SYNOPSIS

Unix:

stunnel [FILE] | -fd N | -help | -version | -sockets | -options

WIN32:

stunnel [ [ -install | -uninstall | -start | -stop |
    -reload | -reopen | -exit ] [-quiet] [FILE] ] |
    -help | -version | -sockets | -options

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 ([email protected])

OPTIONS

FILE

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

-options

Print supported SSL options

-install (Windows NT and later only)

Install NT Service

-uninstall (Windows NT and later only)

Uninstall NT Service

-start (Windows NT and later only)

Start NT Service

-stop (Windows NT and later only)

Stop NT Service

-reload (Windows NT and later only)

Reload the configuration file of the running NT Service

-reopen (Windows NT and later only)

Reopen the log file of the running NT Service

-exit (Win32 only)

Exit an already started stunnel

-quiet (Win32 only)

Don't display any message boxes

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.

An address parameter of an option may be either:

• A port number.
• A colon-separated pair of IP address (either IPv4, IPv6, or domain name) and port number.
• A Unix socket path (Unix only).

GLOBAL OPTIONS

chroot = DIRECTORY (Unix only)

directory to chroot stunnel process

chroot keeps stunnel in a 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.

Several functions of the operating system also need their files to be located within the chroot jail, e.g.:

• Delayed resolver typically needs /etc/nsswitch.conf and /etc/resolv.conf.
• Local time in log files needs /etc/timezone.
• Some other functions may need devices, e.g. /dev/zero or /dev/null.

compression = deflate | zlib

select data compression algorithm

default: no compression

deflate is the standard compression method as described in RFC 1951.

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

debug = [FACILITY.]LEVEL

debugging level

Level is 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 the 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

An example of advanced engine configuration to use the certificate and the corresponding private key from an OpenSC PKCS#11 engine:

    engine = dynamic
    engineCtrl = SO_PATH:/usr/local/lib/engines/libpkcs11.so
    engineCtrl = LOAD
    engineCtrl = MODULE_PATH:opensc-pkcs11.so
    engineCtrl = PIN:123456
    [service]
    engineId = pkcs11
    cert = id_45

engineCtrl = COMMAND[:PARAMETER]

control hardware engine

A special command ``LOAD'' can be used to load engine cryptographic module.

engineDefault = TASK_LIST

set OpenSSL tasks delegated to the current engine

The parameter specifies a comma-separated list of task to be delegated to the current engine.

The following tasks may be available, if supported by the engine: ALL, RSA, DSA, ECDH, ECDSA, DH, RAND, CIPHERS, DIGESTS, PKEY, PKEY_CRYPTO, PKEY_ASN1.

fips = yes | no

Enable or disable FIPS 140-2 mode.

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

default: no (since version 5.00)

foreground = yes | quiet | no (Unix only)

foreground mode

Stay in foreground (don't fork).

With the yes parameter it also logs to stderr in addition to the destinations specified with syslog and output.

default: background in daemon mode

iconActive = ICON_FILE (GUI only)

GUI icon to be displayed when there are established connections

On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.

iconError = ICON_FILE (GUI only)

GUI icon to be displayed when no valid configuration is loaded

On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.

iconIdle = ICON_FILE (GUI only)

GUI icon to be displayed when there are no established connections

On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.

log = append | overwrite

log file handling

This option allows you to choose whether the log file (specified with the output option) is appended or overwritten when opened or re-opened.

default: append

output = FILE

append log messages to a file

/dev/stdout device can be used to send 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 the 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 = SERVICE (Unix only)

stunnel service name

The specified service name is used for syslog and as the inetd mode service name for TCP Wrappers. While this option can technically be specified in the service sections, it is only useful in global options.

default: stunnel

socket = a|l|r:OPTION=VALUE[:VALUE]

Set an option on the accept/local/remote socket

The values for the linger option are l_onof:l_linger. The values for the time are tv_sec:tv_usec.

Examples:

    socket = l:SO_LINGER=1:60
        set one minute timeout for closing local socket
    socket = r:SO_OOBINLINE=yes
        place out-of-band data directly into the
        receive data stream for remote sockets
    socket = a:SO_REUSEADDR=no
        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 a 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 address

If no host specified, defaults to all IPv4 addresses for the local host.

To listen on all IPv6 addresses use:

    accept = :::PORT

CApath = DIRECTORY

Certificate Authority directory

This is the directory in which stunnel will look for certificates when using the verify option. 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 hash algorithm has been changed in OpenSSL 1.0.0. It is required to c_rehash the directory on upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x.

CApath path is relative to the chroot directory if specified.

CAfile = CERT_FILE

Certificate Authority file

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

cert = PEM_FILE

certificate chain PEM file name

The certificates must be in PEM format, and must be from the actual server/client certificate to the self-signed root CA certificate.

A certificate is required in server mode, and optional in client mode.

This parameter is also used as the certificate identifier when a hardware engine is enabled.

checkEmail = EMAIL

email address of the peer certificate subject

Multiple checkEmail options are allowed in a single service section. Certificates are accepted if no checkEmail option was specified, or the email address of the peer certificate matches any of the email addresses specified with checkEmail.

This option requires OpenSSL 1.0.2 or later.

checkHost = HOST

host of the peer certificate subject

Multiple checkHost options are allowed in a single service section. Certificates are accepted if no checkHost option was specified, or the host name of the peer certificate matches any of the hosts specified with checkHost.

This option requires OpenSSL 1.0.2 or later.

checkIP = IP

IP address of the peer certificate subject

Multiple checkIP options are allowed in a single service section. Certificates are accepted if no checkIP option was specified, or the IP address of the peer certificate matches any of the IP addresses specified with checkIP.

This option requires OpenSSL 1.0.2 or later.

ciphers = CIPHER_LIST

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)

config = COMMAND[:PARAMETER]

OpenSSL configuration command

The OpenSSL configuration command is executed with the specified parameter. This allows any configuration commands to be invoked from the stunnel configuration file. Supported commands are described on the SSL_CONF_cmd(3ssl) manual page.

Several config lines can be used to specify multiple configuration commands.

This option requires OpenSSL 1.0.2 or later.

connect = [HOST:]PORT

connect to a remote address

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 option. Note that the CRLs in this directory should be named XXXXXXXX.r0 where XXXXXXXX is the hash value of the CRL.

The hash algorithm has been changed in OpenSSL 1.0.0. It is required to c_rehash the directory on upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x.

CRLpath path is relative to the chroot directory if specified.

CRLfile = CERT_FILE

Certificate Revocation Lists file

This file contains multiple CRLs, used with the verify option.

curve = NID

specify ECDH curve name

To get a list of supported curves use:

    openssl ecparam -list_curves

default: prime256v1

logId = TYPE

connection identifier type

This identifier allows you to distinguish log entries generated for each of the connections.

Currently supported types:

sequential

The numeric sequential identifier is only unique within a single instance of stunnel, but very compact. It is most useful for manual log analysis.

unique

This alphanumeric identifier is globally unique, but longer than the sequential number. It is most useful for automated log analysis.

thread

The operating system thread identifier is neither unique (even within a single instance of stunnel) nor short. It is most useful for debugging software or configuration issues.

default: sequential

debug = 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).

delay = yes | no

delay DNS lookup for the connect option

This option is useful for dynamic DNS, or when DNS is not available during stunnel startup (road warrior VPN, dial-up configurations).

Delayed resolver mode is automatically engaged when stunnel fails to resolve on startup any of the connect targets for a service.

Delayed resolver inflicts failover = prio.

default: no

engineId = ENGINE_ID

select engine ID for the service

engineNum = ENGINE_NUMBER

select engine number for the service

The engines are numbered starting from 1.

exec = EXECUTABLE_PATH

execute a local inetd-type program

exec path is relative to the chroot directory if specified.

The following environmental variables are set on Unix platforms: REMOTE_HOST, REMOTE_PORT, SSL_CLIENT_DN, SSL_CLIENT_I_DN.

execArgs = $0 $1 $2 ...

arguments for exec including the program name ($0)

Quoting is currently not supported. Arguments are separated with an arbitrary amount of whitespace.

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

include = DIRECTORY

include all configuration file parts located in DIRECTORY

The files are included in the ascending alphabetical order of their names.

key = KEY_FILE

private key for the certificate specified with cert option

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

    chmod 600 keyfile

This parameter is also used as the private key identifier when a hardware engine is enabled.

default: the value of the cert option

libwrap = yes | no

Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.

default: no (since version 5.00)

local = HOST

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

sni = SERVICE:SERVER_PATTERN (server mode)

Use the service as a slave service (a name-based virtual server) for Server Name Indication TLS extension (RFC 3546).

service_name specifies the master service that accepts client connections with the accept option. server_name_pattern specifies the host name to be redirected. The pattern may start with the '*' character, e.g. '*.example.com'. Multiple slave services are normally specified for a single master service. The sni option can also be specified more than once within a single slave service.

This service, as well as the master service, may not be configured in client mode.

The connect option of the slave service is ignored when the protocol option is specified, as protocol connects to the remote host before TLS handshake.

Libwrap checks (Unix only) are performed twice: with the master service name after TCP connection is accepted, and with the slave service name during the TLS handshake.

The sni option is only available when compiled with OpenSSL 1.0.0 and later.

sni = SERVER (client mode)

Use the parameter as the value of TLS Server Name Indication (RFC 3546) extension.

The sni option is only available when compiled with OpenSSL 1.0.0 and later.

OCSP = URL

select OCSP responder for certificate verification

OCSPaia = yes | no

validate certificates with their AIA OCSP responders

This option enables stunnel to validate certificates with the list of OCSP responder URLs retrieved from their AIA (Authority Information Access) extension.

OCSPflag = OCSP_FLAG

specify OCSP responder 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

OCSPnonce = yes | no

send and verify the OCSP nonce extension

This option protects the OCSP protocol against replay attacks. Due to its computational overhead, the nonce extension is usually only supported on internal (e.g. corporate) responders, and not on public OCSP responders.

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. stunnel -options lists the options found to be allowed in the current combination of stunnel and the OpenSSL library used to build it.

Several option lines can be used to specify multiple options. An option name can be prepended with a dash (``-'') to disable the option.

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

    options = DONT_INSERT_EMPTY_FRAGMENTS

default:

    options = NO_SSLv2
    options = NO_SSLv3

protocol = PROTO

application protocol to negotiate SSL

This option enables initial, protocol-specific negotiation of the SSL/TLS encryption. The protocol option should not be used with SSL encryption on a separate port.

Currently supported protocols:

cifs

Proprietary (undocummented) extension of CIFS protocol implemented in Samba. Support for this extension was dropped in Samba 3.0.0.

connect

Based on RFC 2817 - Upgrading to TLS Within HTTP/1.1, section 5.2 - Requesting a Tunnel with CONNECT

This protocol is only supported in client mode.

imap

Based on RFC 2595 - Using TLS with IMAP, POP3 and ACAP

nntp

Based on RFC 4642 - Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP)

This protocol is only supported in client mode.

pgsql

Based on http://www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982

pop3

Based on RFC 2449 - POP3 Extension Mechanism

proxy

Haproxy client IP address http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt

smtp

Based on RFC 2487 - SMTP Service Extension for Secure SMTP over TLS

socks

SOCKS versions 4, 4a, and 5 are supported. The SOCKS protocol itself is encapsulated within SSL/TLS encryption layer to protect the final destination address.

http://www.openssh.com/txt/socks4.protocol

http://www.openssh.com/txt/socks4a.protocol

The BIND command of the SOCKS protocol is not supported. The USERID parameter is ignored.

See Examples section for sample configuration files for VPN based on SOCKS encryption.

protocolAuthentication = AUTHENTICATION

authentication type for the protocol negotiations

Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.

Supported authentication types for the 'connect' protocol are 'basic' or 'ntlm'. The default 'connect' authentication type is 'basic'.

Supported authentication types for the 'smtp' protocol are 'plain' or 'login'. The default 'smtp' authentication type is 'plain'.

protocolDomain = DOMAIN

domain for the protocol negotiations

Currently, this option is only supported in the client-side 'connect' protocol.

protocolHost = HOST:PORT

destination address for the protocol negotiations

protocolHost specifies the final SSL server to be connected to by the proxy, and not the proxy server directly connected by stunnel. The proxy server should be specified with the 'connect' option.

Currently the protocol destination address only applies to the 'connect' protocol.

protocolPassword = PASSWORD

password for the protocol negotiations

Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.

protocolUsername = USERNAME

username for the protocol negotiations

Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.

PSKidentity = IDENTITY

PSK identity for the PSK client

PSKidentity can be used on stunnel clients to select the PSK identity used for authentication. This option is ignored in server sections.

default: the first identity specified in the PSKsecrets file.

PSKsecrets = FILE

file with PSK identities and corresponding keys

Each line of the file in the following format:

    IDENTITY:KEY

The key is required to be at least 20 characters long. The file should not be world-readable nor world-writable.

pty = yes | no (Unix only)

allocate a pseudoterminal for 'exec' option

redirect = [HOST:]PORT

redirect SSL client connections on certificate-based authentication failures

This option only works in server mode. Some protocol negotiations are also incompatible with the redirect option.

renegotiation = yes | no

support SSL renegotiation

Applications of the SSL renegotiation include some authentication scenarios, or re-keying long lasting connections.

On the other hand this feature can facilitate a trivial CPU-exhaustion DoS attack:

http://vincent.bernat.im/en/blog/2011-ssl-dos-mitigation.html

Please note that disabling SSL renegotiation does not fully mitigate this issue.

default: yes (if supported by OpenSSL)

reset = yes | no

attempt to use the TCP RST flag to indicate an error

This option is not supported on some platforms.

default: yes

retry = yes | no

reconnect a connect+exec section after it was disconnected

default: no

setgid = GROUP (Unix only)

Unix group id

As a global option: setgid() to the specified group in daemon mode and clear all other groups.

As a service-level option: set the group of the Unix socket specified with ``accept''.

setuid = USER (Unix only)

Unix user id

As a global option: setuid() to the specified user in daemon mode.

As a service-level option: set the owner of the Unix socket specified with ``accept''.

sessionCacheSize = NUM_ENTRIES

session cache size

sessionCacheSize specifies the maximum number of the internal session cache entries.

The value of 0 can be used for unlimited size. It is not recommended for production use due to the risk of a memory exhaustion DoS attack.

sessionCacheTimeout = TIMEOUT

session cache timeout

This is the number of seconds to keep cached SSL sessions.

sessiond = HOST:PORT

address of sessiond SSL cache server

sslVersion = SSL_VERSION

select the SSL protocol version

Supported values: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2

Availability of specific protocols depends on the linked OpenSSL library. Older versions of OpenSSL do not support TLSv1.1 and TLSv1.2. Newer versions of OpenSSL do not support SSLv2.

Obsolete SSLv2 and SSLv3 are currently disabled by default. See the options option documentation for details.

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 to a remote host

TIMEOUTidle = SECONDS

time to keep an idle connection

transparent = none | source | destination | both (Unix only)

enable transparent proxy support on selected platforms

Supported values:

none

Disable transparent proxy support. This is the default.

source

Re-write the address to appear as if a wrapped daemon is connecting from the SSL client machine instead of the machine running stunnel.

This option is currently available in:

Remote mode (connect option) on Linux >=2.6.28

This configuration requires stunnel to be executed as root and without the setuid option.

This configuration 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
    echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter

stunnel must also to be executed as root and without the setuid option.

Remote mode (connect option) on Linux 2.2.x

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

stunnel must also to be executed as root and without the setuid option.

Remote mode (connect option) on FreeBSD >=8.0

This configuration requires additional firewall and routing setup. stunnel must also to be executed as root and without the setuid option.

Local mode (exec option)

This configuration works by pre-loading the libstunnel.so shared library. _RLD_LIST environment variable is used on Tru64, and LD_PRELOAD variable on other platforms.

destination

The original destination is used instead of the connect option.

A service section for transparent destination may look like this:

    [transparent]
    client = yes
    accept = <stunnel_port>
    transparent = destination

This configuration requires iptables setup to work, possibly in /etc/rc.local or equivalent file.

For a connect target installed on the same host:

    /sbin/iptables -t nat -I OUTPUT -p tcp --dport <redirected_port> \
        -m ! --uid-owner <stunnel_user_id> \
        -j DNAT --to-destination <local_ip>:<stunnel_port>

For a connect target installed on a remote host:

    /sbin/iptables -I INPUT -i eth0 -p tcp --dport <stunnel_port> -j ACCEPT
    /sbin/iptables -t nat -I PREROUTING -p tcp --dport <redirected_port> \
        -i eth0 -j DNAT --to-destination <local_ip>:<stunnel_port>

The transparent destination option is currently only supported on Linux.

both

Use both source and destination transparent proxy.

Two legacy options are also supported for backward compatibility:

yes

This option has been renamed to source.

no

This option has been renamed to none.

verify = LEVEL

verify the peer certificate

level 0

Request and ignore the peer certificate.

level 1

Verify the peer certificate if present.

level 2

Verify the peer certificate.

level 3

Verify the peer with locally installed certificate.

level 4

Ignore the CA chain and only verify the peer 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

or in remote mode:

    [imapd]
    accept = 993
    connect = 143

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

    [imap]
    client = yes
    accept = 143
    connect = servername:993

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

To setup SOCKS VPN configure the following client service:

    [socks_client]
    client = yes
    accept = 127.0.0.1:1080
    connect = vpn_server:9080
    verify = 4
    CAfile = stunnel.pem

The corresponding configuration on the vpn_server host:

    [socks_server]
    protocol = socks
    accept = 9080
    cert = stunnel.pem
    key = stunnel.key

Now test your configuration on the client machine with:

    curl --socks4a localhost http://www.example.com/

NOTES

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.

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 also be empty lines after the certificate and the private key. Any 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 or later) 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.

Important note: If /dev/urandom is available, OpenSSL often seeds the PRNG with it while checking the random state. On systems with /dev/urandom OpenSSL is likely to use it even though it is listed at the very bottom of the list above. This is the behaviour of OpenSSL and not stunnel.

DH PARAMETERS

stunnel 4.40 and later contains hardcoded 2048-bit DH parameters. Starting with stunnel 5.18, these hardcoded DH parameters are replaced every 24 hours with autogenerated temporary DH parameters. DH parameter generation may take several minutes.

Alternatively, it is possible to specify static DH parameters in the certificate file, which disables generating temporary DH parameters:

    openssl dhparam 2048 >> stunnel.pem

FILES

@sysconfdir@/stunnel/stunnel.conf

stunnel configuration file

AUTHOR

Michał Trojnara

<[email protected]>