11.1. Network Security Groups (NSGs)
Network Security Groups (NSGs) are a fundamental aspect of network security in Azure’s virtual network infrastructure. They act as a firewall for virtual networks, providing a critical layer of security that controls the traffic to and from network interfaces (NIC), virtual machine (VM) instances, and subnets. NSGs operate by defining a set of security rules that allow or deny network traffic to resources connected to Azure Virtual Networks (VNet). Each rule in an NSG controls traffic based on protocol, direction, source address, destination address, source port, and destination port. By effectively managing these rules and applying NSGs to the appropriate resources, organizations can ensure a granular level of control over their network traffic, which is essential for securing their cloud environment.
Understanding Network Security Groups
An NSG contains multiple security rules, much like a traditional network firewall’s access control list (ACL). Each rule in the NSG defines how inbound or outbound traffic is handled. These rules are processed in ascending priority order, starting from the lowest number (highest priority) to the highest number (lowest priority). The priority system ensures that when traffic is evaluated against the rules in an NSG, the first rule that applies to that type of traffic (as determined by the 5-tuple match) is applied, and subsequent rules are ignored.
NSGs are stateful, meaning that if a connection from a virtual machine to another resource is allowed, the return traffic is automatically allowed, regardless of inbound security rules. Stateful filtering simplifies security architecture as it negates the need for separate rules for return traffic.
Security Rules Explained
Each security rule within an NSG consists of the following properties:
● Direction: Whether the rule applies to inbound or outbound traffic.
● Protocol: The protocol that the rule applies to (TCP, UDP, or Any).
● Source and Destination Port Range: The range of ports that the rule applies to.
● Source and Destination Address Prefixes: The range of source or destination IP addresses that the rule applies to, which can include CIDR blocks, a single IP, or service tags.
● Action: The action that takes place when the rule conditions are met (Allow or Deny).
NSG security rules can be as generic or as specific as required. For instance, one can create a rule to allow all outbound HTTP traffic or a rule that only allows SSH traffic from a specific IP address to a specific virtual machine.
Best Practices for Implementing NSGs
In implementing NSGs, the following best practices should be observed:
● Least Privilege Principle: Only the necessary traffic required for the operation should be allowed; all other traffic should be denied.
● Segregation of Duties: Separate NSGs should be created for different types of traffic or different VM roles to provide granular control and minimize risk.
● Logging and Monitoring: All NSGs should be integrated with Azure Monitor and Azure Log Analytics to record traffic patterns and detect anomalies.
● Rule Clarity: The naming convention for NSG rules should be clear and descriptive to make it easier to understand and manage the rules.
● Rule Prioritization: Properly prioritize rules to ensure traffic is allowed or denied in the correct order.
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