Thank you, Comcast.

Fri Feb 26 14:18:50 UTC 2016

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 swm.pp.se> 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.
> 
> http://www.bitag.org/documents/Port-Blocking.pdf
> 
> 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 swm.pp.se