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Re: Port blocking last resort in fight against virus
From: "John Palmer" <nanog () adns net>
Date: Tue, 12 Aug 2003 12:11:48 -0500

----- Original Message ----- 
From: "Dave Israel" <davei () algx net>
To: "McBurnett, Jim" <jmcburnett () msmgmt com>
Cc: "Jack Bates" <jbates () brightok net>; "Mans Nilsson" <mansaxel () sunet se>; <nanog () merit edu>
Sent: Tuesday, August 12, 2003 12:00
Subject: RE: Port blocking last resort in fight against virus

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.

Yes, some providers however react improperly to certain situations and 
do not listen to their paying customers.

RCN in Chicago is one example. One day, they just started blocking 
outbound port 25 on their network. Now, I use other SMTP servers
other than the RCN one. In my case, they're my servers and all I have to
do is set up my SMTP to listen on an additional port. For others, they
aren't so lucky and may have a legitimate gripe with them for censoring

In the case of 135-139, no one who uses these ports legitimatly should
have a need to use them "in the wild" unless in a tunnel. If a user came to
me complaining about them being blocked, I would ask the user why they
were using them incorrectly and would suggest safer ways to do the same

So, being a good ISP is trying to accomodate the needs of as many 
customers as you can, while being a good net neighbor. This is not
always easy.

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