为何二级域名仍需非顶级DNS服务器？解析顶级DNS返回逻辑

Great question—let’s break down why those registrar-hosted nameservers (like registrar1.companydns.com and registrar2.companydns.com) are a critical part of how DNS works, even though they add an extra recursive step in your dig +trace output.

Core Reasons for Their Existence #

• DNS Hierarchy Depends on Delegation
  The top-level domain (TLD) servers (like .com) can’t possibly store A records for every single second-level domain under them—there are millions of .com domains, and that number grows every day. Instead, TLD servers only store delegation records (NS records) that point to the servers responsible for managing a specific domain’s DNS data. This delegation model is what makes the entire DNS system scalable; it distributes the load across thousands of nameservers instead of forcing TLD servers to handle everything.

• You Need Control Over Your Domain’s Records
  If TLD servers directly returned your A record, you’d have no way to manage your domain’s DNS settings on your own. Want to switch your website’s IP? Add an MX record for your email? Set up a CNAME for a subdomain? You’d have to submit a request to the TLD operator, which would be slow, cumbersome, and impractical. Registrar-provided NS servers act as your domain’s "control panel"—you can edit records anytime, without relying on a third party to make changes for you.

• Redundancy & Reliability
  DNS requires at least two NS servers per domain for redundancy. If the TLD returned only your A record, a single point of failure (like your web server going down) would take your domain offline entirely. With multiple registrar-hosted NS servers, if one goes down, the others can still respond to queries. Plus, most registrars operate geographically distributed NS servers, which improves query speed and uptime for users around the world.

• Support for Advanced DNS Features
  Modern DNS isn’t just about A records. Features like DNSSEC (to prevent hijacking), GeoDNS (to route users to the closest server), dynamic DNS (for changing IPs), and email authentication records (SPF, DKIM) all require a dedicated nameserver that can manage these complex record types. TLD servers don’t handle these—they only handle delegation. Your registrar’s NS servers are built to support these advanced tools to keep your domain secure and functional.

• Scalability for the Global DNS System
  Imagine if every .com domain’s A record was stored on the TLD servers. The sheer volume of data would make updates slow, queries laggy, and the system prone to outages. Delegating to individual NS servers lets the DNS system scale horizontally—each nameserver only manages a subset of domains, keeping the entire network efficient and resilient.

In short, that extra recursive step you see in dig +trace isn’t a flaw—it’s how DNS was designed to work. Those registrar-provided NS servers give you control, add reliability, and keep the entire global DNS system running smoothly.

内容的提问来源于stack exchange，提问作者yoyo