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How network security groups filter network traffic

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You can use an Azure network security group (NSG) to filter network traffic to and from Azure resources in an Azure virtual network. A network security group contains security rules that allow or deny inbound network traffic to, or outbound network traffic from, several types of Azure resources. For each rule, you can specify source and destination, port, and protocol.

You can deploy resources from several Azure services into an Azure virtual network. For a complete list, see Services that can be deployed into a virtual network. You can associate zero, or one, network security group to each virtual network subnet and network interface in a virtual machine. The same network security group can be associated to as many subnets and network interfaces as you choose.

The following picture illustrates different scenarios for how network security groups might be deployed to allow network traffic to and from the internet over TCP port 80:

Diagram of NSG processing.

Reference the previous picture, along with the following text, to understand how Azure processes inbound and outbound rules for network security groups:

Inbound traffic

For inbound traffic, Azure processes the rules in a network security group associated to a subnet first, if there's one, and then the rules in a network security group associated to the network interface, if there's one. This process includes intra-subnet traffic as well.

  • VM1: The security rules in NSG1 are processed, since it's associated to Subnet1 and VM1 is in Subnet1. Unless you've created a rule that allows port 80 inbound, the DenyAllInbound default security rule denies the traffic. This blocked traffic then doesn't get evaluated by NSG2 because it's associated with the network interface. However if NSG1 allows port 80 in its security rule, then NSG2 processes the traffic. To allow port 80 to the virtual machine, both NSG1 and NSG2 must have a rule that allows port 80 from the internet.

  • VM2: The rules in NSG1 are processed because VM2 is also in Subnet1. Since VM2 doesn't have a network security group associated to its network interface, it receives all traffic allowed through NSG1 or is denied all traffic denied by NSG1. Traffic is either allowed or denied to all resources in the same subnet when a network security group is associated to a subnet.

  • VM3: Since there's no network security group associated to Subnet2, traffic is allowed into the subnet and processed by NSG2, because NSG2 is associated to the network interface attached to VM3.

  • VM4: Traffic is blocked to VM4, because a network security group isn't associated to Subnet3, or the network interface in the virtual machine. All network traffic is blocked through a subnet and network interface if they don't have a network security group associated to them.

Outbound traffic

For outbound traffic, Azure processes the rules in a network security group associated to a network interface first, if there's one, and then the rules in a network security group associated to the subnet, if there's one. This process includes intra-subnet traffic as well.

  • VM1: The security rules in NSG2 are processed. The AllowInternetOutbound default security rule in both NSG1 and NSG2 allows the traffic unless you create a security rule that denies port 80 outbound to the internet. If NSG2 denies port 80 in its security rule, it denies the traffic, and NSG1 never evaluates it. To deny port 80 from the virtual machine, either, or both of the network security groups must have a rule that denies port 80 to the internet.

  • VM2: All traffic is sent through the network interface to the subnet, since the network interface attached to VM2 doesn't have a network security group associated to it. The rules in NSG1 are processed.

  • VM3: If NSG2 denies port 80 in its security rule, it denies the traffic. If NSG2 doesn't deny port 80, the AllowInternetOutbound default security rule in NSG2 allows the traffic because there's no network security group associated with Subnet2.

  • VM4: All network traffic is allowed from VM4, because a network security group isn't associated to the network interface attached to the virtual machine, or to Subnet3.

Intra-Subnet traffic

It's important to note that security rules in an NSG associated to a subnet can affect connectivity between VMs within it. By default, virtual machines in the same subnet can communicate based on a default NSG rule allowing intra-subnet traffic. If you add a rule to NSG1 that denies all inbound and outbound traffic, VM1 and VM2 won't be able to communicate with each other.

You can easily view the aggregate rules applied to a network interface by viewing the effective security rules for a network interface. You can also use the IP flow verify capability in Azure Network Watcher to determine whether communication is allowed to or from a network interface. You can use IP flow verify to determine whether a communication is allowed or denied. Additionally, Use IP flow verify to surface the identity of the network security rule responsible for allowing or denying the traffic.

Note

Network security groups are associated to subnets or to virtual machines and cloud services deployed in the classic deployment model, and to subnets or network interfaces in the Resource Manager deployment model. To learn more about Azure deployment models, see Understand Azure deployment models.

Tip

Unless you have a specific reason to, we recommend that you associate a network security group to a subnet, or a network interface, but not both. Since rules in a network security group associated to a subnet can conflict with rules in a network security group associated to a network interface, you can have unexpected communication problems that require troubleshooting.

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