[knot-resolver-users] udp/tcp transport fallback strategy

Hi, it seems kresd 5.7.6 falls back from UDP to TCP transport but never retries over UDP. Impact: udp/53-only services (incorrect but happens in the wild[1]) may become ~permanently (long TTL) unreachable when udp/53 is not 100% reliable. When temporal udp/53 communication error occurs, kresd will fall back to tcp/53, fail to connect, store negative information and return SERVFAIL. Since retries only consider tcp/53 transport, there is no easy way to recover for udp/53-only domains. I believe it would be nice to retry with UDP as well as TCP (other implementations do this) to handle such cases more gently. [iterat][55073123.04] <= rcode: NOERROR [valdtr][55073123.04] <= cached insecure response, going insecure [iterat][55073123.02] 'www.somefooservice.com.' type 'A' new uid was assigned .05, parent uid .00 [select][55073123.05] => id: '37763' choosing from addresses: 0 v4 + 0 v6; names to resolve: 0 v4 + 0 v6; force_resolve: 0; NO6: IPv6 is OK [select][55073123.05] => id: '37763' no suitable transport, zone cut: 'www.somefooservice.com.' [iterat][55073123.05] 'www.somefooservice.com.' type 'A' new uid was assigned .06, parent uid .00 [select][55073123.06] => id: '27070' choosing from addresses: 0 v4 + 0 v6; names to resolve: 0 v4 + 0 v6; force_resolve: 0; NO6: IPv6 is OK [select][55073123.06] => id: '27070' no suitable transport, zone cut: 'www.somefooservice.com.' [resolv][55073123.06] AD: request NOT classified as SECURE [resolv][55073123.06] finished in state: 8, queries: 2, mempool: 81952 B Workaround: cache.max_ttl() - haven't tried cache.clear('FQDN record affected') does NOT work cache.clear('entire TLD affected') does NOT work cache.clear('.') DOES work cache.clear() DOES work [1] It took us some time to reach major mobile device producer and convince them to expose tcp/53 ;) /PM