ISP port blocking practice

[justin at justinshore.com (Justin Shore)]

Zhiyun Qian wrote:
> 1). For any outgoing traffic, if the destination port is 25, then drop 
> the packets.
> 2). For any incoming traffic, if the source port is 25, then drop the 
> packets.

It's been pointed that I glossed over the wording of #2, specifically 
missing the "source port" part of it, thus giving the right answer to 
the wrong question.  :-)

To answer your question, all our tcp/25 filters are based on destination 
port.  I could use source port but really I'm more concerned with my 
customers not running SMTP servers in one direction and them not being 
able to send spam in the other.  Using source port needlessly 
complicates those goals IMHO.  Someone might have a specific reason to 
use it but I don't in my case at least.

Justin