Azure Firewall: Understanding TCP Ping Behavior and Effective Connectivity Testing

In the realm of cloud security, Azure Firewall Premium serves as a robust barrier, meticulously controlling both inbound and outbound traffic to safeguard your virtual environment. A common configuration practice is to block all outbound traffic by default, permitting only specific Fully Qualified Domain Names (FQDNs). However, this stringent setup can lead to perplexing scenarios, especially when standard connectivity tests yield misleading results.

In Azure's hub-and-spoke network architecture, the hub functions as a central point connecting multiple spoke virtual networks (VNets), facilitating efficient management and security of network traffic.
This design allows for centralized control over network traffic, making it essential to understand how to effectively test traffic flow within this topology.
Understanding Traffic Flow in Hub-and-Spoke Architecture
In this setup, the hub VNet typically hosts shared services such as Azure Firewall, VPN gateways, or other network virtual appliances (NVAs). Spoke VNets, which represent isolated workloads or applications, connect to the hub through virtual network peering. This configuration enables spokes to utilize shared resources in the hub while maintaining isolation from each other.
Challenges with Traditional Connectivity Testing Tools
When testing connectivity within this architecture, tools like Test-NetConnection or telnet may indicate successful connections even if the actual traffic is blocked by the firewall. This occurs because such tools perform a TCP handshake that the firewall can respond to directly, leading to false positives in connectivity tests.
Effective Traffic Testing Methods
To accurately assess traffic flow in a hub-and-spoke topology, consider the following approaches:

1. Use Application-Level Testing Tools: Employ tools like curl to initiate actual data requests to the target service. For example, running curl -v https://www.example.com attempts to establish a full connection, providing a more accurate representation of connectivity.
2. Implement Network Security Group (NSG) Flow Logs: Enable NSG flow logs to monitor and analyze traffic patterns within your VNets. This helps in identifying allowed and denied flows, offering insights into the effectiveness of your security rules.
3. Leverage Azure Monitor: Utilize Azure Monitor to set up alerts and analyze logs related to network traffic. This comprehensive monitoring aids in promptly identifying and addressing connectivity issues.

Conclusion
Effectively testing traffic within Azure's hub-and-spoke network architecture requires an understanding of how traditional tools interact with network security devices. By adopting application-level testing tools and leveraging Azure's monitoring capabilities, you can gain accurate insights into your network's connectivity, ensuring a secure and well-functioning environment.