STUNNEL(8)

📦

version 5.82 · 16 versions

5.82 · alpine 3.24, arch rolling (latest)
5.74 · opensuse-leap 16.0
5.76 · nixos 26.05
5.71 · centos-stream 9
5.72 · centos-stream 10
5.80 · nixpkgs unstable
5.80 · ubuntu stonking
5.80 · fedora 43
5.63 · ubuntu jammy
5.75 · fedora 43
5.80 · fedora 44
5.75 · fedora 44
5.72 · ubuntu noble
5.68 · debian bookworm
5.77 · ubuntu resolute
5.74 · debian trixie, ubuntu questing

💖 NAME 💖

okay so this is stunnel — the TLS offloading and load-balancing proxy — and yes bestie, this is the STUNNEL(8) page from the stunnel TLS Proxy manual 💅✨. It's giving secure-wrapper main character energy, and we're gonna read it together.

✨ 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 a 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 paths 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 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 a 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 140-3 mode.

This option allows you to disable entering FIPS mode if stunnel was compiled with FIPS 140-3 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.

Also, heads up bestie — this same parameter is also what you use to provide the root CA certificate you need to validate OCSP stapling when you're in server mode 💅

CAstore = URI_CA — load trusted CA certificates from a resource specified by a URI ✨

This one lets you pull CA certs in from an external source that the OSSL_STORE framework supports — like a PKCS#11 module (hello, hardware token 👋), the system certificate store, or even a remote resource. So many vibes.

It's fully independent of CAfile and CAdir, and it does the same job: it provides the certificates used by verifyChain or verifyPeer, plus it validates OCSP stapling in server mode.

This option requires OpenSSL 3.0 or later, fr fr.

cert = CERT_FILE | URI — the certificate chain file name 🎀

This parameter specifies the file holding the certificates stunnel uses 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. Format-wise it has to be either PEM or P12.

A certificate chain is required in server mode, and optional in client mode.

You can specify the cert option multiple times! The first certificate encountered is used as the server or client certificate, and 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 (a PKCS#11 URI) when a hardware engine or provider is enabled. iykyk.

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 if the email address of the end-entity (leaf) peer certificate matches any of the email addresses you listed with checkEmail.

Multiple checkEmail options are allowed in a single service section — stack 'em.

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 if the host name of the end-entity (leaf) peer certificate matches any of the hosts you listed 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 if the IP address of the end-entity (leaf) peer certificate matches any of the IP addresses you listed 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) 🔐

Heads up: this option does not impact TLSv1.3 ciphersuites, bestie. That's a whole different option.

It's 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 ciphersuite 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 gets executed with the specified parameter. This lets you invoke literally any configuration commands from the stunnel configuration file. The supported commands are described on the SSL_CONF_cmd(3ssl) manual page — go read it, it's giving lore.

You can use several config lines to specify multiple configuration commands.

Use the curves option instead of enabling config = Curves:list_curves to support elliptic curves. (Noted 💅)

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 the host resolves to multiple addresses and/or if multiple connect options are specified, then the remote address is chosen using a round-robin algorithm. Fair and balanced, we love to see it ✨

CRLpath = DIRECTORY — Certificate Revocation Lists directory 📂

This is the directory where stunnel looks for CRLs when using the verifyChain and verifyPeer options, and with CRLcheckChain for full-chain CRL checking. Real talk: the CRLs in this directory should be named XXXXXXXX.r0 where XXXXXXXX is the hash value of the CRL.

The hash algorithm 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. Don't skip this step bestie, it will bite you 😭

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, and with CRLcheckChain for full-chain CRL checking.

CRLcheckChain = yes | no — check CRLs for the full peer certificate chain 🔍

This option requires verifyChain = yes and either CRLfile or CRLpath.

By default, CRL checking is only performed for the end-entity (leaf) peer certificate. With CRLcheckChain = yes, CRL checking is additionally performed for every certificate in the verified issuer chain. Heads up: full-chain mode fails closed if an applicable issuer CRL is missing, expired, or invalid. No vibes-based acceptance here 💀

For a chain like Root CA -> Intermediate CA -> Leaf, an intermediate-issued CRL is needed for the leaf certificate and a root-issued CRL is needed for the intermediate certificate.

default: no

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 lets you tell apart the log entries generated for each of the connections. Iconic, honestly.

Currently supported types:

sequential

The numeric sequential identifier is only unique within a single instance of stunnel, but it's very compact. Most useful for manual log analysis.

unique

This alphanumeric identifier is globally unique, but longer than the sequential number. 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 inetd mode.

default: sequential

debug = 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 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. Like, respectfully — it's a lot 💀

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. Off-by-one is not the vibe 🫶

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 📁✨

Include directives may be nested to a maximum depth of 100 configuration files, including the main configuration file. Exceeding this limit rejects the configuration as a likely recursive include — safety first, bestie.

The files are included in 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 the 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 (a 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 — (the full deets on this one are literally right below, keep scrolling bestie 💅)

select OCSP responder for the end-entity (leaf) peer certificate verification

OCSPaia = yes | no — validate certificates with their AIA OCSP responders ✨ this one lets stunnel validate certificates using the list of OCSP responder URLs it pulled from their AIA (Authority Information Access) extension. iykyk.
OCSPflag = OCSP_FLAG — specify OCSP responder flag 🎀 several OCSPflag lines 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. periodt.
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 the 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. no cap.

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. fr fr.

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, though — that's giving memory exhaustion DoS attack energy and we don't stan that 💀

sessionCacheTimeout = TIMEOUT — session cache timeout, aka the number of seconds to keep cached TLS sessions 💅
sessionResume = yes | no — allow or disallow session resumption. default: yes ✨
sessiond = HOST:PORT — address of the sessiond TLS cache server 🎀
sni = SERVICE_NAME:SERVER_NAME_PATTERN (server mode) — use this service as a secondary service (a name-based virtual server) for the 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. Double the vibe check, double the safety 🫶

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, bestie, look at these:

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. (rip SSLv2, you will not be missed 💀)

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. It's giving "too much" vs "not enough" and both are a problem, ngl 😭

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
transport = tcp | udp — select transport protocol 🚀

Supported values:

tcp — Use standard TLS over TCP (default).
udp — Use DTLS over UDP.

Each datagram opens a new logical connection within the service. The server uses a different connected socket per client, and DTLS cookies protect against DDoS reflection attacks. cookies as armor, iconic 🍪

default: tcp

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. bestie, please migrate, it's for your own good 🙏

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. like, verifyChain alone is not the whole meal, it's the seasoning 🧂

The self-signed root CA certificate needs to be stored in the file you named with <code>CAfile</code>, or in the directory you named with <code>CApath</code> — pick one, bestie, it's gotta live somewhere 💅

default: no

verifyPeer = yes | no — this one checks the end-entity (leaf) peer certificate, like the actual ID of the person at the door, not just their whole family tree 🫶

The end-entity (leaf) peer certificate needs to be stored in the file you named with <code>CAfile</code>, or in the directory you named with <code>CApath</code>.

default: no

✨ RETURN VALUE ✨

stunnel returns zero on success, non-zero on error. Iconic, minimal, no notes 💖

🎀 SIGNALS 🎀

These are the signals you can throw at stunnel in a Unix environment, and what each one does to it:

SIGHUP — forces a reload of the config file, like a soft refresh 🔄

Heads up though: some global options will not get reloaded:

chroot
foreground
pid
setgid
setuid

Also, using the setuid option will prevent stunnel from binding to privileged (<1024) ports while it's reloading the configuration. No cap, that's just how it is 🙈

And when the chroot option is in play, stunnel looks for all its files (the config file, the certificates, the log file, the pid file — the whole entourage) inside the chroot jail 🏰

SIGUSR1 — closes and reopens the stunnel log file. This is literally what log rotation is for, bestie 🗂️
SIGUSR2 — logs the list of active connections, like a little headcount of who's currently in the group chat 👀
SIGTERM, SIGQUIT, SIGINT — shut stunnel down. Say bye 💀

⚠️ Sending any other signal to the server? The result is undefined. So maybe don't, bestie, that's giving chaos 🚫

🌸 EXAMPLES 🌸

Want to wrap TLS around your local imapd service? Slap this in:

[imapd]
accept = 993
exec = /usr/sbin/imapd
execArgs = imapd

Or if you're doing remote mode instead:

[imapd]
accept = 993
connect = 143

Want your local e-mail client to connect to a TLS-enabled imapd on some other server? Point the e-mail client at localhost port 143, then use:

[imap]
client = yes
accept = 143
connect = servername:993

Trying to tunnel to your pppd daemon on port 2020? Something like this does the trick:

[vpn]
accept = 2020
exec = /usr/sbin/pppd
execArgs = pppd local
pty = yes

And if you want to run stunnel in inetd mode to launch your imapd process, you'd use this stunnel.conf. Important: there must be no [service_name] section in it, periodt.

exec = /usr/sbin/imapd
execArgs = imapd

To set up a SOCKS VPN, configure this client service:

[socks_client]
client = yes
accept = 127.0.0.1:1080
connect = vpn_server:9080
verifyPeer = yes
CAfile = stunnel.pem

And the matching config on the vpn_server host:

[socks_server]
protocol = socks
accept = 9080
cert = stunnel.pem
key = stunnel.key

Now go test your setup on the client machine with this:

curl --socks4a localhost http://www.example.com/

Here's an example of a server-mode SNI configuration — this one's kind of the main character of configs 👑:

[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

Now for advanced engine config 🧚 — this example lets you authenticate with private keys stored in the Windows certificate store (Windows only). With the CAPI engine you don't have to manually pick which client key to use: the client key gets selected automatically based on the list of CAs the server trusts. Obsessed, honestly ✨

engine = capi

[service]
engineId = capi
client = yes
accept = 127.0.0.1:8080
connect = example.com:8443

Here's an advanced engine config using the certificate and matching 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

And an advanced engine config pulling the certificate and its 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

Advanced provider config time 🦋 — this one uses the certificate and matching 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 you gotta give a full absolute path in the PKCS11_MODULE_PATH or pkcs11_module parameter. Don't skip this step bestie 😭

💜 NOTES 💜

✨ RESTRICTIONS ✨

stunnel cannot be used for the FTP daemon — like, at all — because of how the FTP protocol works: it utilizes multiple ports for data transfers. There are TLS-enabled versions of FTP and telnet daemons available, though, so you have options 💅

🫶 INETD MODE 🫶

The most common use of stunnel is to listen on a network port and then establish communication with either a new port via the connect option, or a new program via the exec option. But there's this special case where you want some other program to accept the incoming connections and launch stunnel — for example with inetd, xinetd, or tcpserver.

So like, if you've got this line in inetd.conf:

imaps stream tcp nowait root @bindir@/stunnel stunnel @sysconfdir@/stunnel/imaps.conf

In these cases, the inetd-style program is the one responsible for binding the network socket (imaps in the example above) and handing it over to stunnel when a connection shows up. Which means — and this is the key part — you do not want stunnel to have any accept option. All the Service Level Options go in the global options section instead, and there will be no [service_name] section present. Peep the EXAMPLES section for example configurations 👀

🎀 CERTIFICATES 🎀

Every 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 get a certificate and a key is to generate them with the free OpenSSL package. You can find more info on certificate generation on the pages listed below 💖

The .pem file should contain the unencrypted private key and a signed certificate (not a certificate request — different thing, don't get it twisted). 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) so TLS can use good randomness. These sources get loaded in order until enough 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.

Quick note: on Windows machines that don't have any console user interaction (mouse movements, creating windows, etc.), the screen contents are not variable enough to be sufficient — so you should provide a random file for use with the RNDfile flag. Fr fr, don't skip it 🙈

And another one: the file specified with the RNDfile flag should contain random data — meaning it should contain different information each time stunnel is run. This is handled automatically unless the RNDoverwrite flag is used. If you wanna 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's listed at the very bottom of the list above. This is the behaviour of OpenSSL and not stunnel — don't blame our girl 💅

🦋 DH PARAMETERS 🦋

stunnel 4.40 and later contains hardcoded 2048-bit DH parameters. Starting with stunnel 5.18, those hardcoded DH parameters are replaced every 24 hours with autogenerated temporary DH parameters. Fair warning: DH parameter generation may take several minutes. Like, go make a snack 🍓

Alternatively, it's possible to specify static DH parameters in the certificate file, which disables generating temporary DH parameters:

openssl dhparam 2048 >> stunnel.pem

⭐ FILES ⭐

@sysconfdir@/stunnel/stunnel.conf — the stunnel configuration file 📄

💀 BUGS 💀

The execArgs option and the Win32 command line do not support quoting. That's it, that's the bug 🫠

👑 SEE ALSO 👑

https://www.stunnel.org/ — stunnel Application
https://openssl-library.org/ — OpenSSL Library

💖 AUTHOR 💖

Michał Trojnara — Michal.Trojnara@stunnel.org

And that's the whole page, bestie!! 🎉 You just read the STUNNEL(8) man page for stunnel 5.82, dated 2026.09.17 — slay, go configure something iconic ✨💅🎀

📦 where this page comes from

alpine 3.24: stunnel-doc 5.82-r0 · GPL-2.0-or-later WITH OpenSSL-Exception
arch rolling: stunnel 5.82-1 · GPL-2.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms