AWS-ECS - Communication between containers - Unknown host error

Question

I have two Docker containers.

TestWeb (Expose: 80)
TestAPI (Expose: 80)

Testweb container calls TestApi container. Host can communicate with TestWeb container from port 8080. Host can communicate with TestApi using 8081.

I can get TestWeb to call TestApi in my dev box (Windows 10) but when I deploy the code to AWS (ECS) I get "unknown host" exception. Both the containers work just fine and I can call them individually. But when I call a method that internally makes a Rest call using HttpClient to a method in Container2, it gives the error:

An error occurred while sending the request. ---> System.Net.Http.CurlException: Couldn't resolve host name.

Code:

using (var client = new HttpClient())
        {
            try
            {
                string url = "http://testapi/api/Tenant/?i=" + id;
                var response = client.GetAsync(url).Result;
                if (response.IsSuccessStatusCode)
                {
                    var responseContent = response.Content;
                    string responseString = responseContent.ReadAsStringAsync().Result;
                    return responseString;
                }
                return response.StatusCode.ToString();
            }
            catch (HttpRequestException httpRequestException)
            {
                return httpRequestException.Message;
            }
}

The following are the things I have tried:

The two containers (TestWeb, TestAPI) are in the same Task definition in AWS ECS. When I inspect the containers, I get the IP address of each of the containers. I can ping container2 from container1 with their IP address. But I can't ping using container2 name. It gives me "unknown host" error.

Elijah Lynn Elijah Lynn · Accepted Answer · 2019-02-13T04:30:26

It appears ECS doesn't use legit docker-compose under the hood, however, their implementation does support the Compose V2 "links" feature.

Here is a portion of my compose file I just ran on ECS that needed this same functionality AND had the same "could not resolve host" error you were getting. The "links" I added fixed my hostname resolution issue on Elastic Container Service!

version: '3'
services:
  appserver:
    links:
      - database:database
      - socks-proxy:socks-proxy

This allowed my appserver to communicate TO the database and socks-proxy hostnames. The format is "SERVICE:ALIAS" and it is fine to keep them both the same as a default practice.

In your example it would be:

version: '3'
services:
  testapi:
    links:
      - testweb:testweb
  testweb:
    links:
      - testapi:testapi

AWS-ECS - Communication between containers - Unknown host error

2 Answers