SSH on a DCOS+Marathon cluster with Docker

I just wanted to test some latencies from a DCOS cluster.

The first suggestion to run some commands in an existing container comes from DCOS documentation and is dcos task exec. But it only works for containers launched using the Universal Container Runtime. The second suggestion, if your nodes run docker, is to SSH into the nodes and then docker exec a shell in a container to do some tests. Although this means you need to have access to the nodes themselves (private key/password), something a sysadmin won't give you without a very good reason.

El cheapo attempt.

If you have access to the dcos cluster you can always start a new app using the DCOS cli tool. An app with only one container with ssh, curl, and your public ssh key in authorized_keys.

FROM alpine:latest

RUN apk --no-cache add openssh curl && \
    ssh-keygen -A

COPY authorized_keys /root/.ssh/authorized_keys

ENTRYPOINT ["/usr/sbin/sshd", "-De"]

Publish it somewhere on the public internet (I used the public docker registry at hub.docker.com). And prepare a new task file (task.json) to launch it. This task has the id staging-authentication-latencies that we will use later.

{
    "id": "staging-authentication-latencies",
    "container": {
        "type": "DOCKER",
        "docker": {
        "image": "docker.io/graffic/ssh:latest",
        "network": "BRIDGE",
        "portMappings": [
                {
                    "hostPort": 22022,
                    "containerPort": 22,
                    "protocol": "tcp"
                }
        ],
        "privileged": false,
        "force_pull_image": true
        }
    },
    "instances": 1,
    "cpus": 0.5,
    "mem": 512
}

Now we can:

• Start the new app with dcos marathon app add task.json
• See how the deploy is going: dcos marathon app show staging-authentication-latencies

Find the task entry and get the host IP. You will need that to connect via ssh:

{
    "tasks": [
        {
        "appId": "/staging-authentication-latencies",
        "host": "X.X.X"
        }
    ]
}

• SSH into the container: ssh -p 22022 root@host_you_got_from_app_show and run your tests from there.
• Remove the app when you're done: dcos marathon app remove staging-authentication-latencies