uRPF-based Blackhole Routing System Overview
gmaxwell at martin.fl.us
Fri Nov 7 19:29:00 UTC 2003
On Fri, 7 Nov 2003, Robert A. Hayden wrote:
> 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
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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