RaspberryPi/Old Reverse SSH Stunnel

What is stunnel

Nominally, stunnel provides SSL encryption and decryption, which provides services not capable of SSL to communicate securely using SSL. (Example: if a mail server listens for unencrypted mail traffic on port 25, and clients send encrypted mail traffic on port 465, stunnel listens on port 465, passes traffic through stunnel to decrypt it, and then passes it to local port 25.

But this can also be used to wrap arbitrary traffic in SSL. In the case of reverse SSH, this provides a way to "wrap" SSH connections in an SSL layer, to make it through the firewall and past intrusion detection systems.

See also RaspberryPi/Reverse SSH

The Setup

Our stunnel setup will be as follows

The stunnel server will be our Raspberry Pi.

The stunnel client will be our command and control server.

Stunnel on Raspberry Pi

Installing

First, install stunnel

apt-get install -y stunnel4

Edit Configuration File

On the Raspberry Pi, edit the stunnel configuration file:

$ vim /etc/stunnel/stunnel.conf

Add the following information, which will point stunnel to your private key stunnel.pem (we'll cover how to create this next).

More information on this process is here: https://www.digitalocean.com/community/tutorials/how-to-set-up-an-ssl-tunnel-using-stunnel-on-ubuntu

/etc/stunnel/stunnel.conf

client = no
[squid]
accept = 8888
connect = 127.0.0.1:3128
cert = /etc/stunnel/stunnel.pem

Basically, this accepts incoming connections on port 8888. This then reroutes those incoming connections to 127.0.0.1 (localhost) and port 3128, which is where squid runs by default (localhost:3128). So, we're forwarding everything from port 8888 to port 3128.

Note that it is pointing to a certificate file in /etc/stunnel/stunnel.pem, so our next step will be to create this certificate file.

Generate Private Keys/Certificates for SSL

Now you need to generate private keys, so that stunnel has private keys to use when encrypting using SSL.

First, go to the directory where stunnel keeps all of its files:

$ cd /etc/stunnel/

Generate Private Key

Use the openssl library to generate a 2048-bit private RSA key:

$ openssl genrsa -out key.pem 2048

Generating RSA private key, 2048 bit long modulus
.....+++
..+++
e is 65537 (0x10001)

Generate a Self-Signed Certificate

To do SSL, an stunnel server must have an SSL certificate, which requires a private key and a signature. We already generated a private key, so now we generate a certificate, and use our own key to sign it. Do this by running the following:

$ openssl req -new -x509 -key key.pem -out cert.pem -days 365

You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:.
State or Province Name (full name) [Some-State]:.
Locality Name (eg, city) []:.
Organization Name (eg, company) [Internet Widgits Pty Ltd]:.
Organizational Unit Name (eg, section) []:.
Common Name (e.g. server FQDN or YOUR name) []: 10.0.0.243
Email Address []:.

Key and Certificate

Now you have your private key in key.pem and your server's certificate in cert.pem.

$ ls -lh
total 16K
-rw-r--r-- 1 root root  615 Apr 22  2013 README
-rw-r--r-- 1 root root 1.7K Aug  4 18:06 key.pem
-rw-r--r-- 1 root root  964 Aug  4 18:08 cert.pem

put those both into the certificate file that we pointed to above, with our Squid proxy:

$ cat key.pem cert.pem >> /etc/stunnel/stunnel.pem

Configure stunnel to Start on Boot

Now configure stunnel by editing the default stunnel file:

$ vim /etc/default/stunnel4

change enable = 0 to enable = 1

Restart stunnel

you can restart the stunnel service now,

$ /etc/init.d/stunnel4 restart

Add Configuration

Now open /etc/stunnel/stunnel.conf and add the following configuration:

sslVersion = all
options = NO_SSLv2
cert = /etc/stunnel/server.pem
pid = /var/run/stunnel.pid
output = /var/log/stunnel
[openvpn]
client = no
accept = 993
connect = 34567

Open Firewall

Now add a firewall a firewall setting on the Raspbery Pi by creating a file firewall.sh:

iptables -A INPUT -p tcp -dport 993 -j ACCEPT

The next step is to restart the stunnel services:

/etc/init.d/stunnel4 restart

Install Squid

Now the final step is to install Squid proxy on the Raspberry Pi:

apt-get install -y squid3

Stunnel on Command and Control Server

Now we can install an stunnel client on our command and control server.

Install

Install stunnel on our command and control server in the usual manner:

apt-get install -y stunnel4

Private Key

Next, we'll copy the file stunnel.pem, which is the security certificate we created on the Raspberry Pi and will be using to encrypt the SSL traffic, onto our command and control server. This file is IMPORTANT! Without it, you can't decrypt stunnel traffic.