TCP/IP Connection Cutting On Linux Firewalls

Chris Lowth writes "Network security administrators sometimes need to be able to abort TCP/IP connections routed over their firewalls on demand. This would allow them to terminate connections such as SSH tunnels or VPNs left in place by employees over night, abort hacker attacks when they are detected, stop high bandwidth consuming downloads - etc. There are many potential applications. This article describes how a Linux IPTables based firewall/router can be used to send the right combination of TCP/IP packets to both ends of a connection to cause them to abort the conversation. It describes the steps required to perform this task, and introduces a new open-source utility called 'cutter' that automates the process."

Comments

Re:great

[nadadogg]

Well, you could prevent this by assigning a list of "safe" IP addresses that would not call for termination, but merely be logged. This way, unauthorized entry into the network would be stopped, and working from home would be brought to the higher-ups' attention, thereby making you look good :)

Just a thought, really.

Re:Would be handy

[Technician]

I think a fuse function should be included. Anything that saturates your uplink for 5 minutes should drop you off the net. This could be from anything such as a rogue robot, cracked or exploited mail server serving mass SPAM, a fast SQL type virus, or a break-in copying your fileserver. P-P serving lots of copyrighted material would also trip it. This could have a few anoyance false trips, but if fuses are widely used, it could greatly slow the kind of stuff we want off the net anyway. Maybe it could even save your webserver from melting when it's posted on /.

nice first step

[Lumpy]

Give me a web interface showing all the connections and each end's ip address, how about a simple bargraph showing bandwidth use per connection also?

This would be the ultimate-awesome tool for a netadmin. couple this with cutter and you have a great way of managing that traffic!

Re:nice first step

[tensai]

Check out ntop. It watches traffic passively and generates quite a few pretty graphs. It has breakdowns by protocol, machine, time of day. All sorts of stuff. Extremely useful for troubleshooting the "my internet is broken" problems.

tcpkill

[pknut]

The 'cutter' program introduced in the article sounds suspiciously similar to Dug Song's tcpkill program (a member of his dsniff network utilities). In fact, tcpkill appears to be superior because it matches packets via tcpdump expressions, and hence is more versatile.

Re:fuckwit? I don't think so.

[tomhudson]

I tend to distrust session variables (especially those stored in /tmp, and, yes, I know you can designate another directory), so I authenticate users on each access. This has the following added benefits

1. Any changes in permissions are immediately reflected in the user app - not only after they log out
2. Single point of failure - the user validation code, not user validation && session management
3. Shutting down and restarting the server doesn't affect user access between clicks

Don't get me wrong - sessions are fine for those who like them. I'd just rather do things a bit differently. Besides, there's nothing to keep you from maintaining state with one or more of these techniques:

1. keeping state in variables in a database
2. keeping state in a file for that particular user
3. passing state in forms with POST or GET
4. passing state with urls
5. passing state with the carrier-pigeon protocol (for very high latency :-)