IT Professionals

A Question About Internet Connection Sharing

Posted by on Sep 16, 2009 | 3 Comments

Gnomie Chris Papaioannou writes:

Hey, Chris!

Was wondering if you could help me with something, I’ve tried searching but can’t find a certain answer.

I’m going to University on the 20th and moving house in the UK. They only provide you with one RJ45 socket in the room, which is a bit of a pain for most geeks. Means I can only have one device connected at any time. I’ve checked and I’m not allowed to plug in a hub or router to expand the network in any way.

I plan on taking my desktop PC with two network cards built in. They claim that if you leave Internet connection sharing enabled on your NIC then it will interfere with their network and end up with you and possibly others on the same hub as you being disconnected.

I looked into it and it turns out that it’s to do with your PC acting as a rogue DHCP server when ICS is enabled.

However, I’m unsure if this means ICS cannot be enabled on just the NIC you’re hooked up to their network on, or if any NIC with ICS enabled on the whole machine would cause the computer to cause issues.

My plan, to enable me to have more machines (and my iPhone) connected to their network, is to have one NIC on my desktop connected as they require with ICS turned off, and then have the other turned on, providing Internet access to a router in my room so that all those connected to the router have their traffic router through the desktop to get Internet access.

As far as I know, this would work and would only require me to have 1 IP address and 1 MAC address visible to the network, while providing access for multiple devices. All traffic would appear like it is all intended for my one desktop machine, masking the others.

Is my understanding correct — that only my main machine would be visible to the network — and how likely is it that my use of ICS on the NIC not even connected to the network physically would be able to be detected/cause problems?

I can’t only have one device connected at any one time and swap. :(

  • Mark

    That’s an absurd policy. Plug in a NAT router anyway. It’s the solution least likely to cause trouble with the university IT dept.

  • Douglas Hornick

    The network people are just trying to keep traffic per room down to one machine each…..or they are a bunch of goobers. Connecting a router or using ICS will not cause problems with their equipment as far as a supposed “rogue” DHCP server on the line because that is only seen on your side of the network. They will only see traffic for the assigned IP address.

  • Rambozo

    This is a legal problem, not a technical problem. My guess is that they have grown tired of clueless people hooking things up to the network and causing issues. Your ICS solution is an even bigger bucket of worms than if you just connected a properly configured router. ICS is a form of routing anyway. You don’t need to use DHCP(server) at all if you use static IPs on the inside subnet. If you insist on doing this, use a Linux box since “technically” you won’t be running ICS. But it sounds like no matter what you will be in violation of the rules and subject to the consequences of that. Also, you will not be masked from a good network monitor, no matter what. At the packet level it will be clear that something funny is going on, even though it might not be clear just what it is.
    What would they have you do if you have a MacBook Air, with no Ethernet port?