DNS Lookup

What a DNS lookup resolves

A DNS lookup queries the distributed name system for the records attached to a domain. The record type decides what you get back: an A record maps a name to an IPv4 address, AAAA to IPv6, MX names the mail servers and their priority, NS lists the authoritative name servers, TXT holds arbitrary text (SPF, DKIM, domain verification), and CNAME points one name at another as an alias. A single hostname usually has several record types at once.

When you reach for it

You check DNS after moving a site to a new host, when email bounces and you suspect a missing MX or broken SPF, when a CDN swap needs a CNAME verified, or when a certificate authority asks you to prove ownership with a TXT record. It is also the first thing to confirm before blaming the application for an unreachable host.

Example

$ dig example.com A +short
93.184.216.34

$ dig example.com MX +short
10 mail.example.com.
20 backup.example.com.

The number before each MX host is its priority: the lowest value is tried first, and equal values share load. The trailing dot marks a fully qualified name so the resolver does not append a search domain.

Common pitfalls

The one that burns everyone is TTL and propagation. When you change a record, old answers stay cached until the previous TTL expires, so a record with a 24-hour TTL can keep serving the old IP long after you edited it. Lower the TTL a day before a planned migration. Also note a CNAME cannot coexist with other records on the same name, which is why you cannot put a CNAME on a root domain that also needs MX records.

FAQ

Why do two lookups return different IPs? Many domains publish several A records for load balancing, and resolvers rotate them. GeoDNS can also hand different regions different answers.

My change is not showing up, why? You are almost certainly seeing a cached answer. Query an authoritative server directly, or wait out the old TTL before concluding something broke.

What record do I need for a certificate? Usually a TXT record the CA dictates. Once DNS is correct, confirm the cert with the SSL checker.