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) (Recommended)

CyberFOX DNS-over-HTTPS (DoH): The primary workaround for CGNAT environments. DoH encrypts DNS traffic and routes it through a unique per-device or per-profile URL, ensuring filtering policies are applied correctly regardless of shared IP addresses. This enables granular control, per-device reporting, and consistent policy enforcement without relying on source IP identification. See: Roaming Devices

Deploy the CyberFOX DNS Agent (Recommended for managed Windows endpoints)

CyberFOX DNS Agent: A lightweight Windows client installed on endpoints that enforces DNS filtering policies per device, regardless of the network or IP address. Like DoH, it bypasses the IP-based identification problem entirely by binding filtering to the device rather than the network. See: Agent-Based Devices

Switch to IPv6 Where Available (Long-term option)

IPv6 provides a unique public IP for each device, which would eliminate the shared-IP problem at the network level. However, this requires full end-to-end IPv6 support from your ISP and all network infrastructure, and is not a practical immediate solution for most environments. If your ISP supports IPv6 and your infrastructure is ready, enabling it can complement the filtering improvements above — but DoH or the DNS Agent should be the primary fix for CGNAT.