1stunnel(8) stunnel TLS Proxy stunnel(8)
2
6 stunnel - TLS offloading and load-balancing proxy
7
9 Unix:
10 stunnel [FILE] | -fd N | -help | -version | -sockets | -options
11 WIN32:
13 stunnel [ [ -install | -uninstall | -start | -stop |
14 -reload | -reopen | -exit ] [-quiet] [FILE] ] |
15 -help | -version | -sockets | -options
16
18 The stunnel program is designed to work as TLS encryption wrapper
19 between remote clients and local (inetd-startable) or remote servers.
20 The concept is that having non-TLS aware daemons running on your system
21 you can easily set them up to communicate with clients over secure TLS
22 channels.
23 stunnel can be used to add TLS functionality to commonly used Inetd
25 daemons like POP-2, POP-3, and IMAP servers, to standalone daemons like
26 NNTP, SMTP and HTTP, and in tunneling PPP over network sockets without
27 changes to the source code.
28 This product includes cryptographic software written by Eric Young
30 (eay@cryptsoft.com)
31
33 FILE
34 Use specified configuration file
35 -fd N (Unix only)
37 Read the config file from specified file descriptor
38 -help
40 Print stunnel help menu
41 -version
43 Print stunnel version and compile time defaults
44 -sockets
46 Print default socket options
47 -options
49 Print supported TLS options
50 -install (Windows NT and later only)
52 Install NT Service
53 -uninstall (Windows NT and later only)
55 Uninstall NT Service
56 -start (Windows NT and later only)
58 Start NT Service
59 -stop (Windows NT and later only)
61 Stop NT Service
62 -reload (Windows NT and later only)
64 Reload the configuration file of the running NT Service
65 -reopen (Windows NT and later only)
67 Reopen the log file of the running NT Service
68 -exit (Win32 only)
70 Exit an already started stunnel
71 -quiet (Win32 only)
73 Don't display any message boxes
74
76 Each line of the configuration file can be either:
77 • An empty line (ignored).
79 • A comment starting with ';' (ignored).
81 • An 'option_name = option_value' pair.
83 • '[service_name]' indicating a start of a service definition.
85 An address parameter of an option may be either:
87 • A port number.
89 • A colon-separated pair of IP address (either IPv4, IPv6, or domain
91 name) and port number.
92 • A Unix socket path (Unix only).
94 GLOBAL OPTIONS
96 chroot = DIRECTORY (Unix only)
97 directory to chroot stunnel process
98 chroot keeps stunnel in a chrooted jail. CApath, CRLpath, pid and
100 exec are located inside the jail and the patches have to be
101 relative to the directory specified with chroot.
102 Several functions of the operating system also need their files to
104 be located within the chroot jail, e.g.:
105 • Delayed resolver typically needs /etc/nsswitch.conf and
107 /etc/resolv.conf.
108 • Local time in log files needs /etc/timezone.
110 • Some other functions may need devices, e.g. /dev/zero or
112 /dev/null.
113 compression = deflate | zlib
115 select data compression algorithm
116 default: no compression
118 Deflate is the standard compression method as described in RFC
120 1951.
121 debug = [FACILITY.]LEVEL
123 debugging level
124 Level is one of the syslog level names or numbers emerg (0), alert
126 (1), crit (2), err (3), warning (4), notice (5), info (6), or debug
127 (7). All logs for the specified level and all levels numerically
128 less than it will be shown. Use debug = debug or debug = 7 for
129 greatest debugging output. The default is notice (5).
130 The syslog facility 'authpriv' will be used unless a facility name
132 is supplied. (Facilities are not supported on Win32.)
133 Case is ignored for both facilities and levels.
135 EGD = EGD_PATH (Unix only)
137 path to Entropy Gathering Daemon socket
138 Entropy Gathering Daemon socket to use to feed the OpenSSL random
140 number generator.
141 engine = auto | ENGINE_ID
143 select hardware or software cryptographic engine
144 default: software-only cryptography
146 See Examples section for an engine configuration to use the
148 certificate and the corresponding private key from a cryptographic
149 device.
150 engineCtrl = COMMAND[:PARAMETER]
152 control hardware engine
153 engineDefault = TASK_LIST
155 set OpenSSL tasks delegated to the current engine
156 The parameter specifies a comma-separated list of task to be
158 delegated to the current engine.
159 The following tasks may be available, if supported by the engine:
161 ALL, RSA, DSA, ECDH, ECDSA, DH, RAND, CIPHERS, DIGESTS, PKEY,
162 PKEY_CRYPTO, PKEY_ASN1.
163 fips = yes | no
165 enable or disable FIPS 140-2 mode.
166 This option allows you to disable entering FIPS mode if stunnel was
168 compiled with FIPS 140-2 support.
169 default: no (since version 5.00)
171 foreground = yes | quiet | no (Unix only)
173 foreground mode
174 Stay in foreground (don't fork).
176 With the yes parameter it also logs to stderr in addition to the
178 destinations specified with syslog and output.
179 default: background in daemon mode
181 iconActive = ICON_FILE (GUI only)
183 GUI icon to be displayed when there are established connections
184 On Windows platform the parameter should be an .ico file containing
186 a 16x16 pixel image.
187 iconError = ICON_FILE (GUI only)
189 GUI icon to be displayed when no valid configuration is loaded
190 On Windows platform the parameter should be an .ico file containing
192 a 16x16 pixel image.
193 iconIdle = ICON_FILE (GUI only)
195 GUI icon to be displayed when there are no established connections
196 On Windows platform the parameter should be an .ico file containing
198 a 16x16 pixel image.
199 log = append | overwrite
201 log file handling
202 This option allows you to choose whether the log file (specified
204 with the output option) is appended or overwritten when opened or
205 re-opened.
206 default: append
208 output = FILE
210 append log messages to a file
211 /dev/stdout device can be used to send log messages to the standard
213 output (for example to log them with daemontools splogger).
214 pid = FILE (Unix only)
216 pid file location
217 If the argument is empty, then no pid file will be created.
219 pid path is relative to the chroot directory if specified.
221 RNDbytes = BYTES
223 bytes to read from random seed files
224 RNDfile = FILE
226 path to file with random seed data
227 The OpenSSL library will use data from this file first to seed the
229 random number generator.
230 RNDoverwrite = yes | no
232 overwrite the random seed files with new random data
233 default: yes
235 service = SERVICE (Unix only)
237 stunnel service name
238 The specified service name is used for syslog and as the inetd mode
240 service name for TCP Wrappers. While this option can technically
241 be specified in the service sections, it is only useful in global
242 options.
243 default: stunnel
245 syslog = yes | no (Unix only)
247 enable logging via syslog
248 default: yes
250 taskbar = yes | no (WIN32 only)
252 enable the taskbar icon
253 default: yes
255 SERVICE-LEVEL OPTIONS
257 Each configuration section begins with a service name in square
258 brackets. The service name is used for libwrap (TCP Wrappers) access
259 control and lets you distinguish stunnel services in your log files.
260 Note that if you wish to run stunnel in inetd mode (where it is
262 provided a network socket by a server such as inetd, xinetd, or
263 tcpserver) then you should read the section entitled INETD MODE below.
264 accept = [HOST:]PORT
266 accept connections on specified address
267 If no host specified, defaults to all IPv4 addresses for the local
269 host.
270 To listen on all IPv6 addresses use:
272 accept = :::PORT
274 CApath = DIRECTORY
276 Certificate Authority directory
277 This is the directory in which stunnel will look for certificates
279 when using the verifyChain or verifyPeer options. Note that the
280 certificates in this directory should be named XXXXXXXX.0 where
281 XXXXXXXX is the hash value of the DER encoded subject of the cert.
282 The hash algorithm has been changed in OpenSSL 1.0.0. It is
284 required to c_rehash the directory on upgrade from OpenSSL 0.x.x to
285 OpenSSL 1.x.x.
286 CApath path is relative to the chroot directory if specified.
288 CAfile = CA_FILE
290 Certificate Authority file
291 This file contains multiple CA certificates, to be used with the
293 verifyChain and verifyPeer options.
294 cert = CERT_FILE
296 certificate chain file name
297 The parameter specifies the file containing certificates used by
299 stunnel to authenticate itself against the remote client or server.
300 The file should contain the whole certificate chain starting from
301 the actual server/client certificate, and ending with the self-
302 signed root CA certificate. The file must be either in PEM or P12
303 format.
304 A certificate chain is required in server mode, and optional in
306 client mode.
307 This parameter is also used as the certificate identifier when a
309 hardware engine is enabled.
310 checkEmail = EMAIL
312 email address of the peer certificate subject
313 Multiple checkEmail options are allowed in a single service
315 section. Certificates are accepted if no subject checks were
316 specified, or the email address of the peer certificate matches any
317 of the email addresses specified with checkEmail.
318 This option requires OpenSSL 1.0.2 or later.
320 checkHost = HOST
322 host of the peer certificate subject
323 Multiple checkHost options are allowed in a single service section.
325 Certificates are accepted if no subject checks were specified, or
326 the host name of the peer certificate matches any of the hosts
327 specified with checkHost.
328 This option requires OpenSSL 1.0.2 or later.
330 checkIP = IP
332 IP address of the peer certificate subject
333 Multiple checkIP options are allowed in a single service section.
335 Certificates are accepted if no subject checks were specified, or
336 the IP address of the peer certificate matches any of the IP
337 addresses specified with checkIP.
338 This option requires OpenSSL 1.0.2 or later.
340 ciphers = CIPHER_LIST
342 select permitted TLS ciphers (TLSv1.2 and below)
343 This option does not impact TLSv1.3 ciphersuites.
345 A colon-delimited list of the ciphers to allow in the TLS
347 connection, for example DES-CBC3-SHA:IDEA-CBC-MD5.
348 ciphersuites = CIPHERSUITES_LIST
350 select permitted TLSv1.3 ciphersuites
351 A colon-delimited list of TLSv1.3 ciphersuites names in order of
353 preference.
354 This option requires OpenSSL 1.1.1 or later.
356 default:
358 TLS_CHACHA20_POLY1305_SHA256:TLS_AES_256_GCM_SHA384:TLS_AES_128_GCM_SHA256
359 client = yes | no
361 client mode (remote service uses TLS)
362 default: no (server mode)
364 config = COMMAND[:PARAMETER]
366 OpenSSL configuration command
367 The OpenSSL configuration command is executed with the specified
369 parameter. This allows any configuration commands to be invoked
370 from the stunnel configuration file. Supported commands are
371 described on the SSL_CONF_cmd(3ssl) manual page.
372 Several config lines can be used to specify multiple configuration
374 commands.
375 Use curves option instead of enabling config = Curves:list_curves
377 to support elliptic curves.
378 This option requires OpenSSL 1.0.2 or later.
380 connect = [HOST:]PORT
382 connect to a remote address
383 If no host is specified, the host defaults to localhost.
385 Multiple connect options are allowed in a single service section.
387 If host resolves to multiple addresses and/or if multiple connect
389 options are specified, then the remote address is chosen using a
390 round-robin algorithm.
391 CRLpath = DIRECTORY
393 Certificate Revocation Lists directory
394 This is the directory in which stunnel will look for CRLs when
396 using the verifyChain and verifyPeer options. Note that the CRLs in
397 this directory should be named XXXXXXXX.r0 where XXXXXXXX is the
398 hash value of the CRL.
399 The hash algorithm has been changed in OpenSSL 1.0.0. It is
401 required to c_rehash the directory on upgrade from OpenSSL 0.x.x to
402 OpenSSL 1.x.x.
403 CRLpath path is relative to the chroot directory if specified.
405 CRLfile = CRL_FILE
407 Certificate Revocation Lists file
408 This file contains multiple CRLs, used with the verifyChain and
410 verifyPeer options.
411 curves = list
413 ECDH curves separated with ':'
414 Note: This option is supported for server mode sockets only.
416 Only a single curve name is allowed for OpenSSL older than 1.1.1.
418 To get a list of supported curves use:
420 openssl ecparam -list_curves
422 default:
424 X25519:P-256:X448:P-521:P-384 (OpenSSL 1.1.1 or later)
426 prime256v1 (OpenSSL older than 1.1.1)
428 logId = TYPE
430 connection identifier type
431 This identifier allows you to distinguish log entries generated for
433 each of the connections.
434 Currently supported types:
436 sequential
438 The numeric sequential identifier is only unique within a
439 single instance of stunnel, but very compact. It is most
440 useful for manual log analysis.
441 unique
443 This alphanumeric identifier is globally unique, but longer
444 than the sequential number. It is most useful for automated
445 log analysis.
446 thread
448 The operating system thread identifier is neither unique (even
449 within a single instance of stunnel) nor short. It is most
450 useful for debugging software or configuration issues.
451 process
453 The operating system process identifier (PID) may be useful in
454 the inetd mode.
455 default: sequential
457 debug = LEVEL
459 debugging level
460 Level is a one of the syslog level names or numbers emerg (0),
462 alert (1), crit (2), err (3), warning (4), notice (5), info (6), or
463 debug (7). All logs for the specified level and all levels
464 numerically less than it will be shown. The default is notice (5).
465 While the debug = debug or debug = 7 level generates the most
467 verbose output, it is only intended to be used by stunnel
468 developers. Please only use this value if you are a developer, or
469 you intend to send your logs to our technical support. Otherwise,
470 the generated logs will be confusing.
471 delay = yes | no
473 delay DNS lookup for the connect option
474 This option is useful for dynamic DNS, or when DNS is not available
476 during stunnel startup (road warrior VPN, dial-up configurations).
477 Delayed resolver mode is automatically engaged when stunnel fails
479 to resolve on startup any of the connect targets for a service.
480 Delayed resolver inflicts failover = prio.
482 default: no
484 engineId = ENGINE_ID
486 select engine ID for the service
487 engineNum = ENGINE_NUMBER
489 select engine number for the service
490 The engines are numbered starting from 1.
492 exec = EXECUTABLE_PATH
494 execute a local inetd-type program
495 exec path is relative to the chroot directory if specified.
497 The following environmental variables are set on Unix platforms:
499 REMOTE_HOST, REMOTE_PORT, SSL_CLIENT_DN, SSL_CLIENT_I_DN.
500 execArgs = $0 $1 $2 ...
502 arguments for exec including the program name ($0)
503 Quoting is currently not supported. Arguments are separated with
505 an arbitrary amount of whitespace.
506 failover = rr | prio
508 Failover strategy for multiple "connect" targets.
509 rr round robin - fair load distribution
511 prio
513 priority - use the order specified in config file
514 default: prio
516 ident = USERNAME
518 use IDENT (RFC 1413) username checking
519 include = DIRECTORY
521 include all configuration file parts located in DIRECTORY
522 The files are included in the ascending alphabetical order of their
524 names. The recommended filename convention is
525 for global options:
527 00-global.conf
529 for local service-level options:
531 01-service.conf
533 02-service.conf
535 key = KEY_FILE
537 private key for the certificate specified with cert option
538 A private key is needed to authenticate the certificate owner.
540 Since this file should be kept secret it should only be readable by
541 its owner. On Unix systems you can use the following command:
542 chmod 600 keyfile
544 This parameter is also used as the private key identifier when a
546 hardware engine is enabled.
547 default: the value of the cert option
549 libwrap = yes | no
551 Enable or disable the use of /etc/hosts.allow and /etc/hosts.deny.
552 default: no (since version 5.00)
554 local = HOST
556 By default, the IP address of the outgoing interface is used as the
557 source for remote connections. Use this option to bind a static
558 local IP address instead.
559 OCSP = URL
561 select OCSP responder for certificate verification
562 OCSPaia = yes | no
564 validate certificates with their AIA OCSP responders
565 This option enables stunnel to validate certificates with the list
567 of OCSP responder URLs retrieved from their AIA (Authority
568 Information Access) extension.
569 OCSPflag = OCSP_FLAG
571 specify OCSP responder flag
572 Several OCSPflag can be used to specify multiple flags.
574 currently supported flags: NOCERTS, NOINTERN, NOSIGS, NOCHAIN,
576 NOVERIFY, NOEXPLICIT, NOCASIGN, NODELEGATED, NOCHECKS, TRUSTOTHER,
577 RESPID_KEY, NOTIME
578 OCSPnonce = yes | no
580 send and verify the OCSP nonce extension
581 This option protects the OCSP protocol against replay attacks. Due
583 to its computational overhead, the nonce extension is usually only
584 supported on internal (e.g. corporate) responders, and not on
585 public OCSP responders.
586 options = SSL_OPTIONS
588 OpenSSL library options
589 The parameter is the OpenSSL option name as described in the
591 SSL_CTX_set_options(3ssl) manual, but without SSL_OP_ prefix.
592 stunnel -options lists the options found to be allowed in the
593 current combination of stunnel and the OpenSSL library used to
594 build it.
595 Several option lines can be used to specify multiple options. An
597 option name can be prepended with a dash ("-") to disable the
598 option.
599 For example, for compatibility with the erroneous Eudora TLS
601 implementation, the following option can be used:
602 options = DONT_INSERT_EMPTY_FRAGMENTS
604 default:
606 options = NO_SSLv2
608 options = NO_SSLv3
609 Use sslVersionMax or sslVersionMin option instead of disabling
611 specific TLS protocol versions when compiled with OpenSSL 1.1.0 or
612 later.
613 protocol = PROTO
615 application protocol to negotiate TLS
616 This option enables initial, protocol-specific negotiation of the
618 TLS encryption. The protocol option should not be used with TLS
619 encryption on a separate port.
620 Currently supported protocols:
622 cifs
624 Proprietary (undocummented) extension of CIFS protocol
625 implemented in Samba. Support for this extension was dropped
626 in Samba 3.0.0.
627 capwin
629 http://www.capwin.org/ application support
630 capwinctrl
632 http://www.capwin.org/ application support
633 This protocol is only supported in client mode.
635 connect
637 Based on RFC 2817 - Upgrading to TLS Within HTTP/1.1, section
638 5.2 - Requesting a Tunnel with CONNECT
639 This protocol is only supported in client mode.
641 imap
643 Based on RFC 2595 - Using TLS with IMAP, POP3 and ACAP
644 ldap
646 Based on RFC 2830 - Lightweight Directory Access Protocol (v3):
647 Extension for Transport Layer Security
648 nntp
650 Based on RFC 4642 - Using Transport Layer Security (TLS) with
651 Network News Transfer Protocol (NNTP)
652 This protocol is only supported in client mode.
654 pgsql
656 Based on
657 http://www.postgresql.org/docs/8.3/static/protocol-flow.html#AEN73982
658 pop3
660 Based on RFC 2449 - POP3 Extension Mechanism
661 proxy
663 Passing of the original client IP address with HAProxy PROXY
664 protocol version 1
665 https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt
666 smtp
668 Based on RFC 2487 - SMTP Service Extension for Secure SMTP over
669 TLS
670 socks
672 SOCKS versions 4, 4a, and 5 are supported. The SOCKS protocol
673 itself is encapsulated within TLS encryption layer to protect
674 the final destination address.
675 http://www.openssh.com/txt/socks4.protocol
677 http://www.openssh.com/txt/socks4a.protocol
679 The BIND command of the SOCKS protocol is not supported. The
681 USERID parameter is ignored.
682 See Examples section for sample configuration files for VPN
684 based on SOCKS encryption.
685 protocolAuthentication = AUTHENTICATION
687 authentication type for the protocol negotiations
688 Currently, this option is only supported in the client-side
690 'connect' and 'smtp' protocols.
691 Supported authentication types for the 'connect' protocol are
693 'basic' or 'ntlm'. The default 'connect' authentication type is
694 'basic'.
695 Supported authentication types for the 'smtp' protocol are 'plain'
697 or 'login'. The default 'smtp' authentication type is 'plain'.
698 protocolDomain = DOMAIN
700 domain for the protocol negotiations
701 Currently, this option is only supported in the client-side
703 'connect' protocol.
704 protocolHeader = HEADER
706 header for the protocol negotiations
707 Currently, this option is only supported in the client-side
709 'connect' protocol.
710 protocolHost = ADDRESS
712 host address for the protocol negotiations
713 For the 'connect' protocol negotiations, protocolHost specifies
715 HOST:PORT of the final TLS server to be connected to by the proxy.
716 The proxy server directly connected by stunnel must be specified
717 with the connect option.
718 For the 'smtp' protocol negotiations, protocolHost controls the
720 client SMTP HELO/EHLO value.
721 protocolPassword = PASSWORD
723 password for the protocol negotiations
724 Currently, this option is only supported in the client-side
726 'connect' and 'smtp' protocols.
727 protocolUsername = USERNAME
729 username for the protocol negotiations
730 Currently, this option is only supported in the client-side
732 'connect' and 'smtp' protocols.
733 PSKidentity = IDENTITY
735 PSK identity for the PSK client
736 PSKidentity can be used on stunnel clients to select the PSK
738 identity used for authentication. This option is ignored in server
739 sections.
740 default: the first identity specified in the PSKsecrets file.
742 PSKsecrets = FILE
744 file with PSK identities and corresponding keys
745 Each line of the file in the following format:
747 IDENTITY:KEY
749 Hexadecimal keys are automatically converted to binary form. Keys
751 are required to be at least 16 bytes long, which implies at least
752 32 characters for hexadecimal keys. The file should neither be
753 world-readable nor world-writable.
754 pty = yes | no (Unix only)
756 allocate a pseudoterminal for 'exec' option
757 redirect = [HOST:]PORT
759 redirect TLS client connections on certificate-based authentication
760 failures
761 This option only works in server mode. Some protocol negotiations
763 are also incompatible with the redirect option.
764 renegotiation = yes | no
766 support TLS renegotiation
767 Applications of the TLS renegotiation include some authentication
769 scenarios, or re-keying long lasting connections.
770 On the other hand this feature can facilitate a trivial CPU-
772 exhaustion DoS attack:
773 http://vincent.bernat.im/en/blog/2011-ssl-dos-mitigation.html
775 Please note that disabling TLS renegotiation does not fully
777 mitigate this issue.
778 default: yes (if supported by OpenSSL)
780 reset = yes | no
782 attempt to use the TCP RST flag to indicate an error
783 This option is not supported on some platforms.
785 default: yes
787 retry = yes | no
789 reconnect a connect+exec section after it was disconnected
790 default: no
792 securityLevel = LEVEL
794 set the security level
795 The meaning of each level is described below:
797 level 0
799 Everything is permitted.
800 level 1
802 The security level corresponds to a minimum of 80 bits of
803 security. Any parameters offering below 80 bits of security are
804 excluded. As a result RSA, DSA and DH keys shorter than 1024
805 bits and ECC keys shorter than 160 bits are prohibited. All
806 export cipher suites are prohibited since they all offer less
807 than 80 bits of security. SSL version 2 is prohibited. Any
808 cipher suite using MD5 for the MAC is also prohibited.
809 level 2
811 Security level set to 112 bits of security. As a result RSA,
812 DSA and DH keys shorter than 2048 bits and ECC keys shorter
813 than 224 bits are prohibited. In addition to the level 1
814 exclusions any cipher suite using RC4 is also prohibited. SSL
815 version 3 is also not allowed. Compression is disabled.
816 level 3
818 Security level set to 128 bits of security. As a result RSA,
819 DSA and DH keys shorter than 3072 bits and ECC keys shorter
820 than 256 bits are prohibited. In addition to the level 2
821 exclusions cipher suites not offering forward secrecy are
822 prohibited. TLS versions below 1.1 are not permitted. Session
823 tickets are disabled.
824 level 4
826 Security level set to 192 bits of security. As a result RSA,
827 DSA and DH keys shorter than 7680 bits and ECC keys shorter
828 than 384 bits are prohibited. Cipher suites using SHA1 for the
829 MAC are prohibited. TLS versions below 1.2 are not permitted.
830 level 5
832 Security level set to 256 bits of security. As a result RSA,
833 DSA and DH keys shorter than 15360 bits and ECC keys shorter
834 than 512 bits are prohibited.
835 default: 2
837 The securityLevel option is only available when compiled with
839 OpenSSL 1.1.0 and later.
840 requireCert = yes | no
842 require a client certificate for verifyChain or verifyPeer
843 With requireCert set to no, the stunnel server accepts client
845 connections that did not present a certificate.
846 Both verifyChain = yes and verifyPeer = yes imply requireCert =
848 yes.
849 default: no
851 setgid = GROUP (Unix only)
853 Unix group id
854 As a global option: setgid() to the specified group in daemon mode
856 and clear all other groups.
857 As a service-level option: set the group of the Unix socket
859 specified with "accept".
860 setuid = USER (Unix only)
862 Unix user id
863 As a global option: setuid() to the specified user in daemon mode.
865 As a service-level option: set the owner of the Unix socket
867 specified with "accept".
868 sessionCacheSize = NUM_ENTRIES
870 session cache size
871 sessionCacheSize specifies the maximum number of the internal
873 session cache entries.
874 The value of 0 can be used for unlimited size. It is not
876 recommended for production use due to the risk of a memory
877 exhaustion DoS attack.
878 sessionCacheTimeout = TIMEOUT
880 session cache timeout
881 This is the number of seconds to keep cached TLS sessions.
883 sessionResume = yes | no
885 allow or disallow session resumption
886 default: yes
888 sessiond = HOST:PORT
890 address of sessiond TLS cache server
891 sni = SERVICE_NAME:SERVER_NAME_PATTERN (server mode)
893 Use the service as a slave service (a name-based virtual server)
894 for Server Name Indication TLS extension (RFC 3546).
895 SERVICE_NAME specifies the master service that accepts client
897 connections with the accept option. SERVER_NAME_PATTERN specifies
898 the host name to be redirected. The pattern may start with the '*'
899 character, e.g. '*.example.com'. Multiple slave services are
900 normally specified for a single master service. The sni option can
901 also be specified more than once within a single slave service.
902 This service, as well as the master service, may not be configured
904 in client mode.
905 The connect option of the slave service is ignored when the
907 protocol option is specified, as protocol connects to the remote
908 host before TLS handshake.
909 Libwrap checks (Unix only) are performed twice: with the master
911 service name after TCP connection is accepted, and with the slave
912 service name during the TLS handshake.
913 The sni option is only available when compiled with OpenSSL 1.0.0
915 and later.
916 sni = SERVER_NAME (client mode)
918 Use the parameter as the value of TLS Server Name Indication (RFC
919 3546) extension.
920 Empty SERVER_NAME disables sending the SNI extension.
922 The sni option is only available when compiled with OpenSSL 1.0.0
924 and later.
925 socket = a|l|r:OPTION=VALUE[:VALUE]
927 Set an option on the accept/local/remote socket
928 The values for the linger option are l_onof:l_linger. The values
930 for the time are tv_sec:tv_usec.
931 Examples:
933 socket = l:SO_LINGER=1:60
935 set one minute timeout for closing local socket
936 socket = r:SO_OOBINLINE=yes
937 place out-of-band data directly into the
938 receive data stream for remote sockets
939 socket = a:SO_REUSEADDR=no
940 disable address reuse (enabled by default)
941 socket = a:SO_BINDTODEVICE=lo
942 only accept connections on loopback interface
943 sslVersion = SSL_VERSION
945 select the TLS protocol version
946 Supported versions: all, SSLv2, SSLv3, TLSv1, TLSv1.1, TLSv1.2,
948 TLSv1.3
949 Availability of specific protocols depends on the linked OpenSSL
951 library. Older versions of OpenSSL do not support TLSv1.1, TLSv1.2
952 and TLSv1.3. Newer versions of OpenSSL do not support SSLv2.
953 Obsolete SSLv2 and SSLv3 are currently disabled by default.
955 Setting the option
957 sslVersion = SSL_VERSION
959 is equivalent to options
961 sslVersionMax = SSL_VERSION
963 sslVersionMin = SSL_VERSION
964 when compiled with OpenSSL 1.1.0 and later.
966 sslVersionMax = SSL_VERSION
968 maximum supported protocol versions
969 Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
971 all enable protocol versions up to the highest version supported by
973 the linked OpenSSL library.
974 Availability of specific protocols depends on the linked OpenSSL
976 library.
977 The sslVersionMax option is only available when compiled with
979 OpenSSL 1.1.0 and later.
980 default: all
982 sslVersionMin = SSL_VERSION
984 minimum supported protocol versions
985 Supported versions: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2, TLSv1.3
987 all enable protocol versions down to the lowest version supported
989 by the linked OpenSSL library.
990 Availability of specific protocols depends on the linked OpenSSL
992 library.
993 The sslVersionMin option is only available when compiled with
995 OpenSSL 1.1.0 and later.
996 default: TLSv1
998 stack = BYTES (except for FORK model)
1000 CPU stack size of created threads
1001 Excessive thread stack size increases virtual memory usage.
1003 Insufficient thread stack size may cause application crashes.
1004 default: 65536 bytes (sufficient for all platforms we tested)
1006 ticketKeySecret = SECRET
1008 hexadecimal symmetric key used for session ticket confidentiality
1009 protection
1010 Session tickets defined in RFC 5077 provide an enhanced session
1012 resumption capability, where the server-side caching is not
1013 required to maintain per session state.
1014 Combining ticketKeySecret and ticketMacSecret options allow to
1016 resume a negotiated session on other cluster nodes, or to resume a
1017 negotiated session after server restart.
1018 The key is required to be either 16 or 32 bytes long, which implies
1020 exactly 32 or 64 hexadecimal digits. Colons may optionally be used
1021 between two-character hexadecimal bytes.
1022 This option only works in server mode.
1024 The ticketKeySecret option is only available when compiled with
1026 OpenSSL 1.0.0 and later.
1027 Disabling NO_TICKET option is required for the ticket support in
1029 OpenSSL older than 1.1.1, but note that this option is incompatible
1030 with the redirect option.
1031 ticketMacSecret = SECRET
1033 hexadecimal symmetric key used for session ticket integrity
1034 protection
1035 The key is required to be either 16 or 32 bytes long, which implies
1037 exactly 32 or 64 hexadecimal digits. Colons may optionally be used
1038 between two-character hexadecimal bytes.
1039 This option only works in server mode.
1041 The ticketMacSecret option is only available when compiled with
1043 OpenSSL 1.0.0 and later.
1044 TIMEOUTbusy = SECONDS
1046 time to wait for expected data
1047 TIMEOUTclose = SECONDS
1049 time to wait for close_notify (set to 0 for buggy MSIE)
1050 TIMEOUTconnect = SECONDS
1052 time to wait to connect to a remote host
1053 TIMEOUTidle = SECONDS
1055 time to keep an idle connection
1056 transparent = none | source | destination | both (Unix only)
1058 enable transparent proxy support on selected platforms
1059 Supported values:
1061 none
1063 Disable transparent proxy support. This is the default.
1064 source
1066 Re-write the address to appear as if a wrapped daemon is
1067 connecting from the TLS client machine instead of the machine
1068 running stunnel.
1069 This option is currently available in:
1071 Remote mode (connect option) on Linux >=2.6.28
1073 This configuration requires stunnel to be executed as root
1074 and without the setuid option.
1075 This configuration requires the following setup for
1077 iptables and routing (possibly in /etc/rc.local or
1078 equivalent file):
1079 iptables -t mangle -N DIVERT
1081 iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
1082 iptables -t mangle -A DIVERT -j MARK --set-mark 1
1083 iptables -t mangle -A DIVERT -j ACCEPT
1084 ip rule add fwmark 1 lookup 100
1085 ip route add local 0.0.0.0/0 dev lo table 100
1086 echo 0 >/proc/sys/net/ipv4/conf/lo/rp_filter
1087 stunnel must also to be executed as root and without the
1089 setuid option.
1090 Remote mode (connect option) on Linux 2.2.x
1092 This configuration requires the kernel to be compiled with
1093 the transparent proxy option. Connected service must be
1094 installed on a separate host. Routing towards the clients
1095 has to go through the stunnel box.
1096 stunnel must also to be executed as root and without the
1098 setuid option.
1099 Remote mode (connect option) on FreeBSD >=8.0
1101 This configuration requires additional firewall and routing
1102 setup. stunnel must also to be executed as root and
1103 without the setuid option.
1104 Local mode (exec option)
1106 This configuration works by pre-loading the libstunnel.so
1107 shared library. _RLD_LIST environment variable is used on
1108 Tru64, and LD_PRELOAD variable on other platforms.
1109 destination
1111 The original destination is used instead of the connect option.
1112 A service section for transparent destination may look like
1114 this:
1115 [transparent]
1117 client = yes
1118 accept = <stunnel_port>
1119 transparent = destination
1120 This configuration requires iptables setup to work, possibly in
1122 /etc/rc.local or equivalent file.
1123 For a connect target installed on the same host:
1125 /sbin/iptables -t nat -I OUTPUT -p tcp --dport <redirected_port> \
1127 -m ! --uid-owner <stunnel_user_id> \
1128 -j DNAT --to-destination <local_ip>:<stunnel_port>
1129 For a connect target installed on a remote host:
1131 /sbin/iptables -I INPUT -i eth0 -p tcp --dport <stunnel_port> -j ACCEPT
1133 /sbin/iptables -t nat -I PREROUTING -p tcp --dport <redirected_port> \
1134 -i eth0 -j DNAT --to-destination <local_ip>:<stunnel_port>
1135 The transparent destination option is currently only supported
1137 on Linux.
1138 both
1140 Use both source and destination transparent proxy.
1141 Two legacy options are also supported for backward compatibility:
1143 yes This option has been renamed to source.
1145 no This option has been renamed to none.
1147 verify = LEVEL
1149 verify the peer certificate
1150 This option is obsolete and should be replaced with the verifyChain
1152 and verifyPeer options.
1153 level 0
1155 Request and ignore the peer certificate.
1156 level 1
1158 Verify the peer certificate if present.
1159 level 2
1161 Verify the peer certificate.
1162 level 3
1164 Verify the peer against a locally installed certificate.
1165 level 4
1167 Ignore the chain and only verify the peer certificate.
1168 default
1170 No verify.
1171 verifyChain = yes | no
1173 verify the peer certificate chain starting from the root CA
1174 For server certificate verification it is essential to also require
1176 a specific certificate with checkHost or checkIP.
1177 The self-signed root CA certificate needs to be stored either in
1179 the file specified with CAfile, or in the directory specified with
1180 CApath.
1181 default: no
1183 verifyPeer = yes | no
1185 verify the peer certificate
1186 The peer certificate needs to be stored either in the file
1188 specified with CAfile, or in the directory specified with CApath.
1189 default: no
1191
1193 stunnel returns zero on success, non-zero on error.
1194
1196 The following signals can be used to control stunnel in Unix
1197 environment:
1198 SIGHUP
1200 Force a reload of the configuration file.
1201 Some global options will not be reloaded:
1203 • chroot
1205 • foreground
1207 • pid
1209 • setgid
1211 • setuid
1213 The use of the 'setuid' option will also prevent stunnel from
1215 binding to privileged (<1024) ports during configuration reloading.
1216 When the 'chroot' option is used, stunnel will look for all its
1218 files (including the configuration file, certificates, the log file
1219 and the pid file) within the chroot jail.
1220 SIGUSR1
1222 Close and reopen the stunnel log file. This function can be used
1223 for log rotation.
1224 SIGUSR2
1226 Log the list of active connections.
1227 SIGTERM, SIGQUIT, SIGINT
1229 Shut stunnel down.
1230 The result of sending any other signals to the server is undefined.
1232
1234 In order to provide TLS encapsulation to your local imapd service, use:
1235 [imapd]
1237 accept = 993
1238 exec = /usr/sbin/imapd
1239 execArgs = imapd
1240 or in remote mode:
1242 [imapd]
1244 accept = 993
1245 connect = 143
1246 In order to let your local e-mail client connect to a TLS-enabled imapd
1248 service on another server, configure the e-mail client to connect to
1249 localhost on port 119 and use:
1250 [imap]
1252 client = yes
1253 accept = 143
1254 connect = servername:993
1255 If you want to provide tunneling to your pppd daemon on port 2020, use
1257 something like:
1258 [vpn]
1260 accept = 2020
1261 exec = /usr/sbin/pppd
1262 execArgs = pppd local
1263 pty = yes
1264 If you want to use stunnel in inetd mode to launch your imapd process,
1266 you'd use this stunnel.conf. Note there must be no [service_name]
1267 section.
1268 exec = /usr/sbin/imapd
1270 execArgs = imapd
1271 To setup SOCKS VPN configure the following client service:
1273 [socks_client]
1275 client = yes
1276 accept = 127.0.0.1:1080
1277 connect = vpn_server:9080
1278 verifyPeer = yes
1279 CAfile = stunnel.pem
1280 The corresponding configuration on the vpn_server host:
1282 [socks_server]
1284 protocol = socks
1285 accept = 9080
1286 cert = stunnel.pem
1287 key = stunnel.key
1288 Now test your configuration on the client machine with:
1290 curl --socks4a localhost http://www.example.com/
1292 An example server mode SNI configuration:
1294 [virtual]
1296 ; master service
1297 accept = 443
1298 cert = default.pem
1299 connect = default.internal.mydomain.com:8080
1300 [sni1]
1302 ; slave service 1
1303 sni = virtual:server1.mydomain.com
1304 cert = server1.pem
1305 connect = server1.internal.mydomain.com:8081
1306 [sni2]
1308 ; slave service 2
1309 sni = virtual:server2.mydomain.com
1310 cert = server2.pem
1311 connect = server2.internal.mydomain.com:8082
1312 verifyPeer = yes
1313 CAfile = server2-allowed-clients.pem
1314 An example of advanced engine configuration allows for authentication
1316 with private keys stored in the Windows certificate store (Windows
1317 only). With the CAPI engine you don't need to manually select the
1318 client key to use. The client key is automatically selected based on
1319 the list of CAs trusted by the server.
1320 engine = capi
1322 [service]
1324 engineId = capi
1325 client = yes
1326 accept = 127.0.0.1:8080
1327 connect = example.com:8443
1328 An example of advanced engine configuration to use the certificate and
1330 the corresponding private key from a pkcs11 engine:
1331 engine = pkcs11
1333 engineCtrl = MODULE_PATH:opensc-pkcs11.so
1334 engineCtrl = PIN:123456
1335 [service]
1337 engineId = pkcs11
1338 client = yes
1339 accept = 127.0.0.1:8080
1340 connect = example.com:843
1341 cert = pkcs11:token=MyToken;object=MyCert
1342 key = pkcs11:token=MyToken;object=MyKey
1343 An example of advanced engine configuration to use the certificate and
1345 the corresponding private key from a SoftHSM token:
1346 engine = pkcs11
1348 engineCtrl = MODULE_PATH:softhsm2.dll
1349 engineCtrl = PIN:12345
1350 [service]
1352 engineId = pkcs11
1353 client = yes
1354 accept = 127.0.0.1:8080
1355 connect = example.com:843
1356 cert = pkcs11:token=MyToken;object=KeyCert
1357
1359 RESTRICTIONS
1360 stunnel cannot be used for the FTP daemon because of the nature of the
1361 FTP protocol which utilizes multiple ports for data transfers. There
1362 are available TLS-enabled versions of FTP and telnet daemons, however.
1363 INETD MODE
1365 The most common use of stunnel is to listen on a network port and
1366 establish communication with either a new port via the connect option,
1367 or a new program via the exec option. However there is a special case
1368 when you wish to have some other program accept incoming connections
1369 and launch stunnel, for example with inetd, xinetd, or tcpserver.
1370 For example, if you have the following line in inetd.conf:
1372 imaps stream tcp nowait root /usr/bin/stunnel stunnel /etc/stunnel/imaps.conf
1374 In these cases, the inetd-style program is responsible for binding a
1376 network socket (imaps above) and handing it to stunnel when a
1377 connection is received. Thus you do not want stunnel to have any
1378 accept option. All the Service Level Options should be placed in the
1379 global options section, and no [service_name] section will be present.
1380 See the EXAMPLES section for example configurations.
1381 CERTIFICATES
1383 Each TLS-enabled daemon needs to present a valid X.509 certificate to
1384 the peer. It also needs a private key to decrypt the incoming data. The
1385 easiest way to obtain a certificate and a key is to generate them with
1386 the free OpenSSL package. You can find more information on certificates
1387 generation on pages listed below.
1388 The .pem file should contain the unencrypted private key and a signed
1390 certificate (not certificate request). So the file should look like
1391 this:
1392 -----BEGIN RSA PRIVATE KEY-----
1394 [encoded key]
1395 -----END RSA PRIVATE KEY-----
1396 -----BEGIN CERTIFICATE-----
1397 [encoded certificate]
1398 -----END CERTIFICATE-----
1399 RANDOMNESS
1401 stunnel needs to seed the PRNG (pseudo-random number generator) in
1402 order for TLS to use good randomness. The following sources are loaded
1403 in order until sufficient random data has been gathered:
1404 • The file specified with the RNDfile flag.
1406 • The file specified by the RANDFILE environment variable, if set.
1408 • The file .rnd in your home directory, if RANDFILE not set.
1410 • The file specified with '--with-random' at compile time.
1412 • The contents of the screen if running on Windows.
1414 • The egd socket specified with the EGD flag.
1416 • The egd socket specified with '--with-egd-sock' at compile time.
1418 • The /dev/urandom device.
1420 Note that on Windows machines that do not have console user interaction
1422 (mouse movements, creating windows, etc.) the screen contents are not
1423 variable enough to be sufficient, and you should provide a random file
1424 for use with the RNDfile flag.
1425 Note that the file specified with the RNDfile flag should contain
1427 random data -- that means it should contain different information each
1428 time stunnel is run. This is handled automatically unless the
1429 RNDoverwrite flag is used. If you wish to update this file manually,
1430 the openssl rand command in recent versions of OpenSSL, would be
1431 useful.
1432 Important note: If /dev/urandom is available, OpenSSL often seeds the
1434 PRNG with it while checking the random state. On systems with
1435 /dev/urandom OpenSSL is likely to use it even though it is listed at
1436 the very bottom of the list above. This is the behaviour of OpenSSL
1437 and not stunnel.
1438 DH PARAMETERS
1440 stunnel 4.40 and later contains hardcoded 2048-bit DH parameters.
1441 Starting with stunnel 5.18, these hardcoded DH parameters are replaced
1442 every 24 hours with autogenerated temporary DH parameters. DH
1443 parameter generation may take several minutes.
1444 Alternatively, it is possible to specify static DH parameters in the
1446 certificate file, which disables generating temporary DH parameters:
1447 openssl dhparam 2048 >> stunnel.pem
1449
1451 /etc/stunnel/stunnel.conf
1452 stunnel configuration file
1453
1455 The execArgs option and the Win32 command line do not support quoting.
1456
1458 tcpd(8)
1459 access control facility for internet services
1460 inetd(8)
1462 internet 'super-server'
1463 http://www.stunnel.org/
1465 stunnel homepage
1466 http://www.openssl.org/
1468 OpenSSL project website
1469
1471 Michał Trojnara
1472 <Michal.Trojnara@stunnel.org>
14735.62 2022.02.07 stunnel(8)