DNS is the system that translates human-readable domain names, like squidproxies.com, into the numerical IP addresses computers use to locate each other on the network. A web request to a domain name typically starts with a DNS lookup (unless the answer is already cached), and how that lookup is handled, locally or through the proxy, has implications for both performance and privacy.

When using a proxy, DNS resolution can happen either on your own device before the request is sent, or remotely through the proxy server itself. Resolving locally can leak your real location or ISP to any party monitoring your DNS traffic, even while the actual web request is routed through the proxy; this is the basis of a "DNS leak."

A single page load can trigger dozens of separate DNS lookups, one for the main domain and one for each distinct third-party domain (ad networks, APIs, CDNs) it loads resources from.