While I agree with all this, keep in mind this is a school and the proposed solution may not be feasible financially or realistically (Can a student in the computer lab unplug an Ethernet connection and plug their device in? Are the teachers systems in the same room as students?) If it's not then some lesser desirable but "better than nothing" solutions would be to get rid of DHCP and assign all static IPs (with just 80 systems this is possible though not desirable), change those IP ranges to just enough to meet the need for the specified systems and allow only IP addresses with a need for Internet access through the firewall. Is monitoring for defined system's IP addresses going offline possible? A good analysis of needs may surface options. Do the teachers need Internet access during school hours (it is possible via cron to have time-based firewall rules). What about the computer lab? Without knowing the specifics these questions are unanswerable. And we have to keep in mind "relative security" - if they have an "evil genius" student on their hands there will be a way around the best security which can be put in place.
----- Original Message -----
From: "Johnny Hughes" <johnny@xxxxxxxxxx>
To: "centos" <centos@xxxxxxxxxx>
Sent: Monday, September 18, 2017 12:42:34 PM
Subject: Re: Block internet access for some users on the LAN ?
On 09/18/2017 12:23 PM, John R Pierce wrote:
> On 9/18/2017 10:03 AM, Nicolas Kovacs wrote:
>> This year the school's director wants to completely block Internet
>> access for all the student's personal devices.
>
> MAC addresses can easily be forged, IP addresses can easily be changed,
> none of that is secure if its on the same network segment
>
> The student's personal devices should be on a completely different
> 'guest' subnet, enforced by the wireless infrastructure, via use of a
> captive portal and/or WPA2-EAP authentication. Presumably most of
> the schools infrastructure is on ethernet? those ethernet connections
> should be kept physically secure so noone unauthorized can plug/unplug
> anything into the ethernet.
>
> THEN you'd use iptables to enforce access restrictions on this guest
> subnet.
>
>
It would be extremely easy to, for example, try to get to the internet
and fail .. look at my IP address and get my default gateway from my
device (that I own) .. then try manually other network addresses until I
find one that works (with the same gateway). That is, I can easily find
the others segments (like the printers) and take a free address in that
segment. Since the whole network is flat, It will let me out then.
As John says .. if you want to isolate guest accounts, do it with a
completely different network segment that is isolated from things you
don't want them to access. You can then setup rules unique to that
network segment that they can't forge (the gateway is the only way that
segment can get out and all the rules are the same for any IP that will
route from that segment).
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