implementing-gcp-vpc-firewall-rules

Implementing and auditing GCP VPC firewall rules to enforce network segmentation, restrict ingress and egress traffic, apply hierarchical firewall policies across the organization, and monitor firewall rule effectiveness using VPC Flow Logs.

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リポジトリ: AgentFlocks/flocks
ソースの最終更新活動: 2026年5月5日 05:45
検出された SKILL.md の言語: 英語
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name	implementing-gcp-vpc-firewall-rules
description	Implementing and auditing GCP VPC firewall rules to enforce network segmentation, restrict ingress and egress traffic, apply hierarchical firewall policies across the organization, and monitor firewall rule effectiveness using VPC Flow Logs.
domain	cybersecurity
subdomain	cloud-security
tags	["cloud-security","gcp","vpc","firewall-rules","network-security","segmentation"]
version	1.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.IR-01","ID.AM-08","GV.SC-06","DE.CM-01"]

Implementing GCP VPC Firewall Rules

When to Use

When deploying new GCP workloads that require network-level access controls
When auditing existing firewall configurations for overly permissive rules
When implementing zero trust network segmentation within GCP VPC networks
When responding to Security Command Center findings about open firewall rules
When building hierarchical firewall policies across a GCP organization

Do not use for application-layer filtering (use Cloud Armor WAF), for DNS-based filtering (use Cloud DNS response policies), or for VPN/interconnect traffic filtering without understanding that VPC firewall rules apply to traffic within the VPC.

Prerequisites

GCP project with Compute Engine API enabled
IAM roles: roles/compute.securityAdmin for firewall management, roles/compute.networkViewer for auditing
Organization Admin role for hierarchical firewall policies
gcloud CLI authenticated with appropriate permissions
VPC Flow Logs enabled on target subnets for monitoring

Workflow

Step 1: Audit Existing Firewall Rules for Security Gaps

Enumerate all firewall rules and identify overly permissive configurations.

# List all firewall rules in the project
gcloud compute firewall-rules list \
  --format="table(name, network, direction, priority, allowed[].map().firewall_rule().list():label=ALLOWED, sourceRanges, targetTags)"

# Find rules allowing all traffic from 0.0.0.0/0
gcloud compute firewall-rules list \
  --filter="direction=INGRESS AND sourceRanges=0.0.0.0/0" \
  --format="table(name, network, allowed, priority, targetTags)" \
  --sort-by=priority

# Find rules allowing all protocols and ports
gcloud compute firewall-rules list \
  --filter="direction=INGRESS AND allowed[].IPProtocol=all" \
  --format="table(name, network, sourceRanges, targetTags)"

# Find rules with SSH (22) or RDP (3389) open to the internet
gcloud compute firewall-rules list \
  --filter="direction=INGRESS AND sourceRanges=0.0.0.0/0 AND (allowed[].ports=22 OR allowed[].ports=3389)" \
  --format="table(name, network, allowed, sourceRanges)"

# Check for disabled rules
gcloud compute firewall-rules list \
  --filter= \
  --format=

# Create rule allowing HTTPS from the internet to web servers only
gcloud compute firewall-rules create allow-https-web \
  --network=production-vpc \
  --direction=INGRESS \
  --action=ALLOW \
  --rules=tcp:443 \
  --source-ranges=0.0.0.0/0 \
  --target-tags=web-server \
  --priority=1000 \
  --description="Allow HTTPS to web servers from internet"

# Create rule allowing SSH only from bastion host subnet
gcloud compute firewall-rules create allow-ssh-bastion \
  --network=production-vpc \
  --direction=INGRESS \
  --action=ALLOW \
  --rules=tcp:22 \
  --source-ranges=10.0.1.0/24 \
  --target-tags=ssh-allowed \
  --priority=1000 \
  --description="Allow SSH only from bastion subnet"

# Create rule allowing internal communication between app tiers
gcloud compute firewall-rules create allow-app-to-db \
  --network=production-vpc \
  --direction=INGRESS \
  --action=ALLOW \
  --rules=tcp:5432 \
  --source-tags=app-server \
  --target-tags=db-server \
  --priority=1000 \
  --description="Allow PostgreSQL from app tier to database tier"

# Create service-account-based rule (more secure than tags)
gcloud compute firewall-rules create allow-api-internal \
  --network=production-vpc \
  --direction=INGRESS \
  --action=ALLOW \
  --rules=tcp:8080 \
  --source-service-accounts=api-client@project.iam.gserviceaccount.com \
  --target-service-accounts=api-server@project.iam.gserviceaccount.com \
  --priority=1000

# Deny all egress by default (low priority)
gcloud compute firewall-rules create deny-all-egress \
  --network=production-vpc \
  --direction=EGRESS \
  --action=DENY \
  --rules=all \
  --destination-ranges=0.0.0.0/0 \
  --priority=65534 \
  --description="Default deny all egress traffic"

# Allow egress to Google APIs via restricted VIP
gcloud compute firewall-rules create allow-google-apis \
  --network=production-vpc \
  --direction=EGRESS \
  --action=ALLOW \
  --rules=tcp:443 \
  --destination-ranges=199.36.153.4/30 \
  --priority=1000 \
  --description="Allow HTTPS to Google APIs restricted VIP"

# Allow DNS resolution
gcloud compute firewall-rules create allow-dns-egress \
  --network=production-vpc \
  --direction=EGRESS \
  --action=ALLOW \
  --rules=udp:53,tcp:53 \
  --destination-ranges=169.254.169.254/32,8.8.8.8/32,8.8.4.4/32 \
  --priority=1000 \
  --description="Allow DNS resolution to metadata and Google DNS"

# Allow egress to specific external services
gcloud compute firewall-rules create allow-external-apis \
  --network=production-vpc \
  --direction=EGRESS \
  --action=ALLOW \
  --rules=tcp:443 \
  --destination-ranges=PARTNER_CIDR/32 \
  --target-tags=api-client \
  --priority=1000

# Create an organization-level firewall policy
gcloud compute firewall-policies create \
  --organization=ORG_ID \
  --short-name=org-security-policy \
  --description="Organization-wide security firewall policy"

# Add rule to block known malicious IP ranges at org level
gcloud compute firewall-policies rules create 100 \
  --firewall-policy=org-security-policy \
  --organization=ORG_ID \
  --direction=INGRESS \
  --action=deny \
  --src-ip-ranges=THREAT_INTEL_CIDR_1,THREAT_INTEL_CIDR_2 \
  --layer4-configs=all \
  --description="Block known malicious IPs organization-wide"

# Add rule to enforce HTTPS-only ingress at org level
gcloud compute firewall-policies rules create 200 \
  --firewall-policy=org-security-policy \
  --organization=ORG_ID \
  --direction=INGRESS \
  --action=allow \
  --src-ip-ranges=0.0.0.0/0 \
  --layer4-configs=tcp:443 \
  --description="Allow only HTTPS from external sources"

# Associate policy with the organization
gcloud compute firewall-policies associations create \
  --firewall-policy=org-security-policy \
  --organization=ORG_ID

# Enable flow logs on a subnet
gcloud compute networks subnets update production-subnet \
  --region=us-central1 \
  --enable-flow-logs \
  --logging-aggregation-interval=interval-5-sec \
  --logging-flow-sampling=1.0 \
  --logging-metadata=include-all

# Query flow logs in Cloud Logging for denied traffic
gcloud logging read '
  resource.type="gce_subnetwork"
  AND jsonPayload.disposition="DENIED"
  AND timestamp>="2026-02-22T00:00:00Z"
' --limit=50 --format=json

# Find traffic hitting overly permissive rules
gcloud logging read '
  resource.type="gce_subnetwork"
  AND jsonPayload.rule_details.reference:"/firewall-rules/default-allow-"
' --limit=100 --format="table(jsonPayload.connection.src_ip,jsonPayload.connection.dest_ip,jsonPayload.connection.dest_port)"

# Export flow logs to BigQuery for analysis
gcloud logging sinks create vpc-flow-bq \
  bigquery.googleapis.com/projects/PROJECT/datasets/vpc_flow_logs \
  --log-filter='resource.type="gce_subnetwork"'

Term	Definition
VPC Firewall Rule	Stateful network-level access control that allows or denies traffic to and from VM instances based on IP ranges, protocols, ports, and tags
Hierarchical Firewall Policy	Organization or folder-level firewall policy that is evaluated before VPC-level rules and applies across all child projects
Network Tag	Label applied to VM instances that determines which firewall rules apply, used for targeting ingress and egress rules
Service Account Firewall Rule	Firewall rule that targets instances based on their attached service account, providing more secure targeting than mutable network tags
VPC Flow Logs	Network telemetry captured at the subnet level that records traffic metadata for monitoring, forensics, and firewall rule validation
Implied Rules	Default GCP firewall rules that allow egress to all destinations and deny ingress from all sources, with lowest priority (65535)

GCP VPC Firewall Audit Report
================================
Project: production-project
VPC Network: production-vpc
Audit Date: 2026-02-23

RULE INVENTORY:
  Total firewall rules: 34
  Ingress rules: 22
  Egress rules: 12
  Disabled rules: 3

CRITICAL FINDINGS:
[FW-001] SSH Open to Internet
  Rule: default-allow-ssh
  Source: 0.0.0.0/0 -> tcp:22
  Target: All instances (no tags)
  Priority: 65534
  Remediation: Restrict to bastion subnet CIDR

[FW-002] No Egress Restrictions
  Issue: Only implied allow-all-egress rule exists
  Risk: No controls on outbound data exfiltration
  Remediation: Add default-deny egress and explicit allow rules

REMEDIATION ACTIONS COMPLETED:
  Rules deleted: 3 (overly permissive defaults)
  Rules created: 8 (targeted allow rules)
  Egress deny rule: Created at priority 65534
  Flow logs enabled: 6 subnets