Relationship Between Firewall Rules and Routes in GCP

Overview

In cloud computing, Google Cloud Platform (GCP) provides a robust and flexible infrastructure for managing networks, enabling organizations to deploy, scale, and secure their applications efficiently.

This blog delves into the relationship between firewall rules and routes, offering a comprehensive understanding of their functionalities, interplay, and best practices for configuration.

Firewall Rules in GCP

Firewall rules in GCP are policies that define how incoming and outgoing traffic is allowed or denied to and from your virtual machine (VM) instances. They operate at the network level and are configured for Virtual Private Cloud (VPC) networks. These rules apply to all instances within a network unless explicitly overridden by more specific rules.

Key Features of Firewall Rules

1. Direction:
   1. Ingress Rules: Control incoming traffic to instances.
   2. Egress Rules: Control outgoing traffic from instances.
2. Priority:
   1. Rules have a priority value (0 to 65535), with lower numbers indicating higher priority.
   2. Traffic is evaluated against the rules in ascending order of priority.
3. Action:
   1. Allow: Permits the traffic.
   2. Deny: Blocks the traffic.
4. Targets:
   1. Specify the instances to which the rule applies. Targets can be defined using tags, service accounts, or all instances.
5. Source/Destination:
   1. Specify IP ranges, tags, or service accounts for ingress and egress rules.
6. Protocols and Ports:
   1. Define specific protocols (e.g., TCP, UDP, ICMP) and ports for the rule.

Fig . 1 – VPC firewall rules | Cloud NGFW | Google Cloud

Example:

To allow SSH traffic (TCP port 22) from a specific IP range (e.g., 203.0.113.0/24):

Direction: Ingress
Source Range: 203.0.113.0/24
Protocol: TCP
Port: 22
Action: Allow

Routes in GCP

Routes in GCP determine how packets are directed from one instance to another or external destinations. They operate at the network layer and use a combination of destination IP ranges and next-hop configurations to guide traffic.

Key Features of Routes

1. Destination Range:
   10. Specifies the IP range for which the route applies (e.g., 10.0.0.0/16).
2. Priority:
   1. Routes are also prioritized (0 to 65535), with lower numbers indicating higher precedence.
3. Next Hop:
   1. Defines where the traffic should go. Common next-hop types include:
      1. Default Internet Gateway: For traffic destined to the internet.
      2. Instance: For traffic routed to a specific VM instance.

• VPN Tunnel: For traffic routed through a VPN.

1. Peering: For traffic routed through Amazon VPC peering connections.

4. Route Type:
   1. System-generated routes: Automatically created by GCP (e.g., a default route to the internet).
   2. Custom routes: Created manually by users for specific purposes.

Example:

To route traffic for the 10.1.0.0/16 subnet to a specific VM instance:

Destination Range: 10.1.0.0/16
Next Hop: VM Instance (e.g., my-instance)
Priority: 100

Fig. 2 – Dynamic Routing Example

The Interplay Between Firewall Rules and Routes

The relationship between firewall rules and routes is foundational to network traffic management in GCP. Both elements work together to ensure that traffic is directed correctly and securely.

Key Points of Interaction:

1. Traffic Flow Evaluation:
   1. Routes determine where traffic should go.
   2. Firewall rules determine whether the traffic is allowed to proceed.
2. Order of Operations:
   1. When a packet arrives at a VM:
      1. GCP evaluates the route to determine the next hop.
      2. The packet is checked against applicable firewall rules to decide whether it should be allowed or denied.
   2. Default Configurations:
      1. By default, GCP creates certain system routes and firewall rules:
         1. System Routes: Includes a default route for internet-bound traffic (0.0.0.0/0).
         2. Default Firewall Rules: Allow internal communication within the Amazon VPC and block all other traffic.
      2. Overlap Scenarios:
         1. The traffic is denied if a route directs traffic to a destination, but a firewall rule blocks it.
         2. Conversely, the traffic is dropped if a firewall rule allows traffic, but no route exists for the destination.

Practical Example

Suppose you want to allow HTTP traffic from the internet to a web server instance in your Amazon VPC:

1. Route Configuration:
   1. Ensure there is a default internet route (0.0.0.0/0) with a next hop set to the default internet gateway.
2. Firewall Rule Configuration:
   1. Create an ingress rule allowing TCP traffic on port 80 from source 0.0.0.0/0.

Without the route and firewall rule, traffic will not reach the web server.

Best Practices for Configuring Firewall Rules and Routes

1. Follow the Principle of Least Privilege:
   1. Configure firewall rules to allow only necessary traffic. Avoid overly permissive rules, such as wide-open ingress rules.
2. Leverage Tags and Service Accounts:
   1. Use instance tags and service accounts to apply firewall rules selectively, ensuring better management and security.
3. Use Custom Routes Wisely:
   1. Avoid unnecessary custom routes to reduce complexity. Use them only when default routes do not meet your needs.

Common Misconfigurations and Troubleshooting

1. Blocked Traffic Despite Open Routes:
   1. Cause: Firewall rules are too restrictive.
   2. Solution: Check ingress and egress rules for the affected traffic.
2. Traffic Dropped Despite Open Firewall Rules:
   1. Cause: Missing or incorrect routes.
   2. Solution: Verify that routes exist for the destination and that the correct next hop is available.
3. Asymmetric Routing:
   1. Cause: Incorrect route prioritization or misconfigured next hops.
   2. Solution: Ensure consistent route configurations across all VPC subnets.
4. Excessive Open Access:
   1. Cause: Overly permissive firewall rules.
   2. Solution: Restrict rules to specific IP ranges, ports, and protocols.

Conclusion

The interplay between firewall rules and routes in GCP forms the backbone of a secure and efficient network infrastructure. While routes define the paths traffic can take, firewall rules ensure that only authorized traffic flows through these paths. Understanding how these components work together is essential for architects, administrators, and engineers tasked with designing and managing GCP environments.

By following best practices, avoiding common misconfigurations, and continuously monitoring the network, you can leverage the full potential of GCP’s networking capabilities to build secure, scalable, and resilient systems.

Drop a query if you have any questions regarding GCP and we will get back to you quickly.

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