North American Network Operators Group

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RE: Port blocking last resort in fight against virus

  • From: Dave Israel
  • Date: Tue Aug 12 13:03:42 2003

On 8/12/2003 at 12:40:19 -0400, McBurnett, Jim said:
> who in there right mind would pass NB traffic in the wild?

That's the problem; not all customers are in their right mind.  All
they know is that it was working yesterday, and not today, because you
blocked a port.

The question of port blocking for most sizable ISPs comes down to
principle vs principle.  One the one hand, you have the principle of
network invisibility.  You agreed to pass customer traffic, not pass
judgement on it.  If it's a valid IP packet, you'll deliver it.  And
you don't slow down or stop traffic because you're spending cycles
examining packets.*  That's what customers expect.

On the other hand, you have the principle of being a good network
citizen.  You try to keep your tables clean and your peers from
flapping.  You accept valid routes and inform your peers when you get
invalid ones, so they have a chance to fix them.  You are properly
embarrassed when you find a spammer on your network or your name on
the CIDR report.  And you don't spew other people's networks with worm
traffic.  That is what other providers expect.

Port blocking is therefore a quandry: do you stick with your customer
principle, or your provider principle?  I think most of us weigh the
damage of the attack vs the damage of losing the port, and make
individual judgement calls.  It would be nice if there were some
central consensus on when to block ports; then individual providers 
wouldn't need to take abuse from customers or other networks when their
judgement wasn't exactly the same as somebody else's.


* Before the holy war starts, yeah, some hardware doesn't slow down
when blocking ports, and this is only an issue if your hardware isn't
that breed.  My point is that this might be an issue for some hardware,
and that "Buy vendor X" isn't really a solution for everyone.