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=head1 NAME
=encoding utf8
stunnel - TLS offloading and load-balancing proxy
=over 4
=item B<Unix:>
B<stunnel> [S<FILE>] | S<-fd N> | S<-help> | S<-version> | S<-sockets> | S<-options>
=item B<WIN32:>
B<stunnel> [ [ S<-install> | S<-uninstall> | S<-start> | S<-stop> |
S<-reload> | S<-reopen> | S<-exit> ] [S<-quiet>] [S<FILE>] ] |
S<-help> | S<-version> | S<-sockets> | S<-options>
The B<stunnel> program is designed to work as I<SSL> encryption wrapper
between remote clients and local (I<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.
B<stunnel> can be used to add SSL functionality to commonly used I<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 (
=head1 OPTIONS
=over 4
=item B<FILE>
Use specified configuration file
=item B<-fd N> (Unix only)
Read the config file from specified file descriptor
=item B<-help>
Print B<stunnel> help menu
=item B<-version>
Print B<stunnel> version and compile time defaults
=item B<-sockets>
Print default socket options
=item B<-options>
Print supported SSL options
=item B<-install> (Windows NT and later only)
Install NT Service
=item B<-uninstall> (Windows NT and later only)
Uninstall NT Service
=item B<-start> (Windows NT and later only)
Start NT Service
=item B<-stop> (Windows NT and later only)
Stop NT Service
=item B<-reload> (Windows NT and later only)
Reload the configuration file of the running NT Service
=item B<-reopen> (Windows NT and later only)
Reopen the log file of the running NT Service
=item B<-exit> (Win32 only)
Exit an already started stunnel
=item B<-quiet> (Win32 only)
Don't display any message boxes
Each line of the configuration file can be either:
=over 4
=item *
An empty line (ignored).
=item *
A comment starting with ';' (ignored).
=item *
An 'option_name = option_value' pair.
=item *
'[service_name]' indicating a start of a service definition.
An address parameter of an option may be either:
=over 4
=item *
A port number.
=item *
A colon-separated pair of IP address (either IPv4, IPv6, or domain name) and port number.
=item *
A Unix socket path (Unix only).
=over 4
=item B<chroot> = DIRECTORY (Unix only)
directory to chroot B<stunnel> process
B<chroot> keeps B<stunnel> in a chrooted jail. I<CApath>, I<CRLpath>, I<pid>
and I<exec> are located inside the jail and the patches have to be relative
to the directory specified with B<chroot>.
Several functions of the operating system also need their files to be located within the chroot jail, e.g.:
=over 4
=item *
Delayed resolver typically needs /etc/nsswitch.conf and /etc/resolv.conf.
=item *
Local time in log files needs /etc/timezone.
=item *
Some other functions may need devices, e.g. /dev/zero or /dev/null.
=item B<compression> = deflate | zlib
select data compression algorithm
default: no compression
deflate is the standard compression method as described in RFC 1951.
zlib compression of B<OpenSSL 0.9.8> or above is not backward compatible with
B<OpenSSL 0.9.7>.
=item B<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 I<debug = debug> or
I<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.
=item B<EGD> = EGD_PATH (Unix only)
path to Entropy Gathering Daemon socket
Entropy Gathering Daemon socket to use to feed the B<OpenSSL> random number
generator. (Available only if compiled with B<OpenSSL 0.9.5a> or higher)
=item B<engine> = auto | ENGINE_ID
select hardware engine
default: software-only cryptography
Here is an example of advanced engine configuration to read the private key from an
OpenSC engine
=item B<engineCtrl> = COMMAND[:PARAMETER]
control hardware engine
Special commands "LOAD" and "INIT" can be used to load and initialize the
engine cryptogaphic module.
=item B<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,
=item B<fips> = yes | no
Enable or disable FIPS 140-2 mode.
This option allows you to disable entering FIPS mode if B<stunnel> was compiled
with FIPS 140-2 support.
default: no (since version 5.00)
=item B<foreground> = yes | no (Unix only)
foreground mode
Stay in foreground (don't fork) and log to stderr
instead of via syslog (unless I<output> is specified).
default: background in daemon mode
=item B<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.
=item B<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.
=item B<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.
=item B<log> = append | overwrite
log file handling
This option allows you to choose whether the log file (specified with the I<output>
option) is appended or overwritten when opened or re-opened.
default: append
=item B<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).
=item B<pid> = FILE (Unix only)
pid file location
If the argument is empty, then no pid file will be created.
I<pid> path is relative to the I<chroot> directory if specified.
=item B<RNDbytes> = BYTES
bytes to read from random seed files
Number of bytes of data read from random seed files. With SSL versions less
than B<0.9.5a>, also determines how many bytes of data are considered
sufficient to seed the PRNG. More recent B<OpenSSL> versions have a builtin
function to determine when sufficient randomness is available.
=item B<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.
=item B<RNDoverwrite> = yes | no
overwrite the random seed files with new random data
default: yes
=item B<service> = SERVICE (Unix only)
stunnel service name
The specified service name is used for syslog and as the I<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
=item B<setgid> = GROUP (Unix only)
setgid() to the specified group in daemon mode and clear all other groups
=item B<setuid> = USER (Unix only)
setuid() to the specified user in daemon mode
=item B<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.
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
=item B<syslog> = yes | no (Unix only)
enable logging via syslog
default: yes
=item B<taskbar> = yes | no (WIN32 only)
enable the taskbar icon
default: yes
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 B<stunnel> services in your log files.
Note that if you wish to run B<stunnel> in I<inetd> mode (where it
is provided a network socket by a server such as I<inetd>, I<xinetd>,
or I<tcpserver>) then you should read the section entitled I<INETD MODE>
=over 4
=item B<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
=item B<CApath> = DIRECTORY
Certificate Authority directory
This is the directory in which B<stunnel> will look for certificates when using
the I<verify> option. Note that the certificates in this directory should be named
XXXXXXXX.0 where XXXXXXXX is the hash value of the DER encoded subject of the
The hash algorithm has been changed in B<OpenSSL 1.0.0>. It is required to
c_rehash the directory on upgrade from B<OpenSSL 0.x.x> to B<OpenSSL 1.x.x>.
I<CApath> path is relative to the I<chroot> directory if specified.
=item B<CAfile> = CERT_FILE
Certificate Authority file
This file contains multiple CA certificates, used with the I<verify> option.
=item B<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.
=item B<checkEmail> = EMAIL
email address of the peer certificate subject
Multiple I<checkEmail> options are allowed in a single service section.
Certificates are accepted if no I<checkEmail> option was specified, or the
email address of the peer certificate matches any of the email addresses
specified with I<checkEmail>.
=item B<checkHost> = HOST
host of the peer certificate subject
Multiple I<checkHost> options are allowed in a single service section.
Certificates are accepted if no I<checkHost> option was specified, or the host
name of the peer certificate matches any of the hosts specified with
=item B<checkIP> = IP
IP address of the peer certificate subject
Multiple I<checkIP> options are allowed in a single service section.
Certificates are accepted if no I<checkIP> option was specified, or the IP
address of the peer certificate matches any of the IP addresses specified with
=item B<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.
=item B<client> = yes | no
client mode (remote service uses SSL)
default: no (server mode)
=item B<connect> = [HOST:]PORT
connect to a remote address
If no host is specified, the host defaults to localhost.
Multiple I<connect> options are allowed in a single service section.
If host resolves to multiple addresses and/or if multiple I<connect>
options are specified, then the remote address is chosen using a
round-robin algorithm.
=item B<CRLpath> = DIRECTORY
Certificate Revocation Lists directory
This is the directory in which B<stunnel> will look for CRLs when
using the I<verify> option. Note that the CRLs in this directory should
be named XXXXXXXX.r0 where XXXXXXXX is the hash value of the CRL.
The hash algorithm has been changed in B<OpenSSL 1.0.0>. It is required to
c_rehash the directory on upgrade from B<OpenSSL 0.x.x> to B<OpenSSL 1.x.x>.
I<CRLpath> path is relative to the I<chroot> directory if specified.
=item B<CRLfile> = CERT_FILE
Certificate Revocation Lists file
This file contains multiple CRLs, used with the I<verify> option.
=item B<curve> = NID
specify ECDH curve name
To get a list of supported curves use:
openssl ecparam -list_curves
default: prime256v1
=item B<logId> = TYPE
connection identifier type
This identifier allows you to distinguish log entries generated for each of the
Currently supported types:
=over 4
=item I<sequential>
The numeric sequential identifier is only unique within a single instance of
B<stunnel>, but very compact. It is most useful for manual log analysis.
=item I<unique>
This alphanumeric identifier is globally unique, but longer than the sequential
number. It is most useful for automated log analysis.
=item I<thread>
The operating system thread identifier is neither unique (even within a single
instance of B<stunnel>) nor short. It is most useful for debugging software
or configuration issues.
default: sequential
=item B<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 I<debug = debug> or
I<debug = 7> for greatest debugging output. The default is notice (5).
=item B<delay> = yes | no
delay DNS lookup for the I<connect> option
This option is useful for dynamic DNS, or when DNS is not available during
B<stunnel> startup (road warrior VPN, dial-up configurations).
Delayed resolver mode is automatically engaged when stunnel fails to resolve on
startup any of the I<connect> targets for a service.
Delayed resolver inflicts I<failover = prio>.
default: no
=item B<engineId> = ENGINE_ID
select engine ID for the service
=item B<engineNum> = ENGINE_NUMBER
select engine number for the service
The engines are numbered starting from 1.
=item B<exec> = EXECUTABLE_PATH
execute a local inetd-type program
I<exec> path is relative to the I<chroot> directory if specified.
The following environmental variables are set on Unix platforms:
=item B<execArgs> = $0 $1 $2 ...
arguments for I<exec> including the program name ($0)
Quoting is currently not supported.
Arguments are separated with an arbitrary amount of whitespace.
=item B<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
=item B<ident> = USERNAME
use IDENT (RFC 1413) username checking
=item B<include> = DIRECTORY
include all configuration file parts located in DIRECTORY
The files are included in the ascending alphabetical order of their names.
=item B<key> = KEY_FILE
private key for the certificate specified with I<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
default: the value of the I<cert> option
=item B<libwrap> = yes | no
Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.
default: no (since version 5.00)
=item B<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.
=item B<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).
I<service_name> specifies the master service that accepts client connections
with the I<accept> option. I<server_name_pattern> specifies the host name to be
redirected. The pattern may start with the '*' character, e.g. '*'.
Multiple slave services are normally specified for a single master service.
The I<sni> option can also be specified more than once within a single slave
This service, as well as the master service, may not be configured in client
The I<connect> option of the slave service is ignored when the I<protocol> option is
specified, as I<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 I<sni> option is only available when compiled with B<OpenSSL 1.0.0> and later.
=item B<sni> = SERVER (client mode)
Use the parameter as the value of TLS Server Name Indication (RFC 3546)
The I<sni> option is only available when compiled with B<OpenSSL 1.0.0> and later.
=item B<OCSP> = URL
select OCSP server for certificate verification
=item B<OCSPaia> = yes | no
validate certificates with their AIA OCSP responders
This option enables I<stunnel> to validate certificates with the list of
OCSP responder URLs retrieved from their AIA (Authority Information Access)
=item B<OCSPflag> = OCSP_FLAG
specify OCSP server flag
Several I<OCSPflag> can be used to specify multiple flags.
=item B<options> = SSL_OPTIONS
B<OpenSSL> library options
The parameter is the B<OpenSSL> option name as described in the
I<SSL_CTX_set_options(3ssl)> manual, but without I<SSL_OP_> prefix.
I<stunnel -options> lists the options found to be allowed in the
current combination of I<stunnel> and the I<OpenSSL> library used
to build it.
Several I<options> 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 = NO_SSLv2
options = NO_SSLv3
=item B<protocol> = PROTO
application protocol to negotiate SSL
This option enables initial, protocol-specific negotiation of the SSL/TLS
The I<protocol> option should not be used with SSL encryption on a separate port.
Currently supported protocols:
=over 4
=item I<cifs>
Proprietary (undocummented) extension of CIFS protocol implemented in Samba.
Support for this extension was dropped in Samba 3.0.0.
=item I<connect>
Based on RFC 2817 - I<Upgrading to TLS Within HTTP/1.1>, section 5.2 - I<Requesting a Tunnel with CONNECT>
This protocol is only supported in client mode.
=item I<imap>
Based on RFC 2595 - I<Using TLS with IMAP, POP3 and ACAP>
=item I<nntp>
Based on RFC 4642 - I<Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP)>
This protocol is only supported in client mode.
=item I<pgsql>
Based on
=item I<pop3>
Based on RFC 2449 - I<POP3 Extension Mechanism>
=item I<proxy>
Haproxy client IP address
=item I<smtp>
Based on RFC 2487 - I<SMTP Service Extension for Secure SMTP over TLS>
=item I<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.
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
=item B<protocolAuthentication> = basic | ntlm
authentication type for protocol negotiations
Currently the authentication type only applies to the 'connect' protocol.
default: basic
=item B<protocolHost> = HOST:PORT
destination address for protocol negotiations
I<protocolHost> specifies the final SSL server to be connected to by the proxy,
and not the proxy server directly connected by B<stunnel>.
The proxy server should be specified with the 'connect' option.
Currently the protocol destination address only applies to 'connect' protocol.
=item B<protocolPassword> = PASSWORD
password for protocol negotiations
=item B<protocolUsername> = USERNAME
username for protocol negotiations
=item B<PSKidentity> = IDENTITY
PSK identity for the PSK client
I<PSKidentity> can be used on B<stunnel> clients to select the PSK identity
used for authentication. This option is ignored in server sections.
default: the first identity specified in the I<PSKsecrets> file.
=item B<PSKsecrets> = FILE
file with PSK identities and corresponding keys
Each line of the file in the following format:
The key is required to be at least 20 characters long.
The file should not be world-readable nor world-writable.
=item B<pty> = yes | no (Unix only)
allocate a pseudoterminal for 'exec' option
=item B<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 I<redirect> option.
=item B<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:
Please note that disabling SSL renegotiation does not fully mitigate
this issue.
default: yes (if supported by B<OpenSSL>)
=item B<reset> = yes | no
attempt to use the TCP RST flag to indicate an error
This option is not supported on some platforms.
default: yes
=item B<retry> = yes | no
reconnect a connect+exec section after it's disconnected
default: no
=item B<sessionCacheSize> = NUM_ENTRIES
session cache size
I<sessionCacheSize> specifies the maximum number of the internal session cache
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.
=item B<sessionCacheTimeout> = TIMEOUT
session cache timeout
This is the number of seconds to keep cached SSL sessions.
=item B<sessiond> = HOST:PORT
address of sessiond SSL cache server
=item B<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 B<options> option documentation for details.
=item B<stack> = BYTES (except for FORK model)
thread stack size
=item B<TIMEOUTbusy> = SECONDS
time to wait for expected data
=item B<TIMEOUTclose> = SECONDS
time to wait for close_notify (set to 0 for buggy MSIE)
=item B<TIMEOUTconnect> = SECONDS
time to wait to connect to a remote host
=item B<TIMEOUTidle> = SECONDS
time to keep an idle connection
=item B<transparent> = none | source | destination | both (Unix only)
enable transparent proxy support on selected platforms
Supported values:
=over 4
=item I<none>
Disable transparent proxy support. This is the default.
=item I<source>
Re-write the address to appear as if a wrapped daemon is connecting
from the SSL client machine instead of the machine running B<stunnel>.
This option is currently available in:
=over 4
=item Remote mode (I<connect> option) on I<Linux E<gt>=2.6.28>
This configuration requires B<stunnel> to be executed as root and without
the I<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 dev lo table 100
echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter
B<stunnel> must also to be executed as root and without the I<setuid> option.
=item Remote mode (I<connect> option) on I<Linux 2.2.x>
This configuration requires the kernel to be compiled with the I<transparent proxy>
Connected service must be installed on a separate host.
Routing towards the clients has to go through the B<stunnel> box.
B<stunnel> must also to be executed as root and without the I<setuid> option.
=item Remote mode (I<connect> option) on I<FreeBSD E<gt>=8.0>
This configuration requires additional firewall and routing setup.
B<stunnel> must also to be executed as root and without the I<setuid> option.
=item Local mode (I<exec> option)
This configuration works by pre-loading the I<> shared library.
_RLD_LIST environment variable is used on Tru64, and LD_PRELOAD variable on
other platforms.
=item I<destination>
The original destination is used instead of the I<connect> option.
A service section for transparent destination may look like this:
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.
=item I<both>
Use both I<source> and I<destination> transparent proxy.
Two legacy options are also supported for backward compatibility:
=over 4
=item I<yes>
This option has been renamed to I<source>.
=item I<no>
This option has been renamed to I<none>.
=item B<verify> = LEVEL
verify the peer certificate
=over 4
=item level 0
Request and ignore the peer certificate.
=item level 1
Verify the peer certificate if present.
=item level 2
Verify the peer certificate.
=item level 3
Verify the peer with locally installed certificate.
=item level 4
Ignore the CA chain and only verify the peer certificate.
=item 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.
B<stunnel> returns zero on success, non-zero on error.
=head1 SIGNALS
The following signals can be used to control B<stunnel> in Unix environment:
=over 4
=item SIGHUP
Force a reload of the configuration file.
Some global options will not be reloaded:
=over 4
=item *
=item *
=item *
=item *
=item *
The use of the 'setuid' option will also prevent B<stunnel> from binding to privileged
(<1024) ports during configuration reloading.
When the 'chroot' option is used, B<stunnel> will look for all its files (including
the configuration file, certificates, the log file and the pid file) within the chroot
=item SIGUSR1
Close and reopen the B<stunnel> log file.
This function can be used for log rotation.
Shut B<stunnel> down.
The result of sending any other signals to the server is undefined.
In order to provide SSL encapsulation to your local I<imapd> service, use:
accept = 993
exec = /usr/sbin/imapd
execArgs = imapd
or in remote mode:
accept = 993
connect = 143
In order to let your local e-mail client connect to an SSL-enabled I<imapd>
service on another server, configure the e-mail client to connect to localhost
on port 119 and use:
client = yes
accept = 143
connect = servername:993
If you want to provide tunneling to your I<pppd> daemon on port 2020,
use something like:
accept = 2020
exec = /usr/sbin/pppd
execArgs = pppd local
pty = yes
If you want to use B<stunnel> in I<inetd> mode to launch your imapd
process, you'd use this I<stunnel.conf>.
Note there must be no I<[service_name]> section.
exec = /usr/sbin/imapd
execArgs = imapd
To setup SOCKS VPN configure the following client service:
client = yes
accept =
connect = vpn_server:9080
verify = 4
CAfile = stunnel.pem
The corresponding configuration on the vpn_server host:
protocol = socks
accept = 9080
cert = stunnel.pem
key = stunnel.key
Now test your configuration on the client machine with:
curl --socks4a localhost
=head1 NOTES
B<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 B<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 I<exec> option.
However there is a special case when you wish to have
some other program accept incoming connections and
launch B<stunnel>, for example with I<inetd>, I<xinetd>,
or I<tcpserver>.
For example, if you have the following line in I<inetd.conf>:
imaps stream tcp nowait root @bindir@/stunnel stunnel @sysconfdir@/stunnel/imaps.conf
In these cases, the I<inetd>-style program is responsible
for binding a network socket (I<imaps> above) and handing
it to B<stunnel> when a connection is received.
Thus you do not want B<stunnel> to have any I<accept> option.
All the I<Service Level Options> should be placed in the
global options section, and no I<[service_name]> section
will be present. See the I<EXAMPLES> section for example
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 B<OpenSSL> package. You can find more
information on certificates generation on pages listed below.
The order of contents of the I<.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:
[encoded key]
[empty line]
[encoded certificate]
[empty line]
B<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:
=over 4
=item *
The file specified with the I<RNDfile> flag.
=item *
The file specified by the RANDFILE environment variable, if set.
=item *
The file .rnd in your home directory, if RANDFILE not set.
=item *
The file specified with '--with-random' at compile time.
=item *
The contents of the screen if running on Windows.
=item *
The egd socket specified with the I<EGD> flag.
=item *
The egd socket specified with '--with-egd-sock' at compile time.
=item *
The /dev/urandom device.
With recent (B<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 I<RNDfile> flag.
Note that the file specified with the I<RNDfile> flag should contain
random data -- that means it should contain different information
each time B<stunnel> is run. This is handled automatically
unless the I<RNDoverwrite> flag is used. If you wish to update this file
manually, the I<openssl rand> command in recent versions of B<OpenSSL>,
would be useful.
Important note: If /dev/urandom is available, B<OpenSSL> often seeds the PRNG
with it while checking the random state. On systems with /dev/urandom
B<OpenSSL> is likely to use it even though it is listed at the very bottom of
the list above. This is the behaviour of B<OpenSSL> and not B<stunnel>.
B<stunnel> 4.40 and later contains hardcoded 2048-bit DH parameters. Starting
with B<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
=head1 FILES
=over 4
=item F<@sysconfdir@/stunnel/stunnel.conf>
B<stunnel> configuration file
=head1 BUGS
The I<execArgs> option and the Win32 command line do not support quoting.
=head1 SEE ALSO
=over 4
=item L<tcpd(8)>
access control facility for internet services
=item L<inetd(8)>
internet 'super-server'
=item F<>
B<stunnel> homepage
=item F<>
B<OpenSSL> project website
=head1 AUTHOR
=over 4
=item MichaƂ Trojnara
=for comment