DNS
DNS is the system that translates human-readable domain names like penfriend.ai into the numerical IP addresses that computers actually use to find each other on the internet. The acronym stands for Domain Name System. Often described as “the phonebook of the internet” - accurate enough that the metaphor has stuck.
Invisible until something breaks. Then DNS becomes the most important system in the world for the next four hours while you wait for changes to propagate and your customers can’t reach the site.
How a DNS lookup actually works
Every time someone visits penfriend.ai, this happens in milliseconds:
The browser asks the local DNS resolver (usually run by the user’s ISP or a public service like Cloudflare 1.1.1.1) for the IP address of penfriend.ai. If the resolver has the answer cached, it returns immediately. If not, it asks the DNS root servers, which point to the .ai TLD servers, which point to penfriend.ai’s authoritative nameservers, which return the actual IP address. The browser then connects to that IP to load the site.
This whole chain typically completes in under 50 milliseconds. When it doesn’t - when one of those servers is slow or down - every subsequent request waits.
The record types that actually matter
A record. Maps a domain name to an IPv4 address. The most common type.
AAAA record. Same thing for IPv6 addresses.
CNAME. Aliases one domain to another. www.penfriend.ai โ penfriend.ai.
MX records. Where email for the domain should be delivered. Critical for email sending and receiving.
TXT records. Arbitrary text - used for SPF, DKIM, DMARC email authentication, domain verification with services, and various other plumbing.
NS records. Which servers are authoritative for the domain. Defines who answers DNS questions about your zone.
What kills sites via DNS
Three patterns I’ve seen take down real businesses:
TTL set too high before a migration. TTL (time to live) tells resolvers how long to cache an answer. Set to 24 hours, then changing the IP address means up to 24 hours of mixed routing. Lower TTLs to 5 minutes 48 hours before a migration; raise back after.
Domain expiration nobody noticed. Auto-renew failed because the credit card on file expired. Domain lapses. Site disappears. Email stops. Recovery can take days even after payment because of WHOIS and registration cycles.
Nameserver changes that break MX records. Someone moves DNS to a new provider but doesn’t migrate the MX records. Email starts bouncing six hours later. Often goes unnoticed because outbound email continues working - only inbound fails.
An example
A B2B SaaS company switched DNS providers from GoDaddy to Cloudflare on a Friday evening - confident it would propagate over the weekend. The migration moved A records but missed the SPF and DMARC TXT records.
By Monday morning, outbound transactional emails (password resets, signup confirmations, invoice receipts) were going to spam at most major providers because authentication had broken. Customers couldn’t sign up because they never got the confirmation email. Support tickets exploded.
The fix took 20 minutes once someone realised what had happened. Diagnosis took 8 hours because nobody had checked DMARC after the migration. Approximate revenue impact from missed signups: about $14K. The DNS migration itself was correctly done; the omission was a single record type.
DNS rewards completeness and unforgivingly punishes partial work.
Related terms
- Domain - the human-readable name DNS resolves
- Domain Authority - an SEO metric tied to the domain DNS serves
- Canonical URL - the SEO directive that operates on the domain DNS routes to
- CSS - one of many resources DNS lookups underpin the loading of
- BIMI - an email-deliverability protocol that depends on DNS records