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stunnel4

NAME

stunnel - universal SSL tunnel

SYNOPSIS

Unix:

stunnel [<filename>] | -fd n | -help | -version | -sockets

WIN32:

stunnel [ [-install | -uninstall | -start | -stop]
    [-quiet] [<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

-fd n (Unix only)

Read the config file from specified file descriptor

-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

-start (NT/2000/XP only)

Start NT Service

-stop (NT/2000/XP only)

Stop NT Service

-quiet (NT/2000/XP only)

Don't display a message box when successfully installed or uninstalled 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

chroot = directory (Unix only)

directory to chroot stunnel process

chroot keeps stunnel in chrooted jail. CApath, CRLpath, 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.

compression = zlib | rle

select data compression algorithm

default: no compression

zlib compression of OpenSSL 0.9.8 or above is not backward compatible with OpenSSL 0.9.7.

rle compression is currently not implemented by the OpenSSL library.

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 'daemon' 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)

engine = auto | <engine id>

select hardware engine

default: software-only cryptography

There's an example in 'EXAMPLES' section.

engineCtrl = command[:parameter]

control hardware engine

Special commands ``LOAD'' and ``INIT'' can be used to load and initialize the engine cryptogaphic module.

fips = yes | no

Enable or disable FIPS 140-2 mode.

This option allows to disable entering FIPS mode if stunnel was compiled with FIPS 140-2 support.

default: yes

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

output = file

append log messages to a file instead of using syslog

/dev/stdout device can be used to redirect log messages to the standard output (for example to log them with daemontools splogger).

pid = file (Unix only)

pid file location

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

pid path is relative to chroot directory if specified.

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

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
 
 

syslog = yes | no (Unix only)

enable logging via syslog

default: yes

taskbar = yes | no (WIN32 only)

enable the taskbar icon

default: yes

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 entitled 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.

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 DER encoded subject of the cert (the first 4 bytes of the MD5 hash in least significant byte order).

CApath path is relative to chroot directory if specified.

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).

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)

connect = [host:]port

connect to a remote host:port

If no host is specified, the host defaults to localhost.

Multiple connect options are allowed in a single service section.

If host resolves to multiple addresses and/or if multiple connect options are specified, then the remote address is chosen using a round-robin algorithm.

CRLpath = directory

Certificate Revocation Lists directory

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

CRLpath path is relative to chroot directory if specified.

CRLfile = certfile

Certificate Revocation Lists file

This file contains multiple CRLs, used with the verify.

delay = yes | no

delay DNS lookup for 'connect' option

engineNum = engine number

select engine number to read private key

The engines are numbered starting from 1.

exec = executable_path (Unix only)

execute local inetd-type program

exec path is relative to chroot directory if specified.

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

arguments for exec including program name ($0)

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

failover = rr | prio

Failover strategy for multiple ``connect'' targets.

     rr (round robin) - fair load distribution
     prio (priority) - use the order specified in config file
 
 

default: rr

ident = username

use IDENT (RFC 1413) username checking

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

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.

OCSP = url

select OCSP server for certificate verification

OCSPflag = flag

specify OCSP server flag

Several OCSPflag can be used to specify multiple flags.

currently supported flags: NOCERTS, NOINTERN NOSIGS, NOCHAIN, NOVERIFY, NOEXPLICIT, NOCASIGN, NODELEGATED, NOCHECKS, TRUSTOTHER, RESPID_KEY, NOTIME

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
 
 

protocol = proto

application protocol to negotiate SSL

currently supported: cifs, connect, imap, nntp, pop3, smtp, pgsql

protocolAuthentication = auth_type

authentication type for protocol negotiations

currently supported: basic, NTLM

Currently authentication type only applies to 'connect' protocol.

default: basic

protocolHost = host:port

destination address for protocol negotiations

protocolPassword = password

password for protocol negotiations

protocolUsername = username

username for protocol negotiations

pty = yes | no (Unix only)

allocate pseudo terminal for 'exec' option

retry = yes | no (Unix only)

reconnect a connect+exec section after it's disconnected

default: no

session = timeout

session cache timeout

sessiond = host:port

address of sessiond SSL cache server

sslVersion = version

select version of SSL protocol

Allowed options: all, SSLv2, SSLv3, TLSv1

stack = bytes (except for FORK model)

thread stack size

TIMEOUTbusy = seconds

time to wait for expected data

TIMEOUTclose = seconds

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

TIMEOUTconnect = seconds

time to wait to connect a remote host

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 currently available in:

     remote mode (I<connect> option) on Linux >=2.6.28
     remote mode (I<connect> option) 2.2.x
     local mode (I<exec> option)
 
 

Remote mode (either 2.2.x and >=2.6.28) requires stunnel to be executed as root. setuid option will also break this functionality.

Linux >=2.6.28 requires the following setup for iptables and routing (possibly in /etc/rc.local or equivalent file):

     iptables -t mangle -N DIVERT
     iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
     iptables -t mangle -A DIVERT -j MARK --set-mark 1
     iptables -t mangle -A DIVERT -j ACCEPT
     ip rule add fwmark 1 lookup 100
     ip route add local 0.0.0.0/0 dev lo table 100
 
 

Linux 2.2.x requires kernel to be compiled with transparent proxy option. Connected service must be installed on a separate host. Routing towards the clients has to go through the stunnel box.

Local mode works by LD_PRELOADing env.so shared library.

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
 
 

It is important to understand, that this option was solely designed for access control and not for authorization. Specifically for level 2 every non-revoked certificate is accepted regardless of its Common Name. For this reason a dedicated CA should be used with level 2, and not a generic CA commonly used for webservers. Level 3 is preferred for point-to-point connections.

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
 
 

Here is an example of advanced engine configuration to read private key from an OpenSC engine

     engine=dynamic
     engineCtrl=SO_PATH:/usr/lib/opensc/engine_pkcs11.so
     engineCtrl=ID:pkcs11
     engineCtrl=LIST_ADD:1
     engineCtrl=LOAD
     engineCtrl=MODULE_PATH:/usr/lib/pkcs11/opensc-pkcs11.so
     engineCtrl=INIT
 
     [service]
     engineNum=1
     key=id_45
 
 

FILES

stunnel.conf

stunnel configuration file

stunnel.pem

stunnel certificate and private key

BUGS

Option execargs does not support quoting.

RESTRICTIONS

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.

NOTES

INETD MODE

The most common use of stunnel is to listen on a network port and establish communication with either a new port via the connect option, or a new program via the exec option. However there is a special case when you wish to have some other program accept incoming connections and launch stunnel, for example with inetd, xinetd, or tcpserver.

For example, if you have the following line in inetd.conf:

     imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf
 
 

In these cases, the inetd-style program is responsible for binding a network socket (imaps above) and handing it to stunnel when a connection is received. Thus you do not want stunnel to have any accept option. All the Service Level Options should be placed in the global options section, and no [service_name] section will be present. See the EXAMPLES section for example configurations.

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.

The order of contents of the .pem file is 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 RNDfile 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 EGD 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 RNDfile flag.

Note that the file specified with the RNDfile flag should contain random data --- that means it should contain different information each time stunnel is run. This is handled automatically unless the RNDoverwrite 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.

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>