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-boun...@nanog.org] 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 <swm...@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: swm...@swm.pp.se
