Thank you, Comcast.

Keith Medcalf kmedcalf at
Fri Feb 26 14:31:47 UTC 2016

ISP's should block nothing, to or from the customer, unless they make it clear *before* selling the service (and include it in the Terms and Conditions of Service Contract), that they are not selling an Internet connection but are selling a partially functional Internet connection (or a limited Internet Service), and specifying exactly what the built-in deficiencies are.

Deficiencies may include:
  port/protocol blockage toward the customer (destination blocks)
  port/protocol blockage toward the internet (source blocks)
  DNS diddling (filtering of responses, NXDOMAIN redirection/wildcards, etc)
  Traffic Shaping/Policing/Congestion policies, inbound and outbound

Some ISPs are good at this and provide opt-in/out methods for at least the first three on the list.  Others not so much.

> -----Original Message-----
> From: NANOG [mailto:nanog-bounces at] On Behalf Of Maxwell Cole
> Sent: Friday, 26 February, 2016 07:19
> To: Mikael Abrahamsson
> Cc: NANOG list
> Subject: Re: Thank you, Comcast.
> I agree,
> At the very least things like SNMP/NTP should be blocked. I mean how many
> people actually run a legit NTP server out of their home? Dozens? And the
> people who run SNMP devices with the default/common communities aren’t the
> ones using it.
> If the argument is that you need a Business class account to run a mail
> server then I have no problem extending that to DNS servers also.
> Cheers,
> Max
> > On Feb 26, 2016, at 8:55 AM, Mikael Abrahamsson <swmike at>
> wrote:
> >
> > On Fri, 26 Feb 2016, Nick Hilliard wrote:
> >
> >> Traffic from dns-spoofing attacks generally has src port = 53 and dst
> port = random.  If you block packets with udp src port=53 towards
> customers, you will also block legitimate return traffic if the customers
> run their own DNS servers or use opendns / google dns / etc.
> >
> > Sure, it's a very interesting discussion what ports should be blocked or
> not.
> >
> >
> >
> > This mentions on page 3.1, TCP(UDP)/25,135,139 and 445. They've been
> blocked for a very long time to fix some issues, even though there is
> legitimate use for these ports.
> >
> > So if you're blocking these ports, it seems like a small step to block
> UDP/TCP/53 towards customers as well. I can't come up with an argument
> that makes sense to block TCP/25 and then not block port UDP/TCP/53 as
> well. If you're protecting the Internet from your customers
> misconfiguraiton by blocking port 25 and the MS ports, why not 53 as well?
> >
> > This is a slippery slope of course, and judgement calls are not easy to
> make.
> >
> > --
> > Mikael Abrahamsson    email: swmike at

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