ISP port blocking practice

Joshua William Klubi joshua.klubi at
Mon Sep 13 19:10:09 UTC 2010

Most of us tend to do only default settings,it would better if we dig better
into our settings and apply stricter rules to enhance security

Sent from my HTC HD2 on Android

On 13 Sep 2010 13:55, "Brian Johnson" <bjohnson at> wrote:
> Brian J.
>>-----Original Message-----
>>From: Ricky Beam [mailto:jfbeam at]
>>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>
>>> Really? So, since so many ISPs are blocking port 25, there's lots
> less
>>> spam hitting our networks?
>>Less than there could be. It appears a lot less effective because
> there
>>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
> to
>>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
> just
>>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,
> etc...).
> The port 25 blocking simply prevents the largest volume of hosts in an
> ISP network, the users, from being a spam delivery platform.
> - Brian
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