STUNNEL(8)

NAME
stunnel - universal SSL tunnel

SYNOPSIS

Unix:
    stunnel [FILE] | -fd N | -help | -version | -sockets

WIN32:
    stunnel [ [-install | -uninstall | -start | -stop] | -exit]
        [-quiet] [FILE] ] | -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

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

-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

-exit (Win32 only)
    Exit an already started stunnel

-quiet (NT/2000/XP 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 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 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 | rle
    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.

    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

    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

engineCtrl = COMMAND[:PARAMETER]
    control hardware engine

    Special commands "LOAD" and "INIT" can be used to load and initialize the engine cryptogaphic 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 to disable entering FIPS mode if stunnel was compiled with FIPS 140-2 support.

    default: no (since version 5.00)

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

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 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 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)
    use specified string as inetd mode service name for TCP Wrapper library

    default: stunnel

setgid = GROUP (Unix only)
    setgid() to the specified group in daemon mode and clear all other groups

setuid = USER (Unix only)
    setuid() to the specified user 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: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 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:

        connect = :::PORT

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 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 chroot directory if specified.

CAfile = CERT_FILE
    Certificate Authority file

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

cert = PEM_FILE
    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 = 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)

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. 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 chroot directory if specified.

CRLfile = CERT_FILE
    Certificate Revocation Lists file

    This file contains multiple CRLs, used with the verify.

curve = NID
    specify ECDH curve name

    To get a list of supported cuves use:

        openssl ecparam -list_curves

    default: prime256v1

delay = yes | no
    delay DNS lookup for 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 local inetd-type program

    exec path is relative to chroot directory if specified.

execargs = $0 $1 $2 ...
    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 = KEY_FILE
    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

libwrap = yes | no
    Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.

    default: no (since version 5.00)

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.

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 accept option. server_name_pattern specifies the host name to be redirected. The pattern may start with '*' character, e.g. '*.example.com'. Multiple slave services are normally specified for a single master service. 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.

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

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

    Option sni 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.

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

OCSP = URL
    select OCSP server for certificate verification

OCSPflag = OCSP_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

    This option enables initial, protocol-specific negotiation of the SSL/TLS encryption. 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

protocolAuthentication = basic | ntlm
    authentication type for protocol negotiations

    Currently authentication type only applies to the 'connect' protocol.

    default: basic

protocolHost = HOST:PORT
    destination address for protocol negotiations

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

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

protocolPassword = PASSWORD
    password for protocol negotiations

protocolUsername = USERNAME
    username for protocol negotiations

pty = yes | no (Unix only)
    allocate pseudoterminal for 'exec' option

redirect = [HOST:]PORT
    redirect SSL client connections on authentication failures

    This option only works in server mode, and without protocol negotiation.

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 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's disconnected

    default: no

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 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 version of SSL protocol

    Allowed options: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2

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 = 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 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 (connect option) on Linux >=2.6.28
            This configuration requires stunnel to be executed as root and without 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 setuid option.

        Remote mode (connect option) on Linux 2.2.x
            This configuration 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.

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

        Local mode (exec option)
            This configuration works by pre-loading libstunnel.so shared library. _RLD_LIST environment variable is used on Tru64, and LD_PRELOAD variable on other platforms.

    destination
        Original destination is used instead of connect option.

        A service section for transparent destination may look like this:

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

        This configuration requires the following setup for iptables (possibly in /etc/rc.local or equivalent file):

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

        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 options has been renamed to source.

    no  This options has been renamed to none.

verify = LEVEL
    verify peer certificate

    level 0
        Request and ignore peer certificate.

    level 1
        Verify peer certificate if present.

    level 2
        Verify peer certificate.

    level 3
        Verify peer with locally installed certificate.

    level 4
        Ignore CA chain and only verify 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.

SIGNALS
The following signals can be used to control stunnel in Unix environment:

SIGHUP
    Force a reload of the configuration file.

    Some global options will not be reloaded:

    •   chroot

    •   foreground

    •   pid

    •   setgid

    •   setuid

    The use of 'setuid' option will also prevent stunnel from binding privileged (<1024) ports during configuration reloading.

    When 'chroot' option is used, stunnel will look for all its files (including configuration file, certificates, log file and pid file) within the chroot jail.

SIGUSR1
    Close and reopen stunnel log file. This function can be used for log rotation.

SIGTERM, SIGQUIT, SIGINT
    Shut stunnel down.

The result of sending any other signals to the server is undefined.

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

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 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 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.

It is also possible to specify DH parameters in the certificate file:

    openssl dhparam 2048 >> stunnel.pem

DH parameter generation may take several minutes.

FILES

stunnel.conf
    stunnel configuration file

BUGS
Option execargs and Win32 command line does not support quoting.

SEE ALSO

tcpd(8)
    access control facility for internet services

inetd(8)
    internet 'super-server'

http://www.stunnel.org/
    stunnel homepage

http://www.openssl.org/
    OpenSSL project website

AUTHOR

Michał Trojnara
    <Michal.Trojnara@mirt.net>