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RE: On the back of other 'security' posts....
- From: Stephen J. Wilcox
- Date: Sun Aug 31 08:29:41 2003
On Sat, 30 Aug 2003, Terry Baranski wrote:
> > Sure, blocking spoofed traffic in the limited cases where it is feasible at
> > the edge would be a good thing, but, I don't see failure to do so as
> > negligent.
> In what instances is blocking spoofed traffic at the edge not feasible?
> ("Spoofed" as in not sourced from one of the customer's netblocks.)
Where the customer is not a basic end user.. an ISP for example who may be
transiting traffic from netblocks that are not theirs.
We still have the other problem where a lot of large networks are using RFC1918
addresses that do not get NAT'd thus filtering will break pMTU.. this is an
issue in my experience mainly for those who host websites, altho many of those
are filtering their own packets anyway and have broken sites!
> > Where exactly do you think that the duty to care in this
> > matter would come from for said ISP?
> Isn't the edge by far the easiest and most logical place to filter
> spoofed packets? What are the good reasons not to do so?
It is the only place, virtually any source and destination address could pass
across our network core, there is no way to filter at any part of the Internet
> > Again, I just don't see where an ISP can or should be held liable for
> > forwarding what appears to be a correctly formatted datagram with a valid
> > destination address.
> I guess "correctly formatted" is a relative term. When *isn't* a packet
> with a spoofed source IP address guaranteed to be illegitimate? Maybe
> such packets shouldn't be considered "correct".
Edge filtering ok. But in general it is not the job of the IP router to look
into the higher level protocols to determine what the packet does. Look up
papers on why this would be bad, RFC3439 by Randy and Dave Meyer is a good read!