How To Fix NLB Interference with TCP on AWS

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If you’re having trouble connecting to your Minecraft server through the Network Load Balancer (NLB), here are some steps you can take to troubleshoot and fix the issue.

First, understand how the NLB works with TCP connections. It makes a TCP handshake with clients and then forwards the traffic to your EC2 instance. Since your target group shows healthy and you can connect directly to the EC2, it seems the core networking is working fine.

Next, check these common issues:

Make sure the NLB preserves the client’s IP address. By default, it does, but double-check that your EC2 instance’s security group allows incoming traffic from clients’ IPs and not just from the NLB. Even if security groups are open, it’s good to confirm.

Verify that the routing on your EC2 instance is correct. The return traffic should be properly routed back to the clients. Since you can connect directly, this is probably okay but still worth a quick look.

Check your health check settings in the target group. Confirm that it tests the Minecraft service on port 25565, not just whether the instance is reachable.

If your NLB is set up across multiple Availability Zones, ensure that cross-zone load balancing is enabled. This can influence how the traffic is distributed.

Review the idle timeout settings of the NLB. Sometimes, connections can drop if they’re idle for too long. Minecraft connections may be affected if the timeout is too short or if data isn’t flowing regularly.

You might also need to fine-tune TCP keepalive settings on your EC2 instance. Proper keepalive settings help maintain connections through the load balancer.

Don’t forget to check your Network Access Control Lists (ACLs). Make sure they allow traffic on port 25565 for both inbound and outbound directions, including the ephemeral ports used for responses.

Since you can connect directly to the EC2 instance but not through the NLB, and tools like tcpdump don’t show incoming traffic, it suggests the traffic isn’t reaching your server. This points to a possible misconfiguration in the NLB or routing issues between the load balancer and your EC2 instance.

By carefully reviewing these aspects, you’ll likely be able to identify and resolve why the Minecraft connections aren’t working through the NLB.