Why Developers Are Integrating DNS Intelligence

Whether you're building a parental control app, an enterprise security dashboard, a VPN client, or a browser extension, DNS filtering APIs give you programmatic access to domain reputation data, malware feeds, and blocklist queries — without maintaining the underlying threat intelligence yourself.

This guide covers the key API categories available, how to evaluate them, and practical integration patterns.

Categories of DNS Filtering APIs

1. Domain Reputation / Threat Intelligence APIs

These APIs return a risk score or threat category for a given domain or IP. Use them to check domains in real time before connecting.

  • Google Safe Browsing API — Free, checks URLs against Google's database of phishing and malware sites. Well-documented and high throughput.
  • URLhaus API (abuse.ch) — Free public API for checking domains/URLs against active malware distribution records. Great for security tooling.
  • VirusTotal API — Aggregates results from 70+ antivirus engines. The free tier supports limited queries/day; generous for development use.

2. Blocklist Query APIs

These allow you to check whether a domain appears in well-known blocklists:

  • DNSBL (DNS-based Block Lists) — The original standard. You can query any DNSBL directly using standard DNS lookups by reversing the IP and appending the DNSBL hostname. No special SDK required.
  • Spamhaus API — Programmatic access to the Spamhaus domain and IP blocklists used by enterprise mail servers worldwide.

3. DNS-over-HTTPS Resolvers as APIs

DoH resolvers like Cloudflare and Google expose JSON-based DNS query APIs you can call directly from application code:

GET https://cloudflare-dns.com/dns-query?name=example.com&type=A
Accept: application/dns-json

Response includes the resolved IPs and response code. This is useful for building custom DNS resolution logic inside apps without relying on the OS resolver.

Integration Pattern: Real-Time Domain Check Before Connection

  1. User or your app initiates a connection to a domain (e.g., from a link click or API call).
  2. Before opening the socket, send the domain to your chosen reputation API.
  3. Parse the response — if threat score is above threshold, block the connection and alert the user.
  4. Cache results with a short TTL (e.g., 5–10 minutes) to avoid redundant API calls.
  5. Log blocked events for auditing and pattern analysis.

Working with the Google Safe Browsing API

The Lookup API v4 is straightforward. Here's the request structure:

POST https://safebrowsing.googleapis.com/v4/threatMatches:find?key=YOUR_API_KEY
Content-Type: application/json

{
  "client": { "clientId": "yourapp", "clientVersion": "1.0" },
  "threatInfo": {
    "threatTypes": ["MALWARE", "SOCIAL_ENGINEERING"],
    "platformTypes": ["ANY_PLATFORM"],
    "threatEntryTypes": ["URL"],
    "threatEntries": [{ "url": "http://example.com/page" }]
  }
}

An empty matches array means the URL is clean. A populated array returns the threat type and severity.

Rate Limiting & Caching Best Practices

  • Always implement a local cache (Redis or in-memory) with domain results to minimize API calls.
  • Respect rate limits — use exponential backoff on 429 responses.
  • For high-volume use cases, evaluate batch endpoints where available (Safe Browsing supports checking multiple URLs per request).
  • Consider a tiered approach: check your local cache first, then a self-hosted blocklist, then an external API as a fallback.

Self-Hosted Option: AdGuard's Filtering Core

For maximum control and zero external dependency, AdGuard's urlfilter library (open-source, Go) lets you compile and query filter lists locally. It's the same engine powering AdGuard products — you can integrate it into Go applications directly and match URLs against EasyList, EasyPrivacy, or any custom filter list without a network call.

Key Takeaways

  • Start with free-tier APIs (Google Safe Browsing, URLhaus) for development and proof-of-concept work.
  • Use DoH JSON APIs for custom DNS resolution logic in your app.
  • Cache aggressively and batch requests to stay within free tier limits.
  • For production apps processing high query volumes, consider self-hosted filtering engines.