Linux Monitor - DNS Servers Script

#!/bin/bash

# This resource is provided as a convenience for Level users. We cannot 
# guarantee it will work in all environments. Please test before deploying 
# to your production environment. We welcome contributions to our community 
# library

# Level Library
# https://level.io/library/script-linux-monitor-dns-servers

# Expected DNS servers (comma-separated list)
allowed_dns_servers_string="{{cf_dns}}"

# Convert the comma-separated list into an array
IFS=',' read -r -a allowed_dns_servers <<< "$allowed_dns_servers_string"

# Function to check DNS servers
check_dns_servers() {
    # Get current DNS servers from resolv.conf as fallback if nmcli is not installed
    if command -v nmcli &> /dev/null; then
        readarray -t current_dns_servers < <(nmcli dev show | grep 'IP4.DNS' | awk '{print $2}')
    else
        readarray -t current_dns_servers < <(grep "^nameserver" /etc/resolv.conf | awk '{print $2}')
    fi

    echo "Allowed DNS servers: ${allowed_dns_servers[*]}"
    echo "Current DNS servers: ${current_dns_servers[*]}"

    if [[ ${#current_dns_servers[@]} -eq 0 ]]; then
        echo "ALERT: No DNS servers configured"
        exit 1
    fi

    # Compare current DNS servers against allowed list
    for dns in "${current_dns_servers[@]}"; do
        if [[ ! " ${allowed_dns_servers[*]} " =~ " ${dns} " ]]; then
            echo "ALERT: Not all DNS servers are in the allowed list."
            exit 1
        fi
    done

    echo "SUCCESS: DNS servers match the allowed list."
}

# Check DNS servers
check_dns_servers