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DNS - Domain Name System 🧭

Table of Contents

Introduction to DNS 🌐

DNS (Domain Name System) is the backbone of the internet, translating human-readable domain names like example.com into machine-readable IP addresses like 192.0.2.1. This system allows users to access websites using easy-to-remember names rather than numeric IP addresses.

Installing and Configuring DNS 🛠️

To set up a DNS server on Linux, follow these steps:

1. Install the DNS Server Software

Depending on your Linux distribution, the installation command may vary:

  • Debian/Ubuntu:
sudo apt install bind9
  • CentOS/RHEL: 
    sudo yum install bind bind-utils

2. Configure the DNS Server

After installation, the DNS server needs to be configured. This involves editing the main configuration file and creating zone files.

  • Edit /etc/named.conf:
    This file is the main configuration file where server settings and zones are defined. Open it with your preferred text editor:
sudo nano /etc/named.conf
  • Example configuration:
options {
    directory "/var/named";
    allow-query { any; };
    recursion yes;

    # 🧭 Listen on specified IP addresses
    listen-on port 53 { 127.0.0.1; 192.0.2.1; };
};

zone "example.com" IN {
    type master;
    file "/var/named/example.com.zone";
};

zone "2.0.192.in-addr.arpa" IN {
    type master;
    file "/var/named/2.0.192.rev";
};
  • listen-on: The listen-on directive tells the DNS server which IP addresses to listen on for DNS queries. In this example, the server listens on 127.0.0.1 (localhost) and 192.0.2.1.

3. Start and Enable the DNS Service

After configuring, start the DNS service and enable it to start on boot:

sudo systemctl start named
sudo systemctl enable named

Creating a DNS Server with a Shell Script 🤖

Here’s a shell script that automates the setup of a basic DNS server with 10 dummy websites, including the listen-on directive:

#!/bin/bash

# 🛠️ Install BIND DNS server
echo "Installing BIND DNS server..."
sudo apt install -y bind9

# 📝 Backup the original configuration
echo "Backing up the original configuration..."
sudo cp /etc/named.conf /etc/named.conf.backup

# ⚙️ Basic configuration setup
echo "Configuring DNS server..."
sudo tee /etc/named.conf > /dev/null <<EOL
options {
    directory "/var/named";
    allow-query { any; };
    recursion yes;

    # 🧭 Listen on specified IP addresses
    listen-on port 53 { 127.0.0.1; 192.0.2.1; };
};

zone "example.com" IN {
    type master;
    file "/var/named/example.com.zone";
};

zone "2.0.192.in-addr.arpa" IN {
    type master;
    file "/var/named/2.0.192.rev";
};
EOL

# 🗂️ Create zone files
echo "Creating zone files..."

# Forward Zone File
sudo tee /var/named/example.com.zone > /dev/null <<EOL
\$TTL 86400
@   IN  SOA     ns1.example.com. admin.example.com. (
        2024090101  ; Serial
        3600        ; Refresh
        1800        ; Retry
        1209600     ; Expire
        86400 )     ; Minimum TTL
    IN  NS      ns1.example.com.

ns1 IN  A       192.0.2.1
www IN  A       192.0.2.2
site1 IN A      192.0.2.3
site2 IN A      192.0.2.4
site3 IN A      192.0.2.5
site4 IN A      192.0.2.6
site5 IN A      192.0.2.7
site6 IN A      192.0.2.8
site7 IN A      192.0.2.9
site8 IN A      192.0.2.10
site9 IN A      192.0.2.11
site10 IN A     192.0.2.12
EOL

# Reverse Zone File
sudo tee /var/named/2.0.192.rev > /dev/null <<EOL
\$TTL 86400
@   IN  SOA     ns1.example.com. admin.example.com. (
        2024090101  ; Serial
        3600        ; Refresh
        1800        ; Retry
        1209600     ; Expire
        86400 )     ; Minimum TTL
    IN  NS      ns1.example.com.

1   IN  PTR     ns1.example.com.
2   IN  PTR     www.example.com.
3   IN  PTR     site1.example.com.
4   IN  PTR     site2.example.com.
5   IN  PTR     site3.example.com.
6   IN  PTR     site4.example.com.
7   IN  PTR     site5.example.com.
8   IN  PTR     site6.example.com.
9   IN  PTR     site7.example.com.
10  IN  PTR     site8.example.com.
11  IN  PTR     site9.example.com.
12  IN  PTR     site10.example.com.
EOL

# 🔄 Restart DNS service to apply changes
echo "Restarting DNS service..."
sudo systemctl restart named

echo "DNS server setup complete with 10 dummy websites."

Explanation of the Script:

  1. Installation: The script installs the bind9 package, which is the DNS server software.
  2. Configuration: It includes the listen-on directive in /etc/named.conf to specify that the DNS server should listen on 127.0.0.1 and 192.0.2.1.
  3. Zone Files: The script generates two zone files:
  4. Forward Zone File: Maps hostnames (ns1, www, site1, etc.) to IP addresses (192.0.2.1 to 192.0.2.12).
  5. Reverse Zone File: Maps IP addresses back to their corresponding hostnames.
  6. Restart DNS Service: The DNS service is restarted to apply the new configuration and zone files.

This script creates a DNS setup with 10 dummy websites (site1.example.com to site10.example.com) and dummy IP addresses. The DNS server listens on the specified IP addresses (127.0.0.1 and 192.0.2.1) for incoming DNS queries.

Types of Hostname Records 📝

  • PTR Record (IP to Hostname): Maps an IP address to a hostname, typically used for reverse DNS lookups.

Example:

2.0.192.in-addr.arpa. IN PTR www.example.com.
  • A Record (Hostname to IP): Maps a hostname to an IP address.

Example:

www.example.com. IN A 192.0.2.2
  • CNAME Record (Hostname to Hostname): Maps one hostname to another, useful for domain aliasing.

Example:

mail.example.com. IN CNAME www.example.com.

Key Configuration Files 📂

  • /etc/named.conf: The primary configuration file for the BIND DNS server. It includes options, logging settings, and zone declarations.

  • /var/named: The directory where zone files are stored. These files contain DNS records for the domains managed by the server.

Managing the DNS Service ⚙️

Restarting the DNS Service

Whenever you make changes to the configuration, restart the DNS service to apply them:

sudo systemctl restart named

Use systemctl commands to manage the service status, start, stop, and enable it on boot.

Example Zone File with a Table Format 🗂️

Here's an example of how to organize DNS records in a zone file using

a table format for clarity:

Zone File Example: example.com.zone

Website Name IP Address Record Type Record
ns1.example.com 192.0.2.1 A ns1 IN A 192.0.2.1
www.example.com 192.0.2.2 A www IN A 192.0.2.2
site1.example.com 192.0.2.3 A site1 IN A 192.0.2.3
site2.example.com 192.0.2.4 A site2 IN A 192.0.2.4
site3.example.com 192.0.2.5 A site3 IN A 192.0.2.5
site4.example.com 192.0.2.6 A site4 IN A 192.0.2.6
site5.example.com 192.0.2.7 A site5 IN A 192.0.2.7
site6.example.com 192.0.2.8 A site6 IN A 192.0.2.8
site7.example.com 192.0.2.9 A site7 IN A 192.0.2.9
site8.example.com 192.0.2.10 A site8 IN A 192.0.2.10
site9.example.com 192.0.2.11 A site9 IN A 192.0.2.11
site10.example.com 192.0.2.12 A site10 IN A 192.0.2.12