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Using stunnel and SSH to bypass IDS

2013-09-04

Live demo in BSD Now Episode 001 | Originally written by TJ for bsdnow.tv | Last updated: 2014/08/20

NOTE: the author/maintainer of the tutorial(s) is no longer with the show, so the information below may be outdated or incorrect.

Here’s a scenario: say you're on an untrusted network, be it corporate, university or at a foreign hotel. You want to tunnel all your traffic through SSH to your trusted server, right? We can't have those pesky script kiddies sniffing your traffic. But what's this? You can't get out on port 22! You can't get out on that other random port you used for SSH either! They're filtering everything but ports 80 and 443. Sometimes just running SSH on port 443 will let you get past this, but other times there’s deep packet inspection in place to prevent that. Any IDS will be able to easily detect SSH on any port you run it on, so we’ll have to find a way to hide it in plain sight. Enter stunnel. It's a simple tool that lets you encapsulate traffic of any protocol in standard SSL/TLS. Your stream of packets will look exactly like a connection to your gmail or anything else. The setup is pretty simple. You'll need stunnel installed on both your client PC and a remote server with sshd already running. Let's install stunnel on the server, make a quick config and setup your key. For FreeBSD, I'll be using ports.

# cd /usr/ports/security/stunnel
# make config-recursive install clean
# vi /usr/local/etc/stunnel/stunnel.conf

In the config, we're going to put the following:

cert=/etc/ssl/stunnel.pem
pid=/var/run/stunnel.pid
setuid = stunnel
setgid = stunnel
[ssh]
accept = your_server_IP:443
connect = 127.0.0.1:22

Now we'll generate the key:

# cd /etc/ssl
# openssl genrsa 1024 > stunnel.key
# openssl req -new -key stunnel.key -x509 -days 1000 -out stunnel.crt
# cat stunnel.crt stunnel.key > stunnel.pem
# chmod 600 stunnel.pem
# service stunnel onestart

Be sure to allow incoming connections to port 443 in your firewall. Now we move over to the client PC. Install stunnel and set up a similar configuration.

# cd /usr/ports/security/stunnel
# make config-recursive install clean
# vi /usr/local/etc/stunnel/stunnel.conf

In the client config, we put:

pid=/var/run/stunnel.pid
client=yes
setuid = stunnel
setgid = stunnel
[ssh]
accept=443
connect=your_server_IP:443

Start the service and test it out:

# service stunnel onestart
$ ssh -vp 443 youruser@localhost

At this point you should be SSHed into your remote server, but all the traffic is tunneled through SSL. Now, say you want to tunnel all your web browsing traffic through this. With stunnel running on both systems, run this on the client:

$ ssh -Cv -ND localhost:9050 localhost

And set your browser proxy settings to tunnel all traffic through a SOCKS host of localhost:9050

To verify everything is working, fire up Wireshark or tcpdump:

Done! It's recommended that you stop the stunnel service when you're not planning on using it, since all SSH connections appear to come from 127.0.0.1, and it can be annoying if someone is trying to bruteforce your login.

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