I am wondering how reverse proxies can be used to bypass company acceptable use policies?
What can be done to prevent it?
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Reverse proxies can't really be used to circumvent the rules, I believe you are referring to forward proxies. Check this post to learn the difference. (If I'm wrong please post a comment)
You can restrict access by forcing them to use your own forward proxy server internally. Do mind that this is not 100% proof, a more skilled user will get through this with ease by using SOCKS over ssh.
What can you do? What you do whenever someone breaks the rules:
Just make sure when you warn and punish them they are aware what the AUP states and also why some things were banned. Make them aware that there might be security threats for the internal network or to the working environment and that their behavior is not acceptable.
Regardless of either a forward or reverse proxy analysis of the network traffic with something like Wireshark will allow you to detect it, which is the first step to preventing it.
Proxies designed to bypass filtering or access control will exhibit these types of behaviours as to bots and generally anything trying to hide it’s true purpose:
http://www.google.com.proxy.com/or trailing like
Host:header does not match the host name
This obviously could include a lot of legitimate traffic however knowing what type of traffic currently goes in and out your network will allow you to detect any anomalies.
Assuming you are starting from no control on your network I would look to build up the following in to enforce strict compliance:
While you might find you are unable to do all of these, any will definitely help.
Hope that helps.
(Answering for a forward proxy as I agree with Lucas).
Depending on how open the environment, there's not much you can do. If you have to allow ssh out of the network for legitimate purposes, you can't figure out if the traffic is normal ssh (e.g., transferring files for legitimate purposes) or a SOCKS proxy over an ssh tunnel; if the user implemented it correctly (sometimes stuff like DNS requests won't use the proxy and you can tell where people are going). If you have the ability to be more restrictive, you can just block everything but port 80 (maybe allowing port 443 for a few known entities like gmail).
A better solution is clear well-known Acceptable Use Policies with clear consequences for violations. E.g., if employee is caught looking at porn/watching netflix movies/using facebook on company computers on company time in clear violation of rules, have harsh penalties (ranging from severely docking pay to firing). Another alternative is installing remote monitoring application on all company computers and letting an admin remotely look in on any desktop. (Though that seems fairly draconian and probably would create a hostile working environment).
Additionally, if what you are worried about is sharing limited bandwidth and people are say streaming lots of video, it may be worth thinking about implement some sort of network quota; possibly more stringent for encrypted traffic versus port 80 HTTP or HTTPS traffic to known servers (e.g., gmail).
If you are worried about people transferring out confidential data that's another issue. Once an employee has access to data on their computer, you basically need to rely on trusting the employee to not misuse the data (as well as restricting and monitoring who is initially accessing the sensitive data). If they can set up a proxy; they can do other things to take the data off-site. Encrypt the files and email them out or put them on removable storage (USB drive/burned cd/dvd/laptop), etc.