DNS Flag Day: Impact on Networking and DNS Operations

January 2019 - Reading Time: 10 minutes

What Is DNS Flag Day?

DNS Flag Day 2019 is a community-driven initiative by major DNS providers to drop support for non-compliant DNS implementations that break Extension Mechanisms for DNS (EDNS). It marks a pivotal shift toward better interoperability, simpler DNS software, and reduced operational complexity.

Why It Matters

Historically, DNS resolvers implemented numerous workarounds to accommodate broken authoritative servers that fail to comply with EDNS standards. This bloats resolver code and complicates deployments. DNS Flag Day establishes a cutoff point—after which resolvers no longer apply these workarounds.

How DNS Flag Day Affects Networks

On or after January 1, 2019, resolvers (including those from vendors like Google, Cloudflare, Cisco OpenDNS, and ISC’s BIND) stop sending fallback queries to non-EDNS-compliant servers. If your DNS server fails to respond correctly to EDNS, it becomes unreachable to participating resolvers.

Testing Your DNS

Testing compliance is straightforward using the official DNS Flag Day test tool provided at dnsflagday.net. The test identifies EDNS compliance issues and offers suggestions for resolution. Administrators are encouraged to verify their domains and infrastructure well in advance.

Common Compliance Issues

• Firewalls dropping EDNS packets
• DNS software improperly handling EDNS options
• Responses with truncated flags and incorrect behavior under TCP fallback

Steps for Operators

Network and system operators should audit and upgrade their DNS infrastructure. Recommended actions:

• Update DNS server software (e.g., BIND, NSD, Unbound) to recent versions
• Check firewall policies for UDP packet size constraints
• Perform EDNS compliance tests on authoritative zones

Industry Impact

This initiative reflects an industry-wide shift toward protocol enforcement and clean implementation. It prioritizes correctness over legacy support. While it introduces short-term pain for non-compliant systems, long-term gains include improved DNS stability, cleaner resolver stacks, and increased security.

Looking Forward

DNS Flag Day 2019 represents the beginning of a more aggressive approach to DNS protocol enforcement. Future flag days may focus on other problematic behavior or deprecated configurations. Operators should treat this event as a model for how protocol communities will push modernization.

Eduardo Wnorowski is a network infrastructure consultant and Director.
With over 24 years of experience in IT and consulting, he helps organizations maintain stable and secure environments through proactive auditing, optimization, and strategic guidance.
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