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stunnel - universal SSL tunnel


stunnel [-c ⎪ -T] [-D [facility.]level] [-O a⎪l⎪r:option=value[:value]] [-o file] [-C cipherlist] [-p pemfile] [-v level] [-A certfile] [-S sources] [-a directory] [-t timeout] [-u ident_username] [-s setuid_user] [-g setgid_group] [-n protocol] [-P { filename ⎪ '' } ] [-B bytes] [-R randfile] [-W] [-E socket] [-I host] [-d [host:]port [-f] ] [ -r [host:]port ⎪ { -l ⎪ -L } program [-- progname args] ]


The stunnel program is designed to work as SSL encryption wrapper between remote clients and local (inetd-startable) or remote servers. The concept is that having non-SSL aware daemons running on your system you can easily set them up to communicate with clients over secure SSL channels.

stunnel can be used to add SSL 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 (


Print stunnel help menu
Debugging level

Level is a one of the syslog level names or numbers emerg (0), alert (1), crit (2), err (3), warning (4), notice (5), info (6), or debug (7). All logs for the specified level and all levels numerically less than it will be shown. Use -D debug or -D 7 for greatest debugging output. The default is notice (5).

The syslog facility 'daemon' will be used unless a facility name is supplied. (Facilities are not supported on windows.)

Case is ignored for both facilities and levels.

Set an option on accept/local/remote socket

The values for linger option are l_onof:l_linger. The values for time are tv_sec:tv_usec.


-O l:SO_LINGER=1:60 - set one minute timeout for closing local socket

-O r:TCP_NODELAY=1 - turn off the Nagle algorithm for remote sockets

-O r:SO_OOBINLINE=1 - place out-of-band data directly into the receive data stream for remote sockets

-O a:SO_REUSEADDR=0 - disable address reuse (enabled by default)

-O a:SO_BINDTODEVICE=lo - only accept connections on loopback interface

The available options and their defaults are:
Option Accept Local Remote OS default
SO_DEBUG -- -- -- 0
SO_DONTROUTE -- -- -- 0
SO_KEEPALIVE -- -- -- 0
SO_LINGER -- -- -- 0:0
SO_OOBINLINE -- -- -- 0
SO_RCVBUF -- -- -- 87380
SO_SNDBUF -- -- -- 16384
SO_RCVLOWAT -- -- -- 1
SO_SNDLOWAT -- -- -- 1
SO_RCVTIMEO -- -- -- 0:0
SO_SNDTIMEO -- -- -- 0:0
SO_REUSEADDR 1 -- -- 0
IP_TOS -- -- -- 0
IP_TTL -- -- -- 64
TCP_NODELAY -- -- -- 0

Append log messages to a file.
Select permitted SSL ciphers

A colon delimited list of the ciphers to allow in the SSL connection. For example DES-CBC3-SHA:IDEA-CBC-MD5

client mode (remote service uses SSL)

default: server mode

transparent proxy mode

Re-write address to appear as if wrapped daemon is connecting from the SSL client machine instead of the machine running stunnel. Available only on some operating systems (Linux only, we believe) and then only in server mode. Note that this option will not combine with proxy mode (-r) unless the client's default route to the target machine lies through the host running stunnel, which cannot be localhost.

private key and certificate chain PEM file name

A PEM is always needed in server mode (by default located in /etc/stunnel/stunnel.pem). Specifying this flag in client mode will use this key and certificate chain as a client side certificate chain. Using client side certs is optional. The certificates must be in PEM format and must be sorted starting with the certificate to the highest level (root CA).

verify peer certificate
  • level 1 - verify peer certificate if present
  • level 2 - verify peer certificate
  • level 3 - verify peer with locally installed certificate
  • default - no verify
client certificate directory

This is the directory in which stunnel will look for certificates when using the -v options. Note that the certificates in this directory should be named XXXXXXXX.0 where XXXXXXXX is the hash value of the cert.

Certificate Authority file

This file contains multiple CA certificates, used with the -v options.

session cache timeout

default: 300 seconds.

Service name to use for tcpwrappers. If not specified then a tcpwrapper service name will be generated automatically for you. This will also be used when auto-generating pid filenames.
Use IDENT (RFC 1413) username checking
Negotiate SSL with specified protocol

currently supported: smtp, pop3, nntp

Entropy Gathering Daemon socket to use to feed OpenSSL random number generator. (Available only if compiled with OpenSSL 0.9.5a or higher)
File containing random input. The SSL library will use data from this file first to seed the random number generator.
Do not overwrite the random seed files with new random data.
Number of bytes of data read from random seed files. With SSL versions less than 0.9.5a, also determines how many bytes of data are considered sufficient to seed the PRNG. More recent OpenSSL versions have a builtin function to determine when sufficient randomness is available.
IP of the outgoing interface is used as source for remote connections. Use this option to bind a static local IP address, instead.
daemon mode

Listen for connections on [host:]port. If no host specified, defaults to all IP addresses for the local host.

default: inetd mode

foreground mode

Stay in foreground (don't fork) and log to stderr instead of via syslog (unless -o is specified).

default: background in daemon mode

execute local inetd-type program.
open local pty and execute program.
setuid() to username in daemon mode
setgid() to groupname in daemon mode. Clears all other groups.
Pid file location

If the argument is a filename, then that filename will be used for the pid. If the argument is empty ('', not missing), then no pid file will be created.

connect to remote service

If no host specified, defaults to localhost.


In order to provide SSL encapsulation to your local imapd service, use

  stunnel -d 993 -l /usr/sbin/imapd -- imapd

In order to let your local e-mail client connect to a SSL-enabled imapd service on another server, configure the e-mail client to connect to localhost on port 119 and use:

  stunnel -c -d 143 -r servername:993

If you want to provide tunneling to your pppd daemon on port 2020, use something like

  stunnel -d 2020 -L /usr/sbin/pppd -- pppd local


If Stunnel is used to create local processes using the -l or -L options, it will set the following environment variables

The IP address of the remote end of the connection.
The DN (Distinguished Name, aka subject name) of the peer certificate, if a certificate was present and verified.
The Issuer's DN of the peer's certificate, if a certificate was present and verified.


Each SSL 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.

Two things are important when generating certificate-key pairs for stunnel. The private key cannot be encrypted, because the server has no way to obtain the password from the user. To produce an unencrypted key add the -nodes option when running the req command from the openssl kit.

The order of contents of the .pem file is also important. It should contain the unencrypted private key first, then a signed certificate (not certificate request). There should be also empty lines after certificate and private key. Plaintext certificate information appended on the top of generated certificate should be discarded. So the file should look like this:

  [encoded key]
  [empty line]
  [encoded certificate]
  [empty line]


stunnel needs to seed the PRNG (pseudo random number generator) in order for SSL to use good randomness. The following sources are loaded in order until sufficient random data has been gathered:
  • The file specified with the -R 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 -E flag.
  • The egd socket specified with '--with-egd-sock' at compile time.
  • The /dev/urandom device.

With recent (>=OpenSSL 0.9.5a) version of SSL it will stop loading random data automatically when sufficient entropy has been gathered. With previous versions it will continue to gather from all the above sources since no SSL function exists to tell when enough data is available.

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 -R flag.

Note that the file specified with the -R flag should contain random data -- that means it should contain different information each time stunnel is run. This is handled automatically unless the -W flag is used. If you wish to update this file manually, the openssl rand command in recent versions of OpenSSL, would be useful.

One important note -- if /dev/urandom is available, OpenSSL has a habit of seeding the PRNG with it even when checking the random state, so on systems with /dev/urandom you're likely to use it even though it's listed at the very bottom of the list above. This isn't stunnel's behaviour, it's OpenSSLs.


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 SSL enabled versions of FTP and telnet daemons, however.


access control facility for internet services
internet ``super-server''
Stunnel homepage
OpenSSL project website