DNS resolution

What it means

DNS resolution obtains DNS data for a name and record type, often using a recursive resolver that follows referrals and reuses cached answers. For a website, an A or AAAA answer may supply an IP address, but DNS does not retrieve the site's HTML. A browser can connect and make an HTTP request only after the necessary name resolution and other connection steps have succeeded. A cached answer can shorten the lookup; a negative DNS answer is still a DNS result, not an HTTP error page. DNS concepts · Current terminology

Try it

Simulation — compare the path, not a stopwatch. Run the cold lookup to its answer, then repeat it: the resolver now has a usable cache entry. Advance the virtual clock to 299 seconds and then 300 seconds to inspect the ordinary expiry boundary. Other presets start with a cached answer, a known delegation, a nonexistent name, or a name that lacks the requested record type.

The initial no-JavaScript example asks for www.example.com. A with an empty answer cache. Its prepared path reaches the example authority and returns 192.0.2.10. All addresses and answers are illustrative. Back reviews this lookup's steps; it does not reverse a query that already completed. Reset clears the trace, cached answer, clock, record choice and scenario.

Who asks whom?

The application typically uses a local resolver interface, often described as a stub, to ask a recursive resolver to obtain an answer. The recursive resolver may be provided by a network, an organization or a separately configured service. It can answer from information it already has or do further work. The client is not necessarily sending its own query to every server shown in a teaching diagram. Resolver roles

In the cold preset, the resolver starts without a useful final answer or delegation. A root server points it toward the .com servers. A .com authority points it toward the servers authoritative for example.com. The resolver then asks the appropriate authority for the requested data. The first two replies are referrals: directions to another part of the DNS hierarchy, not the web server's address. This example omits address-bootstrap and retry details. Referrals and resolver algorithm

The diagram is a role map, not a latency estimate. Real resolvers may use forwarding, already-cached delegation information, additional answers, retries or parallel work. The cached-delegation preset makes one such shortcut visible without trying to simulate every implementation.

A usable cache changes the path

A cached record has a remaining lifetime, not a universal appointment at which the internet changes. In this lab, an answer saved at time 0 with a 300-second teaching TTL remains usable at time 299 and expires at time 300. Repeating a query while the entry is usable does not restart that entry's lifetime. A newly obtained answer starts its own lifetime.

The scene uses an ordinary expiry-only policy. Real DNS includes additional mechanisms such as serve-stale behavior under specified conditions. Those are intentionally not simulated, so the lab's strict boundary must not be used as a guarantee about every production resolver. Caching and TTL · Serve-stale extension

An address, a missing name and a missing type

The A and AAAA choices ask for different record types: an IPv4 address and an IPv6 address respectively. The prepared ipv4.example.com name has an A record but no AAAA RRset. Asking for AAAA therefore produces the lab's NODATA result, described as NOERROR without that requested data. Asking for A succeeds. The name itself has not disappeared.

The missing.example.com preset instead returns NXDOMAIN. A resolver can cache negative results as well as positive answers. For real authoritative negative answers, the negative-cache lifetime is derived using SOA information; the lab uses a fixed 300 seconds only to keep the comparison readable. It does not calculate an arbitrary zone's negative TTL. Negative answers and caching

A CNAME is another distinct possibility: an alias can require following a different name before obtaining the desired type. This model does not implement alias chains, loops or a DNSSEC validator. A timeout or validation-related failure should not be casually relabelled as proof that the domain does not exist. DNS terminology and response distinctions

In practice

When investigating a lookup, record the exact fully qualified name, record type, resolver, time and response. Compare like with like: an A answer from one resolver and an AAAA result from another are not observations of the same query. An authoritative response and a recursive cached response answer different diagnostic questions.

Keep the next layer separate. A correct DNS answer may point to a destination that cannot serve the intended hostname, complete TLS or return the requested path. Conversely, receiving an actual HTTP 404 means an HTTP participant responded; it is not the same symptom as receiving NXDOMAIN. This distinction prevents changing DNS to solve a missing application route.

The useful mental model

Ask three questions: who has the requested data, what useful information is already cached, and what kind of answer came back? Root, TLD and authoritative servers describe a hierarchy of responsibility. Cache state determines how much of that hierarchy this lookup needs. Neither the number of arrows nor the virtual clock predicts a real visitor's loading time.