stunnel - Unix, Linux Command

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stunnel - universal SSL tunnel


Unix: stunnel [<filename>] | -fd n | -help | -version | -sockets
WIN32: stunnel [ [-install | -uninstall | -start | -stop]
[-quiet] [<filename>] ] | -help | -version | -sockets


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 (


<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


Each line of the configuration file can be either:
o an empty line (ignored)
o a comment starting with ’;’ (ignored)
o an ’option_name = option_value’ pair
o ’[service_name]’ indicating a start of a service definition


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

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)

engine = auto | <engine id> select hardware engine

default: software-only cryptography

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.


    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
taskbar = yes | no (WIN32 only) enable the taskbar icon

default: yes


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 cert.

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 remote host:port

If no host specified, defaults to localhost.

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
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.

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.
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:

protocol = proto application protocol to negotiate SSL

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

protocolCredentials = username:password credentials for protocol negotiations
protocolHost = host:port destination address for protocol negotiations
pty = yes | no (Unix only) allocate pseudo terminal for ’exec’ option
session = timeout session cache timeout
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 only available in local mode (exec option) by LD_PRELOADing 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.

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


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


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

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

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

    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


stunnel.conf stunnel configuration file
stunnel.pem stunnel certificate and private key


Option execargs does not support quoting.


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.



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/sbin/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.


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:

    [encoded key]
    -----END RSA PRIVATE KEY-----
    [empty line]
    [encoded certificate]
    -----END CERTIFICATE-----
    [empty line]


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:
o The file specified with the RNDfile flag.
o The file specified by the RANDFILE environment variable, if set.
o The file .rnd in your home directory, if RANDFILE not set.
o The file specified with ’--with-random’ at compile time.
o The contents of the screen if running on Windows.
o The egd socket specified with the EGD flag.
o The egd socket specified with ’--with-egd-sock’ at compile time.
o 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.


tcpd(8) access control facility for internet services
inetd(8) internet ’super-server’ stunnel homepage stunnel Frequently Asked Questions OpenSSL project website


Michal Trojnara <>

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