Understanding DNS: A Complete Guide -

Introduction to DNS
History of DNS
How DNS Works (Step-by-Step)
Real-Life Examples
How to Use DNS in Linux
DNS Client Configuration (Red Hat & Debian-Based)
Setting Up a DNS Server (BIND)
Troubleshooting and Useful Commands
Conclusion

1. Introduction to DNS

DNS, or Domain Name System, is often described as the “phonebook of the internet.” It translates human-friendly domain names (like www.google.com) into IP addresses (like 142.250.64.100) that computers use to identify each other.

Without DNS, you’d have to remember IP addresses instead of domain names — a logistical nightmare.

2. Brief History of DNS

In the early days of ARPANET, name-to-IP mapping was done via a flat file called hosts.txt.
As networks grew, this method became unsustainable.
In 1983, Paul Mockapetris introduced the Domain Name System (DNS) in RFC 882 and 883.
It became a hierarchical, distributed system, forming the backbone of the modern internet.

3. How DNS Works (Step-by-Step)

Let’s say you open your browser and type www.example.com.

Lookup Flow:

Browser checks local DNS cache.
OS checks /etc/hosts (if on Linux).
Sends query to configured DNS server (e.g., 8.8.8.8).
DNS server checks its cache. If not found:
- Queries root name servers.
- Then TLD servers (e.g., .com).
- Then authoritative name server for example.com.
IP address is returned to the client.
Connection proceeds using the resolved IP.

4. Real-Life Examples of DNS

Scenario	DNS Role
Typing a website URL	Resolves domain to IP
Sending an email	Finds mail server via MX records
Using cloud services	Resolves subdomains, APIs, CNAMEs
Internal networks	Private DNS zones for devices and VMs

5. How to Use DNS in Linux

Check Current DNS Settings:

cat /etc/resolv.conf

Sample output:

nameserver 8.8.8.8
nameserver 1.1.1.1

Query DNS Using `dig`, `nslookup`, and `host`

➤ `dig` (most powerful)

dig google.com

Output (simplified):

;; ANSWER SECTION:
google.com.    300 IN A 142.250.64.100

➤ `nslookup`

nslookup google.com

➤ `host`

host google.com

6. DNS Client Configuration in Linux

Red Hat / CentOS / Rocky (NetworkManager-controlled)

Static DNS:

nmcli con mod "System eth0" ipv4.dns "8.8.8.8 1.1.1.1"
nmcli con up "System eth0"

Confirm:

nmcli dev show | grep DNS

Legacy:

Edit /etc/resolv.conf (though NetworkManager may overwrite this)

Debian / Ubuntu (Netplan or systemd-resolved)

With Netplan:

# /etc/netplan/01-netcfg.yaml
network:
  version: 2
  ethernets:
    eth0:
      dhcp4: no
      addresses: [192.168.1.100/24]
      gateway4: 192.168.1.1
      nameservers:
        addresses: [8.8.8.8, 1.1.1.1]

sudo netplan apply

Check DNS:

systemd-resolve --status

7. Setting Up a DNS Server (BIND)

We’ll use BIND9, the most popular open-source DNS server.

Install on Ubuntu/Debian:

sudo apt install bind9 bind9utils bind9-doc

Install on RHEL/CentOS:

sudo dnf install bind bind-utils

Configure Zone:

Edit /etc/bind/named.conf.local (Debian) or /etc/named.conf (RHEL)

zone "example.com" {
    type master;
    file "/etc/bind/zones/db.example.com";
};

Create /etc/bind/zones/db.example.com:

$TTL 86400
@   IN  SOA ns1.example.com. admin.example.com. (
            2024061101 ; Serial
            3600       ; Refresh
            1800       ; Retry
            604800     ; Expire
            86400 )    ; Minimum TTL

@   IN  NS      ns1.example.com.
@   IN  A       192.168.1.100
www IN  A       192.168.1.100

Start and Enable BIND:

sudo systemctl enable named
sudo systemctl start named

8. Troubleshooting and DNS Tools

Command	Use
dig	Full DNS query
host, nslookup	Quick lookups
systemd-resolve –status	Show system DNS in Ubuntu
resolvectl	Modern DNS client in systemd
tcpdump port 53	Capture DNS traffic
journalctl -u named	Logs for BIND

Example: Test Zone File

named-checkzone example.com /etc/bind/zones/db.example.com

9. Conclusion

DNS is more than just a background service — it’s the nervous system of modern networking. Whether you’re configuring a simple client, debugging an issue, or running your own DNS server, understanding DNS gives you powerful control over connectivity, performance, and even security.

From using tools like dig to configuring resolvers and setting up BIND, this guide has walked you through the full stack of DNS in a Linux environment — both Red Hat and Debian-based systems.

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