stunnel - universal SSL tunnel
stunnel [<filename>] | -fdn | -help | -version | -sockets
stunnel [ [-install | -uninstall | -start | -stop]
[-quiet] [<filename>] ] | -help | -version | -sockets
The stunnel program is designed to work as SSL encryption wrapper
between remote clients and local (inetd-startable) or remote servers.
The concept is that having non-SSL aware daemons running on your system
you can easily set them up to communicate with clients over secure SSL
stunnel can be used to add SSL 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
Use specified configuration file
-fd n (Unix only)
Read the config file from specified file descriptor
Print stunnel help menu
Print stunnel version and compile time defaults
Print default socket options
-install (NT/2000/XP only)
Install NT Service
-uninstall (NT/2000/XP only)
Uninstall NT Service
-start (NT/2000/XP only)
Start NT Service
-stop (NT/2000/XP only)
Stop NT Service
-quiet (NT/2000/XP only)
Don’t display a message box when successfully installed or
uninstalled NT service
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
chroot = directory (Unix only)
directory to chroot stunnel process
chroot keeps stunnel in 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.
To have libwrap (TCP Wrappers) control effective in a chrooted
environment you also have to copy its configuration files
(/etc/hosts.allow and /etc/hosts.deny) there.
compression = zlib | rle
select data compression algorithm
default: no compression
zlib compression of OpenSSL 0.9.8 or above is not backward
compatible with OpenSSL 0.9.7.
rle compression is currently not implemented by the OpenSSL
debug = [facility.]level
Level is a one of the syslog level names or numbers emerg (0),
alert (1), crit (2), err (3), warning (4), notice (5), info (6), or
debug (7). All logs for the specified level and all levels
numerically less than it will be shown. Use debug = debug or debug
= 7 for greatest debugging output. The default is notice (5).
The syslog facility ’daemon’ will be used unless a facility name is
supplied. (Facilities are not supported on Win32.)
Case is ignored for both facilities and levels.
EGD = egd path (Unix only)
path to Entropy Gathering Daemon socket
Entropy Gathering Daemon socket to use to feed OpenSSL random
number generator. (Available only if compiled with OpenSSL 0.9.5a
engine = auto | <engine id>
select hardware engine
default: software-only cryptography
There’s an example in ’EXAMPLES’ section.
engineCtrl = command[:parameter]
control hardware engine
Special commands "LOAD" and "INIT" can be used to load and
initialize the engine cryptogaphic module.
fips = yes | no
Enable or disable FIPS 140-2 mode.
This option allows to disable entering FIPS mode if stunnel was
compiled with FIPS 140-2 support.
foreground = yes | no (Unix only)
Stay in foreground (don’t fork) and log to stderr instead of via
syslog (unless output is specified).
default: background in daemon mode
output = file
append log messages to a file instead of using syslog
/dev/stdout device can be used to redirect log messages to the
standard output (for example to log them with daemontools
pid = file (Unix only)
pid file location
If the argument is empty, then no pid file will be created.
pid path is relative to chroot directory if specified.
RNDbytes = bytes
bytes to read from random seed files
Number of bytes of data read from random seed files. With SSL
versions less than 0.9.5a, also determines how many bytes of data
are considered sufficient to seed the PRNG. More recent OpenSSL
versions have a builtin function to determine when sufficient
randomness is available.
RNDfile = file
path to file with random seed data
The SSL 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
service = servicename
use specified string as the service name
On Unix: inetd mode service name for TCP Wrapper library.
On NT/2000/XP: NT service name in the Control Panel.
setgid = groupname (Unix only)
setgid() to groupname in daemon mode and clears all other groups
setuid = username (Unix only)
setuid() to username in daemon mode
socket = a|l|r:option=value[:value]
Set an option on accept/local/remote socket
The values for linger option are l_onof:l_linger. The values for
time are tv_sec:tv_usec.
socket = l:SO_LINGER=1:60
set one minute timeout for closing local socket
socket = r:TCP_NODELAY=1
turn off the Nagle algorithm for remote sockets
socket = r:SO_OOBINLINE=1
place out-of-band data directly into the
receive data stream for remote sockets
socket = a:SO_REUSEADDR=0
disable address reuse (enabled by default)
socket = a:SO_BINDTODEVICE=lo
only accept connections on loopback interface
syslog = yes | no (Unix only)
enable logging via syslog
taskbar = yes | no (WIN32 only)
enable the taskbar icon
Each configuration section begins with 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 host:port
If no host specified, defaults to all IP addresses for the local
CApath = directory
Certificate Authority directory
This is the directory in which stunnel will look for certificates
when using the verify. 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 first 4 bytes of the MD5 hash
in least significant byte order).
CApath path is relative to chroot directory if specified.
CAfile = certfile
Certificate Authority file
This file contains multiple CA certificates, used with the verify.
cert = pemfile
certificate chain PEM file name
A PEM is always needed in server mode. Specifying this flag in
client mode will use this certificate chain as a client side
certificate chain. Using client side certs is optional. The
certificates must be in PEM format and must be sorted starting with
the certificate to the highest level (root CA).
ciphers = cipherlist
Select permitted SSL ciphers
A colon delimited list of the ciphers to allow in the SSL
connection. For example DES-CBC3-SHA:IDEA-CBC-MD5
client = yes | no
client mode (remote service uses SSL)
default: no (server mode)
connect = [host:]port
connect to a remote host:port
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
CRLpath = directory
Certificate Revocation Lists directory
This is the directory in which stunnel will look for CRLs when
using the verify. Note that the CRLs in this directory should be
named XXXXXXXX.0 where XXXXXXXX is the hash value of the CRL.
CRLpath path is relative to chroot directory if specified.
CRLfile = certfile
Certificate Revocation Lists file
This file contains multiple CRLs, used with the verify.
delay = yes | no
delay DNS lookup for ’connect’ option
engineNum = engine number
select engine number to read private key
The engines are numbered starting from 1.
exec = executable_path (Unix only)
execute local inetd-type program
exec path is relative to chroot directory if specified.
execargs = $0 $1 $2 ... (Unix only)
arguments for exec including program name ($0)
Quoting is currently not supported. Arguments are separated with
arbitrary number of whitespaces.
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
key = keyfile
private key for certificate specified with cert option
Private key is needed to authenticate certificate owner. Since
this file should be kept secret it should only be readable to its
owner. On Unix systems you can use the following command:
chmod 600 keyfile
default: value of cert option
local = host
IP of the outgoing interface is used as source for remote
connections. Use this option to bind a static local IP address,
OCSP = url
select OCSP server for certificate verification
OCSPflag = flag
specify OCSP server flag
Several OCSPflag can be used to specify multiple flags.
currently supported flags: NOCERTS, NOINTERN NOSIGS, NOCHAIN,
NOVERIFY, NOEXPLICIT, NOCASIGN, NODELEGATED, NOCHECKS, TRUSTOTHER,
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.
Several options can be used to specify multiple options.
For example for compatibility with erroneous Eudora SSL
implementation the following option can be used:
options = DONT_INSERT_EMPTY_FRAGMENTS
protocol = proto
application protocol to negotiate SSL
currently supported: cifs, connect, imap, nntp, pop3, smtp, pgsql
protocolAuthentication = auth_type
authentication type for protocol negotiations
currently supported: basic, NTLM
Currently authentication type only applies to ’connect’ protocol.
protocolHost = host:port
destination address for protocol negotiations
protocolPassword = password
password for protocol negotiations
protocolUsername = username
username for protocol negotiations
pty = yes | no (Unix only)
allocate pseudo terminal for ’exec’ option
retry = yes | no (Unix only)
reconnect a connect+exec section after it’s disconnected
session = timeout
session cache timeout
sessiond = host:port
address of sessiond SSL cache server
sslVersion = version
select version of SSL protocol
Allowed options: all, SSLv2, SSLv3, TLSv1
stack = bytes (except for FORK model)
thread stack size
TIMEOUTbusy = seconds
time to wait for expected data
TIMEOUTclose = seconds
time to wait for close_notify (set to 0 for buggy MSIE)
TIMEOUTconnect = seconds
time to wait to connect a remote host
TIMEOUTidle = seconds
time to keep an idle connection
transparent = yes | no (Unix only)
transparent proxy mode
Re-write address to appear as if wrapped daemon is connecting from
the SSL client machine instead of the machine running stunnel.
This option is currently available in:
remote mode (I<connect> option) on Linux >=2.6.28
remote mode (I<connect> option) 2.2.x
local mode (I<exec> option)
Remote mode (either 2.2.x and >=2.6.28) requires stunnel to be
executed as root. setuid option will also break this
Linux >=2.6.28 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
Linux 2.2.x requires kernel to be compiled with transparent proxy
option. Connected service must be installed on a separate host.
Routing towards the clients has to go through the stunnel box.
Local mode works by LD_PRELOADing env.so shared library.
verify = level
verify peer certificate
level 1 - verify peer certificate if present
level 2 - verify peer certificate
level 3 - verify peer with locally installed certificate
default - no verify
It is important to understand, that this option was solely designed
for access control and not for authorization. Specifically for
level 2 every non-revoked certificate is accepted regardless of its
Common Name. For this reason a dedicated CA should be used with
level 2, and not a generic CA commonly used for webservers. Level
3 is preferred for point-to-point connections.
stunnel returns zero on success, non-zero on error.
In order to provide SSL encapsulation to your local imapd service, use
accept = 993
exec = /usr/sbin/imapd
execargs = imapd
If you want to provide tunneling to your pppd daemon on port 2020, use
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]
exec = /usr/sbin/imapd
execargs = imapd
Here is an example of advanced engine configuration to read private key
from an OpenSC engine
stunnel configuration file
stunnel certificate and private key
Option execargs does not support quoting.
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 SSL 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 SSL enabled daemon needs to present a valid X.509 certificate to
the peer. It also needs a private key to decrypt the incoming data. The
easiest way to obtain a certificate and a key is to generate them with
the free OpenSSL package. You can find more information on certificates
generation on pages listed below.
The order of contents of the .pem file is important. It should contain
the unencrypted private key first, then a signed certificate (not
certificate request). There should be also empty lines after
certificate and private key. Plaintext certificate information
appended on the top of generated certificate should be discarded. So
the file should look like this:
-----BEGIN RSA PRIVATE KEY-----
-----END RSA PRIVATE KEY-----
stunnel needs to seed the PRNG (pseudo random number generator) in
order for SSL 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.
With recent (>=OpenSSL 0.9.5a) version of SSL it will stop loading
random data automatically when sufficient entropy has been gathered.
With previous versions it will continue to gather from all the above
sources since no SSL function exists to tell when enough data is
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
One important note -- if /dev/urandom is available, OpenSSL has a habit
of seeding the PRNG with it even when checking the random state, so on
systems with /dev/urandom you’re likely to use it even though it’s
listed at the very bottom of the list above. This isn’t stunnel’s
behaviour, it’s OpenSSLs.
access control facility for internet services
stunnel Frequently Asked Questions
OpenSSL project website