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Re: What if it doesn't affect the ISP? (was Re: What do you want your ISP to block today?)
- From: Iljitsch van Beijnum
- Date: Sun Aug 31 05:45:37 2003
On zaterdag, aug 30, 2003, at 20:54 Europe/Amsterdam, Sean Donelan
Only if it impacts the ISP, which it doesn't most of the time unless
they buy an unfortunate brand of dial-up concentrators.
Impact can be more than ISP equipment getting into trouble. It can also
be congestion or excessive bandwidth use because of incoming abusive
traffic, or infected customers.
Bits are bits, very few of them actually impact the ISP itself. Most
ISPs protect their own infrastructure. Routers are very good at
forwarding bits. Routers have problems filtering bits. Whether it is
spam, viruses or other attacks; its mostly customers or end-users that
bear the brunt of the impact, not the ISP.
Actually this doesn't have to be the paradox it seems to be. If we can
find a way to make sure at the source that the destination welcomes the
communication, we can have both.
The recurring theme is: I don't want my ISP to block anything I do, but
ISPs should block other people from doing things I don't think they
So how long is reasonable for an ISP to give a customer to fix an
infected computer; when you have cases like Slammer where it takes only
a few minutes to infect the entire Internet? Do you wait 72 hours?
or until the next business day? or block the traffic immediately?
Let's first look at the reverse situation: infective traffic comes in.
Customers may take the position that it is in their best interest that
their ISP filters this traffic forever, so that they can't get
infected, regardless of whether they patch their systems or not. But it
isn't realistic to expect ISPs to do this.
Or some major ISPs seem to have the practice of letting the infected
computers continuing attacking as long as it doesn't hurt their
First of all, because in many cases, the vulnerability is in a service
that also has legitimate uses. In some cases this isn't much of a
problem: for instance, with the slammer worm blocking the affected port
didn't really impact the SQL service. Or with filtering blaster,
windows file sharing doesn't work anymore but this isn't a public
service so the people who need it can run it over a secure tunnel of
some kind. However, shutting down port 80 because an HTTP
implementation has a vulnerability wouldn't be acceptable because of
the collateral damage.
Then there are the issues of ISPs being able to do this effectively in
the first place, and effectiveness. If ISPs were to filter everything
forever everywhere, maybe this would be effective, but nearly all
equipment takes a performance hit when it has to filter, and this
usually gets worse as the filters get bigger, and there are limits to
the length of filters. On top of that, there is the management issue:
with 100k ADSL customers, you need to apply filters to 100k interfaces
on hundreds of boxes. So in reality ISPs can only have a limited number
of filter rules in a limited number of places. While this gets rid of
most of the infective traffic for as long as the filter is in place,
this doesn't really protect customers, as when one customer is
infected, the infection can still spread to other customers (most worms
are optimized for this) unless the ISP has put filters on all customer
ports. And we've seen that worms are often carried from location to
location in infected laptops.
And then, when the filter rules have to go (for instance because there
is a new worm du jour) experience shows there is still some infecting
traffic, however long after the initial outbreak, so at some point a
vulnerable system WILL be infected.
Last but not least: if ISPs filter X worms, and then worm X+1 presents
itself which proves unfilterable, things get really bad because users
were depending on ISP action to prevent infection, rather than take
their own measures. This could even lead to legal problems for ISPs.
Bottom line: unless ISPs explicitly want to take on this responsibility
and invest in heavier equipment and very advanced network management,
the best they can do is take the edge off by implementing some
filtering that allows their users a little more time to patch their
Then there is the other side of the coin: infected customers. I mostly
work for content hosters these days, and there the situation is
slightly different from the one that access ISPs are facing, as the
number of customers is much smaller and the bandwidth they have is much
larger. So one customer can do much more damage by either causing
congestion in the local network or by driving up the bandwidth use on
external connections (which is expensive because of the usual 95th
percentile billing). There have been several cases the past year where
my customers shut down ports of infected customers of theirs (sometimes
lowering the port speed to 10 Mbps is a good compromise). But since
this leads to many phone calls, I can imagine that doing this for every
infected customer may be a problem for ISPs with many dial/ADSL/cable
customers. Also, if the bandwidth use isn't too excessive, it may not
always be apparent that a customer is infected.