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Re: Government scrutiny is headed our way

  • From: Owen DeLong
  • Date: Wed Jun 17 20:20:36 1998

> > For those who don't bother filtering "because it's too hard or too
> > complicated", if you don't want or can't afford to put the work into tight
> > ingress filtering on all interfaces, it's really easy to just say "our IP
> > blocks are A, B, and C.  Allow input with source addresses in A, B, or C,
> > deny everything else."  That will at least protect the rest of the
> > internet from your lusers.
> Right.  That's what we do on the dial plant today, because there isn't a
> syntax available on our RAS hardware which says "allow anything with this
> RADIUS assigned or dynamic address block (depending on the account) and deny
> everything else".  So we have to relax the filters to be "allow anything from
> netblocks A, B, and C, block everything else" since the syntax we really
> want isn't available.  
I believe RADIUS has a facility for setting ifilt and ofilt based on the
particular user based on the Framed-Filter-ID.  So, for dynamic users,
you could set a filter that allows the Dynamic IP range of that NAS for
single-host users.  For network users, you could either use blocks like
are listed now, or you could set up a filter per user through the choicenet
features (Livingston).  At the very least, with a rational allocation
policy, it should be possible to limit the filters to some subset of
Every IP we own.

> We do that for all dial and ISDN inbound connections today, and have been 
> for a long time.
Some people don't.  That's the problem.

> Still, that's good enough.  You can't launch a DOS attack against ANOTHER
> provider from our plant this way.  We also have directed broadcasts shut
> off network-wide, so attempts to bounce pingstorms off our internal plant 
> (even to internal targets) don't work either.
> That's the 95th percentile solution, and is a hell of a lot more than most
> other ISPs do.  Most don't do ANY filtering of any kind.  I've tested this
> against accounts on other providers, and most will happily forward packets
> with ANY source address from dial customers.

> I understand the CPU problems filtering ingress on a DS-3 to a customer, 
> for example, if the box has a bunch of other interfaces.  But in that case 
> you should insist (contractually) that the *CUSTOMER* router have the 
> filters on ITS interface which talks to you, and TEST it from time to time.

Actually, with Cisco's running the newer Fast Drop code, filtering on a DS-3
is not that big a deal.  Especially when you consider that this can be done
with a simple access-list.


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