DNS Prefetch is a nicety implemented by some browsers where the browser will preemptively make DNS requests for domains referred to by a given page. If a page has links to other websites, it will make DNS requests for those domains so that the local DNS cache is prefilled. This is nice for users because it improves browser performance, but is also a privacy intrusion, and can make it look like the person visited websites they did not visit. For documentation, see https://helmetjs.github.io/docs/dns-prefetch-control.
Set with:
app.use(helmet.dnsPrefetchControl({ allow: false })); // or true
