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