STUNNEL(8) stunnel STUNNEL(8)
stunnel - universal SSL tunnel
stunnel [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 channels.
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 (email@example.com)
Use specified configuration file
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
Each line of the configuration file can be either:
o an empty line (ignored)
o a comment starting with "#" (ignored)
o an "option_name = option_value" pair
o "[service_name]" indicating a start of a service definition
CApath = directory
Certificate Authority directory
This is the directory in which stunnel will look for certificates when using the ver-
ify. Note that the certificates in this directory should be named XXXXXXXX.0 where
XXXXXXXX is the hash value of the cert.
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).
chroot = directory (Unix only)
directory to chroot stunnel process
chroot keeps stunnel in chrooted jail. CApath, 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.
ciphers = cipherlist
Select permitted SSL ciphers
A colon delimited list of the ciphers to allow in the SSL connection. For example
client = yes | no
client mode (remote service uses SSL)
default: no (server mode)
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 'authpriv' 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 or higher)
foreground = yes | no (Unix only)
Stay in foreground (don't fork) and log to stderr instead of via syslog (unless output
default: background in daemon mode
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
chmod 600 keyfile
default: value of cert option
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
For example for compatibility with erroneous Eudora SSL implementation the following
option can be used:
options = DONT_INSERT_EMPTY_FRAGMENTS
output = file
append log messages to a file instead of using syslog
pid = file (Unix only)
pid file location
If the argument is empty, then no pid file will be created.
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 suffi-
cient 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 genera-
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.
session = timeout
session cache timeout
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
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
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
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 ser-
vices 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 enti-
tiled INETD MODE below.
accept = [host:]port
accept connections on specified host:port
If no host specified, defaults to all IP addresses for the local host.
connect = [host:]port
connect to remote host:port
If no host specified, defaults to localhost.
delay = yes | no
delay DNS lookup for 'connect' option
exec = executable_path (Unix only)
execute local inetd-type program
execargs = $0 $1 $2 ... (Unix only)
arguments for exec including program name ($0)
Quoting is currently not supported. Arguments are speparated with arbitrary number of
ident = username
use IDENT (RFC 1413) username checking
local = host
IP of the outgoing interface is used as source for remote connections. Use this
option to bind a static local IP address, instead.
protocol = proto
Negotiate SSL with specified protocol
currently supported: smtp, pop3, nntp
pty = yes | no (Unix only)
allocate pseudo terminal for 'exec' option
TIMEOUTbusy = seconds
time to wait for expected data
TIMEOUTclose = seconds
time to wait for close_notify (set to 0 for buggy MSIE)
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 only available in
local mode (exec option) by LD_PRELOADing env.so shared library or in remote mode
(connect option) on Linux 2.2 kernel compiled with transparent proxy option and then
only in server mode. Note that this option will not combine with proxy mode (connect)
unless the client's default route to the target machine lies through the host running
stunnel, which cannot be localhost.
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 something like
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
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. How-
ever there is a special case when you wish to have some other program accept incoming con-
nections 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/sbin/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.
Two things are important when generating certificate-key pairs for stunnel. The private
key cannot be encrypted, because the server has no way to obtain the password from the
user. To produce an unencrypted key add the -nodes option when running the req command
from the OpenSSL kit.
The order of contents of the .pem file is also important. It should contain the unen-
crypted 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:
o The file specified with the RNDfile flag.
o The file specified by the RANDFILE environment variable, if set.
o The file .rnd in your home directory, if RANDFILE not set.
o The file specified with '--with-random' at compile time.
o The contents of the screen if running on Windows.
o The egd socket specified with the EGD flag.
o The egd socket specified with '--with-egd-sock' at compile time.
o The /dev/urandom device.
With recent (>=OpenSSL 0.9.5a) version of SSL it will stop loading random data automati-
cally 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
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 manu-
ally, the openssl rand command in recent versions of OpenSSL, would be useful.
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
3rd Berkeley Distribution 2002.12.26 STUNNEL(8)