[79884] in North American Network Operators' Group
Re: BCP for ISP to block worms at PEs and NAS
daemon@ATHENA.MIT.EDU (Steven M. Bellovin)
Sun Apr 17 16:13:50 2005
From: "Steven M. Bellovin" <smb@cs.columbia.edu>
To: "J.D. Falk" <jdfalk@cybernothing.org>
Cc: nanog@merit.edu
In-Reply-To: Your message of "Sun, 17 Apr 2005 13:00:30 PDT."
<20050417200030.GG1174@arctic.org>
Date: Sun, 17 Apr 2005 16:13:06 -0400
Errors-To: owner-nanog@merit.edu
In message <20050417200030.GG1174@arctic.org>, "J.D. Falk" writes:
>
>On 04/17/05, John Kristoff <jtk@northwestern.edu> wrote:
>
>> > deny tcp any any range 135 139
>> > deny udp any any range 135 netbios-ss
>> > deny tcp any any eq 445
>> > deny udp any any eq 1026
>>
>> Similar as before, you are going to be removing some legitimate
>> traffic.
>
> Is this really true? All of the ports listed above are used by
> LAN protocols that were never intended to communicate directly
> across backbone networks -- that's why VPNs were invented.
>
> Or, is your argument that some system somewhere MIGHT ignore the
> offical port numbers allocated by IANA and try to pass some
> other kind of traffic there instead?
The issue is client-side port numbers -- those aren't rules that block
only inbound SYNs. That was clear from another paragraph of
Kristoff's post:
Whatever worm you're trying to mitigate above (sasser?), you will
also be occasionally be taking out TCP sessions that happen to be
using that port. Most commonly where one side uses 5554 as it's
ephemeral port.
The result will be intermittent, undiagnosed failures. "Why isn't that
Internet reliable? I do the same thing twice in a row and the second
time it fails."
--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb
