Internet filtering at the packet level?

[Will - Security Engine <security () the-engine org>]

Ok, I was wondering if it was feasable to filter internet access at the 
packet level.  Here is the scenario.

Small college campus - lets say 500 live on campus.  About half that has 
internet access.  Then you also have the computer lab, with 16 
computers.  Each teacher has a computer in their office as well, and the 
CIS dept has about 30 or so computers in use.

The filtering would be done on a Linux server using TCPDump.  I know how 
to implement flags for content checking (If the phrase "hot monkey sex" 
comes up in a packet, the user is flagged and traffic for that user 
would be logged for a set period of time for reviewing later).  What I 
don't know is how to actually stop the traffic - but we won't worry 
about that for now.

Is there any problems with this?  Is it feasable?  How about just the 
flagging portion of it, rather than the actual content blocking?

I'm a student at a private baptist college that gets it's internet 
access through MOREnet.  They require that we filter the content in 
order to use their services.  Currently we only use a URL keyword and 
blacklist filtering system (from my own tests), but it's obvious that 
anybody who is serious about getting around the filter will have no 
problem (web proxies are stupid easy to set up yourself, and P2P isn't 
filtered).  I'm worried that at some point it will come up that we 
aren't doing a good enough job filtering, so we'd need a new solution. 
I think the packet-based system would be more accurate.  I would be more 
inclined to not actually block the content that gets flagged.  I would 
rather know that the user is accessing content ruled against by the ToS 
and confront them on the issue.

Lets not turn this into a censorship debate please ;)

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