Unlocking Peak DNSSEC Performance in CoreDNS: The 30x Advantage of ECDSA Over RSA

When implementing DNSSEC with CoreDNS, choosing the right cryptographic algorithm for signing can dramatically impact performance. CoreDNS’s dnssec plugin guidance strongly recommends ECDSA, and a recent performance deep-dive reveals precisely why: ECDSA (P-256) can be up to 30 times more CPU-efficient than RSA (RSASHA256/3072) for a common DNSSEC workload.

The Critical Role of DNSSEC Signing

DNSSEC ensures the authenticity and integrity of DNS responses. CoreDNS employs “NSEC black lies” for authenticated denial of existence, meaning that for non-existent domains (NXDOMAIN), it dynamically signs responses. This places cryptographic signing directly in the query path, making the choice of algorithm paramount for CPU utilization and network bandwidth.

RFCs like “Elliptic Curve Digital Signature Algorithm (DSA) for DNSSEC” (RFC 6605) and “Use of SHA-2 Algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC” (RFC 5702) govern these standards. Notably, ECDSA P-256 RRSIGs are a compact 64 bytes, while RSASHA256-3072 RRSIGs are 256 bytes, highlighting an immediate bandwidth saving with ECDSA.

A Head-to-Head Performance Comparison

To quantify the difference, two identical CoreDNS configurations were tested, varying only the DNSSEC key algorithm: RSA (RSASHA256, 3072-bit) and ECDSA (P-256). These two algorithms are considered to offer approximately equivalent cryptographic strength.

The test setup involved generating a large volume of unique, non-existent domain queries to force CoreDNS to perform extensive real-time DNSSEC signing. Throughput, latency, and CPU profiles were meticulously measured.

The Stark Results: ECDSA’s Dominance

The findings were unequivocal:

• CPU Usage: For the same NXDOMAIN-signing workload, RSA consumed approximately 30 times more CPU than ECDSA. Over a 30-second profiling period, RSA accumulated about 176 seconds of CPU samples, compared to just 5.6 seconds for ECDSA.
• Latency: ECDSA queries experienced significantly lower average latency.
  • ECDSA P-256: 0.298 ms
  • RSA-3072: 73.412 ms
• Queries Per Second (QPS): ECDSA handled a much higher query rate.
  • ECDSA P-256: 1999.65 QPS
  • RSA-3072: 1342.79 QPS
• Response Size: ECDSA’s smaller signature size translated to more compact responses, saving bandwidth.
  • ECDSA P-256: 495 bytes
  • RSA-3072: 1135 bytes

Conclusion: Embrace ECDSA for Optimal Performance

The evidence is clear: for online DNSSEC signing within CoreDNS, preferring ECDSA (P-256) keys is the optimal choice. It offers vastly superior CPU efficiency, lower latency, higher throughput, and reduced bandwidth consumption compared to RSA-3072, all while providing equivalent cryptographic strength.

Administrators should monitor metrics such as coredns_dnssec_cache_hits_total, coredns_dnssec_cache_misses_total, and overall CPU saturation to ensure their CoreDNS deployment is performing optimally. For deeper debugging, CoreDNS’s pprof plugin is an invaluable tool.