ECS dns lookups of containers

Question

ECS doesn't appear to provide names for containers when using bridge networking.  I've created a task with a frontend and a database container.  The database container's name is "database".  I would expect the frontend to be able to connect to it by name.  Why would this not work?  It works in docker, and should therefore work in ECS, but it doesn't.

Accepted Answer

If you run multiple containers in the same task that uses bridge networking, you can reference one container from the other by name by configuring the appropriate `links` in the container definition.

For example, if you have a Task Definition with a container named `main` and another called `sidecar`, you can publish the IP address of `sidecar` in the `/etc/hosts` file of the main container like so:

```
        "containerDefinitions": [
            {
                "name": "main",
                "links": [
                    "sidecar"
                ],
                ...
            },
            {
                "name": "sidecar",
                ...
            }
         ]
```

In the `main` container, you will see the IP published:

```
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
172.17.0.4      sidecar sidecar ecs-EcsBridgeContainerNameTestStackTaskDefBF06B5D6-6-sidecar-c2ad99be83f489e00c00
172.17.0.5      main
```

See also the [ECS Task Definition documentation](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html) under "Network Settings".

Answer

Hello,

service discovery is not enabled by default, but you can configure it (it relies on Cloud Map internally). You can read more here:
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service-discovery.html

Answer

If you are using docker-compose locally, I would highly recommend to checkout [ECS Compose-X](https://docs.compose-x.io) (worth looking into the [use-cases documented here](https://labs.compose-x.io)).
It will deal with IAM / Security groups etc between your ECS services and your other AWS resources.
For service to service communication, you can link which service talks to another etc. You can automatically get the services registered in AWS CloudMap for service discovery, and it also has AppMesh support if you want to use service mesh on top of it for extra security/traceability etc.
Now if you don't use compose syntax you can look into Copilot, although the supported use-cases is not as deep, but it does the job.

EDIT: not strictly an answer. More of a recommendation in case that were useful to have some ECS "specialized" tools to deal with your services configuration and deployment.

ECS dns lookups of containers

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