# What is the NLB health check and how does it work?

Lily Armstrong · August 4, 2026

> The NLB health check is designed to determine the availability and responsiveness of backend servers by sending periodic requests to each registered...

The NLB health check is designed to determine the availability and responsiveness of backend servers by sending periodic requests to each registered target.

An NLB (Network Load Balancer) conducts health checks using TCP, HTTP, or HTTPS protocols, depending on the configuration settings of the load balancer.

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In TCP health checks, the NLB attempts to establish a TCP connection on the specified port.

If it fails to establish this connection within a designated timeout, the target is marked as unhealthy.

When configured with HTTP or HTTPS health checks, the health check requests utilize the Host header containing the IP address of the load balancer node instead of the target's IP address.

The frequency of health checks is controlled by the HealthCheckIntervalSeconds setting, where the NLB sends out health check requests at set intervals.

An important aspect of NLB health checks is that they employ a distributed health-checking mechanism that uses a consensus approach to determine target health, often resulting in multiple health check requests being sent to individual targets.

If a target group contains at least one healthy target, the health check for that target group passes, allowing traffic to be sent to that group.

Health checks are crucial for dynamic environments where backend instances might frequently scale in or out, and they ensure that clients are only routed to healthy instances.

The NLB supports TLS listener health checks which perform a connectivity test to ensure that TLS termination is working properly; if the health check fails, it indicates problems in the secure communication setup.

If the NLB health check fails when interfacing with an Application Load Balancer (ALB) target group, the ALB might still consider the instance healthy based on its own health check criteria, which can lead to discrepancies in perceived health.

Unlike TCP health checks that assess connectivity, HTTP(S) checks can be more complex; they typically involve sending an HTTP GET request to a specified path and analyzing the response to determine health status.

AWS Route 53 can leverage the health check results from NLBs to automate DNS responses, only directing traffic to healthy endpoints based on the health check outcomes.

The loading and testing of an NLB health check includes considerations for network ACLs (Access Control Lists) that must allow return traffic to ensure that health check responses are received.

In some cases, load balancers such as Alibaba Cloud's NLB may utilize UDP probe packets for health checks, where a valid ICMP response from the backend server signifies that it is healthy.

If health checks reveal that all targets in a target group are unhealthy, Route 53 will fail the alias record for that target group, impacting DNS resolution for clients seeking those services.

The scalability of NLB health checks can be vast since they can manage hundreds to thousands of targets without any performance degradation in health checking.

It's intriguing that NLB health checks can sometimes receive more requests than configured, which arises from its consensus mechanism ensuring accurate health assessments through redundancy.

The NLB mechanism results in different health monitoring strategies depending on traffic patterns; for instance, heavier traffic may prompt more frequent health checks to ensure that backend servers can cope.

Misconfigurations in security groups or network routes can lead to health check failures, commonly requiring engineers to trace packet flows to diagnose connectivity issues effectively.

The science behind health checks does not only involve straightforward connection attempts, but can also touch on complex networking protocols, load balancing algorithms, and cloud architecture principles that govern modern IT infrastructure design.

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