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)

    Stop NT Service

•   -reload (Windows NT and later only)

    Reload the configuration file of the running NT Service

•   -reopen (Windows NT and later only)

    Reopen the log file of the running NT Service

•   -exit (Win32 only)

    Exit an already started stunnel

•   -quiet (Win32 only)

    Don’t display any message boxes

CONFIGURATION FILE
Each line of the configuration file can be either:

•   An empty line (ignored).

•   A comment starting with `;' (ignored).

•   An `option_name = option_value' pair.

•   `[service_name]' indicating a start of a service definition.

An address parameter of an option may be either:

•   A port number.

•   A colon-separated pair of IP address (either IPv4, IPv6, or domain name) and port number.

•   A Unix socket path (Unix only).

  GLOBAL OPTIONS

•   chroot = DIRECTORY (Unix only)

    directory to chroot stunnel process

    chroot keeps stunnel in a chrooted jail. CApath, CRLpath, pid and exec are located inside the jail and the patches have to be relative to the directory specified with chroot.

    Several functions of the operating system also need their files to be located within the chroot jail, e.g.:

    •   Delayed resolver typically needs /etc/nsswitch.conf and /etc/resolv.conf.

    •   Local time in log files needs /etc/timezone.

    •   Some other functions may need devices, e.g. /dev/zero or /dev/null.

•   compression = deflate | zlib | zstd | brotli

    select data compression algorithm

    default: no compression

    Data compression is available only for TLS 1.2 and earlier. Requires lowering securityLevel to 1 when compiled with OpenSSL 1.1.0 and later.

    The zlib, zstd, and brotli algorithms are disabled in the default OpenSSL configuration.

    The zstd and brotli algorithms were added in OpenSSL 3.2.

    Data compression poses a risk in applications that allow an attacker to inject chosen plaintext.

    Deflate is the standard compression method as described in RFC 1951.

    Warning: TLS compression can enable plaintext-recovery attacks, such as CRIME, when attacker-controlled data is compressed together with secret material.

•   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.

    The debug = debug (or the equivalent <debug = 7>) level produces for the most verbose log output. This logging level is only meant to be understood by stunnel developers, and not by users. Please either use the debug level when requested to do so by an stunnel 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 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.

•   provider = PROVIDER_ID

    Specifies the identifier of the provider to be used. PROVIDER_ID is a unique identifier referring to a specific cryptographic service provider.

    This option requires OpenSSL 3.0 or later.

•   providerParameter = PROVIDER_ID:PARAMETER=VALUE

    Sets a specific parameter for the given provider. PROVIDER_ID identifies the provider, PARAMETER is the parameter name, and VALUE is its value. This option allows customization of the selected cryptographic service provider’s configuration.

    This option requires OpenSSL 3.5 or later.

•   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

•   setEnv = VAR_NAME=VALUE

    Change or add an environment variable for child processes. If VAR_NAME already exists, its value will be updated; otherwise, a new variable will be created. This modification applies only to spawned child processes and does not affect the current environment.

•   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

•   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.

    This parameter can also be used to provide the root CA certificate needed to validate OCSP stapling in server mode.

    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.

    This parameter can also be used to provide the root CA certificate needed to validate OCSP stapling in server mode.

•   CAstore = URI_CA

    load trusted CA certificates from a resource specified by a URI

    This option allows loading CA certificates from an external source supported by the OSSL_STORE framework, such as a PKCS#11 module (e.g., hardware token), the system certificate store, or a remote resource.

    This option can be used independently of CAfile and CAdir, and similarly provides certificates used by verifyChain or verifyPeer, as well as for validating OCSP stapling in server mode.

    This option requires OpenSSL 3.0 or later.

•   cert = CERT_FILE | URI

    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.

    The cert option may be specified multiple times. The first certificate encountered is used as the server or client certificate, while any subsequent certificates are treated as intermediate chain elements. This functionality requires OpenSSL version 1.0.2 or later.

    This parameter is also used as the certificate identifier (PKCS#11 URI) when a hardware engine or provider is enabled.

    Note: The provider requires OpenSSL 3.0 or later

•   checkEmail = EMAIL

    verify the email address of the end-entity (leaf) peer certificate subject

    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.

    The ciphersuites option ignores unknown ciphers when compiled with OpenSSL 3.0 or later.

    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 SSL_CONF_cmd(3ssl) 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 `:'

    Note: This option is supported for server mode sockets only.

    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 | URI

    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 (PKCS#11 URI) when a hardware engine or provider is enabled.

    Note: The provider requires OpenSSL 3.0 or later

    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 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.

•   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 SSL_CTX_set_options(3ssl) manual, but without SSL_OP_ prefix. stunnel -options lists the options found to be allowed in the current combination of stunnel and the OpenSSL library used to build it.

    Several option lines can be used to specify multiple options. An option name can be prepended with a dash ("-") to disable the option.

    For example, for compatibility with the erroneous Eudora 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 (undocumented) extension of CIFS protocol implemented in Samba. Support for this extension was dropped in Samba 3.0.0.

    •   capwin

        https://www.capwin.org/ application support

    •   capwinctrl

        https://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 https://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.

        https://www.openssh.com/txt/socks4.protocol

        https://www.openssh.com/txt/socks4a.protocol

        The BIND command of the SOCKS protocol is not supported. The USERID parameter is ignored.

        See Examples section for sample configuration files for VPN based on SOCKS encryption.

•   protocolAuthentication = AUTHENTICATION

    authentication type for the protocol negotiations

    Currently, this option is only supported in the client-side `connect' and `smtp' protocols.

    Supported authentication types for the `connect' protocol are `basic' or `ntlm'. The default `connect' authentication type is `basic'.

    Supported authentication types for the `smtp' protocol are `plain' or `login'. The default `smtp' authentication type is `plain'.

•   protocolDomain = DOMAIN

    domain for the protocol negotiations

    Currently, this option is only supported in the client-side `connect' protocol.

•   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:

    https://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. Additionally, SSLv3, TLS 1.0, TLS 1.1 are all disabled for OpenSSL 3.0 and later.

    •   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. Compression is disabled. SSL version 3 is also not allowed for OpenSSL older than 3.0.

    •   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. Session tickets are disabled. TLS versions below 1.1 are not permitted for OpenSSL older than 3.0.

    •   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 for OpenSSL older than 3.0.

    •   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 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 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 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):

                    iptables -t mangle -N DIVERT
                    iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
                    iptables -t mangle -A DIVERT -j MARK --set-mark 1
                    iptables -t mangle -A DIVERT -j ACCEPT
                    ip rule add fwmark 1 lookup 100
                    ip route add local 0.0.0.0/0 dev lo table 100
                    echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter

            stunnel must also to be executed as root and without the setuid option.

        •   Remote mode (connect option) on Linux 2.2.x

            This configuration requires the kernel to be compiled with the transparent proxy option. Connected service must be installed on a separate host. Routing towards the clients has to go through the stunnel box.

            stunnel must also to be executed as root and without the setuid option.

        •   Remote mode (connect option) on FreeBSD >=8.0

            This configuration requires additional firewall and routing setup. stunnel must also to be executed as root and without the setuid option.

        •   Local mode (exec option)

            This configuration works by pre-loading the libstunnel.so shared library. _RLD_LIST environment variable is used on Tru64, and LD_PRELOAD variable on other platforms.

    •   destination

        The original destination is used instead of the connect option.

        A service section for transparent destination may look like this:

                [transparent]
                client = yes
                accept = <stunnel_port>
                transparent = destination

        This configuration requires iptables setup to work, possibly in /etc/rc.local or equivalent file.

        For a connect target installed on the same host:

                /sbin/iptables -t nat -I OUTPUT -p tcp --dport <redirected_port> \
                    -m ! --uid-owner <stunnel_user_id> \
                    -j DNAT --to-destination <local_ip>:<stunnel_port>

        For a connect target installed on a remote host:

                /sbin/iptables -I INPUT -i eth0 -p tcp --dport <stunnel_port> -j ACCEPT
                /sbin/iptables -t nat -I PREROUTING -p tcp --dport <redirected_port> \
                    -i eth0 -j DNAT --to-destination <local_ip>:<stunnel_port>

        The transparent destination option is currently only supported on Linux.

    •   both

        Use both source and destination transparent proxy.

    Two legacy options are also supported for backward compatibility:

    •   yes

        This option has been renamed to source.

    •   no

        This option has been renamed to none.

•   verify = LEVEL

    verify the peer certificate

    This option is obsolete and should be replaced with the verifyChain and verifyPeer options.

    •   level 0

        Request and ignore the peer certificate 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:

    •   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 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
        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

An example of advanced provider configuration to use the certificate and the corresponding private key from a `pkcs11prov` provider:

Note: requires OpenSSL 3.0 or later

        setEnv = PKCS11_MODULE_PATH=opensc-pkcs11.dll
        setEnv = PKCS11_PIN:123456
        provider = pkcs11prov
        [service]
        client = yes
        accept = 127.0.0.1:8080
        connect = example.com:843
        cert = pkcs11:token=MyToken;object=MyCert
        key = pkcs11:token=MyToken;object=MyKey

Note: requires OpenSSL 3.5 or later

        provider = pkcs11prov
        providerParameter = pkcs11prov:pkcs11_module=opensc-pkcs11.dll
        providerParameter = pkcs11prov:pin=123456
        [service]
        client = yes
        accept = 127.0.0.1:8080
        connect = example.com:843
        cert = pkcs11:token=MyToken;object=MyCert
        key = pkcs11:token=MyToken;object=MyKey

On Windows, the PKCS#11 library must be placed in the `ossl-modules` directory, or a full absolute path must be provided in `PKCS11_MODULE_PATH` or `pkcs11_module` parameter.

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

•   https://www.stunnel.org/

    stunnel Application

•   https://openssl-library.org/

    OpenSSL Library

AUTHOR

•   Michał Trojnara

    Michal.Trojnara@stunnel.org