ISP port blocking practice

Owen DeLong owen at
Fri Sep 3 12:22:00 UTC 2010

On Sep 2, 2010, at 10:41 PM, Franck Martin wrote:

> Have you heard of the submission port?
Yes... Many of the idiots that block outbound 25 also block outbound 587 and sometimes 465.

> Why Clients of an hotel would run a MTA anyhow?
Huh? I think you misunderstand... The problem is hotels blocking people from submitting outbound
messages to their home MTA, not people trying to run an MTA inside their hotel room. NAT pretty
much guarantees running an MTA inside the hotel room is impossible in most circumstances.
(That might improve when IPv6 starts hitting hotel rooms, but, for now, it's just not there).
Yes, some hotels offer you the option of a public IP (and usually when I take that option, I have
fewer network problems in general. One of the reasons I tend to prefer Hilton brands when possible.)


> ----- Original Message -----
> From: "Jack Bates" <jbates at>
> To: "NANOG list" <nanog at>
> Sent: Friday, 3 September, 2010 4:08:54 PM
> Subject: Re: ISP port blocking practice
> Patrick W. Gilmore wrote:
>>> We should be seeking to stop damaging the network for ineffective anti spam measures (blocking outbound 25 for example) rather than to expand this practice to bidirectional brokenness.
>> Since at least part of your premise ('ineffective anti-spam measures') has been objectively proven false to fact for many years, I guess we can ignore the rest of your note.
> He's right though. tcp/25 blocks are a hack. Easy man's way out. 
> Honestly, it'd be nicer if edge or even core systems could easily handle 
> higher level filtering for things like this. There's plenty of systems 
> that watch traffic patterns and issue blocks based on those patterns.
> I was working with a hotel today concerning just that. They were only 
> doing a generic 500 connections in x period, block mac. They are now 
> adding a tighter rule for 15 tcp/25 connections in 1 minute, block 
> tcp/25 (or mac, doesn't matter to me). Of course, we didn't see valid 
> reasons for mail blasts to be leaving a hotel and 15/minute is plenty of 
> grace for a normal user. At an ISP level, it would work fine, though 
> methods for determining exceptions would have to be planned (though that 
> could easily be handled by customer classifications like everything else).
>> Also, just so everyone doesn't think I'm in favor of "damaging" the network, I would much prefer a completely open 'Net.  Who wouldn't?  Since that is not possible, we have to do what we can to damage the network as little as possible.  Port 25 blocking is completely unnoticeable to something on the order of 5-nines worth of users, and the rest should know how to get around it with a minimum of fuss (including things like "ask your provider to unblock" in many cases).
> Blocking inbound vs outbound is another story, though. Getting people to 
> implement spoof protections is more useful. I'd be interested to see 
> your data for concluding 5-nines of users, or did you just make that up?
> Jack

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