Script to configure the DNS server on Debian-based Linux systems at boot

set_dns.sh

#!/bin/bash

# Set this to your desired DNS server
DNS_SERVER="8.8.8.8"

# Set the path for this script
SCRIPT_PATH="/usr/local/bin/set_dns.sh"

# Path to the resolv.conf file
RESOLV_CONF="/etc/resolv.conf"

# Path to the network interfaces file
INTERFACES_FILE="/etc/network/interfaces"

# Function to set DNS server in resolv.conf
set_dns_resolv_conf() {
    echo "nameserver $DNS_SERVER" | sudo tee $RESOLV_CONF
}

# Function to set DNS server in interfaces file
set_dns_interfaces() {
    if ! grep -q "dns-nameservers" $INTERFACES_FILE; then
        echo "dns-nameservers $DNS_SERVER" | sudo tee -a $INTERFACES_FILE
    else
        sudo sed -i "s/dns-nameservers.*/dns-nameservers $DNS_SERVER/" $INTERFACES_FILE
    fi
}

# Set DNS server
set_dns_resolv_conf
set_dns_interfaces

echo "DNS server has been set to $DNS_SERVER"

# Make the script executable
sudo chmod +x "$SCRIPT_PATH"

# Add the script to /etc/rc.local to run on boot
if ! grep -q "$SCRIPT_PATH" /etc/rc.local; then
    sudo sed -i "/^exit 0/i $SCRIPT_PATH" /etc/rc.local
fi

echo "Script has been added to run on boot"

# If this script is not already in the correct location, move it there
if [ "$0" != "$SCRIPT_PATH" ]; then
    sudo mv "$0" "$SCRIPT_PATH"
    echo "Script has been moved to $SCRIPT_PATH"
fi

This script does the following:

1. Sets the DNS server in /etc/resolv.conf.
2. Adds or updates the DNS server in /etc/network/interfaces.
3. Makes itself executable.
4. Adds itself to /etc/rc.local to run on boot.

To use this script:

1. Save it to a file, for example, /usr/local/bin/set_dns.sh.
2. Replace 8.8.8.8 with your desired DNS server IP address.
3. Replace /usr/local/bin/set_dns.sh with the actual path where you saved the script.
4. Run the script with root privileges: sudo bash /usr/local/bin/set_dns.sh.

After running the script and rebooting, your system should use the specified DNS server.