Denis Kirevby Denis Kirev

How DNS Works: A Deep Dive

Understanding the Domain Name System (DNS), its components, and how it enables internet communication.

5 min read
NETWORKWEBGUIDE
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1. What is DNS?

DNS (Domain Name System) is a system that translates domain names (e.g., google.com) into IP addresses (e.g., 142.250.74.238) needed to establish a connection between a client and a server.

🔹 Why is DNS needed?
Instead of memorizing IP addresses, users enter easy-to-remember human-friendly names, and DNS automatically performs the translation (resolution).

2. How Does a DNS Query Work? 🔄

When you enter google.com in your browser, DNS goes through four lookup stages:

1. Checking Local Cache

The browser and operating system first check if the IP address for google.com is already cached.

2. Querying the Recursive DNS Resolver

If the IP is not found in the cache, the request is sent to a recursive DNS server (usually provided by the ISP, or services like Google DNS 8.8.8.8, Cloudflare 1.1.1.1).

3. Hierarchical DNS Lookup

The DNS server performs sequential queries:

  1. Root servers (.) → Directs the query to the .com server.
  2. TLD servers (.com) → Directs the query to google.com's name server.
  3. Authoritative server for google.com → Returns the final IP address.

4. Returning the IP Address

The recursive resolver receives the IP, caches it, and sends it to the browser, which then establishes a connection with the server.

3. Types of DNS Queries 🕵️‍♂️

  • Recursive – The server continues querying until it finds the IP.
  • Iterative – The server returns the closest available data but does not query further.

4. Main Types of DNS Records 📌

DNS stores not only IP addresses but also other important records.

| Type | Description | Example | | -------- | ------------------------------------------ | ----------------------------------------------------- | | A | IPv4 address of a domain | google.com → 142.250.74.238 | | AAAA | IPv6 address of a domain | google.com → 2607:f8b0::200e | | CNAME | Alias (redirection) | www.example.com → example.com | | MX | Mail servers | mail.example.com → 10.20.30.40 | | TXT | Text data (SPF, DKIM, domain verification) | example.com → "v=spf1 include:_spf.google.com ~all" | | NS | Specifies authoritative DNS servers | example.com → ns1.dnsprovider.com | | SRV | Defines services (e.g., SIP, XMPP) | _sip._tcp.example.com | | PTR | Reverse lookup (IP → Domain) | 40.30.20.10 → mail.example.com |

5. DNS Caching and TTL (Time To Live)

DNS records are not queried every time; they are cached at different levels:

  • Browser (Chrome, Firefox) – stores cache for a few minutes.
  • OS (Windows, macOS, Linux) – stores cache for several hours.
  • Intermediate DNS servers (ISPs) – cache records according to TTL.

How to Check DNS Cache?

# Windows
ipconfig /displaydns

# macOS
sudo killall -HUP mDNSResponder && sudo dscacheutil -flushcache

# Linux
systemd-resolve --statistics

How to Check a Domain's TTL?

nslookup -type=a google.com

OR

dig google.com +noall +answer

👉 TTL = 300 means the record will be cached for 5 minutes.

6. DNS Issues and Their Solutions

1. "DNS_PROBE_FINISHED_NXDOMAIN"

❌ Error: Domain name not found.
✅ Solution:

  • Check if the domain name is entered correctly.
  • Flush the DNS cache (ipconfig /flushdns).
  • Use a different DNS server (8.8.8.8, 1.1.1.1).

2. DNS Record Not Updating

❌ Error: DNS changes are not taking effect.
✅ Solution:

  • Wait up to 48 hours (due to caching).
  • Force clear the DNS cache (flushdns).
  • Use nslookup or dig to check new values.

3. Slow Domain Resolution

❌ Error: Slow DNS response time.
✅ Solution:

  • Use faster DNS servers (Google, Cloudflare).
  • Check TTL and lower it before updating records.
  • Check for ISP (provider) issues.

7. Fast Public DNS Servers (Alternatives to ISP DNS)

| Provider | IPv4 | IPv6 | Features | | -------------- | -------------------- | ---------------------- | ----------------------------- | | Google | 8.8.8.8, 8.8.4.4 | 2001:4860:4860::8888 | Fast, stable | | Cloudflare | 1.1.1.1, 1.0.0.1 | 2606:4700:4700::1111 | Fastest (1ms response) | | OpenDNS | 208.67.222.222 | 2620:0:ccc::2 | Parental control | | Quad9 | 9.9.9.9 | 2620:fe::fe | Security, phishing protection |

📌 How to Change DNS in the System?

  1. Windows: Control Panel → Network Connections → IPv4 Properties.
  2. macOS: System Preferences → Network → Advanced → DNS.
  3. Linux: Edit /etc/resolv.conf.

8. DNS over HTTPS (DoH) and DNS over TLS (DoT)

🔒 Regular DNS queries are unencrypted, allowing ISPs to track them.

✅ Solution:

  • DNS over HTTPS (DoH) – Encrypts DNS queries via HTTPS.
  • DNS over TLS (DoT) – Encrypts DNS at the TCP level.

📌 How to Enable DoH?

  • Google Chrome: Enable "Secure DNS" in settings.
  • Firefox: Enable DoH in settings → Cloudflare DNS.
  • Android: Use 1.1.1.1 App by Cloudflare.

9. DNS vs. CDN (Cloudflare, Akamai, AWS Route 53)

Many companies use CDNs (Content Delivery Networks) to speed up website loading.

✅ CDNs use Anycast DNS, allowing queries to be directed to the nearest server instead of a single fixed IP.

Example with dig:

dig google.com

👉 Google will return different IP addresses depending on your location.

10. Conclusion

✅ DNS is the "address book of the internet," translating domains into IP addresses.
✅ The browser's Event Loop can be affected by DNS queries if caching is not used.
✅ Cloudflare 1.1.1.1 and Google 8.8.8.8 are the best ISP DNS alternatives.
✅ Use DNS over HTTPS (DoH) to protect against query interception.

Last updated: February 13, 2025