Bypassing Firewalls - But How?

In my last post I have elaborated multiple reasons as to why someone would want to bypass a Firewall. Now that this is established, here comes the part where we get to the “how”.

If you don't care about the technology behind any of this, jump to the conclusion.

However, if you are interested in the theory behind Firewall bypassing, go right ahead and read the rest of this article.

Digging your way out

Network Firewalls act as a barrier between your client device and the “bad, scary outside world”. They require in- and outgoing traffic to be passed through them to be able to achieve this. Now the general concept to bypassing a firewall is using a proxy or a VPN server. These are uncensored servers on the outside of the restricted network, which then will act as your gateway to the uncensored Internet.

But “Just use a proxy or a VPN” is obviously not the answer to our question. That would be way too easy. Bypassing Firewalls is a never ending game of cat and mouse, and so it gets quite a bit more complex than that. 

For elaboration's sake, let’s pretend your company has banned access to the NYTimes website for whatever reason. If you want to connect to their website, the Firewall will see that your Computer wants to connect to the NYTimes website. Upon seeing this request, the Firewall will not let it pass to the open internet, but rather reply with a very polite “lol no fuck you”. Using a VPN / Proxy however, you are requesting a connection to a domain like myproxyserver.com, which then will forward your request to the NYTimes website and the answer from their servers back to you. Because after all only the NYTimes website is blocked, but your middlebox is not.

For a very unsophisticated Firewall, all of this might already be enough to bypass it. Any VPN or Proxy Server may suffice. But a more advanced Firewall is able to tell that you tried to bypass it by inspecting your traffic, and stop you from doing so. Or your server might be set up in a way that gets blocked for some other reason that isn't even related to your attempt to bypass the Firewall.

Staying undetected, and overcoming obstacles

To actually escape such Firewalls effectively, what we need to know are the restrictions and methodologies used against us first. Once we have an understanding what could make us fail, we can work towards getting around this.

The following list of two Security measures which a Firewall might put into place is far from complete. These are things I have personally dealt with or that I was otherwise able to find out about during my research. There is probably an infinite amount of stuff that might happen to stop you in your tracks, and just as much to bypass that again.

Blocked ports

A very common practice in Firewalls is to block, or whitelist, certain ports. Many resources, like this, exist about this topic, some targeted specifically at Firewall admins. Blocked ports are a common thing to encounter in any kind of restricted network. Even some ISPs are blocking ports. It's usually not specifically done to stop people from bypassing their Firewall, it’s just an overall used practice, following the principle of least privilege. Avoiding blocked ports is easy, just use one on your VPN / Proxy Server that is used for a very common service, and therefore is very likely not to be blocked.

Examples are:

These ports are almost always unrestricted. I personally use Port 443, but the other ones should work as well.

Protocol Fingerprinting

A slightly more advanced method of restricting users is by identifying the protocol their connection is using. Connecting to port 443, the one commonly associated with https traffic is neat, but if the Firewall notices that your connection clearly is NOT https traffic, that isn’t so neat anymore. In fact, it might make you look even more suspicious. For example, The OpenVPN protocol can often be identified when using default settings. To hide what you are doing, many solutions already exist. I have split them into these two categories:

  1. Make your traffic look like a different protocol
  2. Make your traffic look like random shit (aka Obfuscation)

OpenVPN for example, does use SSL for their authentication, which does look very similar to HTTPS traffic at first glance, but analyzing the traffic can reveal the differences between a SSL Handshake performed by a regular https Website and one made via OpenVPN. There are ways to wrap OpenVPN traffic into different protocols like SSH, or use Obfuscation to combat this.

Obfuscation seems to be the more widespread and also more useful method. A famous tool for that are the ‘Pluggable Transports’ made and used by the TOR Project. The one that is currently in use by TOR is obfs4 also referred to as the ‘Obfourscator’. This obfuscation layer for the TCP protocol does transform the traffic in such a way that it is not easily identifiable anymore. Stuff like packet size, packet timing and more can be randomized, and the payloads are heavily encrypted.

Another project that uses obfuscation to avoid detection is ShadowSocks. Originally made to bypass the GFW, it also is hard to be detected by DPI. By using a pre shared key, every single part of the communication, including the TCP handshake, are always encrypted and randomized. Message length and other factors are being randomized as well. Although there seem to be ways to fingerprint vanilla ShadowSocks traffic with DPI, most notably by the GFW itself, for most networks ShadowSocks is a very effective and surprisingly simple way of bypassing Firewalls. Setting up ShadowSocks-libev Server on a Debian 10 VPS took me a whopping 5 minutes last time I did it.

To tackle even the most sophisticated Firewalls like the GFW, you can also use Shadowsocks Plugins to make it even harder to get detected. These plugins do even more things, such as making your traffic look like actual https instead of just obfuscating it, Domain Fronting and much more. But this is currently only neccessary for the most nefarious types of Firewalls.

Some additional things to consider.

There are some more things you should consider when choosing and setting up a method for bypassing. Some proxies commonly installed in networks may have problems dealing with UDP traffic. So setting your connection to TCP only may help. Also, it might be a good idea to only fire up a connection to your Server when it's actually needed, as being connected to the same server for hours at end will look suspicious too. Especially when a human might take a look into some traffic logs. And as a last tip, changing your MAC Address is good for ban evasion.

Conclusion

Now all of this is a lot of information, but what is the conclusion? What do I actually recommend? Well all things considered, I believe using a ShadowSocks Server that is listening on port 443, TCP only, is a very solid start. When dealing with an especially sophisticated Firewall, the various ShadowSocks Plugins will be of help.

For self-hosting, I recommend to get a KVM-VPS running Debian 10. Shadowsocks-libev is in the debian repos, making it a breeze to install. And a KMS Server has the least amount of problems for proxies and / or VPNs. In my experience at least. 

If you don't want or can’t selfhost mullvad.net offers ShadowSocks bridges. And just before anyone asks, I am not affiliated with them.

Some additional resources

Thanks for reading