Packages and Binaries:
stunnel
Universal SSL tunnel for network daemons
The stunnel program is designed to work as SSL encryption
wrapper between remote client and local (inetd-startable) or
remote server. The concept is that having non-SSL aware daemons
running on your system you can easily setup them to
communicate with clients over secure SSL channel.
stunnel can be used to add SSL functionality to commonly used inetd daemons like POP-2, POP-3 and IMAP servers without any changes in the programs’ code.
This package contains a wrapper script for compatibility with stunnel 3.x
Installed size: 575 KB
How to install: sudo apt install stunnel
Dependencies:
- libc6
- libssl3t64
- libsystemd0
- libwrap0
- netbase
- openssl
- perl
- systemd | systemd-standalone-sysusers | systemd-sysusers
stunnel
TLS offloading and load-balancing proxy
root@kali:~# man stunnel
STUNNEL(8) stunnel TLS Proxy STUNNEL(8)
NAME
stunnel - TLS offloading and load-balancing proxy
SYNOPSIS
* Unix: stunnel [FILE] | -fd N | -help | -version | -sockets | -options
* WIN32: stunnel [ [ -install | -uninstall | -start | -stop | -reload |
-reopen | -exit ] [-quiet] [FILE] ] | -help | -version | -sockets | -op-
tions
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 dae-
mons 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
([email protected])
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/re-
solv.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 | zstd | brotli
select data compression algorithm
default: no compression
Data compression is available only for TLS 1.2 and earlier. Requires
lowering securityLevel to 1 when compiled with OpenSSL 1.1.0 and later.
The zlib, zstd, and brotli algorithms are disabled in the default
OpenSSL configuration.
The zstd and brotli algorithms were added in OpenSSL 3.2.
Data compression poses a risk in applications that allow an attacker to
inject chosen plaintext.
Deflate is the standard compression method as described in RFC 1951.
Warning: TLS compression can enable plaintext-recovery attacks, such as
CRIME, when attacker-controlled data is compressed together with secret
material.
* 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 under-
stood 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 sup-
plied. (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 com-
piled 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 desti-
nations 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 out-
put (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 identi-
fies 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 ran-
dom 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 ser-
vice name for TCP Wrappers. While this option can technically be speci-
fied 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 veri-
fyPeer 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 veri-
fyPeer 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 veri-
fyPeer 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 sup-
ported 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 cer-
tificate. The file must be either in PEM or P12 format.
A certificate chain is required in server mode, and optional in client
mode.
The cert option may be specified multiple times. The first certificate
encountered is used as the server or client certificate, while any sub-
sequent certificates are treated as intermediate chain elements. This
functionality requires OpenSSL version 1.0.2 or later.
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 sub-
ject
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 prefer-
ence.
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 parame-
ter. 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 com-
mands.
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 al-
gorithm.
* CRLpath = DIRECTORY
Certificate Revocation Lists directory
This is the directory in which stunnel will look for CRLs when using the
verifyChain and verifyPeer options, and with CRLcheckChain for
full-chain CRL checking. 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 veri-
fyPeer options, and with CRLcheckChain for full-chain CRL checking.
* CRLcheckChain = yes | no
check CRLs for the full peer certificate chain
This option requires verifyChain = yes and either CRLfile or CRLpath.
By default, CRL checking is only performed for the end-entity (leaf)
peer certificate. With CRLcheckChain = yes, CRL checking is addition-
ally performed for every certificate in the verified issuer chain.
Full-chain mode fails closed if an applicable issuer CRL is missing, ex-
pired, or invalid.
For a chain such as Root CA -> Intermediate CA -> Leaf, an intermedi-
ate-issued CRL is needed for the leaf certificate and a root-issued CRL
is needed for the intermediate certificate.
default: no
* curves = list
ECDH curves separated with `:'
Note: This option is supported for server mode sockets only.
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 in-
stance 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 debug-
ging software or configuration issues.
* process
The operating system process identifier (PID) may be useful in the in-
etd 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 dur-
ing stunnel startup (road warrior VPN, dial-up configurations).
Delayed resolver mode is automatically engaged when stunnel fails to re-
solve 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: RE-
MOTE_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 ar-
bitrary 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 verifi-
cation
* 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 Ac-
cess) 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, NO-
TIME
* 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 sup-
ported 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 combina-
tion 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 implementa-
tion, 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 (undocumented) extension of CIFS protocol implemented in
Samba. Support for this extension was dropped in Samba 3.0.0.
* capwin
https://www.capwin.org/ application support
* capwinctrl
https://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): Exten-
sion 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
https://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 des-
tination address.
https://www.openssh.com/txt/socks4.protocol
https://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 charac-
ters 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 sce-
narios, or re-keying long lasting connections.
On the other hand this feature can facilitate a trivial CPU-exhaustion
DoS attack:
https://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 dis-
abled 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 dis-
abled. 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 connec-
tions 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 connec-
tions 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 speci-
fied 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 li-
brary. 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 li-
brary.
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 li-
brary.
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. Insuffi-
cient 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 pro-
tection
Session tickets defined in RFC 5077 provide an enhanced session resump-
tion capability, where the server-side caching is not required to main-
tain 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 ex-
actly 32 or 64 hexadecimal digits. Colons may optionally be used be-
tween 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 ex-
actly 32 or 64 hexadecimal digits. Colons may optionally be used be-
tween 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
* transport = tcp | udp
select transport protocol
Supported values:
* tcp
Use standard TLS over TCP (default).
* udp
Use DTLS over UDP.
Each datagram opens a new logical connection within the service. The
server uses a different connected socket per client, and DTLS cookies
protect against DDoS reflection attacks.
default: tcp
* 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 with-
out 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 op-
tion.
* 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 op-
tion.
* 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 op-
tion.
* Local mode (exec option)
This configuration works by pre-loading the libstunnel.so shared li-
brary. _RLD_LIST environment variable is used on Tru64, and LD_PRE-
LOAD 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 cer-
tificate 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 lo-
calhost 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] sec-
tion.
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` di-
rectory, 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 estab-
lish 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 @bindir@/stunnel stunnel @sysconfdir@/stunnel/imaps.conf
In these cases, the inetd-style program is responsible for binding a net-
work 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 eas-
iest way to obtain a certificate and a key is to generate them with the
free OpenSSL package. You can find more information on certificates gen-
eration on pages listed below.
The .pem file should contain the unencrypted private key and a signed cer-
tificate (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 stun-
nel 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. Start-
ing 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 cer-
tificate 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
* https://www.stunnel.org/
stunnel Application
* https://openssl-library.org/
OpenSSL Library
AUTHOR
* Micha/ Trojnara
[email protected]
stunnel 5.80 2026.08.04 STUNNEL(8)
stunnel3
Universal SSL tunnel
root@kali:~# stunnel3 --help
/usr/bin/stunnel3 version [unknown] calling Getopt::Std::getopts (version 1.14 [paranoid]),
running under Perl version 5.42.2.
Usage: stunnel3 [-OPTIONS [-MORE_OPTIONS]] [--] [PROGRAM_ARG1 ...]
The following single-character options are accepted:
With arguments: -D -O -o -C -p -v -a -A -t -N -u -n -E -R -B -I -d -s -g -P -r -L -l
Boolean (without arguments): -c -T -W -f
Options may be merged together. -- stops processing of options.
Space is not required between options and their arguments.
[Now continuing due to backward compatibility and excessive paranoia.
See 'perldoc Getopt::Std' about $Getopt::Std::STANDARD_HELP_VERSION.]
[.] stunnel 5.80 on x86_64-pc-linux-gnu platform
[.] Compiled/running with OpenSSL 3.6.3 9 Jun 2026
[.] Threading:PTHREAD Sockets:POLL,IPv6,SYSTEMD TLS:ENGINE,OCSP,PSK,SNI,DTLS Auth:LIBWRAP
[.] Reading configuration from descriptor 3
[.] FIPS provider disabled
[!] Inetd mode: TLS server needs a certificate
[!] Configuration failed
stunnel4
Universal SSL tunnnel for network daemons - compatibility package
The stunnel program is designed to work as SSL encryption
wrapper between remote client and local (inetd-startable) or
remote server. The concept is that having non-SSL aware daemons
running on your system you can easily setup them to
communicate with clients over secure SSL channel.
stunnel can be used to add SSL functionality to commonly used inetd daemons like POP-2, POP-3 and IMAP servers without any changes in the programs’ code.
This package provides the /usr/bin/stunnel4 symlink for compatibility with other packages. As soon as all the reverse dependencies within Debian are adapted to use /usr/bin/stunnel instead, it will become a purely transitional package.
Installed size: 24 KB
How to install: sudo apt install stunnel4
Dependencies:
- stunnel
- systemd | systemd-standalone-sysusers | systemd-sysusers
stunnel4
TLS offloading and load-balancing proxy
root@kali:~# man stunnel4
STUNNEL(8) stunnel TLS Proxy STUNNEL(8)
NAME
stunnel - TLS offloading and load-balancing proxy
SYNOPSIS
* Unix: stunnel [FILE] | -fd N | -help | -version | -sockets | -options
* WIN32: stunnel [ [ -install | -uninstall | -start | -stop | -reload |
-reopen | -exit ] [-quiet] [FILE] ] | -help | -version | -sockets | -op-
tions
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 dae-
mons 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
([email protected])
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/re-
solv.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 | zstd | brotli
select data compression algorithm
default: no compression
Data compression is available only for TLS 1.2 and earlier. Requires
lowering securityLevel to 1 when compiled with OpenSSL 1.1.0 and later.
The zlib, zstd, and brotli algorithms are disabled in the default
OpenSSL configuration.
The zstd and brotli algorithms were added in OpenSSL 3.2.
Data compression poses a risk in applications that allow an attacker to
inject chosen plaintext.
Deflate is the standard compression method as described in RFC 1951.
Warning: TLS compression can enable plaintext-recovery attacks, such as
CRIME, when attacker-controlled data is compressed together with secret
material.
* 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 under-
stood 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 sup-
plied. (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 com-
piled 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 desti-
nations 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 out-
put (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 identi-
fies 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 ran-
dom 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 ser-
vice name for TCP Wrappers. While this option can technically be speci-
fied 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 veri-
fyPeer 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 veri-
fyPeer 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 veri-
fyPeer 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 sup-
ported 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 cer-
tificate. The file must be either in PEM or P12 format.
A certificate chain is required in server mode, and optional in client
mode.
The cert option may be specified multiple times. The first certificate
encountered is used as the server or client certificate, while any sub-
sequent certificates are treated as intermediate chain elements. This
functionality requires OpenSSL version 1.0.2 or later.
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 sub-
ject
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 prefer-
ence.
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 parame-
ter. 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 com-
mands.
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 al-
gorithm.
* CRLpath = DIRECTORY
Certificate Revocation Lists directory
This is the directory in which stunnel will look for CRLs when using the
verifyChain and verifyPeer options, and with CRLcheckChain for
full-chain CRL checking. 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 veri-
fyPeer options, and with CRLcheckChain for full-chain CRL checking.
* CRLcheckChain = yes | no
check CRLs for the full peer certificate chain
This option requires verifyChain = yes and either CRLfile or CRLpath.
By default, CRL checking is only performed for the end-entity (leaf)
peer certificate. With CRLcheckChain = yes, CRL checking is addition-
ally performed for every certificate in the verified issuer chain.
Full-chain mode fails closed if an applicable issuer CRL is missing, ex-
pired, or invalid.
For a chain such as Root CA -> Intermediate CA -> Leaf, an intermedi-
ate-issued CRL is needed for the leaf certificate and a root-issued CRL
is needed for the intermediate certificate.
default: no
* curves = list
ECDH curves separated with `:'
Note: This option is supported for server mode sockets only.
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 in-
stance 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 debug-
ging software or configuration issues.
* process
The operating system process identifier (PID) may be useful in the in-
etd 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 dur-
ing stunnel startup (road warrior VPN, dial-up configurations).
Delayed resolver mode is automatically engaged when stunnel fails to re-
solve 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: RE-
MOTE_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 ar-
bitrary 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 verifi-
cation
* 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 Ac-
cess) 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, NO-
TIME
* 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 sup-
ported 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 combina-
tion 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 implementa-
tion, 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 (undocumented) extension of CIFS protocol implemented in
Samba. Support for this extension was dropped in Samba 3.0.0.
* capwin
https://www.capwin.org/ application support
* capwinctrl
https://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): Exten-
sion 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
https://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 des-
tination address.
https://www.openssh.com/txt/socks4.protocol
https://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 charac-
ters 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 sce-
narios, or re-keying long lasting connections.
On the other hand this feature can facilitate a trivial CPU-exhaustion
DoS attack:
https://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 dis-
abled 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 dis-
abled. 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 connec-
tions 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 connec-
tions 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 speci-
fied 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 li-
brary. 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 li-
brary.
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 li-
brary.
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. Insuffi-
cient 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 pro-
tection
Session tickets defined in RFC 5077 provide an enhanced session resump-
tion capability, where the server-side caching is not required to main-
tain 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 ex-
actly 32 or 64 hexadecimal digits. Colons may optionally be used be-
tween 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 ex-
actly 32 or 64 hexadecimal digits. Colons may optionally be used be-
tween 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
* transport = tcp | udp
select transport protocol
Supported values:
* tcp
Use standard TLS over TCP (default).
* udp
Use DTLS over UDP.
Each datagram opens a new logical connection within the service. The
server uses a different connected socket per client, and DTLS cookies
protect against DDoS reflection attacks.
default: tcp
* 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 with-
out 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 op-
tion.
* 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 op-
tion.
* 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 op-
tion.
* Local mode (exec option)
This configuration works by pre-loading the libstunnel.so shared li-
brary. _RLD_LIST environment variable is used on Tru64, and LD_PRE-
LOAD 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 cer-
tificate 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 lo-
calhost 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] sec-
tion.
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` di-
rectory, 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 estab-
lish 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 @bindir@/stunnel stunnel @sysconfdir@/stunnel/imaps.conf
In these cases, the inetd-style program is responsible for binding a net-
work 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 eas-
iest way to obtain a certificate and a key is to generate them with the
free OpenSSL package. You can find more information on certificates gen-
eration on pages listed below.
The .pem file should contain the unencrypted private key and a signed cer-
tificate (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 stun-
nel 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. Start-
ing 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 cer-
tificate 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
* https://www.stunnel.org/
stunnel Application
* https://openssl-library.org/
OpenSSL Library
AUTHOR
* Micha/ Trojnara
[email protected]
stunnel 5.80 2026.08.04 STUNNEL(8)
Updated on: 2026-Aug-25