Azure - User Defined Routes

Introduction

• User Defined Routes (UDRs) in Azure are custom routing rules that allow you to control the traffic flow within your Azure Virtual Network.
• They enable you to override the default system routes provided by Azure, giving you the flexibility to direct network traffic according to specific needs.

Key Features of User Defined Routes

1. Customization of Traffic Flow:
   • UDRs enable you to dictate how traffic is routed within your virtual network.
   • Useful for directing traffic through network appliances like firewalls, network virtual appliances (NVAs), or VPN gateways for processing.
2. Components of UDRs:
   • Route Table: A collection of custom routes you define.
   • Destination: The address prefix of the destination network, specified in CIDR notation (e.g., 10.0.0.0/16).
   • Next Hop Type: Specifies the next hop for the traffic, such as:
   • Virtual Appliance: Directs traffic to a specific IP address, typically an NVA or firewall.
   • Virtual Network Gateway: Routes traffic to an on-premises network through a VPN gateway.
   • Internet: Sends traffic to the Internet.
   • None: Drops the traffic.
   • VNet Peering: Routes traffic to a peered virtual network.
3. Route Table Association:
   • Route tables can be associated with one or more subnets within a virtual network.
   • This association ensures that the routing rules apply to the traffic within those subnets.

Use Cases for User Defined Routes

1. Forced Tunneling:
   • Ensuring that all internet-bound traffic from a subnet goes through a security appliance for inspection and control.
2. Traffic Isolation:
   • Segregating traffic between different subnets or virtual networks for enhanced security and management.
3. Network Security:
   • Directing traffic through network security appliances to implement custom security policies.
4. Optimized Routing:
   • Optimizing routing paths for better performance or to reduce costs by controlling how traffic flows within your network.

Precedence and Application

• Precedence Over System Routes: UDRs take precedence over default system routes provided by Azure. If there is a conflict between a UDR and a system route, the UDR will be used.
• Flexible Configuration: UDRs allow for complex network configurations, catering to specific business needs and security requirements.

Example Scenario

• Imagine you have a subnet within your virtual network where you want all outbound traffic to pass through a firewall for security inspection.
• You can create a route table with a UDR that directs all traffic from that subnet to the firewall’s IP address as the next hop.
• Then, you associate this route table with the subnet, ensuring all traffic follows the defined route.