We’re running payment processing services on Compute Engine instances and suddenly our outbound API calls to our external payment gateway (Stripe) started failing. The VPC firewall configuration appears correct with an egress rule allowing all outbound traffic, but connections time out at the API call stage.
Our payment gateway integration requires HTTPS calls to api.stripe.com on port 443. We’ve verified the payment gateway endpoint is accessible from outside GCP. The firewall rule has priority 1000 with target tags matching our VM instances.
gcloud compute firewall-rules describe allow-payment-egress
allowed:
- IPProtocol: tcp
ports: ['443']
direction: EGRESS
Payment processing is completely blocked and affecting customer transactions. Any insights on what might be blocking outbound API connectivity despite the firewall rule?
Found it! There’s a priority 900 deny rule for egress that was recently added by our security team. It’s blocking all egress except to specific IP ranges. The allow rule at 1000 never gets evaluated. Need to either adjust the deny rule or create a more specific allow rule with higher priority.
Check if your VPC has Private Google Access enabled and whether you’re trying to route through Cloud NAT. Also verify the network tags are actually applied to your Compute Engine instances - the firewall rule won’t apply if tags don’t match.
Here’s the complete solution addressing your VPC firewall configuration, outbound API connectivity, and payment gateway integration:
Root Cause: Your priority 900 deny-all-egress rule is blocking traffic before your priority 1000 allow rule can be evaluated. GCP firewall rules are processed in priority order (lowest number first), and the first matching rule determines the action.
Solution - VPC Firewall Configuration:
Create a higher priority allow rule specifically for payment gateway traffic:
gcloud compute firewall-rules create allow-stripe-payment-api \
--direction=EGRESS \
--priority=850 \
--network=your-vpc-name \
--action=ALLOW \
--rules=tcp:443 \
--destination-ranges=54.187.174.169/32,54.187.205.235/32,54.187.216.72/32 \
--target-tags=payment-processor \
--enable-logging
Outbound API Connectivity Best Practices:
- Use specific destination IP ranges from Stripe’s published list rather than 0.0.0.0/0
- Set priority between 800-890 to ensure it’s evaluated before the deny rule
- Apply target tags to limit scope to only payment processing instances
- Enable logging initially to verify traffic patterns
Payment Gateway Integration Security:
- Maintain the deny-all-egress rule for defense in depth
- Document the allow rule exception in your security runbook
- Set up Cloud Monitoring alerts for firewall rule changes
- Consider using VPC Service Controls perimeter for additional API access control
- Implement application-level API key rotation and monitoring
Verification Steps:
- Confirm rule priority: `gcloud compute firewall-rules list --sort-by=priority --filter=“network:your-vpc-name”
- Test API connectivity from instance: `curl -v https://api.stripe.com/v1/charges
- Review firewall logs: Check Cloud Logging for connection disposition
- Monitor for 24 hours to ensure payment transactions complete successfully
Additional Recommendations:
Implement Cloud NAT for instances without external IPs to centralize egress IP management and simplify firewall rules. This also provides better security by removing direct external IP exposure while maintaining outbound connectivity.
The key lesson here is understanding GCP firewall rule precedence - always check for conflicting rules across the entire priority spectrum when troubleshooting connectivity issues.
Thanks Mike. Verified the network tags are correctly applied to instances. We’re not using Cloud NAT currently - instances have external IPs. Private Google Access is enabled on the subnet. Could there be an implicit deny rule that takes precedence? The firewall logs show allowed connections but API calls still timeout.