What is DNS? How the Domain Name System Works

What is DNS?

DNS, or Domain Name System, is a globally distributed naming system for the internet. It translates human-readable domain names into IP addresses.

Without DNS, you would need to remember addresses like 142.250.80.46 instead of names like google.com.

DNS was created by Paul Mockapetris in the 1980s and standardized in RFC 1034 and RFC 1035. It replaced the old HOSTS.TXT approach that could not scale with the growing internet.

Why DNS Exists

DNS exists because humans prefer names, while networks route traffic using numbers. It provides:

• Human-friendly names - type github.com instead of an IP address.
• Indirection - a domain can point to a new server without changing every bookmark or link.
• Load balancing - one name can resolve to multiple IP addresses.
• Email routing - MX records tell mail servers where to deliver mail.
• Domain verification - TXT records prove domain ownership for services, certificates, SPF, DKIM, and DMARC.

How DNS Resolution Works

When your browser needs an IP address, a recursive resolver walks the DNS hierarchy for you.

You type: www.example.com

1. Browser cache
   If the answer is cached and not expired, use it.

2. Operating system cache
   The OS checks its DNS cache and hosts file.

3. Recursive resolver
   Your ISP resolver or a public resolver checks its cache.

4. Root name server
   Resolver asks: where are the .com servers?

5. TLD name server
   Resolver asks .com: where are example.com's nameservers?

6. Authoritative name server
   Resolver asks example.com's nameserver: what is www.example.com?

7. Answer
   Resolver caches the result and returns the IP address to your device.

The DNS Hierarchy

DNS is organized like an inverted tree, read from right to left through the domain name.

                    . (root)
                    |
           ---------------------
          .com       .org      .net       top-level domains
           |          |         |
       example    wikipedia   cloud      second-level domains
           |          |         |
         www         en        api       subdomains

Full domain: www.example.com.
The final dot represents the DNS root.

DNS Record Types Explained

DNS stores different record types for different jobs:

A      example.com.       93.184.216.34
AAAA   example.com.       2606:2800:220:1:248:1893:25c8:1946
CNAME  www.example.com.   example.com.
MX     example.com.       10 mail.example.com.
TXT    example.com.       "v=spf1 include:_spf.example.com ~all"
NS     example.com.       ns1.example.com.

DNS Caching and TTL

DNS answers are cached to make lookups faster. The cache duration is controlled by TTL, or Time To Live, measured in seconds.

• Short TTL values, such as 300 seconds, help changes take effect quickly.
• Long TTL values, such as 3600 or 86400 seconds, reduce DNS traffic and improve speed.
• Before a planned migration, lower TTL 24-48 hours ahead of time, then raise it again after the change is stable.

DNS Propagation

DNS propagation is the time it takes for old cached answers to expire across resolvers worldwide.

DNS Security

Classic DNS was not designed with privacy or authentication in mind. Modern extensions improve different parts of the system:

Without DNSSEC:
  Resolver asks: What is example.com?
  Attacker replies: It is 6.6.6.6
  Resolver has no built-in way to verify the answer.

With DNSSEC:
  Resolver checks cryptographic signatures.
  Fake answers fail validation and are rejected.

For best protection, DNSSEC helps with authenticity while DoH or DoT helps with privacy in transit.

Public DNS Resolvers

You can use a public resolver instead of your ISP resolver. Common choices include:

Troubleshooting DNS Issues

DNS problems can make a working server look down. Common causes include:

Useful DNS Commands

These commands are useful for debugging DNS from a terminal:

# nslookup
nslookup example.com
nslookup -type=MX example.com
nslookup example.com 8.8.8.8

# dig
dig example.com A
dig example.com MX
dig @8.8.8.8 example.com
dig +trace example.com
dig +short example.com

# Flush DNS cache on Windows
ipconfig /flushdns