# man > stunnel4 [stunnel(8)](https://www.chedong.com/phpMan.php/man/stunnel/8/markdown) stunnel TLS Proxy [stunnel(8)](https://www.chedong.com/phpMan.php/man/stunnel/8/markdown) ## 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 () ## OPTIONS **FILE** Use specified configuration file ### -fd 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 Install NT Service ### -uninstall Uninstall NT Service ### -start Start NT Service ### -stop Stop NT Service ### -reload Reload the configuration file of the running NT Service ### -reopen Reopen the log file of the running NT Service ### -exit Exit an already started stunnel ### -quiet 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. **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. **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 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 **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) 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** = CA_FILE Certificate Authority file This file contains multiple CA certificates, to 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. 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 subject checks were 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 subject checks were 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 subject checks were 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 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 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 [_S_S_S_S_L_L_____C_C_O_O_N_N_F_F_____c_c_m_m_d_d_(3ssl)](https://www.chedong.com/phpMan.php/man/SSSSLLCCOONNFFccmmdd/3ssl/markdown)_ manual page. Several _config_ lines can be used to specify multiple configuration commands. 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: _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. _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) 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: 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 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 [_S_S_S_S_L_L_____C_C_T_T_X_X_____s_s_e_e_t_t_____o_o_p_p_t_t_i_i_o_o_n_n_s_s_(3ssl)](https://www.chedong.com/phpMan.php/man/SSSSLLCCTTXXsseettooppttiioonnss/3ssl/markdown)_ 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 implemented in Samba. Support for this extension was dropped in Samba 3.0.0. _capwin_ application support _capwinctrl_ 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 _ _pop3_ Based on RFC 2449 - _POP3_ _Extension_ _Mechanism_ _proxy_ Passing of the original client IP address with HAProxy PROXY protocol version 1 _ _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. _ _ 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. **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 **redirect** = [HOST:]PORT redirect TLS 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 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: _ 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 reconnect a connect+exec section after it was disconnected 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 than 256 bits are prohibited. In addition to the level 2 exclusions cipher suites not offering forward secrecy are prohibited. TLS versions below 1.1 are not permitted. Session tickets are disabled. level 4 Security level set to 192 bits of security. As a result RSA, 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 default: yes **sessiond** = HOST:PORT address of sessiond TLS cache server **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. 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 library. Older versions of OpenSSL do not support TLSv1.1, TLSv1.2 and TLSv1.3. Newer versions of OpenSSL do not support SSLv2. Obsolete SSLv2 and SSLv3 are currently disabled by default. Setting the option 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 negotiated session after server restart. 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 _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 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 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): 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 = 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 \ -m ! --uid-owner \ -j DNAT --to-destination : For a connect target installed on a remote host: /sbin/iptables -I INPUT -i eth0 -p tcp --dport -j ACCEPT /sbin/iptables -t nat -I PREROUTING -p tcp --dport \ -i eth0 -j DNAT --to-destination : 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 _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 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. 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: [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 An example server mode SNI configuration: [virtual] ; master service accept = 443 cert = default.pem connect = default.internal.mydomain.com:8080 [sni1] ; slave service 1 sni = virtual:server1.mydomain.com cert = server1.pem connect = server1.internal.mydomain.com:8081 [sni2] ; slave 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 connect = example.com:843 cert = pkcs11:token=MyToken;object=MyCert key = pkcs11:token=MyToken;object=MyKey An example of advanced engine configuration to use the certificate and the corresponding private key from a SoftHSM token: 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/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 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: • 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. 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 _/etc/stunnel/stunnel.conf_ **stunnel** configuration file ## BUGS The _execArgs_ option and the Win32 command line do not support quoting. ## SEE ALSO [**tcpd**(8)](https://www.chedong.com/phpMan.php/man/tcpd/8/markdown) access control facility for internet services [**inetd**(8)](https://www.chedong.com/phpMan.php/man/inetd/8/markdown) internet 'super-server' _ **stunnel** homepage _ **OpenSSL** project website ## AUTHOR Michał Trojnara <<_Michal.Trojnara@stunnel.org_>> 5.63 2022.01.18 [stunnel(8)](https://www.chedong.com/phpMan.php/man/stunnel/8/markdown)