| stunnel(8) | stunnel TLS Proxy | stunnel(8) |
stunnel - TLS offloading and load-balancing proxy
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)
Each line of the configuration file can be either:
An address parameter of an option may be either:
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.:
default: no compression
Deflate is the standard compression method as described in RFC 1951.
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.
Entropy Gathering Daemon socket to use to feed the OpenSSL random number generator.
default: software-only cryptography
See Examples section for an engine configuration to use the certificate and the corresponding private key from a cryptographic device.
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.
This option allows you to disable entering FIPS mode if stunnel was compiled with FIPS 140-2 support.
default: no (since version 5.00)
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
On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.
On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.
On Windows platform the parameter should be an .ico file containing a 16x16 pixel image.
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
/dev/stdout device can be used to send log messages to the standard output (for example to log them with daemontools splogger).
If the argument is empty, then no pid file will be created.
pid path is relative to the chroot directory if specified.
The OpenSSL library will use data from this file first to seed the random number generator.
default: yes
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
default: yes
default: yes
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.
If no host specified, defaults to all IPv4 addresses for the local host.
To listen on all IPv6 addresses use:
accept = :::PORT
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.
The loaded CA certificates will be used with the verifyChain and verifyPeer options. Note that the certificates in this directory should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the DER encoded subject of the cert.
The hash algorithm has been changed in OpenSSL 1.0.0. It is required to c_rehash the directory on upgrade from OpenSSL 0.x.x to OpenSSL 1.x.x or later.
CApath path is relative to the chroot directory if specified.
The loaded CA certificates will be used with the verifyChain and verifyPeer options.
The parameter specifies the file containing certificates used by stunnel to authenticate itself against the remote client or server. The file should contain the whole certificate chain starting from the actual server/client certificate, and ending with the self-signed root CA certificate. The file must be either in PEM or P12 format.
A certificate chain is required in server mode, and optional in client mode.
This parameter is also used as the certificate identifier when a hardware engine is enabled.
Certificates are accepted if no subject checks were specified, or the email address of the peer certificate matches any of the email addresses specified with checkEmail.
Multiple checkEmail options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
Certificates are accepted if no subject checks were specified, or the host name of the peer certificate matches any of the hosts specified with checkHost.
Multiple checkHost options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
Certificates are accepted if no subject checks were specified, or the IP address of the peer certificate matches any of the IP addresses specified with checkIP.
Multiple checkIP options are allowed in a single service section.
This option requires OpenSSL 1.0.2 or later.
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.
A colon-delimited list of TLSv1.3 ciphersuites names in order of preference.
This option requires OpenSSL 1.1.1 or later.
default: TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256
default: no (server mode)
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.
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.
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.
This file contains multiple CRLs, used with the verifyChain and verifyPeer options.
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)
This identifier allows you to distinguish log entries generated for each of the connections.
Currently supported types:
default: sequential
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.
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
The engines are numbered starting from 1.
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.
Quoting is currently not supported. Arguments are separated with an arbitrary amount of whitespace.
default: prio
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
A private key is needed to authenticate the certificate owner. Since this file should be kept secret it should only be readable by its owner. On Unix systems you can use the following command:
chmod 600 keyfile
This parameter is also used as the private key identifier when a hardware engine is enabled.
default: the value of the cert option
default: no (since version 5.00)
This option enables stunnel to validate certificates with the list of OCSP responder URLs retrieved from their AIA (Authority Information Access) extension.
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
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.
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.
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:
This protocol is only supported in client mode.
This protocol is only supported in client mode.
This protocol is only supported in client mode.
http://www.openssh.com/txt/socks4.protocol
http://www.openssh.com/txt/socks4a.protocol
The BIND command of the SOCKS protocol is not supported. The USERID parameter is ignored.
See Examples section for sample configuration files for VPN based on SOCKS encryption.
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'.
Currently, this option is only supported in the client-side 'connect' protocol.
Currently, this option is only supported in the client-side 'connect' protocol.
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.
Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.
Currently, this option is only supported in the client-side 'connect' and 'smtp' protocols.
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.
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.
This option only works in server mode. Some protocol negotiations are also incompatible with the redirect option.
Applications of the TLS renegotiation include some authentication scenarios, or re-keying long lasting connections.
On the other hand this feature can facilitate a trivial CPU-exhaustion DoS attack:
http://vincent.bernat.im/en/blog/2011-ssl-dos-mitigation.html
Please note that disabling TLS renegotiation does not fully mitigate this issue.
default: yes (if supported by OpenSSL)
This option is not supported on some platforms.
default: yes
default: no
The meaning of each level is described below:
The securityLevel option is only available when compiled with OpenSSL 1.1.0 and later.
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
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".
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 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.
This is the number of seconds to keep cached TLS sessions.
default: yes
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.
Empty SERVER_NAME disables sending the SNI extension.
The sni option is only available when compiled with OpenSSL 1.0.0 and later.
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
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.
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
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
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)
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.
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.
Supported values:
This option is currently available in:
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.
stunnel must also to be executed as root and without the setuid 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.
Two legacy options are also supported for backward compatibility:
This option is obsolete and should be replaced with the verifyChain and verifyPeer options.
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
The peer certificate needs to be stored either in the file specified with CAfile, or in the directory specified with CApath.
default: no
stunnel returns zero on success, non-zero on error.
The following signals can be used to control stunnel in Unix environment:
Some global options will not be reloaded:
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.
The result of sending any other signals to the server is undefined.
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
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.
The most common use of stunnel is to listen on a network port and establish communication with either a new port via the connect option, or a new program via the exec option. However there is a special case when you wish to have some other program accept incoming connections and launch stunnel, for example with inetd, xinetd, or tcpserver.
For example, if you have the following line in inetd.conf:
imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf
In these cases, the inetd-style program is responsible for binding a network socket (imaps above) and handing it to stunnel when a connection is received. Thus you do not want stunnel to have any accept option. All the Service Level Options should be placed in the global options section, and no [service_name] section will be present. See the EXAMPLES section for example configurations.
Each 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-----
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:
Note that on Windows machines that do not have console user interaction (mouse movements, creating windows, etc.) the screen contents are not variable enough to be sufficient, and you should provide a random file for use with the RNDfile flag.
Note that the file specified with the RNDfile flag should contain random data -- that means it should contain different information each time stunnel is run. This is handled automatically unless the RNDoverwrite flag is used. If you wish to update this file manually, the openssl rand command in recent versions of OpenSSL, would be useful.
Important note: If /dev/urandom is available, OpenSSL often seeds the PRNG with it while checking the random state. On systems with /dev/urandom OpenSSL is likely to use it even though it is listed at the very bottom of the list above. This is the behaviour of OpenSSL and not stunnel.
stunnel 4.40 and later contains hardcoded 2048-bit DH parameters. 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
The execArgs option and the Win32 command line do not support quoting.
| 2022.12.20 | 5.68 |