ISP port blocking practice

Owen DeLong owen at
Mon Sep 6 03:06:29 UTC 2010

On Sep 5, 2010, at 6:18 PM, Jon Lewis wrote:

> On Sun, 5 Sep 2010, Claudio Lapidus wrote:
>>> 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...
>> That won't be probably the case. Here recently we conducted a rather
>> comprehensive analysis on dns activity from subscribers, and we've
>> found that in IP ranges that already have outgoing 25 blocked we were
>> still getting complaints about originating spam. It turned out that
>> the bots also know how to send through webmail, so port 25 blocking
>> renders ineffective there.
> Anti-spam is a never ending arms race.  Originally, the default config for most SMTP servers was to relay for anyone.  10 years ago, sending spam through open SMTP relays was quite common.   Eventually, the default changed, nearly all SMTP relays now restrict access by either client IP or password authentication, and the spammers adapted to open proxies.  Today, nobody in their right mind sets up an open HTTP proxy, because if they do, it'll be found and abused by spammers in no time.  These too have mostly been eliminated, so the spammers had to adapt again, this time to botted end user systems.
> Getting rid of the vast majority of open relays and open proxies didn't solve the spam problem, but there'd be more ways to send spam if those methods were still generally available.  The idea that doing away with open relays and proxies was ineffective, so we may as well not have done and should go back to deploying open relays and open proxies it is silly.
Doing away with open relays and open proxies didn't really interfere with
legitimate traffic on a meaningful level.

Blocking outbound SMTP is causing such problems.

If a better job was done of blocking only 25, perhaps this would be less so.

Unfortunately, many hotel networks and such are doing one or more of the

Blocking ALL SMTP ports (25, 465, 587)
Blocking SSH in some cases (fortunately rare, rendering the SMTP thing mostly easy to work around)
Blocking IMAPs (while leaving IMAP open?!?)
Blocking POP3s (while leaving POP3 open?!?)
Blocking just about everything except 80 and 443

The absolute worst ones are proxying ALL SMTP traffic to their server whether it is the
address you tried to relay through or not. Generally the ones that have done this have
cited the complaints they got from outright blocking SMTP as the reason they felt the
need to do so. When I pointed out that not blocking SMTP and only blocking 25 could
be a viable alternative, they basically laughed at me.

The question isn't just what is or isn't effective, or, even how much it reduces spam
complaints. There is also the question of how much legitimate traffic suffers collateral
damage in your spam mitiigation techniques.


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