North American Network Operators Group
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Re: uRPF-based Blackhole Routing System Overview

  • From: Greg Maxwell
  • Date: Fri Nov 07 14:45:48 2003 
On Fri, 7 Nov 2003, Robert A. Hayden wrote:

[snip]
> One final note.  This system is pretty useless for modem pools, VPN
> concentrators, and many DHCP implementations.  The dynamic IP nature of
> these setups means you will just kill legitimate traffic next time someone
> gets the IP.  You can attempt to correlate your detection with the time
> they were handed out, of course, in the hopes you find them.

Another approach to address this type of problem is the source spoofing
preventing dynamic-acls support that some vendors have been adding to
their products. I don't know if it's in anyone's production code-trains
yet.

The basic idea is that your switch snoops DHCP traffic to the port and
generates an ACL based on the address assigned to the client. Removing a
host is as simple as configuring your DHCP server to ignore it's requests
and perhaps sending a crafty packet (custom written DECLINE) to burp the
existing ACL out of the switch.

Vendor F calls this feature "Source IP Port Security", I'm not sure what
vendor C calls it.

Since this is a layer 2 feature you can configure it far out on the edge
and not just at the router.