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Network security groups

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You can use an Azure network security group to filter network traffic between 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.

This article describes the properties of a network security group rule, the default security rules that are applied, and the rule properties that you can modify to create an augmented security rule.

Security rules

A network security group contains as many rules as desired, within Azure subscription limits. Each rule specifies the following properties:

Property Explanation
Name A unique name within the network security group. The name can be up to 80 characters long. It must begin with a word character, and it must end with a word character or with '_'. The name may contain word characters or '.', '-', '_'.
Priority A number between 100 and 4096. Rules are processed in priority order, with lower numbers processed before higher numbers, because lower numbers have higher priority. Once traffic matches a rule, processing stops. As a result, any rules that exist with lower priorities (higher numbers) that have the same attributes as rules with higher priorities aren't processed.
Azure default security rules are given the highest number with the lowest priority to ensure that custom rules are always processed first.
Source or destination Any, or an individual IP address, classless inter-domain routing (CIDR) block (10.0.0.0/24, for example), service tag, or application security group. If you specify an address for an Azure resource, specify the private IP address assigned to the resource. Network security groups are processed after Azure translates a public IP address to a private IP address for inbound traffic, and before Azure translates a private IP address to a public IP address for outbound traffic. Fewer security rules are needed when you specify a range, a service tag, or application security group. The ability to specify multiple individual IP addresses and ranges (you can't specify multiple service tags or application groups) in a rule is referred to as augmented security rules. Augmented security rules can only be created in network security groups created through the Resource Manager deployment model. You can't specify multiple IP addresses and IP address ranges in network security groups created through the classic deployment model.
If source points to the subnet 10.0.1.0/24 (where VM1 is located) and destination points to the subnet 10.0.2.0/24 (where VM2 is located), this indicates the purpose of NSG is to filter network traffic for VM2 and the NSG is associated with the network interface of VM2.
Protocol TCP, UDP, ICMP, ESP, AH, or Any. The ESP and AH protocols aren't currently available via the Azure portal but can be used via ARM templates.
Direction Whether the rule applies to inbound, or outbound traffic.
Port range You can specify an individual or range of ports. For example, you could specify 80 or 10000-10005. Specifying ranges enables you to create fewer security rules. Augmented security rules can only be created in network security groups created through the Resource Manager deployment model. You can't specify multiple ports or port ranges in the same security rule in network security groups created through the classic deployment model.
Action Allow or deny

Security rules are evaluated and applied based on the five-tuple (source, source port, destination, destination port, and protocol) information. You can't create two security rules with the same priority and direction. A flow record is created for existing connections. Communication is allowed or denied based on the connection state of the flow record. The flow record allows a network security group to be stateful. If you specify an outbound security rule to any address over port 80, for example, it's not necessary to specify an inbound security rule for the response to the outbound traffic. You only need to specify an inbound security rule if communication is initiated externally. The opposite is also true. If inbound traffic is allowed over a port, it's not necessary to specify an outbound security rule to respond to traffic over the port.

Existing connections may not be interrupted when you remove a security rule that allowed the connection. Modifying network security group rules will only affect new connections. When a new rule is created or an existing rule is updated in a network security group, it will only apply to new connections. Existing connections are not reevaluated with the new rules.

There are limits to the number of security rules you can create in a network security group. For details, see Azure limits.

Default security rules

Azure creates the following default rules in each network security group that you create:

Inbound

AllowVNetInBound
Priority Source Source ports Destination Destination ports Protocol Access
65000 VirtualNetwork 0-65535 VirtualNetwork 0-65535 Any Allow
AllowAzureLoadBalancerInBound
Priority Source Source ports Destination Destination ports Protocol Access
65001 AzureLoadBalancer 0-65535 0.0.0.0/0 0-65535 Any Allow
DenyAllInbound
Priority Source Source ports Destination Destination ports Protocol Access
65500 0.0.0.0/0 0-65535 0.0.0.0/0 0-65535 Any Deny

Outbound

AllowVnetOutBound
Priority Source Source ports Destination Destination ports Protocol Access
65000 VirtualNetwork 0-65535 VirtualNetwork 0-65535 Any Allow
AllowInternetOutBound
Priority Source Source ports Destination Destination ports Protocol Access
65001 0.0.0.0/0 0-65535 Internet 0-65535 Any Allow
DenyAllOutBound
Priority Source Source ports Destination Destination ports Protocol Access
65500 0.0.0.0/0 0-65535 0.0.0.0/0 0-65535 Any Deny

In the Source and Destination columns, VirtualNetwork, AzureLoadBalancer, and Internet are service tags, rather than IP addresses. In the protocol column, Any encompasses TCP, UDP, and ICMP. When creating a rule, you can specify TCP, UDP, ICMP or Any. 0.0.0.0/0 in the Source and Destination columns represents all addresses. Clients like Azure portal, Azure CLI, or PowerShell can use * or any for this expression.

You can't remove the default rules, but you can override them by creating rules with higher priorities.

Augmented security rules

Augmented security rules simplify security definition for virtual networks, allowing you to define larger and complex network security policies, with fewer rules. You can combine multiple ports and multiple explicit IP addresses and ranges into a single, easily understood security rule. Use augmented rules in the source, destination, and port fields of a rule. To simplify maintenance of your security rule definition, combine augmented security rules with service tags or application security groups. There are limits to the number of addresses, ranges, and ports that you can specify in a rule. For details, see Azure limits.

Service tags

A service tag represents a group of IP address prefixes from a given Azure service. It helps to minimize the complexity of frequent updates on network security rules.

For more information, see Azure service tags. For an example on how to use the Storage service tag to restrict network access, see Restrict network access to PaaS resources.

Application security groups

Application security groups enable you to configure network security as a natural extension of an application's structure, allowing you to group virtual machines and define network security policies based on those groups. You can reuse your security policy at scale without manual maintenance of explicit IP addresses. To learn more, see Application security groups.

Azure platform considerations

  • Virtual IP of the host node: Basic infrastructure services like DHCP, DNS, IMDS, and health monitoring are provided through the virtualized host IP addresses 168.63.129.16 and 169.254.169.254. These IP addresses belong to Microsoft and are the only virtualized IP addresses used in all regions for this purpose. By default, these services aren't subject to the configured network security groups unless targeted by service tags specific to each service. To override this basic infrastructure communication, you can create a security rule to deny traffic by using the following service tags on your Network Security Group rules: AzurePlatformDNS, AzurePlatformIMDS, AzurePlatformLKM. Learn how to diagnose network traffic filtering and diagnose network routing.

  • Licensing (Key Management Service): Windows images running in virtual machines must be licensed. To ensure licensing, a request is sent to the Key Management Service host servers that handle such queries. The request is made outbound through port 1688. For deployments using default route 0.0.0.0/0 configuration, this platform rule will be disabled.

  • Virtual machines in load-balanced pools: The source port and address range applied are from the originating computer, not the load balancer. The destination port and address range are for the destination computer, not the load balancer.

  • Azure service instances: Instances of several Azure services, such as HDInsight, Application Service Environments, and Virtual Machine Scale Sets are deployed in virtual network subnets. For a complete list of services you can deploy into virtual networks, see Virtual network for Azure services. Before applying a network security group to the subnet, familiarize yourself with the port requirements for each service. If you deny ports required by the service, the service won't function properly.

  • Sending outbound email: Microsoft recommends that you utilize authenticated SMTP relay services (typically connected via TCP port 587, but often others, as well) to send email from Azure Virtual Machines. SMTP relay services specialize in sender reputation, to minimize the possibility that third-party email providers reject messages. Such SMTP relay services include, but aren't limited to, Exchange Online Protection and SendGrid. Use of SMTP relay services is in no way restricted in Azure, regardless of your subscription type.

    If you created your Azure subscription prior to November 15, 2017, in addition to being able to use SMTP relay services, you can send email directly over TCP port 25. If you created your subscription after November 15, 2017, you may not be able to send email directly over port 25. The behavior of outbound communication over port 25 depends on the type of subscription you have, as follows:

    • Enterprise Agreement: For VMs that are deployed in standard Enterprise Agreement subscriptions, the outbound SMTP connections on TCP port 25 won't be blocked. However, there's no guarantee that external domains will accept the incoming emails from the VMs. If your emails are rejected or filtered by the external domains, you should contact the email service providers of the external domains to resolve the problems. These problems aren't covered by Azure support.

      For Enterprise Dev/Test subscriptions, port 25 is blocked by default. It's possible to have this block removed. To request to have the block removed, go to the Can't send email (SMTP-Port 25) section of the Diagnose and Solve settings page for the Azure Virtual Network resource in the Azure portal and run the diagnostic. This will exempt the qualified enterprise dev/test subscriptions automatically.

      After the subscription is exempted from this block and the VMs are stopped and restarted, all VMs in that subscription are exempted going forward. The exemption applies only to the subscription requested and only to VM traffic that is routed directly to the internet.

    • Pay-as-you-go: Outbound port 25 communication is blocked from all resources. No requests to remove the restriction can be made, because requests aren't granted. If you need to send email from your virtual machine, you have to use an SMTP relay service.

    • MSDN, Azure Pass, Azure in Open, Education, and Free trial: Outbound port 25 communication is blocked from all resources. No requests to remove the restriction can be made, because requests aren't granted. If you need to send email from your virtual machine, you have to use an SMTP relay service.

    • Cloud service provider: Outbound port 25 communication is blocked from all resources. No requests to remove the restriction can be made, because requests aren't granted. If you need to send email from your virtual machine, you have to use an SMTP relay service.

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