Carrier Grade NAT (CGNAT) and DNS Filtering Issues

Carrier Grade NAT (CGNAT) is a technique used by Internet Service Providers (ISPs) to conserve IPv4 addresses by allowing multiple customers to share a single public IP address. While CGNAT helps mitigate IPv4 exhaustion, it introduces several challenges, especially in the context of DNS filtering and network security.

What is Carrier Grade NAT (CGNAT)?

CGNAT, also known as Large Scale NAT (LSN), is a type of Network Address Translation where the ISP assigns private IP addresses to customer devices and translates them to a shared public IP address at the ISP level.

Key Characteristics:

• Multiple users share a single public IP.
• NAT occurs at the ISP level, not just within the home or business network.
• Often used in mobile networks and some residential broadband services.

Issues CGNAT Causes with DNS Filtering

DNS filtering relies on identifying and blocking or redirecting DNS queries based on the source IP address or domain name. CGNAT complicates this process in several ways:

Loss of Source IP Granularity

Since multiple users share a single public IP, DNS filtering systems may not accurately identify which user made a specific request.

Rate Limiting and Blocking

DNS filtering services may rate-limit or block requests from a CGNAT-shared IP due to perceived abuse or high traffic volume.

Inconsistent Policy Enforcement

Filtering policies tied to IP addresses may apply incorrectly to multiple users behind the same CGNAT IP.

Logging and Auditing Challenges

Logs may not accurately reflect individual user activity, which can complicate compliance and security audits.

Workarounds and Solutions

Use DNS-over-HTTPS (DoH)

CyberFOX DNS-over-HTTPS (DoH): A secure DNS resolution method that encrypts DNS traffic and ensures filtering policies are applied even when traditional DNS is blocked or intercepted by ISPs.This will enable granular control, centralized management, and consistent policy enforcement across all devices.

Deploy Client-Based DNS Filtering

CyberFOX DNS Agent: A lightweight client installed on endpoints that enforces DNS filtering policies per device, regardless of the network or IP address. This will allow for granular control, centralized management, and consistent policy enforcement across all devices.

Switch to IPv6 Where Available

IPv6 provides unique public IPs for each device, eliminating the need for CGNAT. Check if your ISP supports IPv6 and enable it on your router and devices.