Check your network headers for X-DNS-Prefetch-Control.
');\n});\n\napp.listen(PORT, () => {\n console.log(`Server running on http://localhost:${PORT}`);\n console.log('X-DNS-Prefetch-Control header should be set.');\n});","lang":"typescript","description":"Demonstrates setting the X-DNS-Prefetch-Control header using the middleware, defaulting to 'off', and notes the modern Helmet.js integration."},"warnings":[{"fix":"Migrate to `helmet` and configure `xDnsPrefetchControl` option: `app.use(helmet({ xDnsPrefetchControl: { allow: false } }));`","message":"The standalone `dns-prefetch-control` package is effectively abandoned, with its GitHub repository archived. Its functionality has been integrated directly into the `helmet` package. Users should migrate to using `helmet` directly for this feature.","severity":"deprecated","affected_versions":">=0.1.0"},{"fix":"The default setting of `allow: false` (or just `app.use(dnsPrefetchControl())`) disables prefetching, prioritizing privacy. Carefully evaluate performance gains against privacy concerns before enabling.","message":"Enabling DNS prefetching (`allow: true`) can improve page load performance by proactively resolving domain names. However, it can also have privacy implications, as it causes browsers to make DNS requests for links a user might not actually click, potentially revealing browsing interests to DNS servers and network observers.","severity":"gotcha","affected_versions":">=0.1.0"},{"fix":"Be aware that reliance on non-standard headers can lead to inconsistent behavior in edge cases or with future browser updates. Monitor browser compatibility if this header is critical to your application's privacy or performance strategy.","message":"The `X-DNS-Prefetch-Control` header is a non-standard header, though widely supported by modern browsers. Its behavior might vary slightly across different browser implementations.","severity":"gotcha","affected_versions":">=0.1.0"},{"fix":"Focus DNS prefetching only on key third-party domains used early in the page load and avoid prefetching low-priority or rarely used domains. The middleware controls the HTTP header, which typically applies broadly, so fine-tuning often requires manual `` tags for specific domains if `allow: true` is used.","message":"Overusing DNS prefetch hints (e.g., via `` or excessively broad prefetching) can paradoxically degrade performance by overwhelming the browser's internal queue or causing unnecessary lookups, especially for domains that are not actually used.","severity":"gotcha","affected_versions":">=0.1.0"}],"env_vars":null,"last_verified":"2026-04-22T00:00:00.000Z","next_check":"2026-07-21T00:00:00.000Z","problems":[{"fix":"Ensure correct import syntax: `import dnsPrefetchControl from 'dns-prefetch-control';` for ESM, or `const dnsPrefetchControl = require('dns-prefetch-control');` for CommonJS. If using TypeScript and `esModuleInterop` is `false`, you might need `import * as dnsPrefetchControl from 'dns-prefetch-control';`.","cause":"This usually occurs when attempting to use an ESM `import` statement for a CommonJS module that exports a function directly (e.g., `module.exports = function`) without correctly accessing its default property, or if using a named import for a default export.","error":"TypeError: dnsPrefetchControl is not a function"},{"fix":"Verify `app.use(dnsPrefetchControl(...))` is placed early in your middleware chain. If using `helmet`, ensure `xDnsPrefetchControl` is configured within the `helmet` options object and avoid redundant standalone `dns-prefetch-control` usage. Use browser developer tools to inspect response headers.","cause":"The middleware might not be applied correctly to the Express application, or other middleware might be overwriting the header. Also, if using the standalone package alongside `helmet`, `helmet` might take precedence.","error":"X-DNS-Prefetch-Control header is not being set or has an unexpected value"}],"ecosystem":"npm","meta_description":null}