stunnel(8) - version 3
SYNOPSIS
stunnel [-c | -T] [-D [facility.]level] [-O
a|l|r:option=value[:value]] [-o file] [-C cipherlist] [-p
pemfile] [-v level] [-A certfile] [-S sources] [-a
directory] [-t timeout] [-u ident_username]
[-s setuid_user] [-g setgid_group] [-n protocol]
[-P { dir/ | filename | none } ] [-B bytes] [-R randfile]
[-W] [-E socket] [-I host] [-d [host:]port [-f] ]
[ -r [host:]port | { -l | -L } program [--
progname args] ]
DESCRIPTION
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 (eay@cryptsoft.com)
OPTIONS
-h Print stunnel help menu
-V Print stunnel version and compile time defaults
-D 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. Use -D debug or -D 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 windows.)
Case is ignored for both facilities and levels.
-O a|l|r:option=value[:value]
Set an option on accept/local/remote socket
-O a:SO_REUSEADDR=0 - disable address reuse (enabled
by default)
-O a:SO_BINDTODEVICE=lo - only accept connections on
loopback interface
See 'stunnel -V' for options supported on your system
and their default values.
-o file
Append log messages to a file.
-C 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
-c client mode (remote service uses SSL)
default: server mode
-T 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. Available only on some
operating systems (Linux only, we believe) and then
only in server mode. Note that this option will not
combine with proxy mode (-r) unless the client's
default route to the target machine lies through the
host running stunnel, which cannot be localhost.
-p pemfile
private key and certificate chain PEM file name
A PEM is always needed in server mode (see stunnel -V
output for default). Specifying this flag in client
mode will use this key and 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).
-v level
verify peer certificate
o level 1 - verify peer certificate if present
o level 2 - verify peer certificate
o level 3 - verify peer with locally installed
See stunnel -V output for default.
-A certfile
Certificate Authority file
This file contains multiple CA certificates, used with
the -v options. This file will override the stunnel
compile-time default, if any.
See stunnel -V output for default.
-S sources
Certificate source defaults
Both stunnel and the SSL library used to compile
stunnel have default locations to look for your
certificate file (-A option) and certificate
directories (-a option). The -S flag allows you to
control which of these default sources, if any, should
be used.
0 = ignore all default sources
1 = use ssl library defaults
2 = use stunnel defaults
3 = use both ssl library and stunnel defaults
(See stunnel -V output for the default.)
Note that the -A and -a flags overwrite, not
supplement, the stunnel defaults, whereas the ssl
library defaults, if enabled, are used in addition to
the other sources.
In general, to avoid hurting one's brain, use -S 0 and
explicitly set -A and/or -a as desired.
-t timeout
session cache timeout
default: 300 seconds.
-N servicename
Service name to use for tcpwrappers. If not specified
then a tcpwrapper service name will be generated
automatically for you. This will also be used when
auto-generating pid filenames.
-u ident_username
Use IDENT (RFC 1413) username checking
-n proto
Negotiate SSL with specified protocol
data.
-B bytes
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.
-I host
IP of the outgoing interface is used as source for
remote connections. Use this option to bind a static
local IP address, instead.
-d [host:]port
daemon mode
Listen for connections on [host:]port. If no host
specified, defaults to all IP addresses for the local
host.
default: inetd mode
-f foreground mode
Stay in foreground (don't fork) and log to stderr
instead of via syslog (unless -o is specified).
default: background in daemon mode
-l program [-- programname [arg1 arg2 arg3...] ]
execute local inetd-type program.
-L program [-- programname [arg1 arg2 arg3...] ]
open local pty and execute program.
-s username
setuid() to username in daemon mode
-g groupname
setgid() to groupname in daemon mode. Clears all other
groups.
-P { dir/ | file | none }
Pid file location
If the argument is a pathname ending in a slash, then
a pid file named "stunnel.servicename.pid" will be
created in the specified directory. If the argument
is a filename (no trailing slash), then that filename
will be used for the pid. If the argument is 'none',
port 2020, use something like
stunnel -d 2020 -L /usr/sbin/pppd -- pppd local
CERTIFICATES
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:
-----BEGIN RSA PRIVATE KEY-----
[encoded key]
-----END RSA PRIVATE KEY-----
[empty line]
-----BEGIN CERTIFICATE-----
[encoded certificate]
-----END CERTIFICATE-----
[empty line]
RANDOMNESS
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 -R 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
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 -R flag.
Note that the file specified with the -R flag should
contain random data -- that means it should contain
different information each time stunnel is run. This is
handled automatically unless the -W 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 OpenSSLs.
LIMITATIONS
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.
SEE ALSO
the tcpd(8) manpage
access control facility for internet services
the inetd(8) manpage
internet ``super-server''
http://stunnel.mirt.net/
Stunnel homepage
http://www.stunnel.org/
Stunnel Frequently Asked Questions
http://www.openssl.org/
OpenSSL project website
AUTHOR
Michal Trojnara
<Michal.Trojnara@mirt.net>
3rd Berkeley Distribution STUNNEL.OUT(1)
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