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Re: Cisco says attacks are due to operational practices

  • From: Tom Beeson
  • Date: Thu Feb 10 20:58:27 2000

At 02:29 PM 2/10/00 -0800, Sean Donelan wrote:

In an InteractiveWeek article the head of Cisco's security products group
says the attacks are an operational problem not a technical problem.

>Routers from Cisco and other vendors have the ability to detect the signature
>patterns of a denial-of-service attack, and the routers can filter out that
>traffic, Farnsworth said.
>"The router knows which sources are legitimate or not and drops on the floor
>anything suspicious," Farnsworth said. "Generally speaking, ingress filtering
>and committed rates are effective in terms of preventing [malicious] traffic
>from ever showing up, or filtering it to a reasonable rate."
I would agree with Farnsworth. Cisco routers do have some of the richest filtering mechanisms available. Though the configuration is best done at the edge routers (not on any core backbone). Considering the huge number of end sites out there, this is labor intensive. If you were a medium or large ISP that had 250+ end sites behind you, then you would have to go out and reprogram over 250+ routers. Placing the same changes on the core routers is impractical since it would be very CPU and Memory intensive and just as big a pain to administer. We made a decision over a year ago to start making changes on as many of our customer end sites as possible.

We wrote an in-house perl script to take a Cisco router configuration and build inbound and outbound filters. These filters are then applied to the serial interface that connects to our network and toward the Internet. The inbound filter prevents outsiders from spoofing a LAN IP address aimed at that specific site. The outbound filter prevents someone on the LAN from sending spoofed packets (bogus source IP addresses) from getting to the Internet. Our script also adds the "no IP directed-broadcast", "no service udp-small-servers", and "no service tcp-small-servers" commands. We also add restricted telnet access and other security related commands. The suggested modifications are written to a text file and can be further edited and TFTP'd to the router at the discretion of the engineer.

We routinely manage a large majority our customer's routers. For our customers who manage their own routers, we urge them to add these filets and work to make them aware of any security changes they should make. Education of the customer becomes key to stopping spoofed packets from leaving your network. :-)

We turned on logging on a few sites where we suspected some suspicious activity and actually logged a number of spoofed packets that were caught in the anti-spoofing filters. The bad news is that we have not caught the actual persons sending out the spoofed packets. We suspect that these guys may have moved on somewhere else. If you are willing to commit the resources and time setting anti-spoofing filters on all your end site routers, it is a very worthwhile thing.

-- Tom Beeson

My 2 cents worth. Views are my own and not necessarily that of the company for which I work.

Tom Beeson Oso Grande Technologies
Network Engineer A New Mexico Technet Co.
(505) 345-1748

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