stunnel(8)
NAME
stunnel - TLS offloading and load-balancing proxy
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
(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
-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 verifyChain or verifyPeer options. 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, to be used with the
verifyChain and verifyPeer options.
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 verifyChain and verifyPeer options. 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 verifyChain and
verifyPeer options.
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_NAME:SERVER_NAME_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_NAME (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
requireCert = yes | no
require a client certificate for verifyChain or verifyPeer
With requireCert set to no, the stunnel server accepts client
connections that did not present a certificate.
Both verifyChain = yes and verifyPeer = yes imply requireCert =
yes.
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
This option is obsolete and should be replaced with the verifyChain
and verifyPeer options.
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 against a locally installed certificate.
level 4
Ignore the chain and only verify the peer certificate.
default
No verify.
verifyChain = yes | no
verify the peer certificate chain starting from the root CA
For server certificate verification it is essential to also require
a specific certificate with checkHost or verifyPeer.
The self-signed root CA certificate needs to be stored either in
the file specified with CAfile, or in the directory specified with
CApath.
default: no
verifyPeer = yes | no
verify the peer certificate
The peer certificate needs to be stored either in the file
specified with CAfile, or in the directory specified with CApath.
default: no
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 the 'setuid' option will also prevent stunnel from
binding to privileged (<1024) ports during configuration reloading.
When the 'chroot' option is used, stunnel will look for all its
files (including the configuration file, certificates, the log file
and the pid file) within the chroot jail.
SIGUSR1
Close and reopen the 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
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
verifyPeer = yes
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
BUGS
The execArgs option and the Win32 command line do 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@stunnel.org>
Free and Open Source Software