stunnel - TLS offloading and load-balancing proxy
- Unix:
-
stunnel [FILE] | -fd N | -help | -version |
-sockets | -options
- WIN32:
-
stunnel [ [ -install | -uninstall | -start | -stop |
-reload | -reopen | -exit ] [-quiet] [FILE] ] |
-help | -version | -sockets | -options
The
stunnel program is designed to work as
TLS encryption wrapper
between remote clients and local (
inetd-startable) or remote servers.
The concept is that having non-TLS aware daemons running on your system you
can easily set them up to communicate with clients over secure
TLS
channels.
stunnel can be used to add
TLS functionality to commonly used
Inetd daemons like POP-2, POP-3, and IMAP servers, to standalone
daemons like NNTP, SMTP and HTTP, and in tunneling PPP over network sockets
without changes to the source code.
This product includes cryptographic software written by Eric Young
(
[email protected])
- 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
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).
-
chroot = DIRECTORY (Unix only)
- directory to chroot stunnel process
chroot keeps stunnel in a chrooted jail. CApath,
CRLpath, pid and exec are located inside the jail and
the patches have to be relative to the directory specified with
chroot.
Several functions of the operating system also need their files to be
located within the chroot jail, e.g.:
- •
- Delayed resolver typically needs /etc/nsswitch.conf and
/etc/resolv.conf.
- •
- Local time in log files needs /etc/timezone.
- •
- Some other functions may need devices, e.g. /dev/zero or
/dev/null.
-
compression = deflate | zlib
- select data compression algorithm
default: no compression
Deflate is the standard compression method as described in RFC 1951.
-
debug = [FACILITY.]LEVEL
- debugging level
Level is one of the syslog level names or numbers emerg (0), alert (1), crit
(2), err (3), warning (4), notice (5), info (6), or debug (7). All logs
for the specified level and all levels numerically less than it will be
shown.
The debug = debug (or the equivalent <debug = 7>) level
produces for the most verbose log output. This logging level is only meant
to be understood by stunnel developers, and not by users. Please either
use the debug level when requested to do so by an stunnel developer, or
when you intend to get confused.
The default logging level is notice (5).
The syslog 'daemon' facility will be used unless a facility name is
supplied. (Facilities are not supported on Win32.)
Case is ignored for both facilities and levels.
-
EGD = EGD_PATH (Unix only)
- path to Entropy Gathering Daemon socket
Entropy Gathering Daemon socket to use to feed the OpenSSL random
number generator.
-
engine = auto | ENGINE_ID
- select hardware or software cryptographic engine
default: software-only cryptography
See Examples section for an engine configuration to use the certificate and
the corresponding private key from a cryptographic device.
-
engineCtrl = COMMAND[:PARAMETER]
- control hardware engine
-
engineDefault = TASK_LIST
- set OpenSSL tasks delegated to the current engine
The parameter specifies a comma-separated list of task to be delegated to
the current engine.
The following tasks may be available, if supported by the engine: ALL, RSA,
DSA, ECDH, ECDSA, DH, RAND, CIPHERS, DIGESTS, PKEY, PKEY_CRYPTO,
PKEY_ASN1.
-
fips = yes | no
- enable or disable FIPS 140-2 mode.
This option allows you to disable entering FIPS mode if stunnel was
compiled with FIPS 140-2 support.
default: no (since version 5.00)
-
foreground = yes | quiet | no (Unix only)
- foreground mode
Stay in foreground (don't fork).
With the yes parameter it also logs to stderr in addition to the
destinations specified with syslog and output.
default: background in daemon mode
-
iconActive = ICON_FILE (GUI only)
- GUI icon to be displayed when there are established
connections
On Windows platform the parameter should be an .ico file containing a 16x16
pixel image.
-
iconError = ICON_FILE (GUI only)
- GUI icon to be displayed when no valid configuration is
loaded
On Windows platform the parameter should be an .ico file containing a 16x16
pixel image.
-
iconIdle = ICON_FILE (GUI only)
- GUI icon to be displayed when there are no established
connections
On Windows platform the parameter should be an .ico file containing a 16x16
pixel image.
-
log = append | overwrite
- log file handling
This option allows you to choose whether the log file (specified with the
output option) is appended or overwritten when opened or re-opened.
default: append
-
output = FILE
- append log messages to a file
/dev/stdout device can be used to send log messages to the standard output
(for example to log them with daemontools splogger).
-
pid = FILE (Unix only)
- pid file location
If the argument is empty, then no pid file will be created.
pid path is relative to the chroot directory if
specified.
-
RNDbytes = BYTES
- bytes to read from random seed files
-
RNDfile = FILE
- path to file with random seed data
The OpenSSL library will use data from this file first to seed the random
number generator.
-
RNDoverwrite = yes | no
- overwrite the random seed files with new random data
default: yes
-
service = SERVICE (Unix only)
- stunnel service name
The specified service name is used for syslog and as the inetd mode
service name for TCP Wrappers. While this option can technically be
specified in the service sections, it is only useful in global options.
default: stunnel
-
syslog = yes | no (Unix only)
- enable logging via syslog
default: yes
-
taskbar = yes | no (WIN32 only)
- enable the taskbar icon
default: yes
Each configuration section begins with a service name in square brackets. The
service name is used for libwrap (TCP Wrappers) access control and lets you
distinguish
stunnel services in your log files.
Note that if you wish to run
stunnel in
inetd mode (where it is
provided a network socket by a server such as
inetd,
xinetd, or
tcpserver) then you should read the section entitled
INETD MODE
below.
-
accept = [HOST:]PORT
- accept connections on specified address
If no host specified, defaults to all IPv4 addresses for the local host.
To listen on all IPv6 addresses use:
accept = :::PORT
-
CAengine =
ENGINE-SPECIFIC_CA_CERTIFICATE_IDENTIFIER
- load a trusted CA certificate from an engine
The loaded CA certificates will be used with the verifyChain and
verifyPeer options.
Multiple CAengine options are allowed in a single service section.
Currently supported engines: pkcs11, cng.
-
CApath = CA_DIRECTORY
- load trusted CA certificates from a directory
The loaded CA certificates will be used with the verifyChain and
verifyPeer options. Note that the certificates in this directory
should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the DER
encoded subject of the cert.
The hash algorithm has been changed in OpenSSL 1.0.0. It is required
to c_rehash the directory on upgrade from OpenSSL 0.x.x to
OpenSSL 1.x.x or later.
CApath path is relative to the chroot directory if
specified.
-
CAfile = CA_FILE
- load trusted CA certificates from a file
The loaded CA certificates will be used with the verifyChain and
verifyPeer options.
-
cert = CERT_FILE
- certificate chain file name
The parameter specifies the file containing certificates used by
stunnel to authenticate itself against the remote client or server.
The file should contain the whole certificate chain starting from the
actual server/client certificate, and ending with the self-signed root CA
certificate. The file must be either in PEM or P12 format.
A certificate chain is required in server mode, and optional in client mode.
This parameter is also used as the certificate identifier when a hardware
engine is enabled.
-
checkEmail = EMAIL
- email address of the peer certificate subject
Certificates are accepted if no subject checks were specified, or the email
address of the 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
- host of the peer certificate subject
Certificates are accepted if no subject checks were specified, or the host
name of the 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
- IP address of the peer certificate subject
Certificates are accepted if no subject checks were specified, or the IP
address of the peer certificate matches any of the IP addresses specified
with checkIP.
Multiple checkIP options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
-
ciphers = CIPHER_LIST
- select permitted TLS ciphers (TLSv1.2 and below)
This option does not impact TLSv1.3 ciphersuites.
A colon-delimited list of the ciphers to allow in the TLS connection, for
example DES-CBC3-SHA:IDEA-CBC-MD5.
-
ciphersuites = CIPHERSUITES_LIST
- select permitted TLSv1.3 ciphersuites
A colon-delimited list of TLSv1.3 ciphersuites names in order of preference.
This option requires OpenSSL 1.1.1 or later.
default:
TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256
-
client = yes | no
- client mode (remote service uses TLS)
default: no (server mode)
-
config = COMMAND[:PARAMETER]
-
OpenSSL configuration command
The OpenSSL configuration command is executed with the specified
parameter. This allows any configuration commands to be invoked from the
stunnel configuration file. Supported commands are described on the
SSL_CONF_cmd(3ssl) manual page.
Several config lines can be used to specify multiple configuration
commands.
Use curves option instead of enabling config =
Curves:list_curves to support elliptic curves.
This option requires OpenSSL 1.0.2 or later.
-
connect = [HOST:]PORT
- connect to a remote address
If no host is specified, the host defaults to localhost.
Multiple connect options are allowed in a single service section. If
host resolves to multiple addresses and/or if multiple connect
options are specified, then the remote address is chosen using a
round-robin algorithm.
-
CRLpath = DIRECTORY
- Certificate Revocation Lists directory
This is the directory in which stunnel will look for CRLs when using
the verifyChain and verifyPeer options. Note that the CRLs
in this directory should be named XXXXXXXX.r0 where XXXXXXXX is the hash
value of the CRL.
The hash algorithm has been changed in OpenSSL 1.0.0. It is required
to c_rehash the directory on upgrade from OpenSSL 0.x.x to
OpenSSL 1.x.x.
CRLpath path is relative to the chroot directory if
specified.
-
CRLfile = CRL_FILE
- Certificate Revocation Lists file
This file contains multiple CRLs, used with the verifyChain and
verifyPeer options.
-
curves = list
- ECDH curves separated with ':'
Only a single curve name is allowed for OpenSSL older than 1.1.1.
To get a list of supported curves use:
openssl ecparam -list_curves
default:
X25519:P-256:X448:P-521:P-384 (OpenSSL 1.1.1 or later)
prime256v1 (OpenSSL older than 1.1.1)
-
logId = TYPE
- connection identifier type
This identifier allows you to distinguish log entries generated for each of
the connections.
Currently supported types:
- sequential
- The numeric sequential identifier is only unique within a
single instance of stunnel, but very compact. It is most useful for
manual log analysis.
- unique
- This alphanumeric identifier is globally unique, but longer
than the sequential number. It is most useful for automated log
analysis.
- thread
- The operating system thread identifier is neither unique
(even within a single instance of stunnel) nor short. It is most
useful for debugging software or configuration issues.
- process
- The operating system process identifier (PID) may be useful
in the inetd mode.
-
debug = LEVEL
- debugging level
Level is a one of the syslog level names or numbers emerg (0), alert (1),
crit (2), err (3), warning (4), notice (5), info (6), or debug (7). All
logs for the specified level and all levels numerically less than it will
be shown. The default is notice (5).
While the debug = debug or debug = 7 level generates the most
verbose output, it is only intended to be used by stunnel developers.
Please only use this value if you are a developer, or you intend to send
your logs to our technical support. Otherwise, the generated logs
will be confusing.
-
delay = yes | no
- delay DNS lookup for the connect option
This option is useful for dynamic DNS, or when DNS is not available during
stunnel startup (road warrior VPN, dial-up configurations).
Delayed resolver mode is automatically engaged when stunnel fails to resolve
on startup any of the connect targets for a service.
Delayed resolver inflicts failover = prio.
default: no
-
engineId = ENGINE_ID
- select engine ID for the service
-
engineNum = ENGINE_NUMBER
- select engine number for the service
The engines are numbered starting from 1.
-
exec = EXECUTABLE_PATH
- execute a local inetd-type program
exec path is relative to the chroot directory if specified.
The following environmental variables are set on Unix platforms:
REMOTE_HOST, REMOTE_PORT, SSL_CLIENT_DN, SSL_CLIENT_I_DN.
-
execArgs = $0 $1 $2 ...
- arguments for exec including the program name ($0)
Quoting is currently not supported. Arguments are separated with an
arbitrary amount of whitespace.
-
failover = rr | prio
- Failover strategy for multiple "connect"
targets.
- rr
- round robin - fair load distribution
- prio
- priority - use the order specified in config file
-
ident = USERNAME
- use IDENT (RFC 1413) username checking
-
include = DIRECTORY
- include all configuration file parts located in DIRECTORY
The files are included in the ascending alphabetical order of their names.
The recommended filename convention is
for global options:
00-global.conf
for local service-level options:
01-service.conf
02-service.conf
-
key = KEY_FILE
- private key for the certificate specified with cert
option
A private key is needed to authenticate the certificate owner. Since this
file should be kept secret it should only be readable by its owner. On
Unix systems you can use the following command:
chmod 600 keyfile
This parameter is also used as the private key identifier when a hardware
engine is enabled.
default: the value of the cert option
-
libwrap = yes | no
- Enable or disable the use of /etc/hosts.allow and
/etc/hosts.deny.
default: no (since version 5.00)
-
local = HOST
- By default, the IP address of the outgoing interface is
used as the source for remote connections. Use this option to bind a
static local IP address instead.
-
OCSP = URL
- select OCSP responder for certificate verification
-
OCSPaia = yes | no
- validate certificates with their AIA OCSP responders
This option enables stunnel to validate certificates with the list of
OCSP responder URLs retrieved from their AIA (Authority Information
Access) extension.
-
OCSPflag = OCSP_FLAG
- specify OCSP responder flag
Several OCSPflag can be used to specify multiple flags.
currently supported flags: NOCERTS, NOINTERN, NOSIGS, NOCHAIN, NOVERIFY,
NOEXPLICIT, NOCASIGN, NODELEGATED, NOCHECKS, TRUSTOTHER, RESPID_KEY,
NOTIME
-
OCSPnonce = yes | no
- send and verify the OCSP nonce extension
This option protects the OCSP protocol against replay attacks. Due to its
computational overhead, the nonce extension is usually only supported on
internal (e.g. corporate) responders, and not on public OCSP
responders.
-
options = SSL_OPTIONS
-
OpenSSL library options
The parameter is the OpenSSL option name as described in the
SSL_CTX_set_options(3ssl) manual, but without
SSL_OP_ prefix. stunnel -options lists the options found to
be allowed in the current combination of stunnel and the
OpenSSL library used to build it.
Several option lines can be used to specify multiple options. An
option name can be prepended with a dash ("-") to disable the
option.
For example, for compatibility with the erroneous Eudora TLS implementation,
the following option can be used:
options = DONT_INSERT_EMPTY_FRAGMENTS
default:
options = NO_SSLv2
options = NO_SSLv3
Use sslVersionMax or sslVersionMin option instead of disabling
specific TLS protocol versions when compiled with OpenSSL 1.1.0 or
later.
-
protocol = PROTO
- application protocol to negotiate TLS
This option enables initial, protocol-specific negotiation of the TLS
encryption. The protocol option should not be used with TLS
encryption on a separate port.
Currently supported protocols:
- cifs
- Proprietary (undocummented) extension of CIFS protocol
implemented in Samba. Support for this extension was dropped in Samba
3.0.0.
- capwin
- http://www.capwin.org/ application support
- capwinctrl
- http://www.capwin.org/ application support
This protocol is only supported in client mode.
- connect
- Based on RFC 2817 - Upgrading to TLS Within
HTTP/1.1, section 5.2 - Requesting a Tunnel with CONNECT
This protocol is only supported in client mode.
- imap
- Based on RFC 2595 - Using TLS with IMAP, POP3 and
ACAP
- ldap
- Based on RFC 2830 - Lightweight Directory Access
Protocol (v3): Extension for Transport Layer Security
- nntp
- Based on RFC 4642 - Using Transport Layer Security (TLS)
with Network News Transfer Protocol (NNTP)
This protocol is only supported in client mode.
- pgsql
- Based on
http://www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982
- pop3
- Based on RFC 2449 - POP3 Extension Mechanism
- proxy
- Passing of the original client IP address with HAProxy
PROXY protocol version 1
https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt
- smtp
- Based on RFC 2487 - SMTP Service Extension for Secure
SMTP over TLS
- socks
- SOCKS versions 4, 4a, and 5 are supported. The SOCKS
protocol itself is encapsulated within TLS encryption layer to protect the
final destination address.
http://www.openssh.com/txt/socks4.protocol
http://www.openssh.com/txt/socks4a.protocol
The BIND command of the SOCKS protocol is not supported. The USERID
parameter is ignored.
See Examples section for sample configuration files for VPN based on SOCKS
encryption.
-
protocolAuthentication = AUTHENTICATION
- authentication type for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' and
'smtp' protocols.
Supported authentication types for the 'connect' protocol are 'basic' or
'ntlm'. The default 'connect' authentication type is 'basic'.
Supported authentication types for the 'smtp' protocol are 'plain' or
'login'. The default 'smtp' authentication type is 'plain'.
-
protocolDomain = DOMAIN
- domain for the protocol negotiations
Currently, this option is only supported in the client-side 'connect'
protocol.
-
protocolHeader = HEADER
- header for the protocol negotiations
Currently, this option is only supported in the client-side 'connect'
protocol.
-
protocolHost = ADDRESS
- host address for the protocol negotiations
For the 'connect' protocol negotiations, protocolHost specifies
HOST:PORT of the final TLS server to be connected to by the proxy. The
proxy server directly connected by stunnel must be specified with
the connect option.
For the 'smtp' protocol negotiations, protocolHost controls the
client SMTP HELO/EHLO value.
-
protocolPassword = PASSWORD
- password for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' and
'smtp' protocols.
-
protocolUsername = USERNAME
- username for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' and
'smtp' protocols.
-
PSKidentity = IDENTITY
- PSK identity for the PSK client
PSKidentity can be used on stunnel clients to select the PSK
identity used for authentication. This option is ignored in server
sections.
default: the first identity specified in the PSKsecrets file.
-
PSKsecrets = FILE
- file with PSK identities and corresponding keys
Each line of the file in the following format:
IDENTITY:KEY
Hexadecimal keys are automatically converted to binary form. Keys are
required to be at least 16 bytes long, which implies at least 32
characters for hexadecimal keys. The file should neither be world-readable
nor world-writable.
-
pty = yes | no (Unix only)
- allocate a pseudoterminal for 'exec' option
-
redirect = [HOST:]PORT
- redirect TLS client connections on certificate-based
authentication failures
This option only works in server mode. Some protocol negotiations are also
incompatible with the redirect option.
-
renegotiation = yes | no
- support TLS renegotiation
Applications of the TLS renegotiation include some authentication scenarios,
or re-keying long lasting connections.
On the other hand this feature can facilitate a trivial CPU-exhaustion DoS
attack:
http://vincent.bernat.im/en/blog/2011-ssl-dos-mitigation.html
Please note that disabling TLS renegotiation does not fully mitigate this
issue.
default: yes (if supported by OpenSSL)
-
reset = yes | no
- attempt to use the TCP RST flag to indicate an error
This option is not supported on some platforms.
default: yes
-
retry = yes | no
- reconnect a connect+exec section after it was disconnected
default: no
-
securityLevel = LEVEL
- set the security level
The meaning of each level is described below:
- level 0
- Everything is permitted.
- level 1
- The security level corresponds to a minimum of 80 bits of
security. Any parameters offering below 80 bits of security are excluded.
As a result RSA, DSA and DH keys shorter than 1024 bits and ECC keys
shorter than 160 bits are prohibited. All export cipher suites are
prohibited since they all offer less than 80 bits of security. SSL version
2 is prohibited. Any cipher suite using MD5 for the MAC is also
prohibited.
- level 2
- Security level set to 112 bits of security. As a result
RSA, DSA and DH keys shorter than 2048 bits and ECC keys shorter than 224
bits are prohibited. In addition to the level 1 exclusions any cipher
suite using RC4 is also prohibited. SSL version 3 is also not allowed.
Compression is disabled.
- level 3
- Security level set to 128 bits of security. As a result
RSA, DSA and DH keys shorter than 3072 bits and ECC keys shorter than 256
bits are prohibited. In addition to the level 2 exclusions cipher suites
not offering forward secrecy are prohibited. TLS versions below 1.1 are
not permitted. Session tickets are disabled.
- level 4
- Security level set to 192 bits of security. As a result
RSA, DSA and DH keys shorter than 7680 bits and ECC keys shorter than 384
bits are prohibited. Cipher suites using SHA1 for the MAC are prohibited.
TLS versions below 1.2 are not permitted.
- level 5
- Security level set to 256 bits of security. As a result
RSA, DSA and DH keys shorter than 15360 bits and ECC keys shorter than 512
bits are prohibited.
- default: 2
The
securityLevel option is only available when compiled with
OpenSSL
1.1.0 and later.
-
requireCert = yes | no
- require a client certificate for verifyChain or
verifyPeer
With requireCert set to no, the stunnel server accepts
client connections that did not present a certificate.
Both verifyChain = yes and verifyPeer = yes imply
requireCert = yes.
default: no
-
setgid = GROUP (Unix only)
- Unix group id
As a global option: setgid() to the specified group in daemon mode
and clear all other groups.
As a service-level option: set the group of the Unix socket specified with
"accept".
-
setuid = USER (Unix only)
- Unix user id
As a global option: setuid() to the specified user in daemon mode.
As a service-level option: set the owner of the Unix socket specified with
"accept".
-
sessionCacheSize = NUM_ENTRIES
- session cache size
sessionCacheSize specifies the maximum number of the internal
session cache entries.
The value of 0 can be used for unlimited size. It is not recommended for
production use due to the risk of a memory exhaustion DoS attack.
-
sessionCacheTimeout = TIMEOUT
- session cache timeout
This is the number of seconds to keep cached TLS sessions.
-
sessionResume = yes | no
- allow or disallow session resumption
default: yes
-
sessiond = HOST:PORT
- address of sessiond TLS cache server
-
sni = SERVICE_NAME:SERVER_NAME_PATTERN (server
mode)
- Use the service as a secondary service (a name-based
virtual server) for Server Name Indication TLS extension (RFC 3546).
SERVICE_NAME specifies the primary service that accepts client
connections with the accept option. SERVER_NAME_PATTERN
specifies the host name to be redirected. The pattern may start with the
'*' character, e.g. '*.example.com'. Multiple secondary services are
normally specified for a single primary service. The sni option can
also be specified more than once within a single secondary service.
This service, as well as the primary service, may not be configured in
client mode.
The connect option of the secondary service is ignored when the
protocol option is specified, as protocol connects to the
remote host before TLS handshake.
Libwrap checks (Unix only) are performed twice: with the primary service
name after TCP connection is accepted, and with the secondary service name
during the TLS handshake.
The sni option is only available when compiled with OpenSSL
1.0.0 and later.
-
sni = SERVER_NAME (client mode)
- Use the parameter as the value of TLS Server Name
Indication (RFC 3546) extension.
Empty SERVER_NAME disables sending the SNI extension.
The sni option is only available when compiled with OpenSSL
1.0.0 and later.
-
socket = a|l|r:OPTION=VALUE[:VALUE]
- Set an option on the accept/local/remote socket
The values for the linger option are l_onof:l_linger. The values for the
time are tv_sec:tv_usec.
Examples:
socket = l:SO_LINGER=1:60
set one minute timeout for closing local socket
socket = r:SO_OOBINLINE=yes
place out-of-band data directly into the
receive data stream for remote sockets
socket = a:SO_REUSEADDR=no
disable address reuse (enabled by default)
socket = a:SO_BINDTODEVICE=lo
only accept connections on loopback interface
-
sslVersion = SSL_VERSION
- select the TLS protocol version
Supported versions: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
Availability of specific protocols depends on the linked OpenSSL library.
Older versions of OpenSSL do not support TLSv1.1, TLSv1.2 and TLSv1.3.
Newer versions of OpenSSL do not support SSLv2.
Obsolete SSLv2 and SSLv3 are currently disabled by default.
Setting the option
sslVersion = SSL_VERSION
is equivalent to options
sslVersionMax = SSL_VERSION
sslVersionMin = SSL_VERSION
when compiled with OpenSSL 1.1.0 and later.
-
sslVersionMax = SSL_VERSION
- maximum supported protocol versions
Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
all enable protocol versions up to the highest version supported by
the linked OpenSSL library.
Availability of specific protocols depends on the linked OpenSSL library.
The sslVersionMax option is only available when compiled with
OpenSSL 1.1.0 and later.
default: all
-
sslVersionMin = SSL_VERSION
- minimum supported protocol versions
Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
all enable protocol versions down to the lowest version supported by
the linked OpenSSL library.
Availability of specific protocols depends on the linked OpenSSL library.
The sslVersionMin option is only available when compiled with
OpenSSL 1.1.0 and later.
default: TLSv1
-
stack = BYTES (except for FORK model)
- CPU stack size of created threads
Excessive thread stack size increases virtual memory usage. Insufficient
thread stack size may cause application crashes.
default: 65536 bytes (sufficient for all platforms we tested)
-
ticketKeySecret = SECRET
- hexadecimal symmetric key used for session ticket
confidentiality protection
Session tickets defined in RFC 5077 provide an enhanced session resumption
capability, where the server-side caching is not required to maintain per
session state.
Combining ticketKeySecret and ticketMacSecret options allow to
resume a negotiated session on other cluster nodes, or to resume a
negotiated session after server restart.
The key is required to be either 16 or 32 bytes long, which implies exactly
32 or 64 hexadecimal digits. Colons may optionally be used between
two-character hexadecimal bytes.
This option only works in server mode.
The ticketKeySecret option is only available when compiled with
OpenSSL 1.0.0 and later.
Disabling NO_TICKET option is required for the ticket support in
OpenSSL older than 1.1.1, but note that this option is incompatible with
the redirect option.
-
ticketMacSecret = SECRET
- hexadecimal symmetric key used for session ticket integrity
protection
The key is required to be either 16 or 32 bytes long, which implies exactly
32 or 64 hexadecimal digits. Colons may optionally be used between
two-character hexadecimal bytes.
This option only works in server mode.
The ticketMacSecret option is only available when compiled with
OpenSSL 1.0.0 and later.
-
TIMEOUTbusy = SECONDS
- time to wait for expected data
-
TIMEOUTclose = SECONDS
- time to wait for close_notify (set to 0 for buggy
MSIE)
-
TIMEOUTconnect = SECONDS
- time to wait to connect to a remote host
-
TIMEOUTidle = SECONDS
- time to keep an idle connection
-
transparent = none | source | destination | both
(Unix only)
- enable transparent proxy support on selected platforms
Supported values:
- none
- Disable transparent proxy support. This is the
default.
- source
- Re-write the address to appear as if a wrapped daemon is
connecting from the TLS client machine instead of the machine running
stunnel.
This option is currently available in:
- Remote mode (connect option) on Linux
>=2.6.28
- This configuration requires stunnel to be executed
as root and without the setuid option.
This configuration requires the following setup for iptables and routing
(possibly in /etc/rc.local or equivalent file):
iptables -t mangle -N DIVERT
iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
iptables -t mangle -A DIVERT -j MARK --set-mark 1
iptables -t mangle -A DIVERT -j ACCEPT
ip rule add fwmark 1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100
echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter
stunnel must also to be executed as root and without the
setuid option.
- Remote mode (connect option) on Linux
2.2.x
- This configuration requires the kernel to be compiled with
the transparent proxy option. Connected service must be installed
on a separate host. Routing towards the clients has to go through the
stunnel box.
stunnel must also to be executed as root and without the
setuid option.
- Remote mode (connect option) on FreeBSD
>=8.0
- This configuration requires additional firewall and routing
setup. stunnel must also to be executed as root and without the
setuid option.
- Local mode (exec option)
- This configuration works by pre-loading the
libstunnel.so shared library. _RLD_LIST environment variable is
used on Tru64, and LD_PRELOAD variable on other platforms.
- destination
- The original destination is used instead of the
connect option.
A service section for transparent destination may look like this:
[transparent]
client = yes
accept = <stunnel_port>
transparent = destination
This configuration requires iptables setup to work, possibly in
/etc/rc.local or equivalent file.
For a connect target installed on the same host:
/sbin/iptables -t nat -I OUTPUT -p tcp --dport <redirected_port> \
-m ! --uid-owner <stunnel_user_id> \
-j DNAT --to-destination <local_ip>:<stunnel_port>
For a connect target installed on a remote host:
/sbin/iptables -I INPUT -i eth0 -p tcp --dport <stunnel_port> -j ACCEPT
/sbin/iptables -t nat -I PREROUTING -p tcp --dport <redirected_port> \
-i eth0 -j DNAT --to-destination <local_ip>:<stunnel_port>
The transparent destination option is currently only supported on
Linux.
- both
- Use both source and destination transparent
proxy.
Two legacy options are also supported for backward compatibility:
- yes
- This option has been renamed to source.
- no
- This option has been renamed to none.
-
verify = LEVEL
- verify the peer certificate
This option is obsolete and should be replaced with the verifyChain
and verifyPeer options.
- level 0
- Request and ignore the peer certificate.
- level 1
- Verify the peer certificate if present.
- level 2
- Verify the peer certificate.
- level 3
- Verify the peer against a locally installed
certificate.
- level 4
- Ignore the chain and only verify the peer certificate.
- default
- No verify.
-
verifyChain = yes | no
- verify the peer certificate chain starting from the root 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 peer certificate
The peer certificate needs to be stored either in the file specified with
CAfile, or in the directory specified with CApath.
default: no
stunnel returns zero on success, non-zero on error.
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.
In order to provide TLS encapsulation to your local
imapd service, use:
[imapd]
accept = 993
exec = /usr/sbin/imapd
execArgs = imapd
or in remote mode:
[imapd]
accept = 993
connect = 143
In order to let your local e-mail client connect to a TLS-enabled
imapd
service on another server, configure the e-mail client to connect to localhost
on port 119 and use:
[imap]
client = yes
accept = 143
connect = servername:993
If you want to provide tunneling to your
pppd daemon on port 2020, use
something like:
[vpn]
accept = 2020
exec = /usr/sbin/pppd
execArgs = pppd local
pty = yes
If you want to use
stunnel in
inetd mode to launch your imapd
process, you'd use this
stunnel.conf. Note there must be no
[service_name] section.
exec = /usr/sbin/imapd
execArgs = imapd
To setup SOCKS VPN configure the following client service:
[socks_client]
client = yes
accept = 127.0.0.1:1080
connect = vpn_server:9080
verifyPeer = yes
CAfile = stunnel.pem
The corresponding configuration on the vpn_server host:
[socks_server]
protocol = socks
accept = 9080
cert = stunnel.pem
key = stunnel.key
Now test your configuration on the client machine with:
curl --socks4a localhost http://www.example.com/
An example server mode SNI configuration:
[virtual]
; primary service
accept = 443
cert = default.pem
connect = default.internal.mydomain.com:8080
[sni1]
; secondary service 1
sni = virtual:server1.mydomain.com
cert = server1.pem
connect = server1.internal.mydomain.com:8081
[sni2]
; secondary service 2
sni = virtual:server2.mydomain.com
cert = server2.pem
connect = server2.internal.mydomain.com:8082
verifyPeer = yes
CAfile = server2-allowed-clients.pem
An example of advanced engine configuration allows for authentication with
private keys stored in the Windows certificate store (Windows only). With the
CAPI engine you don't need to manually select the client key to use. The
client key is automatically selected based on the list of CAs trusted by the
server.
engine = capi
[service]
engineId = capi
client = yes
accept = 127.0.0.1:8080
connect = example.com:8443
An example of advanced engine configuration to use the certificate and the
corresponding private key from a pkcs11 engine:
engine = pkcs11
engineCtrl = MODULE_PATH:opensc-pkcs11.so
engineCtrl = PIN:123456
[service]
engineId = pkcs11
client = yes
accept = 127.0.0.1:8080
connect = example.com:843
cert = pkcs11:token=MyToken;object=MyCert
key = pkcs11:token=MyToken;object=MyKey
An example of advanced engine configuration to use the certificate and the
corresponding private key from a SoftHSM token:
engine = pkcs11
engineCtrl = MODULE_PATH:softhsm2.dll
engineCtrl = PIN:12345
[service]
engineId = pkcs11
client = yes
accept = 127.0.0.1:8080
connect = example.com:843
cert = pkcs11:token=MyToken;object=KeyCert
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.
The most common use of
stunnel is to listen on a network port and
establish communication with either a new port via the connect option, or a
new program via the
exec option. However there is a special case when
you wish to have some other program accept incoming connections and launch
stunnel, for example with
inetd,
xinetd, or
tcpserver.
For example, if you have the following line in
inetd.conf:
imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf
In these cases, the
inetd-style program is responsible for binding a
network socket (
imaps above) and handing it to
stunnel when a
connection is received. Thus you do not want
stunnel to have any
accept option. All the
Service Level Options should be placed in
the global options section, and no
[service_name] section will be
present. See the
EXAMPLES section for example configurations.
Each TLS-enabled daemon needs to present a valid X.509 certificate to the peer.
It also needs a private key to decrypt the incoming data. The easiest way to
obtain a certificate and a key is to generate them with the free
OpenSSL package. You can find more information on certificates
generation on pages listed below.
The
.pem file should contain the unencrypted private key and a signed
certificate (not certificate request). So the file should look like this:
-----BEGIN RSA PRIVATE KEY-----
[encoded key]
-----END RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----
[encoded certificate]
-----END CERTIFICATE-----
stunnel needs to seed the PRNG (pseudo-random number generator) in order
for TLS to use good randomness. The following sources are loaded in order
until sufficient random data has been gathered:
- •
- The file specified with the RNDfile flag.
- •
- The file specified by the RANDFILE environment variable, if
set.
- •
- The file .rnd in your home directory, if RANDFILE not
set.
- •
- The file specified with '--with-random' at compile
time.
- •
- The contents of the screen if running on Windows.
- •
- The egd socket specified with the EGD flag.
- •
- The egd socket specified with '--with-egd-sock' at compile
time.
- •
- The /dev/urandom device.
Note that on Windows machines that do not have console user interaction (mouse
movements, creating windows, etc.) the screen contents are not variable enough
to be sufficient, and you should provide a random file for use with the
RNDfile flag.
Note that the file specified with the
RNDfile flag should contain random
data -- that means it should contain different information each time
stunnel is run. This is handled automatically unless the
RNDoverwrite flag is used. If you wish to update this file manually,
the
openssl rand command in recent versions of
OpenSSL, would be
useful.
Important note: If /dev/urandom is available,
OpenSSL often seeds the
PRNG with it while checking the random state. On systems with /dev/urandom
OpenSSL is likely to use it even though it is listed at the very bottom
of the list above. This is the behaviour of
OpenSSL and not
stunnel.
stunnel 4.40 and later contains hardcoded 2048-bit DH parameters.
Starting with
stunnel 5.18, these hardcoded DH parameters are replaced
every 24 hours with autogenerated temporary DH parameters. DH parameter
generation may take several minutes.
Alternatively, it is possible to specify static DH parameters in the certificate
file, which disables generating temporary DH parameters:
openssl dhparam 2048 >> stunnel.pem
- /etc/stunnel/stunnel.conf
-
stunnel configuration file
The
execArgs option and the Win32 command line do not support quoting.
-
tcpd(8)
- access control facility for internet services
-
inetd(8)
- internet 'super-server'
- http://www.stunnel.org/
-
stunnel homepage
- http://www.openssl.org/
-
OpenSSL project website
- Michał Trojnara
- <[email protected]>