Need security advice - Sharing Internet connectivity to a third party

jimieee · 12-22-2003, 06:23 AM

Hi,

Just recently my boss promised the owner of our premises that we would provide her with Internet Connectivity through our network for free as a gesture of goodwill. Whilst I'm all for sharing I want to be sure that I've taken all the security issues into account. Maybe somebody could give me some advice on simple security procedures I could use?

At the moment, we have a Linux Router, with DHCP server that "provides" us with connectivity. Inside we have one physical private network and at the moment our landlady is attached to this with us all on the same subnet. The landlady has two Windoze machines (XP and 98), which as far as I can tell are used for Kazaa and playing games.

What I'd really like to do is have some kind of barrier between their network and ours, because at the moment they can browse our SMB workgroup at will and if they get a worm or anything my Windows clients will be vulnerable. Can I divide us up by putting us on different subnets? If so is their a good tutorial for doing this? I'd quite like to use webmin to set it all up.

Cheers!

~James~

chort · 12-22-2003, 10:06 AM

One easy thing you could do is buy one of those SoHo routers (perhaps you have a spare one already?) that does NAT, DHCP, etc... They go for about $100 USD, I would guess maybe 80 pounds sterling (where the heck is my pounds sign?). You could plug that into your network and give it a WAN IP from your network (such as 192.168.0.5), then configure the LAN on the backend of that router to be some different subnet (such as 172.16.0.0/24). Of course, all that does is block the broadcast traffic from your Samba machines, and blocks any other type of "chatter" going *to* her machines, but she can still access things freely (would have to know where to look tho, because Windows networking will be masked).

The other option would be to put an extra NIC in your Linux router and give it another subnet (172.16.0.0/24). You could hook a switch up to that NIC and use that for the landlady. You can then use iptables and prevent any traffic on that NIC from going to your other internal NIC. i.e. from the landlady NIC, traffic can only go OUT, not "sideways".