Part 4: ECS Networking Pitfalls That Can Take Your Service Down

This is Part 4 of 5 in my series on keeping ECS deployments rock-solid — covering best practices, hidden pitfalls, and the sneaky issues that cause downtime.

Sometimes your ECS service isn’t broken — it’s just stranded on the wrong network.
I’ve had deployments where everything “looked” fine in ECS, but the app couldn’t reach the internet, the ALB couldn’t reach the tasks, or database connections failed — all because of networking misconfigurations.

Here are 3 networking mistakes I’ve run into (and fixed) when working with ECS.

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1️⃣ Wrong Subnet Placement

The Problem:
If you place your ECS tasks in public subnets without proper security controls, you risk exposure.
On the flip side, if they’re in private subnets without a NAT gateway, they might not reach the internet for updates or API calls.

Fix:

• For internet-facing apps, put the ALB in public subnets and ECS tasks in private subnets.

• Add a NAT gateway so tasks in private subnets can make outbound calls.

• Double-check subnet associations in the ECS service configuration.

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2️⃣ Security Group Over-Lockdown

The Problem:
Security groups are great for locking things down, but too much lockdown can block even legitimate traffic.
I’ve seen apps fail because the database’s security group didn’t allow inbound from ECS, or the ALB couldn’t connect to the container port.

Fix:

• Allow ECS tasks to connect to any required databases, caches, or APIs via the right ports.

• Make sure the ALB security group is allowed inbound to the ECS task security group on the container port.

• Use security group IDs instead of IP ranges for cleaner, more secure rules.

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3️⃣ Missing Route Table Entries

The Problem:
If the route tables for your subnets aren’t set up correctly, requests can’t reach the ALB — or the tasks can’t talk back.
This can silently break your service without obvious error messages in ECS.

Fix:

• Public subnets should have a route to the internet gateway.

• Private subnets that need outbound internet should have a route to the NAT gateway.

• Use VPC flow logs to debug dropped traffic.

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💡 Bonus Tip:
If your ECS service depends on external APIs, always test that outbound calls work in staging before going live.
Networking issues often show up only when you try to integrate with the outside world.

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Final Thought

ECS services live and die by their network configuration.
Get subnets, security groups, and route tables right — and you’ll save yourself from some of the most frustrating “it works here, but not there” problems in AWS.