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Re: BCP for ISP to block worms at PEs and NAS

  • From: Steven M. Bellovin
  • Date: Sun Apr 17 16:14:25 2005

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