[knot-dns-users] Inter-operability issues with other DNS Servers

Ahoj, I've searched through the archives and read documentation+RFCs to no avail, so I hope you can help out. I run the authoritative DNS servers for an associative ISP in Barcelona (eXO.cat), and we are running them in FreeBSD using Knot DNS (2.9.5, but .6 and .7's changelogs do not point to a similar issue being solved). We have since then gotten a few reports from different parties of "DNS issues", I am reasonably sure to have pinpointed this down to badly configured DNS resolvers, but our weight is really too tiny to force any change; and the doubt remains as to whether or not this is on us. Without access to the resolvers it's also a tad tricky for us to reproduce. Things do work beautifully with pretty much all of the internet. This appears to be related to: https://tools.ietf.org/html/rfc7873#section-5.2.3 (sometimes in connection with 5.2.4) Maybe someone with more experience and running at a larger scale has pointers on this topic. Specifically, I have observed: Case A: 1. (Probably Bind) Resolver contacts Knot DNS with Client Cookie and old Server Cookie 2. Knot DNS responds BADCOOKIE with the provided Client Cookie, Old Server Cookie, and adds the new server cookie. 3. Resolver contacts Knot DNS with the same Client Cookie and old Server Cokie 4. 2 and 3 repeat for a long time. 5. Domains end up not resolving. Case B: https://github.com/matrix-org/synapse/issues/8581 Their supplier says the DNS server "replies with BADCOOKIE to UDP queries", as if that were a bad thing; but from reading RFC7873, I understand that it is expected documented behaviour and the client ought to try again with the given Server Cookie. They say: "AIUI the resolver does retry, but again receives a BADCOOKIE response." I sadly don't have the tcpdump for those, but it could be just like case A again. These are the relevant bits from the config: server: rundir: "/var/run/knot" user: knot:knot listen: [ 0.0.0.0@53, ::@53 ] log: - target: syslog any: info statistics: append: off database: storage: "/var/db/knot" mod-cookies: - id: default badcookie-slip: 1 mod-rrl: - id: default rate-limit: 200 # Requests per second template: - id: default storage: "/var/db/knot/primary" semantic-checks: on disable-any: on serial-policy: dateserial file: "%s.zone" global-module: mod-cookies/default global-module: mod-rrl/default global-module: mod-stats - id: secondary storage: "/var/db/knot/secondary" semantic-checks: on disable-any: on serial-policy: dateserial file: "%s.zone" module: mod-cookies/default module: mod-rrl/default module: mod-stats - id: signed storage: "/var/db/knot/primary" dnssec-signing: on zonefile-load: difference semantic-checks: on disable-any: on serial-policy: dateserial file: "%s.zone" module: mod-cookies/default module: mod-rrl/default module: mod-stats We are not using anycast clusters or anything like that. A quick solution would be to disable cookies or to experiment with noudp, but that's something we'd like to avoid. The name servers in question are: - ns3.exo.cat - ns4.exo.cat - ns1.unchat.cat (not association, but identical setup) All tests I've ran against the servers and the associated domains (e.g. exo.cat + unchat.cat) appear to be fine. There is an odd one with https://dnsviz.net/d/unchat.cat/dnssec/, but other tests like https://www.zonemaster.net/result/d530cb253298b56c do not report any issues. Thank you in advance for any pointers you may have (and for Knot DNS!), -- Evilham