A “DNS Server Spoofed Request Amplification DDoS” vulnerability indicates that your DNS server can be exploited to participate in Distributed Denial of Service (DDoS) attacks. Attackers can send DNS queries with a spoofed source IP address (the victim’s IP) to your server, prompting it to respond to the victim with amplified traffic, thereby overwhelming the victim’s resources.

Understanding the Vulnerability:

In this scenario, your DNS server is misconfigured to allow recursive queries from any source. Recursive DNS servers resolve domain names by querying other DNS servers on behalf of the requester. If unrestricted, they can be abused to generate large responses to small queries, amplifying the traffic directed at a victim.

Steps to Mitigate the Vulnerability:

1. Disable Recursion for External Queries:
   • BIND DNS Server:Modify your named.conf file to include:

    options {
        recursion no;
        allow-query { trusted_networks; };
    };

   Replace trusted_networks with the IP ranges that should have access to your DNS server.

   • Microsoft DNS Server:
     • Open the DNS Manager.
     • Right-click the DNS server name and select “Properties.”
     • Go to the “Advanced” tab.
     • Check the “Disable recursion” box.
2. Restrict Queries to Authorized Clients:
   • BIND DNS Server:Define an access control list (ACL) for trusted clients and update your named.conf:

    acl "trusted_clients" {
        192.168.1.0/24;
        10.0.0.0/8;
        // Add other trusted IP ranges
    };
   
    options {
        allow-query { trusted_clients; };
        allow-recursion { trusted_clients; };
    };

   • Microsoft DNS Server:
     • In the DNS Manager, right-click the specific zone and select “Properties.”
     • Go to the “Security” tab.
     • Configure permissions to allow only trusted users and computers.
3. Implement Response Rate Limiting (RRL):
   • BIND DNS Server:Add the following to your options block:

    rate-limit {
        responses-per-second 5;
        window 5;
    };

   This configuration limits the number of responses per second to a single client, mitigating potential abuse.

   • Microsoft DNS Server:As of Windows Server 2016, RRL can be configured using PowerShell:

    Add-DnsServerResponseRateLimiting -ResponsesPerSecond 5 -WindowInSeconds 5

   Adjust parameters as needed.

4. Regularly Update DNS Software:
   • Ensure your DNS server software is up-to-date to benefit from the latest security patches and features.
5. Monitor DNS Traffic:
   • Implement logging and monitoring to detect unusual query patterns or spikes in traffic, which could indicate abuse.

By configuring your DNS server to restrict unauthorized access and implementing rate limiting, you can prevent it from being exploited in amplification attacks. Regular monitoring and updates further enhance your server’s security posture.