Provided by: stunnel4_4.53-1.1ubuntu1_amd64 
NAME
stunnel - universal SSL tunnel
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 { filename ⎪ '' } ] [-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
-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
The values for linger option are l_onof:l_linger. The values for time are tv_sec:tv_usec.
Examples:
-O l:SO_LINGER=1:60 - set one minute timeout for closing local socket
-O r:TCP_NODELAY=1 - turn off the Nagle algorithm for remote sockets
-O r:SO_OOBINLINE=1 - place out-of-band data directly into the receive data stream for remote sockets
-O a:SO_REUSEADDR=0 - disable address reuse (enabled by default)
-O a:SO_BINDTODEVICE=lo - only accept connections on loopback interface
The available options and their defaults are:
Option Accept Local Remote OS default
SO_DEBUG -- -- -- 0
SO_DONTROUTE -- -- -- 0
SO_KEEPALIVE -- -- -- 0
SO_LINGER -- -- -- 0:0
SO_OOBINLINE -- -- -- 0
SO_RCVBUF -- -- -- 87380
SO_SNDBUF -- -- -- 16384
SO_RCVLOWAT -- -- -- 1
SO_SNDLOWAT -- -- -- 1
SO_RCVTIMEO -- -- -- 0:0
SO_SNDTIMEO -- -- -- 0:0
SO_REUSEADDR 1 -- -- 0
SO_BINDTODEVICE -- -- -- --
IP_TOS -- -- -- 0
IP_TTL -- -- -- 64
TCP_NODELAY -- -- -- 0
-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 (by default located in /etc/stunnel/stunnel.pem). 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
• level 1 - verify peer certificate if present
• level 2 - verify peer certificate
• level 3 - verify peer with locally installed certificate
• default - no verify
-a directory
client certificate directory
This is the directory in which stunnel will look for certificates when using the -v options. Note
that the certificates in this directory should be named XXXXXXXX.0 where XXXXXXXX is the hash value
of the cert.
-A certfile
Certificate Authority file
This file contains multiple CA certificates, used with the -v options.
-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
currently supported: smtp, pop3, nntp
-E socket
Entropy Gathering Daemon socket to use to feed OpenSSL random number generator. (Available only if
compiled with OpenSSL 0.9.5a or higher)
-R filename
File containing random input. The SSL library will use data from this file first to seed the random
number generator.
-W Do not overwrite the random seed files with new random 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 { file ⎪ '' }
Pid file location
If the argument is a filename, then that filename will be used for the pid. If the argument is empty
('', not missing), then no pid file will be created.
-r [host:]port
connect to remote service
If no host specified, defaults to localhost.
EXAMPLES
In order to provide SSL encapsulation to your local imapd service, use
stunnel -d 993 -l /usr/sbin/imapd -- imapd
If you want to provide tunneling to your pppd daemon on port 2020, use something like
stunnel -d 2020 -L /usr/sbin/pppd -- pppd local
ENVIRONMENT
If Stunnel is used to create local processes using the -l or -L options, it will set the following
environment variables
REMOTE_HOST
The IP address of the remote end of the connection.
SSL_CLIENT_DN
The DN (Distinguished Name, aka subject name) of the peer certificate, if a certificate was present
and verified.
SSL_CLIENT_I_DN
The Issuer's DN of the peer's certificate, if a certificate was present and verified.
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:
• The file specified with the -R 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 -E 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 -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
tcpd(8) access control facility for internet services
inetd(8)
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>
2003-08-01 STUNNEL(8)