I'm not going to talk about the legitimate and illegitimate uses of these. Many more devices are used and abused for nefarious ways, but we don't see the postal system implement mandatory written filtering; nor do we see (legal!) mandatory monitoring and filtering of the telephone/cellular network.
One common way of working around URL filters in the workplace, schools and libraries is to use an anonymizer/proxy service on the internet. This is how many schoolchildren log onto facebook and myspace. Their use is dangerous (as you're typically giving the service your facebook/myspace/hotmail/gmail/etc credentials!) but again, there are plenty of legitimate and safe uses for them.
The problem is constructing filters which block access through these anonymizer/proxy services. Some of them will include the original URL in the request - they're easyish to block. Others will encrypt/obfuscate the URL so a normal filter won't work. There are plenty of tricks which are pulled; describing them will take a long time.
A growing number of these anonymizer/proxy services are using SSL encryption to totally hide what is going on (ie, blocking not only the URL, but the content itself.) This is just not possible to break without some intrusive additions to the users' computer. Let's not go there.
So, there really is only a few ways to combat this:
- You create complicated rules for each anonymizer/proxy service which attempts to track and decode the URL, and filter on that; or
- You create complicated fingerprints to identify types of traffic which indicate the user of an anonymizer/proxy service, and filter on that; or
- You just block any and all proxy anonymizer/proxy sites.
- 1 is difficult and longwinded. A lot of effort would have to be spent to continuously update the set of rules as new proxy services come on board designed to thwart these services.
- 2 is just as difficult and longwinded - and it becomes possible that these fingerprints will identify legitimate sites as proxy services and filter traffic incorrectly.
- 3 is what the majority of current content filters do. They don't bother trying to filter what people are doing with anonymizer/proxy services; they just blanket filter all of them.
Now, as I've mentioned, plenty of new anonymizer/proxy services pop up every day. I'd hazard a guess and suggest that the majority of them are run by shady, nefarious people who see the value in logging your access credentials to popular webmail/social networking sites and selling them to third parties.
The real concern - I've seen more than one user log onto their internet banking and work sites using these anonymizer/proxy services because they're so used to using them, they forget not to. Imagine, for a moment, that gambling sites are blocked and users turn to anonymizer/proxy services to gamble online. They use their credit card details. Ruh roh.
This is another example of the arms race which filtering companies deal with every day.
New anonymizer/proxy services are created every day - many specifically to allow users to bypass country-level filtering. Many of them may be logging and selling your authentication credentials to third parties. Users will simply begin using new anonymizer/proxy services as they creep up to work around any filtering which may be put in place. There is a non-trivial amount of effort required to keep track of all of these sites and noone will ever be 100% effective.
A large amount of effort will be needed to filter these services and perfectly legitimate uses will be blocked.
You don't want to push users to begin using anonymizing/proxy services - that is a battle that you won't win.
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