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Plan for landing zone network segmentation

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This section explores key recommendations to deliver highly secure internal network segmentation within a landing zone to drive a network Zero Trust implementation.

Design considerations

  • The Zero Trust model assumes a breached state and verifies each request as though it originates from an uncontrolled network.

  • An advanced Zero Trust network implementation employs fully distributed ingress and egress cloud micro-perimeters and deeper micro-segmentation.

  • Network security groups (NSGs) can use Azure service tags to facilitate connectivity to Azure platform as a service (PaaS) solutions.

  • Application security groups (ASGs) don't span or provide protection across virtual networks.

  • Use NSG flow logs to inspect traffic that flows through a network point with an NSG attached.

  • Virtual network flow logs provide capabilities that are similar to NSG flow logs but cover a wider range of use cases. They also simplify the scope of traffic monitoring because you can enable logging at the virtual network level.

Design recommendations

  • Delegate subnet creation to the landing zone owner. This will enable them to define how to segment workloads across subnets (for example, a single large subnet, multitier application, or network-injected application). The platform team can use Azure Policy to ensure that an NSG with specific rules (such as deny inbound SSH or RDP from internet, or allow/block traffic across landing zones) is always associated with subnets that have deny-only policies.

  • Use NSGs to help protect traffic across subnets, as well as east/west traffic across the platform (traffic between landing zones).

  • The application team should use application security groups at the subnet-level NSGs to help protect multitier VMs within the landing zone.

    Diagram that shows how application security group works.

  • Use NSGs and application security groups to micro-segment traffic within the landing zone and avoid using a central NVA to filter traffic flows.

  • Enable virtual network flow logs and use traffic analytics to gain insights into ingress and egress traffic flows. Enable flow logs on all critical virtual networks and subnets in your subscriptions, for example virtual networks and subnets that contain Windows Server Active Directory domain controllers or critical data stores. Additionally, you can use flow logs to detect and investigate potential security incidents, compliance and monitoring, and to optimize usage.

  • Use NSGs to selectively allow connectivity between landing zones.

  • For Virtual WAN topologies, route traffic across landing zones via Azure Firewall if your organization requires filtering and logging capabilities for traffic flowing across landing zones.

  • If your organization decides to implement forced tunneling (advertise default route) to on-premises, we recommend incorporating the following outbound NSG rules to deny egress traffic from VNets directly to the internet should the BGP session drop.

Note

Rule priorities will need to be adjusted based on your existing NSG rule set.

Priority Name Source Destination Service Action Remark
100 AllowLocal Any VirtualNetwork Any Allow Allow traffic during normal operations. With forced tunneling enabled, 0.0.0.0/0 is considered part of the VirtualNetwork tag as long as BGP is advertising it to the ExpressRoute or VPN Gateway.
110 DenyInternet Any Internet Any Deny Deny traffic directly to the internet if the 0.0.0.0/0 route is withdrawn from the routes advertised (for example, due to an outage or misconfiguration).

Caution

Azure PaaS services that can be injected into a virtual network maybe not compatible with forced tunneling. Control plane operations may still require direct connectivity to specific public IP addresses for the service to operate correctly. It's recommended to check the specific service documentation for networking requirements and eventually exempt the service subnet from the default route propagation. Service Tags in UDR can be used to bypass default route and redirect control plane traffic only, if the specific service tag is available.