uRPF-based Blackhole Routing System Overview

Scott McGrath mcgrath at fas.harvard.edu
Wed Nov 12 16:17:06 UTC 2003

Vendor C calls it DHCP snooping and to the best of my knowledge it is only 
available under IOS not CatOS

                            Scott C. McGrath

On Fri, 7 Nov 2003, Greg Maxwell wrote:

> 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.

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