Using Stunnel to Encrypt Unsecure Connections

So you’re a fully signed up ‘tin foil hatter’ and you have an old app that communicates over the internet but doesn’t use SSL?

Not to worry, there is a solution for you and its called stunnel.

Stunnel is available for both Linux and Windows, and simply put creates an SSL tunnel from one machine or server to another.

So why use stunnel? Well stunnel…

… provides an extra layer of security to an internet based application.
… secures data previously transmitted in plain text.
… protects your exploitable services by hiding them behind stunnel.

So what else can it be used for? Well…

… you could use it to fool certain websites into thinking you are in a different location.
… it can be used as a simple ‘bouncer’ for pretty much anything!


In my example I wanted to provide a little security to my simple ‘Munin’ monitoring solution (I’ll write something about that at a later date). Munin checks the servers status by ‘telnet’ to the munin-node running on the target server/pc, so if you are monitoring a remote server you have data about your system flying about everywhere! Also it means that there is a naked telnet port open on your server, which although you can lock it down by only allowing connections from a specific IP etc. doesn’t work to well if your on a dynamic IP and still leaves unencrypted data flying about the ether. Ok so in this case security isn’t that vital and none of the data is particularly sensitive. But if it uses TCP and you want to provide an extra layer of protection stunnel can do it.

As an added bonus stunnel is extremely easy to setup.

The rest of this post is based around a CentOS installation. But its pretty much the same on Debian/Ubuntu… just replace ‘yum’ with ‘apt-get’. The Windows install is even easier and the configs are in the same format as the Linux confs so you can easily adapt the instructions bellow!

Firstly lets install stunnel on the client machine (In the case of my example this is actually my ‘server’ machine in Munin, but for example if you wanted to secure SMTP communications this would be the machine sending the emails).

Although in this post I’ve used Munin as an example it really is easy to modify these instructions to fit any service. Simply change the ports!

Install using Yum:

yum install stunnel

(Now I cant remember exactly but I think stunnel is not on the default CentOS repos… so you can add the RPMforge repos like I tend to do:

Install from source:

tar zxf stunnel-4.22.tar.gz
cd stunnel-4.22
make install


On CentOS the default location for the stunnel.conf is /etc/stunnel/ so open this file in your editor of choice:

vi /etc/stunnel/stunnel.conf

Lets set the following options:

#chroot = /var/run/stunnel
setuid = nobody
setgid = nobody
pid = /
client = yes

And create a service:

accept = 9090
connect = 8080

Now as you can see here I’ve set the listener to but you could set this to all interfaces or a specific one by omitting or replacing the The ‘connect’ setting is the servers hostname or IP address and the port that the stunnel ‘server’ is listening on. Another example of using stunnel could be to direct all web requests onto another server using a secure layer:

accept = 80
connect =

You would then setup the ‘server’ stunnel to listen on 8080 and connect to the local (or even a remote!) web server.

Ok thats it for the client side for now. Lets look at the server:

Install stunnel as per the installation instructions above.

On CentOS the default location for the stunnel.conf is /etc/stunnel/ so open this file in your editor of choice:

vi /etc/stunnel/stunnel.conf

Set the following options:

cert = /etc/stunnel/stunnel.pem
chroot = /var/run/stunnel
setuid = nobody
setgid = nobody
pid = /

And create a service:

accept = 9090
connect = 8080

accept = 8080
connect =

With a bit of messing around with DNS or hosts files we could use stunnel to bounce our connection to any server for example! But that’s not really what we’re covering in this article and I’m guessing if you have your reasons for doing something like that you can figure it out on your own.

OK we’re nearly ready to start stunnel. But the observant amongst you will have noticed that stunnel.pem certificate file that we set in the server options doesn’t exist! So lets create one now!

openssl req -new -x509 -days 3650 -nodes -out /etc/stunnel/stunnel.pem -keyout /etc/stunnel/stunnel.pem

Right now lets start stunnel, on both machines simply run the following command:

/usr/sbin/stunnel /etc/stunnel/stunnel.conf

Now lets test it! If the service we’ve setup can be talked to with telnet (eg. Munin or SMTP) then we can test this very simply from the client machine:

telnet localhost:4948

You should get the following back:

Connected to localhost (
Escape character is ‘^]’.
# munin node at

(Now its a little confusing because ‘Connected to localhost’ is actually the response from the munin-node on the remote server!)

As you can see your telnet session has gone into stunnel locally, been transmitted from stunnel on the local machine to stunnel on the server and then from stunnel into the Munin node on the server! Magic!

Stunnel startup script:

Now you can either start stunnel every time your machine starts up manually, add it to the crontab (if you try and start stunnel again and its already running the second instance will just close, but it leaves a mess in your /var/log/secure so don’t do it to often) or use a simple startup script like this one I ( used ) to use:

    if [ -f /var/run/stunnel/ ]; then
        ps aux |grep -v grep |grep $pid |grep stunnel > /dev/null
        if [ $? = 0 ]; then
            echo “Server is already running !!”
            echo “Pid file exists but process not found … trying to start stunnel”
            /usr/sbin/stunnel /etc/stunnel/stunnel.conf
        rm -f /tmp/stunnelrun > /dev/null
        echo “Pid file not found. Starting stunnel.”
        /usr/sbin/stunnel  /etc/stunnel/stunnel.conf

This is a very simple script I knocked up in a few mins. I later replaced it with an init script that I wrote when I had a little more time. But I’m going to post that as another article as I have a bit more to say about that (including some adaptation to make general init scripts for various programs)

To use the above script to start stunnel do the following:

cd /usr/local/sbin
vi stunnel-run

Paste in the above code.
Save and exit.

chmod +x stunnel-run

Test by doing the following:


Now add it to the crontab:

crontab -e

Insert the following line:

*/15 * * * * /usr/local/sbin/stunnel-run 2>&1 > /dev/null

Now your system will run the stunnel-run script every 15mins, checking if stunnel is running and starting stunnel if it is not running. You could simply start stunnel every 15mins as it will exit if it finds it can not use the ports its been assigned, but that leaves a mess in your secure log.

Some of you I’m sure are thinking ‘why use stunnel you could use a vpn or xyz’. Well yes, there are many ways of skinning this cat, stunnel is a simple and versatile one but not the ultimate jack of all trades. Use it where you see fit!

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