DNS Flag Day & Nameserver Compliance Inspector
Audit your authoritative nameserver infrastructure for modern DNS Flag Day standards. Inspect EDNS version 0 support, 1232-byte buffer sizing, packet fragmentation defense, and TCP port 53 fallback.
Frequently Asked Questions
What was DNS Flag Day and why was it enacted by major DNS operators?
DNS Flag Day was an industry-wide initiative led by major DNS software vendors (BIND, PowerDNS, Knot, NLnet Labs) and public resolver operators (Google, Cloudflare, Quad9) to remove legacy backward-compatibility workarounds for non-standard authoritative DNS servers that failed to support EDNS (RFC 6891).
Why does DNS Flag Day 2020 mandate an EDNS buffer size of 1232 bytes?
Traditional 4096-byte UDP buffers caused large DNS responses to exceed the typical 1500-byte Ethernet MTU, triggering IP packet fragmentation. Firewalls and carrier routers routinely drop fragmented IP packets, causing mysterious DNS timeouts. Setting the EDNS buffer to 1232 bytes fits safely inside IPv6 minimum MTU (1280 bytes) without fragmentation.
Why is TCP port 53 fallback mandatory for all nameservers?
When a DNS response exceeds the 1232-byte buffer limit, the authoritative nameserver must set the TC (Truncation) bit. The querying resolver then immediately re-establishes the query over TCP port 53. If a firewall blocks TCP port 53, large DNSSEC-signed responses or large TXT records fail completely.
How do non-compliant nameservers affect website visitors?
Resolvers that encounter broken EDNS implementations may experience 5 to 10 second timeout delays before falling back, leading to slow page loads, intermittent NXDOMAIN errors, and degraded Core Web Vitals.