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Re: TCP session disconnection caused by Code Red?

  • From: Daniel Senie
  • Date: Mon Aug 06 18:41:19 2001

At 06:14 PM 8/6/01, Eric A. Hall wrote:

Alex Bligh wrote:

> 1. RFC826 appears to mandate only positive ARP caching. I can't
>    see a reason why negative ARP caching shouldn't work this
>    way:
>    Keep only one ARP request in flight at a time. Retry ARPs
>    a maximum of [5] times, separated by at least [1] second.
>    After that, cache non-existance of a h/w address for that
>    IP address for normal positive caching time.

The immediate problem with this is that it requires a *MUCH* larger ARP
cache. Rather than needing enough memory for a couple of thousand active
entries (the current norm for middle-of-the road routers), you need enough
room for every possible address on every attached segment.

[unsubstantiated conjecture] This may be what's killing the cable networks.
If they are making room in the NAS ARP caches for the addresses that are
being probed, then they are making room by flushing the "real" ARP entries,
resulting in a constant flush/load cycle. [/uc, but exemplary of the problem
with negative ARP caching.]
Adding to this conjecture, I'm seeing VERY high ARP rates (arp broadcast packets) arriving via the cable modem in my office. Also seeing a high rate of Code Red type attacks attempted at the machines attached. Firewall is just catching and logging them.
Daniel Senie [email protected]
Amaranth Networks Inc.