Troubleshooting communication between containers and the external network

Packets can only pass between containers if the ip_forward parameter is set to 1. Usually, you will simply leave the Docker server at its default setting of --ip-forward=true and Docker will set ip_forward to 1 for you when the server starts up. To check the settings, use the following command:

    # cat /proc/sys/net/ipv4/ip_forward
0  
# echo 1 > /proc/sys/net/ipv4/ip_forward 
# cat /proc/sys/net/ipv4/ip_forward 
1

By enabling ip-forward, users can make communication between containers and the external world possible; it will also be needed for inter-container communication if you are in a multiple bridge setup:

ip-forward = true forwards all the packets to/from the container to the external network

Docker will not delete or modify any pre-existing rules from Docker filter chain. This allows users to create rules to restrict access to containers. Docker uses docker0 bridge for packet flow between all containers...