stunnel - universal SSL tunnel
stunnel [<filename>] | -fd n | -help | -version | -sockets
stunnel [ [-install | -uninstall | -start | -stop] | -exit] [-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 (eay@cryptsoft.com)
Use specified configuration file
Read the config file from specified file descriptor
Print stunnel help menu
Print stunnel version and compile time defaults
Print default socket options
Install NT Service
Uninstall NT Service
Start NT Service
Stop NT Service
Exit an already started stunnel
Don't display any message boxes
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).
directory to chroot stunnel process
chroot keeps stunnel in chrooted jail. CApath, CRLpath, pid and exec are located inside the jail and the patches have to be relative to the directory specified with chroot.
Several functions of the operating system also need their files to be located within chroot jail, e.g.:
Delayed resolver typically needs /etc/nsswitch.conf and /etc/resolv.conf.
Local time in log files needs /etc/timezone.
Some other functions may need devices, e.g. /dev/zero or /dev/null.
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.
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.
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)
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
control hardware engine
Special commands "LOAD" and "INIT" can be used to load and initialize the engine cryptogaphic module.
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 mode
Stay in foreground (don't fork) and log to stderr instead of via syslog (unless output is specified).
default: background in daemon mode
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 location
If the argument is empty, then no pid file will be created.
pid path is relative to chroot directory if specified.
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.
path to file with random seed data
The SSL library will use data from this file first to seed the random number generator.
overwrite the random seed files with new random data
default: yes
use specified string as inetd mode service name for TCP Wrapper library
default: stunnel
setgid() to groupname in daemon mode and clears all other groups
setuid() to username in daemon mode
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
enable logging via syslog
default: yes
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 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
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.
Certificate Authority file
This file contains multiple CA certificates, used with the verify.
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).
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 mode (remote service uses SSL)
default: no (server mode)
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.
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.
Certificate Revocation Lists file
This file contains multiple CRLs, used with the verify.
specify ECDH curve name
To get a list of supported cuves use:
openssl ecparam -list_curves
default: prime256v1
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).
select engine number to read private key
The engines are numbered starting from 1.
execute local inetd-type program
exec path is relative to chroot directory if specified.
arguments for exec including program name ($0)
Quoting is currently not supported. Arguments are separated with arbitrary number of whitespaces.
Failover strategy for multiple "connect" targets.
rr (round robin) - fair load distribution
prio (priority) - use the order specified in config file
default: rr
use IDENT (RFC 1413) username checking
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
Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.
default: yes
IP of the outgoing interface is used as source for remote connections. Use this option to bind a static local IP address, instead.
Use the service as a slave service (a name-based virtual server) for Server Name Indication TLS extension (RFC 3546).
service_name specifies the master service that accepts client connections with accept option. server_name_pattern specifies the host name to be redirected. The pattern may start with '*' character, e.g. '*.example.com'. Multiple slave services are normally specified for a single master service. sni option can also be specified more than once within a single slave service.
This service, as well as the master service, may not be configured in client mode.
connect option of the slave service is ignored when protocol option is specified, as protocol connects remote host before TLS handshake.
Libwrap checks (Unix only) are performed twice: with master service name after TCP connection is accepted, and with slave service name during TLS handshake.
Option sni is only available when compiled with OpenSSL 1.0.0 and later.
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.
select OCSP server for certificate verification
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
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
application protocol to negotiate SSL
This option enables initial, protocol-specific negotiation of the SSL/TLS encryption. protocol option should not be used with SSL encryption on a separate port.
Currently supported protocols:
Proprietary (undocummented) extension of CIFS protocol implemented in Samba. Support for this extension was dropped in Samba 3.0.0.
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.
Based on RFC 2595 - Using TLS with IMAP, POP3 and ACAP
Based on RFC 4642 - Using Transport Layer Security (TLS) with Network News Transfer Protocol (NNTP)
This protocol is only supported in client mode.
Based on http://www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982
Based on RFC 2449 - POP3 Extension Mechanism
Haproxy client IP address http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt
Based on RFC 2487 - SMTP Service Extension for Secure SMTP over TLS
authentication type for protocol negotiations
currently supported: basic, NTLM
Currently authentication type only applies to the 'connect' protocol.
default: basic
destination address for protocol negotiations
protocolHost specifies the final SSL server to be connected by the proxy, and not the proxy server directly connected by stunnel. The proxy server should be specified with the 'connect' option.
Currently protocol destination address only applies to 'connect' protocol.
password for protocol negotiations
username for protocol negotiations
allocate pseudo terminal for 'exec' option
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)
attempt to use TCP RST flag to indicate an error
This option is not supported on some platforms.
default: yes
reconnect a connect+exec section after it's disconnected
default: no
session cache size
sessionCacheSize specifies the maximum number of the internal session cache entries.
The value of 0 can be used for unlimited size. It is not recommended for production use due to the risk of memory exhaustion DoS attack.
session cache timeout
This is the number of seconds to keep cached SSL sessions.
address of sessiond SSL cache server
select version of SSL protocol
Allowed options: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2
thread stack size
time to wait for expected data
time to wait for close_notify (set to 0 for buggy MSIE)
time to wait to connect a remote host
time to keep an idle connection
enable transparent proxy support on selected platforms
Supported values:
Disable transparent proxy support. This is the default.
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:
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.
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.
This configuration requires additional firewall and routing setup. stunnel must also to be executed as root and without setuid 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.
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.
Use both source and destination transparent proxy.
Two legacy options are also supported for backward compatibility:
This options has been renamed to source.
This options has been renamed to none.
verify peer certificate
Request and ignore peer certificate.
Verify peer certificate if present.
Verify peer certificate.
Verify peer with locally installed certificate.
Ignore CA chain and only verify peer certificate.
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.
stunnel returns zero on success, non-zero on error.
The following signals can be used to control stunnel in Unix environment:
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.
Close and reopen stunnel log file. This function can be used for log rotation.
Shut stunnel down.
The result of sending any other signals to the server is undefined.
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
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/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.
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]
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.
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.
stunnel configuration file
Option execargs and Win32 command line does not support quoting.
access control facility for internet services
internet 'super-server'
stunnel homepage
OpenSSL project website