Docker Swarm overlay network - containers can’t communicate

When using Docker Swarm with overlay networks, if containers are unable to communicate with each other, there could be a few potential causes. Here are some troubleshooting steps you can follow:

1. Check network connectivity: Ensure that the containers can communicate with each other over the network. Verify that there are no network connectivity issues or firewall rules blocking the communication between containers within the overlay network.

2. Verify overlay network creation: Confirm that the overlay network is created correctly. Use the docker network ls command to list all the networks and ensure that the overlay network exists.

3. Check container placement: If you are using service deployments in Docker Swarm, check the placement constraints and ensure that the services are scheduled on nodes that are part of the same Swarm cluster and have network connectivity.

4. Inspect containers and networks: Use the docker inspect command to inspect the containers and overlay network. Check the IP addresses assigned to the containers and verify that they are in the same subnet as the overlay network.

   bashCopy code
   # Inspect a container
   
   docker inspect <container_name>
   
   
   
   # Inspect the overlay network
   
   docker network inspect <network_name>
   
   

   Ensure that the containers have the correct IP addresses assigned and that the overlay network configuration matches your expectations.

5. Check DNS resolution: If you are using service names or container names for communication between containers, ensure that DNS resolution is working correctly. Verify that you can resolve the service or container names to their IP addresses within the overlay network.

6. Restart containers: Try restarting the affected containers to see if that resolves the communication issue. Sometimes, restarting containers can help reestablish network connections and resolve transient issues.

7. Inspect container logs: Check the logs of the containers that are having trouble communicating. Look for any error messages or warnings related to network connectivity or service discovery.

8. Update Docker: Ensure that you are using the latest version of Docker and that all nodes in the Swarm cluster are running compatible versions. Update Docker to the latest stable release if necessary.

By following these troubleshooting steps, you can identify and resolve common issues that might prevent containers from communicating within a Docker Swarm overlay network.