NAME¶
stunnel - universal SSL tunnel
SYNOPSIS¶
- Unix:
- stunnel [<filename>] | -fd n | -help |
-version | -sockets
- WIN32:
- stunnel [ [-install | -uninstall | -start | -stop] |
-exit]
[-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
- -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.
- 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.
- 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
/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 = servicename (Unix only)
- use specified string as inetd mode service name for
TCP Wrapper library
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: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 = address
- 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 = 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 = address
- 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 = certfile
- 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).
- engineNum = engine number
- select engine number to read private key
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 = 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
- libwrap = yes | no
- Enable or disable the use of /etc/hosts.allow and
/etc/hosts.deny.
default: yes
- 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_name:server_name (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 specifies the
host name to be redirected. 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_name (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 = 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 (e.g. starttls
or stls)
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 = 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 = 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) 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.
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 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>