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download PDF document: stunnel.8.pdf

stunnel(8) stunnel TLS Proxy 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 TLS encryption wrapper between remote clients and local (inetd-startable) or remote servers. The concept is that having non-TLS aware daemons running on your system you can easily set them up to communicate with clients over secure TLS channels.
stunnel can be used to add TLS 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 TLS 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)
-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:
o An empty line (ignored).
o A comment starting with ';' (ignored).
o An 'option_name = option_value' pair.
o '[service_name]' indicating a start of a service definition.
An address parameter of an option may be either:
o A port number.
o A colon-separated pair of IP address (either IPv4, IPv6, or domain name) and port number.
o 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.:
o Delayed resolver typically needs /etc/nsswitch.conf and /etc/resolv.conf.
o Local time in log files needs /etc/timezone.
o 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.
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 developer, or when you intend to get confused.
The default logging level is notice (5).
The syslog 'daemon' facility 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.
engine = auto | ENGINE_ID select hardware or software cryptographic engine
default: software-only cryptography
See Examples section for an engine configuration to use the certificate and the corresponding private key from a cryptographic device.
engineCtrl = COMMAND[:PARAMETER] control hardware engine
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
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
RNDfile = FILE path to file with random seed data
The OpenSSL 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
syslog = yes | no (Unix only) enable logging via syslog
default: yes
taskbar = yes | no (WIN32 only) 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
CAengine = ENGINE-SPECIFIC_CA_CERTIFICATE_IDENTIFIER load a trusted CA certificate from an engine
The loaded CA certificates will be used with the verifyChain and verifyPeer options.
Multiple CAengine options are allowed in a single service section.
Currently supported engines: pkcs11, cng.
CApath = CA_DIRECTORY load trusted CA certificates from a directory
The loaded CA certificates will be used with the verifyChain and 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 or later.
CApath path is relative to the chroot directory if specified.
CAfile = CA_FILE load trusted CA certificates from a file
The loaded CA certificates will be used with the verifyChain and verifyPeer options.
cert = CERT_FILE certificate chain file name
The parameter specifies the file containing certificates used by stunnel to authenticate itself against the remote client or server. The file should contain the whole certificate chain starting from the actual server/client certificate, and ending with the self- signed root CA certificate. The file must be either in PEM or P12 format.
A certificate chain is required in server mode, and optional in client mode.
Certificates are accepted if no subject checks were specified, or the email address of the end-entity (leaf) peer certificate matches any of the email addresses specified with checkEmail.
Multiple checkEmail options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
checkHost = HOST verify the host of the end-entity (leaf) peer certificate subject
Certificates are accepted if no subject checks were specified, or the host name of the end-entity (leaf) peer certificate matches any of the hosts specified with checkHost.
Multiple checkHost options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
checkIP = IP verify the IP address of the end-entity (leaf) peer certificate subject
Certificates are accepted if no subject checks were specified, or the IP address of the end-entity (leaf) peer certificate matches any of the IP addresses specified with checkIP.
Multiple checkIP options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
ciphers = CIPHER_LIST select permitted TLS ciphers (TLSv1.2 and below)
This option does not impact TLSv1.3 ciphersuites.
A colon-delimited list of the ciphers to allow in the TLS connection, for example DES-CBC3-SHA:IDEA-CBC-MD5.
ciphersuites = CIPHERSUITES_LIST select permitted TLSv1.3 ciphersuites
A colon-delimited list of TLSv1.3 ciphersuites names in order of preference.
This option requires OpenSSL 1.1.1 or later.
default: TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256
client = yes | no client mode (remote service uses TLS)
default: no (server mode)
config = COMMAND[:PARAMETER] OpenSSL configuration command

Use curves option instead of enabling config = Curves:list_curves to support elliptic curves.
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 = CRL_FILE Certificate Revocation Lists file
This file contains multiple CRLs, used with the verifyChain and verifyPeer options.
curves = list ECDH curves separated with ':'
Only a single curve name is allowed for OpenSSL older than 1.1.1.
To get a list of supported curves use:
openssl ecparam -list_curves
default:
X25519:P-256:X448:P-521:P-384 (OpenSSL 1.1.1 or later)
prime256v1 (OpenSSL older than 1.1.1)
logId = TYPE connection identifier type
This identifier allows you to distinguish log entries generated for each of the connections.
Currently supported types:
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.
process The operating system process identifier (PID) may be useful in the inetd mode.
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. The default is notice (5).
While the debug = debug or debug = 7 level generates the most verbose output, it is only intended to be used by stunnel developers. Please only use this value if you are a developer, or you intend to send your logs to our technical support. Otherwise, the generated logs will be confusing.
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) rr round robin - fair load distribution
prio priority - use the order specified in config file
default: prio
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. The recommended filename convention is
for global options:
00-global.conf
for local service-level options:
01-service.conf
02-service.conf
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.
OCSP = URL select OCSP responder for the end-entity (leaf) peer 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 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.
OCSPrequire = yes | no require a conclusive OCSP response
Disable this option to allow a connection even though no conclusive OCSP response was retrieved from stapling and a direct request to the OCSP responder.
default: yes
options = SSL_OPTIONS OpenSSL library options
The parameter is the OpenSSL option name as described in the SSSSLL_CCTTXX_sseett_ooppttiioonnss(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 TLS implementation, the following option can be used:
options = DONT_INSERT_EMPTY_FRAGMENTS
default:
options = NO_SSLv2 options = NO_SSLv3
Use sslVersionMax or sslVersionMin option instead of disabling specific TLS protocol versions when compiled with OpenSSL 1.1.0 or later.
protocol = PROTO application protocol to negotiate TLS
This option enables initial, protocol-specific negotiation of the TLS encryption. The protocol option should not be used with TLS encryption on a separate port.
Currently supported protocols:
cifs Proprietary (undocummented) extension of CIFS protocol http://www.capwin.org/ application support
This protocol is only supported in client mode.
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
ldap Based on RFC 2830 - Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security
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 Passing of the original client IP address with HAProxy PROXY protocol version 1 https://www.haproxy.org/download/1.8/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 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.
protocolDomain = DOMAIN domain for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' protocol.
protocolHeader = HEADER header for the protocol negotiations
Currently, this option is only supported in the client-side 'connect' protocol.
protocolHost = ADDRESS host address for the protocol negotiations
For the 'connect' protocol negotiations, protocolHost specifies HOST:PORT of the final TLS server to be connected to by the proxy. The proxy server directly connected by stunnel must be specified with the connect option.
For the 'smtp' protocol negotiations, protocolHost controls the client SMTP HELO/EHLO value.
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
Hexadecimal keys are automatically converted to binary form. Keys are required to be at least 16 bytes long, which implies at least 32 characters for hexadecimal keys. The file should neither be world-readable nor world-writable.
pty = yes | no (Unix only) allocate a pseudoterminal for 'exec' option
renegotiation = yes | no support TLS renegotiation
Applications of the TLS 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 TLS 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 | DELAY reconnect a connect+exec section after it was disconnected
The DELAY value specifies the number of milliseconds before retrying. "retry = yes" has the same effect as "retry = 1000".
default: no
securityLevel = LEVEL set the security level
The meaning of each level is described below:
level 0 Everything is permitted.
level 1 The security level corresponds to a minimum of 80 bits of security. Any parameters offering below 80 bits of security are excluded. As a result RSA, DSA and DH keys shorter than 1024 bits and ECC keys shorter than 160 bits are prohibited. All export cipher suites are prohibited since they all offer less than 80 bits of security. SSL version 2 is prohibited. Any cipher suite using MD5 for the MAC is also prohibited.
level 2 Security level set to 112 bits of security. As a result RSA, DSA and DH keys shorter than 2048 bits and ECC keys shorter than 224 bits are prohibited. In addition to the level 1 exclusions any cipher suite using RC4 is also prohibited. SSL version 3 is also not allowed. Compression is disabled.
level 3 Security level set to 128 bits of security. As a result RSA, DSA and DH keys shorter than 3072 bits and ECC keys shorter DSA and DH keys shorter than 7680 bits and ECC keys shorter than 384 bits are prohibited. Cipher suites using SHA1 for the MAC are prohibited. TLS versions below 1.2 are not permitted.
level 5 Security level set to 256 bits of security. As a result RSA, DSA and DH keys shorter than 15360 bits and ECC keys shorter than 512 bits are prohibited.
default: 2
The securityLevel option is only available when compiled with OpenSSL 1.1.0 and later.
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 TLS sessions.
sessionResume = yes | no allow or disallow session resumption Use the service as a secondary service (a name-based virtual server) for Server Name Indication TLS extension (RFC 3546).
SERVICE_NAME specifies the primary 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 secondary services are normally specified for a single primary service. The sni option can also be specified more than once within a single secondary service.
This service, as well as the primary service, may not be configured in client mode.
The connect option of the secondary 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 primary service name after TCP connection is accepted, and with the secondary 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.
Empty SERVER_NAME disables sending the SNI extension.
The sni option is only available when compiled with OpenSSL 1.0.0 and later.
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
sslVersion = SSL_VERSION select the TLS protocol version
Supported versions: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
Availability of specific protocols depends on the linked OpenSSL sslVersion = SSL_VERSION
is equivalent to options
sslVersionMax = SSL_VERSION sslVersionMin = SSL_VERSION
when compiled with OpenSSL 1.1.0 and later.
sslVersionMax = SSL_VERSION maximum supported protocol versions
Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
all enable protocol versions up to the highest version supported by the linked OpenSSL library.
Availability of specific protocols depends on the linked OpenSSL library.
The sslVersionMax option is only available when compiled with OpenSSL 1.1.0 and later.
default: all
sslVersionMin = SSL_VERSION minimum supported protocol versions
Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
all enable protocol versions down to the lowest version supported by the linked OpenSSL library.
Availability of specific protocols depends on the linked OpenSSL library.
The sslVersionMin option is only available when compiled with OpenSSL 1.1.0 and later.
default: TLSv1
stack = BYTES (except for FORK model) CPU stack size of created threads
Excessive thread stack size increases virtual memory usage. Insufficient thread stack size may cause application crashes.
default: 65536 bytes (sufficient for all platforms we tested)
ticketKeySecret = SECRET hexadecimal symmetric key used for session ticket confidentiality protection
Session tickets defined in RFC 5077 provide an enhanced session resumption capability, where the server-side caching is not required to maintain per session state.
Combining ticketKeySecret and ticketMacSecret options allow to resume a negotiated session on other cluster nodes, or to resume a
The ticketKeySecret option is only available when compiled with OpenSSL 1.0.0 and later.
Disabling NO_TICKET option is required for the ticket support in OpenSSL older than 1.1.1, but note that this option is incompatible with the redirect option.
ticketMacSecret = SECRET hexadecimal symmetric key used for session ticket integrity protection
The key is required to be either 16 or 32 bytes long, which implies exactly 32 or 64 hexadecimal digits. Colons may optionally be used between two-character hexadecimal bytes.
This option only works in server mode.
The ticketMacSecret option is only available when compiled with OpenSSL 1.0.0 and later.
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
TIMEOUTocsp = SECONDS time to wait to connect an OCSP responder
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 TLS 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):

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.

level 0 Request and ignore the peer certificate chain.
level 1 Verify the peer certificate chain if present.
level 2 Verify the peer certificate chain.
level 3 Verify the peer certificate chain and the end-entity (leaf) peer certificate against a locally installed certificate.
level 4 Ignore the peer certificate chain and only verify the end- entity (leaf) peer certificate against a locally installed 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 checkIP.
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 end-entity (leaf) peer certificate
The end-entity (leaf) 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:
o chroot
o foreground
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.
SIGUSR2 Log the list of active connections.
SIGTERM, SIGQUIT, SIGINT Shut stunnel down.
The result of sending any other signals to the server is undefined.
EXAMPLES In order to provide TLS 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 a TLS-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:
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/
An example server mode SNI configuration:
[virtual] ; primary service accept = 443 cert = default.pem connect = default.internal.mydomain.com:8080
[sni1] ; secondary service 1 sni = virtual:server1.mydomain.com cert = server1.pem connect = server1.internal.mydomain.com:8081
[sni2] ; secondary service 2 sni = virtual:server2.mydomain.com cert = server2.pem connect = server2.internal.mydomain.com:8082 verifyPeer = yes CAfile = server2-allowed-clients.pem
An example of advanced engine configuration allows for authentication with private keys stored in the Windows certificate store (Windows only). With the CAPI engine you don't need to manually select the client key to use. The client key is automatically selected based on the list of CAs trusted by the server.
engine = capi
[service] engineId = capi client = yes accept = 127.0.0.1:8080 connect = example.com:8443
An example of advanced engine configuration to use the certificate and the corresponding private key from a pkcs11 engine:
engine = pkcs11 engineCtrl = MODULE_PATH:opensc-pkcs11.so engineCtrl = PIN:123456
[service] engineId = pkcs11 client = yes accept = 127.0.0.1:8080 engine = pkcs11 engineCtrl = MODULE_PATH:softhsm2.dll engineCtrl = PIN:12345
[service] engineId = pkcs11 client = yes accept = 127.0.0.1:8080 connect = example.com:843 cert = pkcs11:token=MyToken;object=KeyCert
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 TLS-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/local/bin/stunnel stunnel /usr/local/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 TLS-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 .pem file should contain the unencrypted private key and a signed certificate (not certificate request). So the file should look like this:
-----BEGIN RSA PRIVATE KEY----- [encoded key] -----END RSA PRIVATE KEY----- -----BEGIN CERTIFICATE----- [encoded certificate] -----END CERTIFICATE-----
RANDOMNESS stunnel needs to seed the PRNG (pseudo-random number generator) in order for TLS to use good randomness. The following sources are loaded in order until sufficient random data has been gathered:
o The file specified with the RNDfile flag. o The contents of the screen if running on Windows.
o The egd socket specified with the EGD flag.
o The egd socket specified with '--with-egd-sock' at compile time.
o The /dev/urandom device.
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 /usr/local/etc/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 Michal Trojnara