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@swm.pp.se