Linux / Unix Command: stunnel

Command Library

NAME

stunnel - universal SSL tunnel  

SYNOPSIS

stunnel [filename] | -help | -version | -sockets  

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

[filename]

Use specified configuration file

-help

Print stunnel help menu

-version

Print stunnel version and compile time defaults

-sockets

Print default socket options

-install (NT/2000/XP only)

Install NT Service

-uninstall (NT/2000/XP only)

Uninstall NT Service

 

CONFIGURATION FILE

Each line of the configuration file can be either:

*

an empty line (ignored)

*

a comment starting with ``#'' (ignored)

*

an ``option_name = option_value'' pair

*

``[service_name]'' indicating a start of a service definition

 

GLOBAL OPTIONS

CApath = directory

Certificate Authority directory

This is the directory in which stunnel will look for certificates when using the verify. Note that the certificates in this directory should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the cert.

CAfile = certfile

Certificate Authority file

This file contains multiple CA certificates, used with the verify.

cert = pemfile

certificate chain PEM file name

A PEM is always needed in server mode. Specifying this flag in client mode will use this 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).

chroot = directory (Unix only)

directory to chroot stunnel process

chroot keeps stunnel in chrooted jail. CApath, pid and exec are located inside the jail and the patches have to be relative to the directory specified with chroot.

To have libwrap (TCP Wrappers) control effective in a chrooted environment you also have to copy its configuration files (/etc/hosts.allow and /etc/hosts.deny) there.

ciphers = 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

client = yes | no

client mode (remote service uses SSL)

default: no (server mode)

debug = [facility.]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 debug = debug or debug = 7 for greatest debugging output. The default is notice (5).

The syslog facility 'authpriv' will be used unless a facility name is supplied. (Facilities are not supported on Win32.)

Case is ignored for both facilities and levels.

EGD = egd path (Unix only)

path to Entropy Gathering Daemon socket

Entropy Gathering Daemon socket to use to feed OpenSSL random number generator. (Available only if compiled with OpenSSL 0.9.5a or higher)

foreground = yes | no (Unix only)

foreground mode

Stay in foreground (don't fork) and log to stderr instead of via syslog (unless output is specified).

default: background in daemon mode

key = keyfile

private key for certificate specified with cert option

Private key is needed to authenticate certificate owner. Since this file should be kept secret it should only be readable to its owner. On Unix systems you can use the following command:

    chmod 600 keyfile

default: value of cert option

options = SSL_options

OpenSSL library options

The parameter is the OpenSSL option name as described in the SSL_CTX_set_options(3ssl) manual, but without SSL_OP_ prefix. Several options can be used to specify multiple options.

For example for compatibility with erroneous Eudora SSL implementation the following option can be used:

    options = DONT_INSERT_EMPTY_FRAGMENTS

output = file

append log messages to a file instead of using syslog

pid = file (Unix only)

pid file location

If the argument is empty, then no pid file will be created.

RNDbytes = bytes

bytes to read from random seed files

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.

RNDfile = file

path to file with random seed data

The SSL library will use data from this file first to seed the random number generator.

RNDoverwrite = yes | no

overwrite the random seed files with new random data

default: yes

service = servicename

use specified string as the service name

On Unix: inetd mode service name for TCP Wrapper library.

On NT/2000/XP: NT service name in the Control Panel.

default: stunnel

session = timeout

session cache timeout

setgid = groupname (Unix only)

setgid() to groupname in daemon mode and clears all other groups

setuid = username (Unix only)

setuid() to username in daemon mode

socket = 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:

    socket = l:SO_LINGER=1:60
        set one minute timeout for closing local socket
    socket = r:TCP_NODELAY=1
        turn off the Nagle algorithm for remote sockets
    socket = r:SO_OOBINLINE=1
        place out-of-band data directly into the
        receive data stream for remote sockets
    socket = a:SO_REUSEADDR=0
        disable address reuse (enabled by default)
    socket = a:SO_BINDTODEVICE=lo
        only accept connections on loopback interface

verify = 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

 

SERVICE-LEVEL OPTIONS

Each configuration section begins with service name in square brackets. The service name is used for libwrap (TCP Wrappers) access control and lets you distinguish stunnel services in your log files.

Note that if you wish to run stunnel in inetd mode (where it is provided a network socket by a server such as inetd, xinetd, or tcpserver) then you should read the section entitiled INETD MODE below.

accept = [host:]port

accept connections on specified host:port

If no host specified, defaults to all IP addresses for the local host.

connect = [host:]port

connect to remote host:port

If no host specified, defaults to localhost.

delay = yes | no

delay DNS lookup for 'connect' option

exec = executable_path (Unix only)

execute local inetd-type program

execargs = $0 $1 $2 ... (Unix only)

arguments for exec including program name ($0)

Quoting is currently not supported. Arguments are speparated with arbitrary number of whitespaces.

ident = username

use IDENT (RFC 1413) username checking

local = host

IP of the outgoing interface is used as source for remote connections. Use this option to bind a static local IP address, instead.

protocol = proto

Negotiate SSL with specified protocol

currently supported: smtp, pop3, nntp

pty = yes | no (Unix only)

allocate pseudo terminal for 'exec' option

TIMEOUTbusy = seconds

time to wait for expected data

TIMEOUTclose = seconds

time to wait for close_notify (set to 0 for buggy MSIE)

TIMEOUTidle = seconds

time to keep an idle connection

transparent = yes | no (Unix only)

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. This option is only available in local mode (exec option) by LD_PRELOADing env.so shared library or in remote mode (connect option) on Linux 2.2 kernel compiled with transparent proxy option and then only in server mode. Note that this option will not combine with proxy mode (connect) unless the client's default route to the target machine lies through the host running stunnel, which cannot be localhost.

 

RETURN VALUE

stunnel returns zero on success, non-zero on error.  

EXAMPLES

In order to provide SSL encapsulation to your local imapd service, use

    [imapd]
    accept = 993
    exec = /usr/sbin/imapd
    execargs = imapd

If you want to provide tunneling to your pppd daemon on port 2020, use something like

    [vpn]
    accept = 2020
    exec = /usr/sbin/pppd
    execargs = pppd local
    pty = yes

If you want to use stunnel in inetd mode to launch your imapd process, you'd use this stunnel.conf. Note there must be no [service_name] section.

    exec = /usr/sbin/imapd
    execargs = imapd

 

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

 

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