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Hosting and deploy

What is DNS, and how does it work?

DNS turns a domain name into the address of the server behind it. How a lookup works, the record types you will meet, TTL and propagation, and common mistakes.

DNS, the Domain Name System, is the internet's directory: it translates a domain name such as example.com into the IP address of the server that hosts it, and stores other records such as where email goes. When you connect a domain to a host, you are editing DNS records. Changes spread as cached answers expire, according to each record's TTL.

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By Michal Měřínský. Audit methodology.

How a lookup works

When you type a domain, your device asks a recursive resolver, usually run by your internet provider or a service such as 1.1.1.1 or 8.8.8.8. If the resolver has no cached answer, it asks the root servers which servers handle .com, asks those which name servers handle example.com, and asks those for the record. The answer is cached for as long as its TTL says, so the next lookup is instant.

The records you will meet

  • A: points a name to an IPv4 address.
  • AAAA: points a name to an IPv6 address.
  • CNAME: makes one name an alias of another, such as www pointing to your host's address. It cannot be used on the bare domain in standard DNS.
  • ALIAS, ANAME or CNAME flattening: provider features that allow alias-like behavior on the bare domain.
  • MX: where email for the domain is delivered.
  • TXT: free text, used for domain verification and email authentication such as SPF and DMARC.
  • NS: which name servers are authoritative for the domain.
  • CAA: which certificate authorities may issue certificates for it.

TTL and propagation

Each record has a TTL, time to live, in seconds. Resolvers keep a cached answer that long. When you change a record, people whose resolvers cached the old answer keep seeing it until the TTL runs out. That delay is what people call propagation. Before a planned change, lower the TTL to a few minutes a day in advance, then raise it again afterwards.

Check your records

dig +short example.com A
dig +short www.example.com CNAME
dig +short example.com TXT

On Windows, nslookup does the same job. Online lookup tools show answers from several locations, which helps during changes.

Common mistakes

  • Changing name servers when you only needed to change one record, which moves the whole zone and can break email.
  • Deleting MX or TXT records while setting up a website, which breaks email delivery and authentication.
  • Leaving old A records pointing at a server you no longer control, which can let someone else serve content on your subdomain.
  • Forgetting the www record or the bare domain, so one of them does not resolve.

Before changing DNS for a domain that sends email, copy every existing record somewhere safe.

What a scan can and cannot tell you

WebAuditOS sees DNS through its results: whether the domain resolves to a site served over HTTPS, whether HTTP redirects to it, and whether the certificate matches the name.

It does not audit your DNS zone, check email records such as SPF, DKIM and DMARC, or look for dangling records on other subdomains.

If a scan cannot reach your site at all, check DNS first. A missing or wrong A or CNAME record looks like a dead site.

Short answers

How long does DNS propagation take? As long as the old record's TTL, often minutes to a few hours, occasionally up to a day or two.

What is the difference between a domain and DNS? The domain is the name you register. DNS is the system that says where that name points.

Should I use my registrar's DNS or Cloudflare's? Either works. Choose one place to manage records and keep it there.

Why does my site work for me but not for others? Their resolver may still have the old record cached.

In the product

The checks this maps to

Every check named here runs on every scan, on every plan - what a plan changes is how many pages are crawled, how long the history is kept and what you can do with the result. These are the ids the findings carry, so you can match a line in a report to the explanation above.

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