Ubuntu Manpage: stunnel - universal SSL tunnel

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

       In  order  to  let  your  local  e-mail  client connect to a SSL-enabled imapd service on another server,
       configure the e-mail client to connect to localhost on port 119 and use:

         stunnel -c -d 143 -r servername:993

       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.

AUTHOR

           Michal Trojnara
                   <Michal.Trojnara@stunnel.org>

                                                   2003-08-01                                         STUNNEL(8)