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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.