Tip

Who's going outside your network?


Who's going outside your network?
Tom Lancaster

Intrusion Detection Systems are getting quite a bit of attention these days. They're getting more sophisticated and more expensive. They're also proving they're worth the effort. However, these systems specialize in detecting well-known attacks against your network that are primarily inbound and based on lower-layers of the IP stack, such as attacks that exploit vulnerabilities in fragmentation and ICMP. Out of the box, they aren't much help against a far greater threat: your users.

The problem is that your users often do undesirable or even dangerous things out of ignorance, and all this happens above the application layer, which looks "normal" to the Intrusion Detection Systems. For example, they may click 'yes' and allow Windows Update to upgrade their system, which may disrupt backward compatibility with an existing corporate system. Proxy servers, firewalls or an Intrusion Detection System would rarely block this type of traffic.

A cheap and easy way to solve this problem, although it isn't foolproof, is to put bogus entries in user's hosts file. The hosts file resolves domain names to IP addresses just like DNS, but it is always checked before DNS. So for instance, if you wanted to prevent users from accessing Microsoft's Windows Update, you could place an entry

127.0.0.1 windowsupdate.Microsoft.com

You can easily block access to any site, but this method isn't foolproof if users are smart enough to figure out the IP address or if they have permissions to edit their hosts file (obviously you can remove edit permission to the hosts file if you think users can dope this out).

If you want to take this one step further, set up a Windows server with Netmon and point all your hosts entries to its address. For instance, if the server running Netmon is 192.168.1.35, then use:

192.168.1.35 windowsupdate.Microsoft.com

This will cause the users' computers to send the traffic to your Netmon server, which can act as a logging device and allow you to capture and analyze everything.

In this way, you can maintain consistency in your desktop environment. If you use a system like Microsoft's SMS to roll out upgrades, consider creating a simple package with your custom hosts file for your users.


Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.


This was first published in November 2001

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