ISP port blocking practice
bjohnson at drtel.com
Mon Sep 13 13:54:35 UTC 2010
>From: Ricky Beam [mailto:jfbeam at gmail.com]
>Sent: Friday, September 03, 2010 9:30 PM
>To: Owen DeLong; Patrick W. Gilmore
>Cc: NANOG list
>Subject: Re: ISP port blocking practice
>On Fri, 03 Sep 2010 08:12:01 -0400, Owen DeLong <owen at delong.com>
>> Really? So, since so many ISPs are blocking port 25, there's lots
>> spam hitting our networks?
>Less than there could be. It appears a lot less effective because
>are so many ISPs not doing any blocking. Both of my residential
>connections are open, and always have been. (even dialup was unblocked.
>which I always found odd since the UUNET wholesale dialup agreement
>requires the RADIUS response contain a packet filter limiting port 25
>your mail server(s).)
>If I block port 25 on my network, no spam will originate from it.
>(probablly) The spammers will move on to a network that doesn't block
>their crap. As long as there are such open networks, spam will be
>rampant. If, overnight, every network filtered port 25, spam would all
>but disappear. But spam would not completely disappear -- it would
>be coming from known mailservers :-) thus enters outbound scanning and
>the frustrated user complaints from poorly tuned systems...
This is what we (network admins) get paid to do! If we are running a
server that is a security risk to the net, then we can't complain when
it gets filtered. It is our job to do our due diligence and ensure our
servers are not spam hot-beds or open relays (or other bad stuff,
The port 25 blocking simply prevents the largest volume of hosts in an
ISP network, the users, from being a spam delivery platform.
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