I have created a four subnet VPC, where two subnets are private and two public. Instances in the private subnet has only private IP's and reach the the internet through an IGW/NAT GW.

I can successfully create an ECS clusters spanning my public subnets, but when I create a cluster in the backend subnets, the cluster instances are unable to register in the cluster.

I suspect network issues, and I am debugging with Flowlogs. In the mean time, I was hoping someone could shed some light over what ports that need to be open, or other configuration involved in ECS cluster management.

  • I don't know much about ECS. Try opening all ports / routes to your public subnet, if it makes a difference that could be interesting and useful diagnosis information. – Tim Feb 13 '17 at 8:01
  • My analysis so far shows a lot of blocked UDP traffic from cluster instances to hosts that are not a part of my infra. – Glenn Bech Feb 13 '17 at 9:13

I suspect this is port related, as it's fairly standard that things in a cluster need to communicate. The two articles below should answer this question for you. It seems to me the following are probably required

  • Amazon ECS agent ports 51678 and 51679 (protocol unspecified)
  • TCP 2376 and 2377 (docker)
  • TCP / UDP 7946 (docker)
  • UDP 4789 (docker)
  • Ephemeral ports 49153 to 65535 (protocol unspecified)

Note that I know little about ECS and have simply done a couple of Google searches and read documentation. ECS is based on Docker so I looked at that. Some experimentation will be required.

Digital Ocean has a good article on Docker Ports.

TCP port 2376 for secure Docker client communication. This port is required for Docker Machine to work. Docker Machine is used to orchestrate Docker hosts. TCP port 2377. This port is used for communication between the nodes of a Docker Swarm or cluster. It only needs to be opened on manager nodes. TCP and UDP port 7946 for communication among nodes (container network discovery). UDP port 4789 for overlay network traffic (container ingress networking).

Then the Amazon documentation mentions some of the same ports.

The default ephemeral port range is 49153 to 65535, and this range is used for Docker versions prior to 1.6.0. For Docker version 1.6.0 and later, the Docker daemon tries to read the ephemeral port range from /proc/sys/net/ipv4/ip_local_port_range; if this kernel parameter is unavailable, the default ephemeral port range is used. You should not attempt to specify a host port in the ephemeral port range, because these are reserved for automatic assignment. In general, ports below 32768 are outside of the ephemeral port range.

The default reserved ports are 22 for SSH, the Docker ports 2375 and 2376, and the Amazon ECS container agent ports 51678 and 51679. Any host port that was previously specified in a running task is also reserved while the task is running (after a task stops, the host port is released).The current reserved ports are displayed in the remainingResources of DescribeContainerInstances output, and a container instance may have up to 100 reserved ports at a time, including the default reserved ports (automatically assigned ports do not count toward the 100 reserved ports limit).

up vote 1 down vote accepted

It's important to remember that the Network ACL for the subnet that contains the NAT gateway needs a rule for inbound SSL (443). If not, no outbound SSL can be done from the private subnet. Network traffic from my private subnet to http://cloudformation.eu-central-1.amazonaws.com/ was blocked on the subnet level.

This made the shell command, running on the back-end instances, cfn-signal to block. The cfn-signal command's duty is to report back to Cloud formation that an instance is up and running, so that the script may continue to create dependent resources.

I thought of this as an ECS problem, not a Cloudformation problem.

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