Fine-Grained Outbound Control: How Google Cloud Cloud NAT Rules Enhance Security and Compliance

In a standard Cloud NAT setup, all outbound traffic from a VPC shares the same pool of public IP addresses. While simple, this creates several issues in complex or regulated environments:

• Strict Whitelisting Issues: Many critical external services like payment processors, partner APIs, and security systems enforce strict firewall policies, requiring whitelisting of specific public IPs. Without rules, your private VMs would use rotating or shared IPs, forcing whitelisting of all or risking connection failures.
• Lack of Traffic Segmentation: All internet-bound traffic looks the same to the outside world. There’s no way to easily segment or audit for a specific purpose.
• Audit and Compliance Gaps: In regulated industries, outbound connections must sometimes be traced back to specific applications or environments. When all traffic uses a unified IP pool, this traceability is lost on the public internet, complicating audit trails and regulatory compliance.

Cloud NAT rules solve these problems by introducing conditional logic to the NAT process. A rule defines a match condition (based on source or destination) and a corresponding action (which specific NAT IP address to use). Once NAT rules are defined, each packet is tested against them. If a packet meets the condition of a rule, the specified action is executed.

The key solutions offered by Cloud NAT rules are (Only IPv4 destination addresses are supported):

Source-Based Traffic Segmentation ( Preview): In source-based rules, packets are matched by their source IP address ( the internal IP range of a specific subnet).

“IF the traffic is coming from the10.10.20.0/24 subnet (our testing environment), THEN use the public NAT IP203.0.113.50 ."

This is invaluable for traffic segmentation. It allows you to assign a dedicated, easily identifiable public IP to specific applications, environments (like Dev/Test/Prod), or high-priority subnets, making auditing and security monitoring significantly easier.

Sample source-based rules

Destination-Based IP Whitelisting: This is the most common and powerful application. In destination-based rules, packets are matched by their destination IP address.

“IF the destination IP is within the198.51.100.0/24 range (our partner's API), THEN use the dedicated public NAT IP203.0.113.20 ."

This ensures that only specific public IPs are used for whitelisted services, with all other traffic defaulting to the NAT IP pool. It solves the whitelisting issue without compromising general connectivity.

Sample destination-based rules

Enhanced Control and Flexibility

Rules introduce a priority system, allowing network administrators to define policies with precision, ensuring that the most specific and critical rules are evaluated first. This introduces a policy-driven, declarative approach to managing outbound access for privately addressed resources.

To use NAT rules, follow these general steps:

1. Ensure your NAT gateway is using manual IP allocation (rather than automatic).
2. Disable endpoint-independent mapping, since it is incompatible with NAT rules.
3. Define your rules, giving them a priority, specifying whether they match by source or destination, and indicate which external IPs to use. .
4. Deploy rules via the Cloud Console, gcloud CLI, or IaC tools.
5. Monitor, update, or delete rules as your policies evolve.

Minimal gcloud example (source-based rule):

gcloud beta compute routers nats rules create 1000 \
  --router=MY_ROUTER \
  --region=MY_REGION \
  --nat=MY_NAT \
  --match='inIpRange(source.ip, "10.10.1.0/24")' \
  --source-nat-active-ips=EXTERNAL_IP_1 \
  --project=MY_GCP_PROJECT

You can similarly update or delete rules using gcloud compute routers nats rules update/delete commands.

• Carefully design priorities to avoid unintended overlaps.
• Always define a default fallback rule, ensuring coverage for unmatched traffic.
• Monitor active ports and NAT capacity to prevent exhaustion.
• Audit regularly to confirm that rule-based segmentation aligns with compliance requirements.