Aside from the BCPs currently being discussed for ingress filtering, I would be very interested in seeing what this traffic looked like from the perspective of your DNS servers' logs.

I assume you're talking about customer facing recursive/caching resolvers, and not authoritative-only nameservers. 

Considering that one can run an instance of an anycasted recursive nameserver, under heavy load for a very large number of clients, on a $600 1U server these days... I wonder what exactly the threat model is.

Reverse amplification of DNS traffic returning to the spoofed IPs for DoS purposes, such as to cause the nameserver to DoS a single /32 endpoint IP being targeted, as in common online gaming disputes? 

What volume of pps or Mbps would appear as spurious traffic as a result of this attack?



On Tue, Oct 13, 2020 at 3:14 PM Brian Knight via NANOG <nanog@nanog.org> wrote:
We recently received an email notice from a group of security
researchers who are looking at the feasibility of attacks using spoofed
traffic.  Their methodology, in broad strokes, was to send traffic to
our DNS servers with a source IP that looked like it came from our
network.  Their attacks were successful, and they included a summary of
what they found.  So this message has started an internal conversation
on what traffic we should be filtering and how.

This security test was not about BCP 38 for ingress traffic from our
customers, nor was it about BGP ingress filtering.  This tested our
ingress filtering from the rest of the Internet.

It seems to me like we should be filtering traffic with spoofed IPs on
our transit, IX, and peering links.  I have done this filtering in the
enterprise world extensively, and it's very helpful to keep out bad
traffic.  BCP 84 also discusses ingress filtering for SP's briefly and
seems to advocate for it.

We have about 15 IP blocks allocated to us.  With a network as small as
ours, I chose to go with a static ACL approach to start the
conversation.  I crafted a static ACL, blocking all ingress traffic
sourced from any of our assigned IP ranges.  I made sure to include:

* Permit entries for P-t-P WAN subnets on peering links
* Permit entries for IP assignments to customers running multi-homed BGP
* The "permit ipv4 any any" at the end :)

The questions I wanted to ask the SP community are:

* What traffic filtering do you do on your transits, on IX ports, and
your direct peering links?
* How is it accomplished?  Through static ACL or some flavor of uRPF?
* If you use static ACLs, what is the administrative overhead like? 
What makes it easy or difficult to update?
* How did you test your filters when they were implemented?

Thanks a lot,

-Brian