DNS Lookup

DNS is the directory that turns a name people can remember into an address a machine can use. Every name has a set of records, each with a type that says what kind of answer it holds. Pick a type, type a domain and the tool asks a public resolver and shows what came back, including the TTL of each answer. A full URL works too — only the host name is used.

A records hold an IPv4 address and AAAA records hold an IPv6 one; those are the two that actually get a browser to a server. CNAME is an alias saying "ask about this other name instead", which is why a CNAME cannot sit alongside other records on the same name and cannot be used on a bare domain. NS lists the nameservers that are authoritative for the zone, and it is the first thing to check when a change you made is not showing up: if the NS records point at a provider you no longer use, you have been editing the wrong copy of the zone. SOA holds the zone's administrative data, the primary nameserver and the responsible contact, and is a quick way to confirm which system really owns the zone.

MX records tell other mail servers where to deliver mail for the domain, and each one carries a preference number in front of the host name. Lower is preferred: a sending server tries the lowest number first, and only moves to the next when it cannot connect. Equal numbers mean the load is spread between those hosts. So "10 aspmx.l.google.com" being tried before "20 alt1.aspmx.l.google.com" is by design, not a fault. If the MX list is empty, mail for the domain is usually delivered to the A record instead, which is almost never what a modern setup intends.

TXT records are free text, and email authentication has quietly taken them over. An SPF record starts with v=spf1 and lists the servers allowed to send mail using your domain, ending in ~all (soft fail) or -all (hard fail); there must be exactly one SPF record per domain, and a second one silently breaks both. DKIM public keys live on a name like selector._domainkey.example.com, so you have to know the selector your mail provider uses to look one up. DMARC lives on _dmarc.example.com and tells receiving servers what to do when SPF and DKIM disagree with the visible From address, plus where to send reports. TXT is also where domain-ownership verification strings from Google, Microsoft and others end up.

Every answer carries a TTL, the number of seconds a resolver may keep it cached. That is why a DNS change does not take effect everywhere at once: caches all over the world hold the old answer until their copy expires. Lowering the TTL a day before a planned migration is the standard trick. Note that this tool asks a public resolver rather than your own, so a cached answer here can differ from what your computer currently sees. Only the name and record type you choose are sent to the server; no file is uploaded.