Open Resolver
A recursive DNS resolver that accepts queries from any source IP on the internet, instead of restricting service to its operator's own users.
An open resolver is a recursive DNS server that answers queries from anyone, not just its operator's customers or internal users. Some are intentional (Cloudflare 1.1.1.1, Google 8.8.8.8, Quad9 9.9.9.9), but most are misconfigured home routers, cable modems, or hosting servers. The problem is that open resolvers are the abused infrastructure in DNS amplification DDoS attacks: attackers send small spoofed queries and the resolver reflects large answers at the victim. The Open Resolver Project and Shadowserver still track hundreds of thousands of unintentionally open resolvers; closing them is one of the standard hardening tasks for any DNS deployment.