Since the 1st of February multiple major DNS (defined) resolvers removed resolver workarounds. The resolvers involved in the initiative include ISC, Cloudflare, Facebook, Cisco, Google (among others).
The workarounds were removed to stop DNS queries not compliant with the following official Requests for Comments (RFC) 1035 and 2671 from being completed(resolved). In more depth; the DNS Flag day page explains these workarounds are being removed due to:
The current DNS is unnecessarily slow and inefficient because of efforts to accommodate a few DNS systems that are not in compliance with DNS standards established two decades ago.
To ensure further sustainability of the system it is time to end these accommodations and remediate the non-compliant systems. This change will make most DNS operations slightly more efficient, and also allow operators to deploy new functionality, including new mechanisms to protect against DDoS attacks.
It appears that DNS amplification and DNS flood attacks are the threats attempting to be mitigated with these changes. A full list of the types of DDoS (defined) attacks is available from the following Cloudflare page (at the end of that page):
It will be interesting to see the effect of these changes on the DNS infrastructure when it is again targeted by botnets (defined) (e.g. made up of Internet of Things (IoT)(defined) or compromised systems or by other means. Such botnets can make use a command and control (C2) (defined) infrastructure.