[j-nsp] Block port 0 traffic

John Kristoff jtk at northwestern.edu
Fri Oct 22 12:10:31 EDT 2004

On Thu, 14 Oct 2004 22:43:34 -0400 (EDT)
"Brian W. Gemberling" <bwgember at ptptech.com> wrote:

> You can use the from
> port 0;
> that should match either or, or both.

While others have mentioned fragments, please note that UDP messages
with source port equal to zero are normal and common.  Many UDP-based
apps will set their source port to zero when they do not expect a reply
from the receiver.  From RFC 768:

  Source Port is an optional field, when meaningful, it indicates the port
  of the sending  process,  and may be assumed  to be the port  to which a
  reply should  be addressed  in the absence of any other information.  If
  not used, a value of zero is inserted.


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