"you will also block legitimate return traffic if the customers run their own DNS servers or use opendns / google dns / etc." I'm fine with that. Residential customers shouldn't be running DNS servers anyway and as far as the outside resolvers to go, ehhhh... I see the case for OpenDNS given that you can use it to filter (though that's easily bypassed), but not really for any others. ----- Mike Hammett Intelligent Computing Solutions http://www.ics-il.com Midwest-IX http://www.midwest-ix.com ----- Original Message ----- From: "Nick Hilliard" <nick@foobar.org> To: "Mikael Abrahamsson" <swmike@swm.pp.se> Cc: "NANOG list" <nanog@nanog.org> Sent: Friday, February 26, 2016 7:17:30 AM Subject: Re: Thank you, Comcast. Mikael Abrahamsson wrote:
Why isn't UDP/53 blocked towards customers? I know historically there were resolvers that used UDP/53 as source port for queries, but is this the case nowadays?
I know providers that have blocked UDP/53 towards customers as a countermeasure to the amplification attacks. As far as I heard, there were no customer complaints.
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. Nick