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What happens when an application looks up a hostname?
The application generally relies on a stub resolver provided by the operating system or runtime. The stub sends a query to a recursive resolver—often configured by the network, device, or application. The recursive resolver does the work of finding an answer on the client’s behalf and returns an answer or an error. RFC 1034 describes these resolver roles and the referral-based lookup process.
The exact path depends on the name, requested record type, available delegation and cache state. The steps below describe a typical lookup when the recursive resolver does not already have usable data or referrals cached.
- The client asks its stub resolver. The application requests a particular record type for a name—for example, an A record. The stub forwards the question to its configured recursive resolver.
- The recursive resolver checks its cache. If it has an unexpired answer it can use, it can return that data without contacting the hierarchy again. Cached delegation information may also let it skip some earlier steps in a lookup.
- The resolver asks a root server for a referral. If it needs to continue from the top of the hierarchy, it asks a root server where to find the relevant top-level domain’s servers. The root server refers it onward; it does not ordinarily supply the requested host record.
- The resolver asks a top-level-domain server for another referral. The server for the relevant TLD directs the resolver to the name servers authoritative for the domain.
- The resolver asks an authoritative server for the requested data. That server responds from its zone data. The response may contain the requested record, an alias such as a CNAME that leads to more lookups, or an error.
- The resolver returns the result to the client. The recursive resolver sends the answer or failure back to the stub, which makes the result available to the application.
This is a logical sequence, not a requirement to contact a root server for every lookup. Cache state can supply an answer directly or provide enough delegation data to start farther down the hierarchy.
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What do recursive resolvers and authoritative name servers do?
A recursive resolver accepts a client’s question and tries to obtain a complete answer for it. It may reuse cached records, follow referrals, and return either data or an error. In this context, “recursive” describes the service the resolver provides to its client; the resolver’s own queries to DNS servers commonly follow referrals step by step.
An authoritative name server serves DNS data for a zone. It is the source the resolver reaches for records in that zone, rather than a general-purpose cache performing the whole lookup for the client. Delegations connect the hierarchy: a parent zone points the resolver toward the name servers for a child zone.
The distinction matters during troubleshooting. A recursive resolver can return data it previously cached, while an authoritative server’s current zone data may already have changed. Asking one does not necessarily reveal what the other would return.
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Which DNS record is being requested?
A resource record has an owner name, type, class, TTL, and type-specific data; RFC 1035 specifies the core DNS message and record format. Engineers most often encounter these types:
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- A: IPv4 address data.
- AAAA: IPv6 address data.
- CNAME: an alias from one name to another name.
- NS: name-server information used in DNS delegation and zone data.
The record type is part of the question. Getting an answer to an A query does not establish that an AAAA query will also succeed or return corresponding data. A response can also include a CNAME before the requested address data, so an application’s apparent “one lookup” may involve more than one name.
What does DNS TTL mean?
A record’s time to live (TTL) is the maximum time a cache may retain that record. The zone administrator sets the TTL for the data; a TTL of zero prohibits caching. As a cached record ages, the remaining TTL decreases. A resolver can reuse the record while it remains valid rather than repeating the lookup.
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TTL affects change propagation, but it is not a command that flushes existing caches. If an administrator lowers a record’s TTL before a planned change, caches that fetch the new TTL can retain the record for less time. A cache that already holds the old record under a longer TTL may continue using it until that existing lifetime expires. Thus, changing authoritative data does not make every recursive resolver return the new value immediately.
Short TTLs can reduce the time a newly fetched answer remains cached, but they also reduce the opportunity to reuse cached answers. TTL is therefore a trade-off between cache reuse and how long a cached copy can persist after an update.
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Start by identifying the observation point. A result from an application, a local stub, a recursive resolver, and an authoritative server may differ because they involve different query types, cache states, or stages of the lookup.
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- Identify the resolver that answered. Check which recursive resolver the client actually used; do not assume it is the resolver configured elsewhere in the environment.
- Record the exact name and type. Distinguish A from AAAA, and note whether the response contains a CNAME.
- Inspect the returned data and remaining TTL. A remaining TTL can help indicate how long a cached record may still be reused; it does not by itself identify where the record was first obtained.
- Check the response code and outcome. Separate an answer from a name error or temporary failure rather than treating every unsuccessful lookup as the same problem.
- Compare recursive and authoritative answers where appropriate. A difference can be consistent with a cached answer that has not expired, so verify the relevant TTL and resolver state before concluding that authoritative changes failed.
These checks make a report such as “DNS is stale” more actionable: it can be narrowed to a particular resolver, query type, response, and cache lifetime.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What changes with DNS over HTTPS?
DNS over HTTPS (DoH) carries DNS queries and responses through HTTP exchanges over HTTPS. RFC 8484 defines the mapping and focuses on communication between a DNS client, such as a stub resolver, and a recursive resolver. The DNS question and answer retain DNS semantics; HTTPS changes the transport between those endpoints, not the naming hierarchy or the role of authoritative servers.
| Mechanism | What it changes or protects | What it does not establish |
|---|---|---|
| Classic DNS | Uses the DNS message format specified by RFC 1035 and is commonly transported over UDP or TCP. | By itself, the transport does not provide the HTTPS protection described for DoH. |
| DNS over HTTPS (DoH) | Carries DNS messages over HTTPS between the client and the chosen DoH resolver, protecting that connection from on-path observation or interference in ways traditional unencrypted transport does not. | Does not make all DNS activity private, hide queries from the resolver, or prove that DNS data is authentic. |
| DNSSEC | Addresses authenticity of DNS data through DNSSEC validation. | Is not a replacement for encrypted transport between a client and resolver. |
Choosing DoH also changes the client’s resolver relationship: queries go to the selected DoH provider, which receives them. Network operators may have different visibility or control over this traffic than over traditional DNS, while the provider still sees the requests. RFC 8484 discusses correlation risks across network and HTTP layers; DoH should not be treated as anonymity.
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HTTP caching has DNS-specific limits in DoH. RFC 8484 says an HTTP response’s freshness lifetime must not exceed the smallest TTL in its Answer section and recommends making the two equal. A DoH client also accounts for the HTTP Age header when determining the remaining DNS TTL. Consequently, HTTP freshness cannot legitimately extend DNS answer validity beyond the record TTL.
Is DoH the same thing as DNSSEC?
No. DoH protects the transport interaction between the client and DoH resolver; DNSSEC validation addresses whether DNS data is authentic. An HTTPS connection does not, on its own, validate the returned DNS data. The protocols can be used together because they address different layers of the problem.
RFC 8484, an IETF standards-track document published in October 2018, states: “DNSSEC and DoH are independent and fully compatible protocols, each solving different problems.”
Can a resolver answer after a record’s TTL expires?
Some resolvers can serve stale DNS data as a resiliency measure. RFC 8767, published in December 2020, standardizes resolver behavior for serving stale data to improve resilience. It specifies that a stale record returned in a response must have a TTL greater than zero and recommends 30 seconds for that returned TTL.
This is a defined mechanism, not a guarantee that every resolver serves stale answers. In an incident, an expired authoritative or cached record therefore does not prove that every resolver will immediately have no answer; the specific resolver’s behavior matters.
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