Docker: able to telnet to remote machines from host but not from container

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We have a couple docker containers deployed on ECS. The application inside the container uses remote service, so it needs to access them using their 10.X.X.X private IPs.

We are using Docker 1.13 with CentOS 7 and docker/alpine as our base image. We are also using netwokMode: host for our containers. The problem comes when we can successfully run telnet 10.X.X.X 9999 from the host machine but if we run the same command from inside the container, it just hangs and it's not able to connect.

In addition, we have net.ipv4.ip_forward enabled in the host machines (where the container runs) but disabled in the remote machine.

Not sure what could be the issue, maybe iptables?

amazon-ecs docker networking
1 Answers

I have spent the day with the same problem (tried with both network mode 'bridge' and 'host'), and it looks like an issue with using busybox's telnet inside ECS - Alpine's telnet is a symlink to busybox. I don't know enough about busybox/networking to suggest what the root cause is, but I was able to prove the network path was clear by using other tools.

My 'go to' for testing a network path is using netcat as follows. The 'success' or 'failure' message varies from version to version, but a refusal or a timeout (-w#) is pretty obvious. All netcat does here is request a socket - it doesn't actually talk to the listening application, so you need something else to test that.

nc -vz -w2 HOST PORT

My problem today was troubleshooting an app's mongo connection. nc showed the path was clear, but telnet had the same issue as you reported. I ended up installing the mongo client and checking with that, and I could connect properly.

If you need to actually run commands over telnet from inside your ECS container, perhaps try installing a different telnet tool and avoiding the busybox inbuilt one.

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