What is a DNS lookup?
The process of translating a hostname (like example.com) into the IP address a client uses to actually connect.
Also known as: DNS resolution
A DNS lookup walks a chain of resolvers. Your machine asks its configured resolver (often the ISP's, or 1.1.1.1 / 8.8.8.8). If the answer is not cached, the resolver asks a root server which TLD server handles .com, then asks that TLD server which authoritative server handles example.com, then asks that authoritative server for the A or AAAA record. The chain answers are cached at every level using TTLs.
For HTTP requests, DNS happens before TCP. A typical browser request waits on DNS (often a few ms from a warm cache, sometimes hundreds of ms cold), then TCP handshake, then TLS handshake, then the actual GET. DNS is invisible until it breaks, at which point every site you visit "looks down" because nothing resolves.
Crawlers and brand-data tools care about DNS for a few reasons. The presence of MX records signals a domain that handles email; the SOA gives the authoritative nameserver, which often reveals the registrar or DNS provider. A missing or recently-changed A record can flag a domain that has expired, parked, or migrated to a new host.
In the wild
dig stripe.comto see the live A and AAAA records- A status page running periodic DNS lookups against multiple resolvers to detect propagation delays
- A crawler skipping a domain whose authoritative nameservers all fail to respond