address of the end-entity
(leaf) peer certificate matches any of the email addresses specified with checkEmail.
Multiple checkEmail options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
checkHost = HOST
verify the host of the end-entity (leaf) peer certificate subject
Certificates are accepted if no subject checks were specified, or the host name of the end-entity
(leaf) peer certificate matches any of the hosts specified with checkHost.
Multiple checkHost options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
checkIP = IP
verify the IP address of the end-entity (leaf) peer certificate subject
Certificates are accepted if no subject checks were specified, or the IP address of the end-entity
(leaf) peer certificate matches any of the IP addresses specified with checkIP.
Multiple checkIP options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
ciphers = CIPHER_LIST
select permitted TLS ciphers (TLSv1.2 and below)
This option does not impact TLSv1.3 ciphersuites.
A colon-delimited list of the ciphers to allow in the TLS connection, for example
DES-CBC3-SHA:IDEA-CBC-MD5.
ciphersuites = CIPHERSUITES_LIST
select permitted TLSv1.3 ciphersuites
A colon-delimited list of TLSv1.3 ciphersuites names in order of preference.
This option requires OpenSSL 1.1.1 or later.
default: TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256
client = yes | no
client mode (remote service uses TLS)
default: no (server mode)
config = COMMAND[:PARAMETER]
OpenSSL configuration command
The OpenSSL configuration command is executed with the specified parameter. This allows any
configuration commands to be invoked from the stunnel configuration file. Supported commands are
described on the SSL_CONF_cmd(3ssl) manual page.
Several config lines can be used to specify multiple configuration commands.
Use curves option instead of enabling config = Curves:list_curves to support elliptic curves.
This option requires OpenSSL 1.0.2 or later.
connect = [HOST:]PORT
connect to a remote address
If no host is specified, the host defaults to localhost.
Multiple connect options are allowed in a single service section. If host resolves to multiple
addresses and/or if multiple connect options are specified, then the remote address is chosen using a
round-robin algorithm.
CRLpath = DIRECTORY
Certificate Revocation Lists directory
This is the directory in which stunnel will look for CRLs when using the verifyChain and verifyPeer
options. Note that the CRLs in this directory should be named XXXXXXXX.r0 where XXXXXXXX is the hash
value of the CRL.
The hash algorithm has been changed in OpenSSL 1.0.0. It is required to c_rehash the directory on
upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x.
CRLpath path is relative to the chroot directory if specified.
CRLfile = CRL_FILE
Certificate Revocation Lists file
This file contains multiple CRLs, used with the verifyChain and verifyPeer options.
curves = list
ECDH curves separated with ':'
Only a single curve name is allowed for OpenSSL older than 1.1.1.
To get a list of supported curves use:
openssl ecparam -list_curves
default:
X25519:P-256:X448:P-521:P-384 (OpenSSL 1.1.1 or later)
prime256v1 (OpenSSL older than 1.1.1)
logId = TYPE
connection identifier type
This identifier allows you to distinguish log entries generated for each of the connections.
Currently supported types:
sequential
The numeric sequential identifier is only unique within a single instance of stunnel, but very
compact. It is most useful for manual log analysis.
unique
This alphanumeric identifier is globally unique, but longer than the sequential number. It is
most useful for automated log analysis.
thread
The operating system thread identifier is neither unique (even within a single instance of
stunnel) nor short. It is most useful for debugging software or configuration issues.
process
The operating system process identifier (PID) may be useful in the inetd mode.
default: sequential
debug = LEVEL
debugging level
Level is a one of the syslog level names or numbers emerg (0), alert (1), crit (2), err (3), warning
(4), notice (5), info (6), or debug (7). All logs for the specified level and all levels numerically
less than it will be shown. The default is notice (5).
While the debug = debug or debug = 7 level generates the most verbose output, it is only intended to
be used by stunnel developers. Please only use this value if you are a developer, or you intend to
send your logs to our technical support. Otherwise, the generated logs will be confusing.
delay = yes | no
delay DNS lookup for the connect option
This option is useful for dynamic DNS, or when DNS is not available during stunnel startup (road
warrior VPN, dial-up configurations).
Delayed resolver mode is automatically engaged when stunnel fails to resolve on startup any of the
connect targets for a service.
Delayed resolver inflicts failover = prio.
default: no
engineId = ENGINE_ID
select engine ID for the service
engineNum = ENGINE_NUMBER
select engine number for the service
The engines are numbered starting from 1.
exec = EXECUTABLE_PATH
execute a local inetd-type program
exec path is relative to the chroot directory if specified.
The following environmental variables are set on Unix platforms: REMOTE_HOST, REMOTE_PORT,
SSL_CLIENT_DN, SSL_CLIENT_I_DN.
execArgs = $0 $1 $2 ...
arguments for exec including the program name ($0)
Quoting is currently not supported. Arguments are separated with an arbitrary amount of whitespace.
failover = rr | prio
Failover strategy for multiple "connect" targets.
rr round robin - fair load distribution
prio
priority - use the order specified in config file
default: prio
ident = USERNAME
use IDENT (RFC 1413) username checking
include = DIRECTORY
include all configuration file parts located in DIRECTORY
The files are included in the ascending alphabetical order of their names. The recommended filename
convention is
for global options:
00-global.conf
for local service-level options:
01-service.conf
02-service.conf
key = KEY_FILE
private key for the certificate specified with cert option
A private key is needed to authenticate the certificate owner. Since this file should be kept secret
it should only be readable by its owner. On Unix systems you can use the following command:
chmod 600 keyfile
This parameter is also used as the private key identifier when a hardware engine is enabled.
default: the value of the cert option
libwrap = yes | no
Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.
default: no (since version 5.00)
local = HOST
By default, the IP address of the outgoing interface is used as the source for remote connections.
Use this option to bind a static local IP address instead.
OCSP = URL
select OCSP responder for the end-entity (leaf) peer certificate verification
OCSPaia = yes | no
validate certificates with their AIA OCSP responders
This option enables stunnel to validate certificates with the list of OCSP responder URLs retrieved
from their AIA (Authority Information Access) extension.
OCSPflag = OCSP_FLAG
specify OCSP responder flag
Several OCSPflag can be used to specify multiple flags.
currently supported flags: NOCERTS, NOINTERN, NOSIGS, NOCHAIN, NOVERIFY, NOEXPLICIT, NOCASIGN,
NODELEGATED, NOCHECKS, TRUSTOTHER, RESPID_KEY, NOTIME
OCSPnonce = yes | no
send and verify the OCSP nonce extension
This option protects the OCSP protocol against replay attacks. Due to its computational overhead,
the nonce extension is usually only supported on internal (e.g. corporate) responders, and not on
public OCSP responders.
OCSPrequire = yes | no
require a conclusive OCSP response
Disable this option to allow a connection even though no conclusive OCSP response was retrieved from
stapling and a direct request to the OCSP responder.
default: yes
options = SSL_OPTIONS
OpenSSL library options
The parameter is the OpenSSL option name as described in the SSL_CTX_set_options(3ssl) manual, but
without SSL_OP_ prefix. stunnel -options lists the options found to be allowed in the current
combination of stunnel and the OpenSSL library used to build it.
Several option lines can be used to specify multiple options. An option name can be prepended with a
dash ("-") to disable the option.
For example, for compatibility with the erroneous Eudora TLS implementation, the following option can
be used:
options = DONT_INSERT_EMPTY_FRAGMENTS
default:
options = NO_SSLv2
options = NO_SSLv3
Use sslVersionMax or sslVersionMin option instead of disabling specific TLS protocol versions when
compiled with OpenSSL 1.1.0 or later.
protocol = PROTO
application protocol to negotiate TLS
This option enables initial, protocol-specific negotiation of the TLS encryption. The protocol
option should not be used with TLS encryption on a separate port.
Currently supported protocols:
cifs
Proprietary (undocummented) extension of CIFS protocol implemented in Samba. Support for this
extension was dropped in Samba 3.0.0.
capwin
http://www.capwin.org/ application support
capwinctrl
http://www.capwin.org/ application support
This protocol is only supported in client mode.
connect
Based on RFC 2817 - Upgrading to TLS Within HTTP/1.1, section 5.2 - Requesting a Tunnel with
CONNECT
This protocol is only supported in client mode.
imap
Based on RFC 2595 - Using TLS with IMAP, POP3 and ACAP
ldap
Based on RFC 2830 - Lightweight Directory Access Protocol (v3): Extension for Transport Layer
Security
nntp
Based on RFC 4642 - Using Transport Layer Security (TLS) with Network News Transfer Protocol
(NNTP)
This protocol is only supported in client mode.
pgsql
Based on http://www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982
pop3
Based on RFC 2449 - POP3 Extension Mechanism
proxy
Passing of the original client IP address with HAProxy PROXY protocol version 1
https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt
smtp
Based on RFC 2487 - SMTP Service Extension for Secure SMTP over TLS
socks
SOCKS versions 4, 4a, and 5 are supported. The SOCKS protocol itself is encapsulated within TLS
encryption layer to protect the final destination address.
http://www.openssh.com/txt/socks4.protocol
http://www.openssh.com/txt/socks4a.protocol
The BIND command of the SOCKS protocol is not supported. The USERID parameter is ignored.
See Examples section for sample configuration files for VPN based on SOCKS encryption.
protocolAuthentication = AUTHENTICATION
authentication type for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.
Supported authentication types for the 'connect' protocol are 'basic' or 'ntlm'. The default
'connect' authentication type is 'basic'.
Supported authentication types for the 'smtp' protocol are 'plain' or 'login'. The default 'smtp'
authentication type is 'plain'.
protocolDomain = DOMAIN
domain for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' protocol.
protocolHeader = HEADER
header for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' protocol.
protocolHost = ADDRESS
host address for the protocol negotiations
For the 'connect' protocol negotiations, protocolHost specifies HOST:PORT of the final TLS server to
be connected to by the proxy. The proxy server directly connected by stunnel must be specified with
the connect option.
For the 'smtp' protocol negotiations, protocolHost controls the client SMTP HELO/EHLO value.
protocolPassword = PASSWORD
password for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.
protocolUsername = USERNAME
username for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.
PSKidentity = IDENTITY
PSK identity for the PSK client
PSKidentity can be used on stunnel clients to select the PSK identity used for authentication. This
option is ignored in server sections.
default: the first identity specified in the PSKsecrets file.
PSKsecrets = FILE
file with PSK identities and corresponding keys
Each line of the file in the following format:
IDENTITY:KEY
Hexadecimal keys are automatically converted to binary form. Keys are required to be at least 16
bytes long, which implies at least 32 characters for hexadecimal keys. The file should neither be
world-readable nor world-writable.
pty = yes | no (Unix only)
allocate a pseudoterminal for 'exec' option
redirect = [HOST:]PORT
redirect TLS client connections on certificate-based authentication failures
This option only works in server mode. Some protocol negotiations are also incompatible with the
redirect option.
renegotiation = yes | no
support TLS renegotiation
Applications of the TLS renegotiation include some authentication scenarios, or re-keying long
lasting connections.
On the other hand this feature can facilitate a trivial CPU-exhaustion DoS attack:
http://vincent.bernat.im/en/blog/2011-ssl-dos-mitigation.html
Please note that disabling TLS renegotiation does not fully mitigate this issue.
default: yes (if supported by OpenSSL)
reset = yes | no
attempt to use the TCP RST flag to indicate an error
This option is not supported on some platforms.
default: yes
retry = yes | no | DELAY
reconnect a connect+exec section after it was disconnected
The DELAY value specifies the number of milliseconds before retrying. "retry = yes" has the same
effect as "retry = 1000".
default: no
securityLevel = LEVEL
set the security level
The meaning of each level is described below:
level 0
Everything is permitted.
level 1
The security level corresponds to a minimum of 80 bits of security. Any parameters offering below
80 bits of security are excluded. As a result RSA, DSA and DH keys shorter than 1024 bits and ECC
keys shorter than 160 bits are prohibited. All export cipher suites are prohibited since they all
offer less than 80 bits of security. SSL version 2 is prohibited. Any cipher suite using MD5 for
the MAC is also prohibited.
level 2
Security level set to 112 bits of security. As a result RSA, DSA and DH keys shorter than 2048
bits and ECC keys shorter than 224 bits are prohibited. In addition to the level 1 exclusions any
cipher suite using RC4 is also prohibited. SSL version 3 is also not allowed. Compression is
disabled.
level 3
Security level set to 128 bits of security. As a result RSA, DSA and DH keys shorter than 3072
bits and ECC keys shorter than 256 bits are prohibited. In addition to the level 2 exclusions
cipher suites not offering forward secrecy are prohibited. TLS versions below 1.1 are not
permitted. Session tickets are disabled.
level 4
Security level set to 192 bits of security. As a result RSA, DSA and DH keys shorter than 7680
bits and ECC keys shorter than 384 bits are prohibited. Cipher suites using SHA1 for the MAC are
prohibited. TLS versions below 1.2 are not permitted.
level 5
Security level set to 256 bits of security. As a result RSA, DSA and DH keys shorter than 15360
bits and ECC keys shorter than 512 bits are prohibited.
default: 2
The securityLevel option is only available when compiled with OpenSSL 1.1.0 and later.
requireCert = yes | no
require a client certificate for verifyChain or verifyPeer
With requireCert set to no, the stunnel server accepts client connections that did not present a
certificate.
Both verifyChain = yes and verifyPeer = yes imply requireCert = yes.
default: no
setgid = GROUP (Unix only)
Unix group id
As a global option: setgid() to the specified group in daemon mode and clear all other groups.
As a service-level option: set the group of the Unix socket specified with "accept".
setuid = USER (Unix only)
Unix user id
As a global option: setuid() to the specified user in daemon mode.
As a service-level option: set the owner of the Unix socket specified with "accept".
sessionCacheSize = NUM_ENTRIES
session cache size
sessionCacheSize specifies the maximum number of the internal session cache entries.
The value of 0 can be used for unlimited size. It is not recommended for production use due to the
risk of a memory exhaustion DoS attack.
sessionCacheTimeout = TIMEOUT
session cache timeout
This is the number of seconds to keep cached TLS sessions.
sessionResume = yes | no
allow or disallow session resumption
default: yes
sessiond = HOST:PORT
address of sessiond TLS cache server
sni = SERVICE_NAME:SERVER_NAME_PATTERN (server mode)
Use the service as a secondary service (a name-based virtual server) for Server Name Indication TLS
extension (RFC 3546).
SERVICE_NAME specifies the primary service that accepts client connections with the accept option.
SERVER_NAME_PATTERN specifies the host name to be redirected. The pattern may start with the '*'
character, e.g. '*.example.com'. Multiple secondary services are normally specified for a single
primary service. The sni option can also be specified more than once within a single secondary
service.
This service, as well as the primary service, may not be configured in client mode.
The connect option of the secondary service is ignored when the protocol option is specified, as
protocol connects to the remote host before TLS handshake.
Libwrap checks (Unix only) are performed twice: with the primary service name after TCP connection is
accepted, and with the secondary service name during the TLS handshake.
The sni option is only available when compiled with OpenSSL 1.0.0 and later.
sni = SERVER_NAME (client mode)
Use the parameter as the value of TLS Server Name Indication (RFC 3546) extension.
Empty SERVER_NAME disables sending the SNI extension.
The sni option is only available when compiled with OpenSSL 1.0.0 and later.
socket = a|l|r:OPTION=VALUE[:VALUE]
Set an option on the accept/local/remote socket
The values for the linger option are l_onof:l_linger. The values for the time are tv_sec:tv_usec.
Examples:
socket = l:SO_LINGER=1:60
set one minute timeout for closing local socket
socket = r:SO_OOBINLINE=yes
place out-of-band data directly into the
receive data stream for remote sockets
socket = a:SO_REUSEADDR=no
disable address reuse (enabled by default)
socket = a:SO_BINDTODEVICE=lo
only accept connections on loopback interface
sslVersion = SSL_VERSION
select the TLS protocol version
Supported versions: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
Availability of specific protocols depends on the linked OpenSSL library. Older versions of OpenSSL
do not support TLSv1.1, TLSv1.2 and TLSv1.3. Newer versions of OpenSSL do not support SSLv2.
Obsolete SSLv2 and SSLv3 are currently disabled by default.
Setting the option
sslVersion = SSL_VERSION
is equivalent to options
sslVersionMax = SSL_VERSION
sslVersionMin = SSL_VERSION
when compiled with OpenSSL 1.1.0 and later.
sslVersionMax = SSL_VERSION
maximum supported protocol versions
Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
all enable protocol versions up to the highest version supported by the linked OpenSSL library.
Availability of specific protocols depends on the linked OpenSSL library.
The sslVersionMax option is only available when compiled with OpenSSL 1.1.0 and later.
default: all
sslVersionMin = SSL_VERSION
minimum supported protocol versions
Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
all enable protocol versions down to the lowest version supported by the linked OpenSSL library.
Availability of specific protocols depends on the linked OpenSSL library.
The sslVersionMin option is only available when compiled with OpenSSL 1.1.0 and later.
default: TLSv1
stack = BYTES (except for FORK model)
CPU stack size of created threads
Excessive thread stack size increases virtual memory usage. Insufficient thread stack size may cause
application crashes.
default: 65536 bytes (sufficient for all platforms we tested)
ticketKeySecret = SECRET
hexadecimal symmetric key used for session ticket confidentiality protection
Session tickets defined in RFC 5077 provide an enhanced session resumption capability, where the
server-side caching is not required to maintain per session state.
Combining ticketKeySecret and ticketMacSecret options allow to resume a negotiated session on other
cluster nodes, or to resume a negotiated session after server restart.
The key is required to be either 16 or 32 bytes long, which implies exactly 32 or 64 hexadecimal
digits. Colons may optionally be used between two-character hexadecimal bytes.
This option only works in server mode.
The ticketKeySecret option is only available when compiled with OpenSSL 1.0.0 and later.
Disabling NO_TICKET option is required for the ticket support in OpenSSL older than 1.1.1, but note
that this option is incompatible with the redirect option.
ticketMacSecret = SECRET
hexadecimal symmetric key used for session ticket integrity protection
The key is required to be either 16 or 32 bytes long, which implies exactly 32 or 64 hexadecimal
digits. Colons may optionally be used between two-character hexadecimal bytes.
This option only works in server mode.
The ticketMacSecret option is only available when compiled with OpenSSL 1.0.0 and later.
TIMEOUTbusy = SECONDS
time to wait for expected data
TIMEOUTclose = SECONDS
time to wait for close_notify (set to 0 for buggy MSIE)
TIMEOUTconnect = SECONDS
time to wait to connect a remote host
TIMEOUTidle = SECONDS
time to keep an idle connection
TIMEOUTocsp = SECONDS
time to wait to connect an OCSP responder
transparent = none | source | destination | both (Unix only)
enable transparent proxy support on selected platforms
Supported values:
none
Disable transparent proxy support. This is the default.
source
Re-write the address to appear as if a wrapped daemon is connecting from the TLS client machine
instead of the machine running stunnel.
This option is currently available in:
Remote mode (connect option) on Linux >=2.6.28
This configuration requires stunnel to be executed as root and without the setuid option.
This configuration requires the following setup for iptables and routing (possibly in
/etc/rc.local or equivalent file):
iptables -t mangle -N DIVERT
iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
iptables -t mangle -A DIVERT -j MARK --set-mark 1
iptables -t mangle -A DIVERT -j ACCEPT
ip rule add fwmark 1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100
echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter
stunnel must also to be executed as root and without the setuid option.
Remote mode (connect option) on Linux 2.2.x
This configuration requires the kernel to be compiled with the transparent proxy option.
Connected service must be installed on a separate host. Routing towards the clients has to
go through the stunnel box.
stunnel must also to be executed as root and without the setuid option.
Remote mode (connect option) on FreeBSD >=8.0
This configuration requires additional firewall and routing setup. stunnel must also to be
executed as root and without the setuid option.
Local mode (exec option)
This configuration works by pre-loading the libstunnel.so shared library. _RLD_LIST
environment variable is used on Tru64, and LD_PRELOAD variable on other platforms.
destination
The original destination is used instead of the connect option.
A service section for transparent destination may look like this:
[transparent]
client = yes
accept = <stunnel_port>
transparent = destination
This configuration requires iptables setup to work, possibly in /etc/rc.local or equivalent file.
For a connect target installed on the same host:
/sbin/iptables -t nat -I OUTPUT -p tcp --dport <redirected_port> \
-m ! --uid-owner <stunnel_user_id> \
-j DNAT --to-destination <local_ip>:<stunnel_port>
For a connect target installed on a remote host:
/sbin/iptables -I INPUT -i eth0 -p tcp --dport <stunnel_port> -j ACCEPT
/sbin/iptables -t nat -I PREROUTING -p tcp --dport <redirected_port> \
-i eth0 -j DNAT --to-destination <local_ip>:<stunnel_port>
The transparent destination option is currently only supported on Linux.
both
Use both source and destination transparent proxy.
Two legacy options are also supported for backward compatibility:
yes This option has been renamed to source.
no This option has been renamed to none.
verify = LEVEL
verify the peer certificate
This option is obsolete and should be replaced with the verifyChain and verifyPeer options.
level 0
Request and ignore the peer certificate chain.
level 1
Verify the peer certificate chain if present.
level 2
Verify the peer certificate chain.
level 3
Verify the peer certificate chain and the end-entity (leaf) peer certificate against a locally
installed certificate.
level 4
Ignore the peer certificate chain and only verify the end-entity (leaf) peer certificate against
a locally installed certificate.
default
No verify.
verifyChain = yes | no
verify the peer certificate chain starting from the root CA
For server certificate verification it is essential to also require a specific certificate with
checkHost or checkIP.
The self-signed root CA certificate needs to be stored either in the file specified with CAfile, or
in the directory specified with CApath.
default: no
verifyPeer = yes | no
verify the end-entity (leaf) peer certificate
The end-entity (leaf) peer certificate needs to be stored either in the file specified with CAfile,
or in the directory specified with CApath.
default: no
RETURN VALUE
stunnel returns zero on success, non-zero on error.
SIGNALS
The following signals can be used to control stunnel in Unix environment:
SIGHUP
Force a reload of the configuration file.
Some global options will not be reloaded:
• 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
NOTES
RESTRICTIONS
stunnel cannot be used for the FTP daemon because of the nature of the FTP protocol which utilizes
multiple ports for data transfers. There are available TLS-enabled versions of FTP and telnet daemons,
however.
INETD MODE
The most common use of stunnel is to listen on a network port and establish communication with either a
new port via the connect option, or a new program via the exec option. However there is a special case
when you wish to have some other program accept incoming connections and launch stunnel, for example with
inetd, xinetd, or tcpserver.
For example, if you have the following line in inetd.conf:
imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf
In these cases, the inetd-style program is responsible for binding a network socket (imaps above) and
handing it to stunnel when a connection is received. Thus you do not want stunnel to have any accept
option. All the Service Level Options should be placed in the global options section, and no
[service_name] section will be present. See the EXAMPLES section for example configurations.
CERTIFICATES
Each TLS-enabled daemon needs to present a valid X.509 certificate to the peer. It also needs a private
key to decrypt the incoming data. The easiest way to obtain a certificate and a key is to generate them
with the free OpenSSL package. You can find more information on certificates generation on pages listed
below.
The .pem file should contain the unencrypted private key and a signed certificate (not certificate
request). So the file should look like this:
-----BEGIN RSA PRIVATE KEY-----
[encoded key]
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
[encoded certificate]
-----END CERTIFICATE-----
RANDOMNESS
stunnel needs to seed the PRNG (pseudo-random number generator) in order for TLS to use good randomness.
The following sources are loaded in order until sufficient random data has been gathered:
• 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
Michał Trojnara
<Michal.Trojnara@stunnel.org>
5.72 2024.04.01 stunnel(8)