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)
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 cert.
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).
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.)
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.
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
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 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,
For example, if you have the following line in inetd.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
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 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:
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 available.
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.
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