DNS flag day – 1/2/19

The current DNS is unnecessarily slow and suffers from inability to deploy new features. To remediate these problems, vendors of DNS software and also big public DNS providers are going to remove certain workarounds on February 1st, 2019.

This change affects only sites which operate software which is not following published standards.


On or around Feb 1st, 2019, major open source resolver vendors will release updates that implement stricter EDNS handling. Specifically, the following versions introduce this change:

  • BIND 9.13.3 (development) and 9.14.0 (production)
  • Knot Resolver already implemented stricter EDNS handling in all current versions
  • PowerDNS Recursor 4.2.0
  • Unbound 1.9.0

Also public DNS providers listed below will disable workarounds.


Minimal working setup which will allow your domain to survive 2019 DNS flag day must not have timeout result in any of plain DNS and EDNS version 0 tests implemented in ednscomp tool. Please note that this minimal setup is still not standards compliant and will cause other issues sooner or later. For this reason we strongly recommend you to get full EDNS compliance (all tests ok) instead of doing just minimal cleanup otherwise you will have to face new issues later on.


Firewalls must not drop DNS packets with EDNS extensions, including unknown extensions. Modern DNS software may deploy new extensions (e.g. DNS cookies to protect from DoS attacks). Firewalls which drop DNS packets with such extensions are making the situation worse for everyone, including worsening DoS attacks and inducing higher latency for DNS traffic.

DNS software developers

The main change is that DNS software from vendors named above will interpret timeouts as sign of a network or server problem. Starting February 1st, 2019 there will be no attempt to disable EDNS as reaction to a DNS query timeout.

This effectively means that all DNS servers which do not respond at all to EDNS queries are going to be treated as dead.

Source: DNS flag day

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