Browser-Based Dns Diagnostics Via Cors-Enabled Json Api

Issue 81 Edition 2026-03-22 4 min read

Not accepted General

Sources: 1 • Confidence: Medium • Updated: 2026-04-12 10:18

Key takeaways

Cloudflare's 1.1.1.1 DNS resolver provides a CORS-enabled JSON API.
Cloudflare's 1.1.1.2 resolver is positioned as blocking malware.
Cloudflare's 1.1.1.3 resolver is positioned as blocking both malware and adult content.
The author used Claude Code to build a UI that runs DNS queries against Cloudflare's 1.1.1.1, 1.1.1.2, and 1.1.1.3 resolvers.

Cloudflare's 1.1.1.1 DNS resolver provides a CORS-enabled JSON API.
The author used Claude Code to build a UI that runs DNS queries against Cloudflare's 1.1.1.1, 1.1.1.2, and 1.1.1.3 resolvers.

Cloudflare's 1.1.1.2 resolver is positioned as blocking malware.
Cloudflare's 1.1.1.3 resolver is positioned as blocking both malware and adult content.

What is the exact URL, request format, and response schema for the CORS-enabled JSON API used for DNS queries?
What rate limits, quotas, or acceptable-use constraints apply to browser-originated requests to the JSON DNS API?
How do 1.1.1.2 and 1.1.1.3 represent blocking in responses (for example, special IPs, NXDOMAIN, or other signaling)?
What is the observed difference in results between the three resolvers for a controlled set of known domains, and how stable are those differences over time?
Is the referenced UI available for inspection, and does it run entirely client-side without a proxy service?

If Cloudflare provides a CORS-enabled JSON DNS endpoint, browser-only diagnostic tools could proliferate, potentially increasing developer reliance on Cloudflare resolver infrastructure for testing and comparisons.
Differentiated resolver policies such as malware-only and malware-plus-adult could support lightweight DNS-layer filtering adoption via simple resolver switching, implying demand for policy-based DNS options.
A working client-side UI that compares results across 1.1.1.1, 1.1.1.2, and 1.1.1.3 suggests a low-friction workflow for auditing DNS policy outcomes, which could increase scrutiny of filtering accuracy and consistency.

Public documentation of the CORS-enabled JSON API with stable URL, request format, and response schema that supports sustained browser-originated querying without proxies.
Clear, consistent signaling of blocked outcomes for 1.1.1.2 and 1.1.1.3, such as standardized response codes or IP behaviors, enabling deterministic interpretation in client tools.
Published rate limits or acceptable-use terms that explicitly allow interactive browser usage at meaningful volumes, supporting production-grade client-side diagnostics.

CORS policy becomes restrictive or the JSON endpoint is deprecated, forcing browser tools to use backend proxies and reducing the cited advantage.
Rate limits or quotas are too low for practical browser-based diagnostics, or terms discourage such usage, limiting adoption of client-only workflows.
Blocking behavior for 1.1.1.2 and 1.1.1.3 is inconsistent or opaque across domains or over time, making comparisons unreliable and reducing utility of resolver selection for policy outcomes.