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 (email@example.com)
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.
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
SO_BINDTODEVICE -- -- -- --
IP_TOS -- -- -- 0
IP_TTL -- -- -- 64
TCP_NODELAY -- -- -- 0
A colon delimited list of the ciphers to allow in the SSL connection. For example DES-CBC3-SHA:IDEA-CBC-MD5
default: server 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.
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).
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.
This file contains multiple CA certificates, used with the -v options.
default: 300 seconds.
currently supported: smtp, pop3, nntp
Listen for connections on [host:]port. If no host specified, defaults to all IP addresses for the local host.
default: inetd mode
Stay in foreground (don't fork) and log to stderr instead of via syslog (unless -o is specified).
default: background in daemon mode
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.
If no host specified, defaults to localhost.
stunnel -d 993 -l /usr/sbin/imapd -- imapd
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
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:
-----BEGIN RSA PRIVATE KEY----- [encoded key] -----END RSA PRIVATE KEY----- [empty line] -----BEGIN CERTIFICATE----- [encoded certificate] -----END CERTIFICATE----- [empty line]
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.