Stunnel

Related articles
Transport Layer Security
stunnel
(“Secure Tunnel”) is a
cross-platform application used to provide a universal TLS/SSL tunneling service. It is a sort of a proxy designed to add TLS encryption functionality to existing clients and servers without any changes in the programs' code. It is designed for security, portability, and scalability (including load-balancing), making it suitable for large deployments. It uses
OpenSSL
, and is distributed under GNU GPL version 2 or later with OpenSSL exception.
Stunnel input can only be TCP packets. Its
FAQ
has some workarounds for UDP.
WireGuard
also has UDP capabilities. Conceptually, stunnel can be thought of as a generalization of HTTPS, or a lightweight tunnel. Basically, it just wraps/unwraps the data within TLS/SSL. It can be used with traditional network interfaces and TCP/IP stack.
Authentication can also be used by the server to allow access only to approved clients.
Installation
Install
the
stunnel
package.
Depending on your usage, you might also need to
edit the provided systemd units
to better
handle dependencies
. In order for stunnel to start up automatically at system boot you must
enable
it.
Configuration
The main configuration file is read from
/etc/stunnel/stunnel.conf
. It is an ini-style file. It is composed from a global section followed by one or more service sections.
Tip
/etc/stunnel/stunnel.conf-sample
has examples for a few configurations.
A client is one to accept non-TLS encrypted data. Stunnel will encrypt its data with TLS and connect to the stunnel server. The stunnel server accepts TLS encrypted data and decrypts it. It then connects to where the data should be sent to.
The default
debug
value is
notice
, which is very verbose. After verifying correct operation, it is worth to explicitly set a lower value in the configuration file.
/etc/stunnel/stunnel.conf
debug = err
For better security, it is advised to explicitly set an appropriate uid and gid, other than root, for the global section and the per service sections. The configuration tokens
setuid
and
setgid
are available for this purpose.
Byte order mark (BOM)
The configuration file should have a UTF-8
byte order mark (BOM)
, at the beginning of the file. A BOM is the unicode character U+FEFF. Its UTF-8 representation is the (hexadecimal) byte sequence 0xEF, 0xBB, 0xBF. Creating a file with these bytes at its beginning can be done by
# echo -e '\x
ef
\x
bb
\x
bf
; BOM composed of non printable characters. It is here, before the semicolon!' > /etc/stunnel/stunnel.conf
To test if those bytes appear, one can use
% od --address-radix=n --format=x1c --read-bytes=8 /etc/stunnel/stunnel.conf
ef  bb  bf
3b  20  42  4f  4d
 357 273 277   ;       B   O   M
Note that when printing the file to the screen, such as with
cat
, or when editing the file with a text editor, the BOM bytes are usually not displayed. They should be there, though. Which is why, after editing is completed, you might want to verify that they are still there with the above
od
or similar command.
Tip
When using
Kate
, add a comment containing
kate: bom on;
in the first or last 10 lines and the UTF-8 BOM will be automatically added to the file.
Pre-shared key
At least one of the client and the server, and optionally both, should be authenticated. Either a pre-shared secret or a key and certificate pair can be used for authentication. A pre-shared secret has to be transferred to all involved machines beforehand by other means, such as
SCP and SFTP
. When such transfer is acceptable, pre-shared key is the fastest method. Its speed might help to mitigate attacks. A simple configuration for a single server with a single client that are using a pre-shared secret is:
Client:
/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!
setuid = stunnel
setgid = stunnel

[trivial client]
client     = yes
accept     = 127.0.0.1:
src_port
connect    =
server_host
:
server_port
debug      = 3
PSKsecrets = /etc/stunnel/psk.txt
setuid     = stunnel
setgid     = stunnel
Server:
/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!
setuid = stunnel
setgid = stunnel

[trivial server]
accept     =
server_port
connect    =
dst_port
debug      = 3
PSKsecrets = /etc/stunnel/psk.txt
setuid     = stunnel
setgid     = stunnel
where
/etc/stunnel/psk.txt
could be created on one machine by
# openssl rand -base64 -out /etc/stunnel/psk.txt 180
# sed --in-place --null-data 's/\n//g;1s/^/psk:/' /etc/stunnel/psk.txt
# chmod 640 /etc/stunnel/psk.txt
and copied to the other machine by secure means before starting stunnel. The
permissions
for each
psk.txt
file should be set appropriately, neither world-readable nor world-writable. The psk string from the
sed
command is just a random name for the sake of the example. Do read
stunnel(8)
.
Certificates
Per the upstream examples, such as those in
/etc/stunnel/stunnel.conf-sample
, there  is an option to get stunnel to run with certificates. With proper stunnel configurations, the certificates used need not be
trusted by the system
. However, when the certificates chain is not trusted by the system, the clients and optionally the server must approve it before using it, for example, by holding a copy of the whole chain including the root certificates.
Following is an example to allow only a limited number of clients to actually use the traditional TCP version of the
echo service
. In this example, even if the external TCP port of the echo server on port 12345 portrayed here is open to anyone, the server will deny any client whose certificates it does not have. In effect, with such usage of certificates, the clients' certificates effectively act like pre-shared secrets.
Client:
/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!

[traditional echo client]
CAfile       = /etc/stunnel/server_req.pem
accept       = ::1:54321
cert         = /etc/stunnel/clients_req.pem
client       = yes
connect      = Server_IPv6:12345
debug        = 5
key          = /etc/stunnel/clients_key.pem
setgid       = stunnel
setuid       = stunnel
verifyChain  = yes
verifyPeer   = yes
Server:
/etc/stunnel/stunnel.conf
; BOM composed of non printable characters. It is here, before the semicolon!

[traditional echo server]
CAfile       = /etc/stunnel/clients_req.pem
accept       = Server_IPv6:12345
cert         = /etc/stunnel/server_req.pem
connect      = ::1:7
debug        = 5
key          = /etc/stunnel/server_key.pem
setgid       = stunnel
setuid       = stunnel
verifyChain  = yes
verifyPeer   = yes
The required certificates can be generated as
self-signed certificates
. For example, by
# openssl req -new -x509 -subj "/C=GB/ST=LONDON/O=TOWER OF LONDON/CN=DOMAINS" -key <(openssl genpkey -algorithm ED448) -noenc -days 365 -out server_req.pem -keyout server_key.pem
And a similar command can be used to generate the certificate for each client. When it is acceptable for all the clients to share a single certificate, this reduces to
# openssl req -new -x509 -subj "/C=CA/ST=ONTARIO/O=TOWER OF LONDON ASSOCIATION/CN=CA BRANCH" -key <(openssl genpkey -algorithm ED448) -noenc -days 365 -out clients_req.pem -keyout clients_key.pem
Test it:
$ echo traditional echo protocol | ncat ::1 54321
traditional echo protocol
See also
Wikipedia:stunnel
Debian:Pan#SSL encryption
Paranoid Penguin - Rehabilitating Clear-Text Network Applications with Stunnel
How To Encrypt Traffic to Redis with Stunnel on Ubuntu 16.04
. Despite the title, by changing the ports the article is useful for other services too. It also contains links to articles about other options, such as
spiped
or a
VPN
, at the bottom of the article.