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How to isolate a computer from the network


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#1 technics

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Posted 12 August 2008 - 08:07

I have a total of three PCs going through a router and I often use them to share files etc, but due to a recent virus attack I would like to isolate one of the PCs from the network so that it can't spread viruses etc through the network.

I'm guessing that once isolated it should still be able to use the printer (which is also plugged into the router) and continue to access the internet as well.

Any help would be much appreciated.


#2 +Brandon Live

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Posted 12 August 2008 - 08:15

Unplug the other computers from the router?

I guess it's unclear what you mean by "isolate." It sounds like you want them on separate routers.

#3 popisdead uk

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Posted 12 August 2008 - 08:17

Take it out of the same domain/workgroup. That will stop it sharing files, but I'm not sure of the top of my head if it will stop you sharing a printer.

#4 OP technics

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Posted 12 August 2008 - 08:19

thanks for the reply.

what i meant was that it no longer has the ability to send/receive information to the other computers as it did when I had the recent virus problem. I've tried to change the workgroup but it still seems to be able to see/view/change the other computers information.

hope this clears what i mean!

#5 +BudMan

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Posted 12 August 2008 - 12:08

Take it out of the same domain/workgroup. That will stop it sharing files

And where did you get that tidbit of misinformation from??

What workgroup your in has NOTHING to do with file sharing, and to be honest either does the domain really -- as long as you auth to the machine with a valid account you can access resources. A domain just makes it easier to give permissions since there is a central userbase.

And this clearly does not isolate the machine from possible viruses or exploits to the ports they are listening on, ie the file sharing ports, etc. Not having a valid account to auth with will not always protect you from a virus/exploit - but it will protect you from that machines virus from infecting every file it has permission too, depending on what account its logged in as, etc.

So blocking the account the other machine logs in with from having file share access would provide some minor protection. But if using SFS, thats not possible -- would need to have pro with the ability to give different accounts different access to files/shares, etc. But that would not protect you from an exploit using the file sharing ports, etc.

If you router does not allow you to setup vlans and put access control lists between them. Then your going to need to run software filewalls on the machines or put them behind a nat compared to the other machine. You would then need to do a port forward to allow printer access. Quite often this is port 9100, it would depend exactly how your sharing your printer - be it connected to a machine or on a stand alone printer server device, or if the printer has its own network card, etc.

Treating a machine(s) as hostile on the same local network is what software firewalls are good for! If you run software firewalls on your machines -- just use them to block access from that one machine.

One way as mentioned to put a line of protection between that machine and your other machines is to put the other machines behind another nat router.

example layout;
doublenat.png

But I would REALLY NOT suggest this -- since the double nat comes with quite a few of its own headaches. Your best bet is to get a router that supports lan to lan firewall rules. There are few out there -- but not recalling any model numbers off the top of my head.. Would have to look around for one that does -- but they are out there for sure. What router do you have now?

#6 popisdead uk

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Posted 12 August 2008 - 12:20

My mistake, I didn't quite understand the question. It was early when I replied. You can get down off that soapbox of yours now :laugh:

#7 +BudMan

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Posted 12 August 2008 - 12:26

Believe me -- I would love to get off the soapbox about FUD -- but it seems no matter how many times you state FACT and discredit FUD, it still manages to stay around. And the misconceptions about what a workgroup is and does just never seems to go way -- Arrrghhh ;)

I would love to know what you thought the question was asking -- that suggesting a different workgroup or domain would isolate it.

#8 DavidM

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Posted 12 August 2008 - 12:41

I have a total of three PCs going through a router and I often use them to share files etc, but due to a recent virus attack I would like to isolate one of the PCs from the network so that it can't spread viruses etc through the network.

I'm guessing that once isolated it should still be able to use the printer (which is also plugged into the router) and continue to access the internet as well.

Any help would be much appreciated.

So you want to stop computer (1) from giving it's virus to computers (2) and (3) on the network, but still want it to spread the virus to the internet?

You do know they make good FREE anti-virus software don't you? Right here on Neowin.

#9 popisdead uk

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Posted 12 August 2008 - 13:15

Believe me -- I would love to get off the soapbox about FUD -- but it seems no matter how many times you state FACT and discredit FUD, it still manages to stay around. And the misconceptions about what a workgroup is and does just never seems to go way -- Arrrghhh ;)

I would love to know what you thought the question was asking -- that suggesting a different workgroup or domain would isolate it.


I honestly don't know mate. It was early this morning when I read it. I think I thought he meant, .... I have no idea. Really.

If I had read it properly I would have seen what he was trying to do, and I would have ignored it.

#10 OP technics

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Posted 12 August 2008 - 13:42

thanks for your reply guys. it's been much appreciated.

#11 +BudMan

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Posted 12 August 2008 - 14:46

Follow up: Router that supports lan to lan firewall rules, yes I know this model is EOL, but its an example of what I was talking about with lan to lan rules on a home priced router.

lan_lan_firewall.jpg

Also if your router will run dd-wrt or openwrt you can setup vlans and use iptables to deny specific traffic between the vlans, etc. Not sure if tomato can do this?

So you can accomplish what you want fairly simple with home priced network equipment to be sure. I do not believe the current web ui to dd-wrt supports lan to lan firewall rules? But can be done from the command line after a bit of reading for sure. Or I do believe you can run firewall builder on it. http://www.dd-wrt.co...irewall_Builder which would give you a gui in building your rules.

Another option would be to run a linux distro as your router, ipcop I know allows for multiple segments and then rules between them. Any of the router distro's should be able to do something as basic as this.

What I would suggest is if your current router does not support either dd-wrt or lan to lan rules is since you would need to buy another router to use the double nat method anyway -- is purchase one that allows for lan to lan filtering, or one that supports 3rd party firmware that will allow you to do it.

#12 OP technics

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Posted 12 August 2008 - 20:16

thanks for that.. (just going out atm and will read your post when i return)

i just had a quick idea, which is probably very silly but thought i might ask anyway. would disabling/removing file and printer sharing for Microsoft networks in the local area connection properties help solve the problem i'm having?

#13 OP technics

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Posted 12 August 2008 - 20:16

thanks for that.. (just going out atm and will read your post when i return)

i just had a quick idea, which is probably very silly but thought i might ask anyway. would disabling/removing file and printer sharing for Microsoft networks in the local area connection properties help solve the problem i'm having?

#14 +BudMan

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Posted 12 August 2008 - 20:27

On which machine? On the machine your worried about, or the other machines. If turn off file and print sharing on the machine your saying is hostile -- that does not prevent some type of infection from doing anything -- just that it would not be able to use the built in file and print sharing to do it.

But without know exactly which infection your looking to protect against.. You would need to understand its method of spreading to know if that could stop it.

Now doing it on the 3 other machines would prevent them from listing, and or anything from accessing file shares, etc.. EVEN yourself for you legit use, etc.. Its a pretty harsh fix -- and this does not mean that they would still be safe from any type of infection the other machine might pick up that uses say a rpc exploit, to compromise the other machines.

You need to isolate the friendly machines from the hostile machine. This can be done with firewalls on each of the friendly machines, or with firewall between them (the lan to lan rules example) or by putting the friendly machines behind a nat to the network the hostile machine is on, etc. etc.

#15 OP technics

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Posted 13 August 2008 - 10:47

got it! will play around with the firewall's for an immediate fix.

many thanks for all your help.



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