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¶
AUTHOR¶
- Michal Trojnara
- <Michal.Trojnara@mirt.net>